Open Cluster M26 (also known as Messier Object 26 or NGC 6694) is an w:open cluster in the w:constellation w:Scutum. It was discovered by w:Charles Messier in w:1764.

M26 spans about 22 w:light years across and is at a distance of 5,000 light years from the w:Earth. The brightest star is of magnitude 11.9 and the age of this cluster has been calculated to be 89 million years. An interesting feature of M26 is a region of low w:star density near the nucleus, most likely caused by an obscuring cloud of w:interstellar matter between us and the cluster.

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The Dumbbell Nebula (also known as Messier 27, M 27, or NGC 6853) is a w:planetary nebula (PN) in the w:constellation w:Vulpecula, at a distance of about 1,360 w:light years.

This object was the first w:planetary nebula to be discovered; by w:Charles Messier in 1764. At its brightness of w:visual magnitude 7.5 and its diameter of about 8 w:arcminutes, it is easily visible in binoculars, and a popular observing target in amateur telescopes.


w:ESO image showing extended structure and central star
Credit: ESO

This PN appears to be shaped like an w:prolate spheroid and is viewed from our perspective along the plane of its w:equator. In 1992, Moreno-Corral et al. computed that the rate of expansion in the plane of the sky of this PN was no more than 2″.3 per century. From this, an upper limit to the age of 14,600 yr may be determined. In 1970, Bohuski, Smith, and Weedman found an expansion velocity of 31 km/s. Given its w:semi-minor axis w:radius of 1.01 ly, this implies that the kinematic age of the nebula is some 9,800 years.[4][5]


HST closeup of knots in M 27
Credit: C.R. O’Dell (Vanderbilt University)

Like many nearby planetary nebulae, the Dumbbell contains knots. Its central region is marked by a pattern of dark and bright cusped knots and their associated dark tails (see picture). The knots vary in appearance from symmetric objects with tails to rather irregular tail-less objects. Similarly to the w:Helix Nebula and the w:Eskimo Nebula, the heads of the knots have bright cusps which are local w:photoionization fronts.[5]

Central star

The central star, a w:white dwarf, is estimated to have a radius which is 0.055 ± 0.02 R which gives it a size larger than any other known white dwarf.[2] The central star mass was estimated in 1999 by Napiwotzki to be 0.56 ± 0.01 M.[2]


^a Radius = distance × sin(angular size / 2) = 1,240+180−140[3] * sin(8′.0 / 2) = 1.44+0.21−0.16 ly
^b Semi minor axis = distance × sin(minor axis size / 2) = 1,240+180−140[3] * sin(5′.6 / 2) = 1.01+0.15−0.11 ly
^c Kinematic age = semi-minor axis / expansion rate = 1.01+0.15−0.11b ly / 31 km/s = 9.56+1.42−1.04×1012 km / 31[4] km/s = 3.08+0.46−0.34×1011 s = 9,800+1,500−1,100 yr
^d 7.5 apparent magnitude – 5 * (log10(420+50−70 pc distance) – 1) = -0.6+0.4−0.3 absolute magnitude


  1. abcdefg “SIMBAD Astronomical Database”. Results for M 27. Retrieved 2007-01-03. 
  2. abc Benedict, G. Fritz; McArthur, B. E.; Fredrick, L. W.; Harrison, T. E.; Skrutskie, M. F.; Slesnick, C. L.; Rhee, J.; Patterson, R. J.; Nelan, E.; Jefferys, W. H.; van Altena, W.; Montemayor, T.; Shelus, P. J.; Franz, O. G.; Wasserman, L. H.; Hemenway, P. D.; Duncombe, R. L.; Story, D.; Whipple, A. L.; Bradley, A. J. (2003). “Astrometry with The Hubble Space Telescope: A Parallax of the Central Star of the Planetary Nebula NGC 6853”. The Astronomical Journal 126 (5): 2549–2556. doi:10.1086/378603.….126.2549B. 
  3. abc Harris, Hugh C.; Dahn, Conard C.; Canzian, Blaise; Guetter, Harry H.; Leggett, S. K.; Levine, Stephen E.; Luginbuhl, Christian B.; Monet, Alice K. B.; Monet, David G.; Pier, Jeffrey R.; Stone, Ronald C.; Tilleman, Trudy; Vrba, Frederick J.; Walker, Richard L. (February 2007). “Trigonometric Parallaxes of Central Stars of Planetary Nebulae”. The Astronomical Journal 133 (2): 631–638. doi:10.1086/510348.….133..631H. 
  4. abc O’Dell, C. R.; Balick, B.; Hajian, A. R.; Henney, W. J.; Burkert, A. (2002). “Knots in Nearby Planetary Nebulae”. The Astronomical Journal 123 (6): 3329–3347. doi:10.1086/340726.….123.3329O. 
  5. ab O’dell, C. R.; Balick, B.; Hajian, A. R.; Henney, W. J.; Burkert, A. (2003). “Knots in Planetary Nebulae”. Winds, Bubbles, and Explosions: a conference to honor John Dyson, Pátzcuaro, Michoacán, México, September 9-13, 2002 (Eds. S. J. Arthur & W. J. Henney) Revista Mexicana de Astronomía y Astrofísica (Serie de Conferencias) ( 15: 29–33.…29O. 

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Messier 28 (also known as M28 or NGC 6626) is a w:globular cluster in the w:constellation Sagittarius. It was discovered by w:Charles Messier in w:1764.

M28 is at a distance of about 18,000 to 19,000 w:light-years away from w:Earth. 18 RR Lyrae type w:variable stars have been observed in this cluster. In w:1986, M28 became the first globular cluster where a millisecond pulsar was discovered (by the w:Lovell Telescope at w:Jodrell Bank Observatory).[3]

External links


Messier 29 (also known as M 29 or NGC 6913) is an w:open cluster in the w:Cygnus constellation. It was discovered by w:Charles Messier in w:1764, and can be seen from w:Earth by using w:binoculars.

External links


Messier 30 (also known as M30 or NGC 7099) is a w:globular cluster in the w:Capricornus constellation. It was discovered by w:Charles Messier in w:1764. M30 is at a distance of about 26,000 w:light-years away from w:Earth.

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The Andromeda Galaxy (also known as Messier 31, M31, or NGC 224; often referred to as the Great Andromeda Nebula in older texts) is a spiral galaxy approximately 2.5 million light-years away[4] in the constellation Andromeda. It is the nearest spiral galaxy to our own, the Milky Way. As it is visible as a faint smudge on a moonless night, it is one of the farthest objects visible to the naked eye, and can be seen even from urban areas with binoculars. It is named after the princess Andromeda (Greek: Ανδρομέδη – Andromédē) in Greek mythology. Andromeda is the largest galaxy of the w:Local Group, which consists of the Andromeda Galaxy, the Milky Way Galaxy, the Triangulum Galaxy, and about 30 other smaller galaxies. Although the largest, it may not be the most massive, as recent findings suggest that the Milky Way contains more dark matter and may be the most massive in the grouping.[7] The 2006 observations by the Spitzer Space Telescope revealed that M31 contains one trillion (1012) stars, greatly exceeding the number of stars in our own galaxy.[8]
While the 2006 estimates put the mass of the Milky Way to be ~80% of the mass of Andromeda, which is estimated to be 7.1×1011 [solar masses,[2] a 2009 study concluded that Andromeda and the Milky Way are about equal in mass.[9]

At an apparent magnitude of 4.4, the Andromeda Galaxy is notable for being one of the brightest Messier objects,[10] making it easily visible to the naked eye even when viewed from areas with moderate light pollution. Although it appears more than six times as wide as the full moon when photographed through a larger telescope, only the brighter central region is visible with the naked eye.

Observation history

The earliest recorded observation of the Andromeda Galaxy was in 964 CE by the Persian astronomer, Abd al-Rahman al-Sufi (Azophi),[11] who described it as a “small cloud” in his Book of Fixed Stars. Other star charts of that period have it labeled as the Little Cloud.[11]
The first description of the object based on telescopic observation was given by Simon Marius[11] in 1612. Charles Messier catalogued it as object M31 in 1764 and incorrectly credited Marius as the discoverer, unaware of Al Sufi’s earlier work. In 1785, the astronomer William Herschel noted a faint reddish hue in the core region of the M31. He believed it to be the nearest of all the “great nebulae” and, based on the color and magnitude of the nebula, he incorrectly estimated that it was no more than 2,000 times the distance of w:Sirius.[12]

w:William Huggins in 1864 observed the w:spectrum of M31 and noted that it differed from a gaseous nebula.[13] The spectra of M31 displayed a w:continuum of frequencies, superimposed with dark absorption lines that help identify the chemical composition of an object. The Andromeda nebula was very similar to the spectra of individual stars, and from this it was deduced that M31 had a stellar nature. In 1885, a supernova (known as “S Andromedae”) was seen in M31, the first and so far only one observed in that galaxy. At the time M31 was considered to be a nearby object, so the cause was thought to be a much less luminous and unrelated event called a nova, and was named accordingly “Nova 1885”.[14]

The first photographs of M31 were taken in 1887 by w:Isaac Roberts from his private observatory in w:Sussex, England. The long-duration exposure allowed the spiral structure of the galaxy to be seen for the first time.[15] However, at the time this object was commonly believed to be a nebula within our galaxy, and Roberts mistakenly believed that M31 and similar spiral nebulae were actually solar systems being formed, with the satellites nascent planets. The w:radial velocity of this object with respect to our w:solar system was measured in 1912 by w:Vesto Slipher at the w:Lowell Observatory, using w:spectroscopy. The result was the largest velocity recorded at that time, at 300 kilometres per second (186 miles/s.), moving in the direction of the Sun.[16]

M31 in proximity to Pegasus constellation

Island universe

In 1917, w:Heber Curtis observed a nova within M31. Searching the photographic record, 11 more novae were discovered. Curtis noticed that these novae were, on average, 10 magnitudes fainter than those that occurred within our Galaxy. As a result he was able to come up with a distance estimate of 500,000 light-years. He became a proponent of the so-called “island universes” hypothesis, which held that spiral nebulae were actually independent galaxies.[17]

In 1920 the w:Great Debate between w:Harlow Shapley and w:Heber Curtis took place, concerning the nature of the w:Milky Way, spiral nebulae, and the dimensions of the w:universe. To support his claim that Great Andromeda Nebula (M31) was an external galaxy, Curtis also noted the appearance of dark lanes resembling the dust clouds in our own Galaxy, as well as the significant w:Doppler shift. In 1922 w:Ernst Öpik presented a very elegant and simple astrophysical method to estimate the distance of M31, his result (450 kpc) put Andromeda Nebula far outside our Galaxy.[18]w:Edwin Hubble settled the debate in 1925 when he identified extragalactic w:Cepheid variable stars for the first time on astronomical photos of M31. These were made using the 2.5 metre (100 in) w:Hooker telescope, and they enabled the distance of Great Andromeda Nebula to be determined. His measurement demonstrated conclusively that this feature was not a cluster of stars and gas within our Galaxy, but an entirely separate galaxy located a significant distance from our own.[19]

This galaxy plays an important role in galactic studies, since it is the nearest giant spiral (although not the nearest galaxy). In 1943, w:Walter Baade was the first person to resolve stars in the central region of the Andromeda Galaxy. Based on his observations of this galaxy, he was able to discern two distinct populations of stars based on their w:metallicity, naming the young, high velocity stars in the disk Type I and the older, red stars in the bulge Type II. This nomenclature was subsequently adopted for stars within the Milky Way, and elsewhere. (The existence of two distinct populations had been noted earlier by w:Jan Oort.)[20] Dr. Baade also discovered that there were two types of Cepheid variables, which resulted in a doubling of the distance estimate to M31, as well as the remainder of the Universe.[21]

Radio emission from the Andromeda Galaxy was first detected by w:Grote Reber in 1940. The first radio maps of the galaxy were made in the 1950s by John Baldwin and collaborators at the Cambridge Radio Astronomy Group.[22] The core of the Andromeda Galaxy is called 2C 56 in the 2C radio astronomy catalogue. In 2009, the first planet may have been discovered in the Andromeda Galaxy. This candidate was detected using a technique called w:microlensing, which is caused by the deflection of light by a massive object.[23]


The Andromeda Galaxy is approaching the w:Sun at about 300 kilometers per second (186 miles/s.), so it is one of the few w:blue shifted galaxies. The Andromeda Galaxy and the Milky Way are approaching one another at a speed of 100 to 140 kilometers per second (62–87 miles/s.; 223,200–313,200mph).[24] The collision is predicted to occur in about 2.5 billion years. In that case the two galaxies will likely merge to form a giant w:elliptical galaxy.[25] However, Andromeda’s tangential velocity with respect to the Milky Way is only known to within about a factor of two, which creates uncertainty about the details of the collision.[26] Such events are frequent among the galaxies in w:galaxy groups. The fate of the w:Earth and the w:Solar System in the event of a collision are presently unknown, but there is a small chance that the Solar System could be ejected from the Milky Way or join Andromeda.[27]

The measured distance to the Andromeda Galaxy was doubled in 1953 when it was discovered that there is another, dimmer type of w:Cepheid. In the 1990s, measurements of both standard w:red giants as well as w:red clump stars from the w:Hipparcos satellite measurements were used to calibrate the Cepheid distances.[28][29]

Recent distance estimates

At least four distinct techniques have been used to measure distances to the Andromeda Galaxy.

In 2003, using the infrared w:surface brightness fluctuations (I-SBF) and adjusting for the new period-luminosity value of Freedman et al. 2001 and using a metallicity correction of -0.2 mag dex−1 in (O/H), an estimate of 2.57 ± 0.06 Mly (787 ± 18 kpc) was derived.

Using the Cepheid variable method, an estimate of 2.51 ± 0.13 Mly (770 ± 40 kpc) was achieved in 2004.[3][2]

In 2005, a group of astronomers consisting of w:Ignasi Ribas (CSIC, IEEC) and his colleagues announced the discovery of an eclipsing binary star in the Andromeda Galaxy. The binary star, designated M31VJ00443799+4129236,c has two luminous and hot blue stars of types O and B. By studying the eclipses of the stars, which occur every 3.54969 days, the astronomers were able to measure their sizes. Knowing the sizes and temperatures of the stars they were able to measure the w:absolute magnitude of the stars. When the visual and absolute magnitudes are known, the distance to the star can be measured. The stars lie at the distance of 2.52 ± 0.14 Mly (770 ± 40 kpc) and the whole Andromeda Galaxy at about 2.5 Mly.[4] This new value is in excellent agreement with the previous, independent Cepheid-based distance value.

Andromeda is close enough that the w:Tip of the Red Giant Branch (TRGB) method may also be used to estimate its distance. The estimated distance to M31 using this technique in 2005 yielded 2.56 ± 0.08 Mly (785 ± 25 kpc).[5]

Averaged together, all these distance measurements give a combined distance estimate of 2.54 ± 0.06 Mly (778 ± 17 kpc).a Based upon the above distance, the diameter of M31 at the widest point is estimated to be 141 ± 3 kly.d

Mass and luminosity estimates

Mass estimates for the Andromeda halo (including w:dark matter) give a value of approximately 1.23×1012 M[30] (or 1.2 million million w:solar masses) compared to 1.9×1012 M for the Milky Way. Thus M31 may be less massive than our own galaxy, although the error range is still too large to say for certain. Even so, the masses of the Milky Way and M31 are comparable, and M31’s spheroid actually has a higher stellar density than that of the Milky Way.[31]

In particular, M31 appears to have significantly more common stars than the Milky Way, and the estimated w:luminosity of M31, ~2.6×1010 L, is about 25% higher than that of our own galaxy.[32] However the rate of star formation in the Milky Way is much higher, with M31 only producing about one solar mass per year compared to 3–5 solar masses for the Milky Way. The rate of w:supernovae in the Milky Way is also double that of M31.[33] This suggests that M31 has experienced a great star formation phase in its past, but is now relatively w:quiescent, whereas the Milky Way is experiencing more active star formation.[32] Should this continue, the luminosity in the Milky Way may overtake that of M31 in the future.


Based on its appearance in visible light, the Andromeda galaxy is classified as an SA(s)b galaxy in the de Vaucouleurs-Sandage extended classification system of spiral galaxies.[1] However, data from the w:2MASS survey showed that the bulge of M31 has a box-like appearance, which implies that the galaxy is actually a barred galaxy with the bar viewed almost directly along its long axis.[34]

In 2005, astronomers used the w:Keck telescopes to show that the tenuous sprinkle of stars extending outward from the galaxy is actually part of the main disk itself.[35] This means that the spiral disk of stars in Andromeda is three times larger in diameter than previously estimated. This constitutes evidence that there is a vast, extended stellar disk that makes the galaxy more than 220,000 light-years in diameter. Previously, estimates of Andromeda’s size ranged from 70,000 to 120,000 light-years across.

The galaxy is inclined an estimated 77° relative to the Earth (where an angle of 90° would be viewed directly from the side). Analysis of the cross-sectional shape of the galaxy appears to demonstrate a pronounced, S-shaped warp, rather than just a flat disk.[36] A possible cause of such a warp could be gravitational interaction with the satellite galaxies near M31. The galaxy M33 could be responsible for some warp in M31’s arms, though more precise distances and radial velocities are required.

Spectroscopic studies have provided detailed measurements of the rotational velocity of M31 at various radii from the core. In the vicinity of the core, the rotational velocity climbs to a peak of 225 kilometres per second (140 miles/s.) at a radius of 1,300 w:light-years, then descends to a minimum at 7,000 light-years where the rotation velocity may be as low as 50 kilometres per second (31 miles/s.). Thereafter the velocity steadily climbs again out to a radius of 33,000 light-years, where it reaches a peak of 250 kilometres per second (155 miles/s.). The velocities slowly decline beyond that distance, dropping to around 200 kilometres per second (124 miles/s.) at 80,000 light-years. These velocity measurements imply a concentrated mass of about 6×109 M in the nucleus. The total mass of the galaxy increases w:linearly out to 45,000 light-years, then more slowly beyond that radius.[37]

The w:spiral arms of Andromeda are outlined by a series of w:H II regions that Baade described as resembling “beads on a string”. They appear to be tightly wound, although they are more widely spaced than in our galaxy.[38] Rectified images of the galaxy show a fairly normal spiral galaxy with the arms wound up in a clockwise direction. There are two continuous trailing arms that are separated from each other by a minimum of about 13,000 light-years. These can be followed outward from a distance of roughly 1,600 light-years from the core. The most likely cause of the spiral pattern is thought to be interaction with M32. This can be seen by the displacement of the neutral hydrogen clouds from the stars.[39]

In 1998, images from the w:European Space Agency’s w:Infrared Space Observatory demonstrated that the overall form of the Andromeda galaxy may be transitioning into a w:ring galaxy. The gas and dust within Andromeda is generally formed into several overlapping rings, with a particularly prominent ring formed at a radius of 32,000 light-years from the core.[40] This ring is hidden from visible light images of the galaxy because it is composed primarily of cold dust.

Close examination of the inner region of Andromeda showed a smaller dust ring that is believed to have been caused by the interaction with M32 more than 200 million years ago. Simulations show that the smaller galaxy passed through the disk of Andromeda along the latter’s polar axis. This collision stripped more than half the mass from the smaller M32 and created the ring structures in Andromeda.[41]

Studies of the extended halo of M31 show that it is roughly comparable to that of the Milky Way, with stars in the halo being generally “metal-poor”, and increasingly so with greater distance.[42] This evidence indicates that the two galaxies have followed similar evolutionary paths. They are likely to have accreted and assimilated about 1–200 low-mass galaxies during the past 12 billion years.[43] The stars in the extended halos of M31 and the Milky Way may extend nearly one-third the distance separating the two galaxies.


HST image of Andromeda galaxy core showing possible double structure. w:NASA/w:ESA photo.

Artist’s concept of Andromeda galaxy core showing a view across a mysterious disk of young, blue stars encircling a supermassive black hole. w:NASA/w:ESA photo.

M31 is known to harbor a dense and compact star cluster at its very center. In a large telescope it creates a visual impression of a star embedded in the more diffuse surrounding bulge. The luminosity of the nucleus is in excess of the most luminous globular clusters.[citation needed]

In 1991 w:Tod R. Lauer used WFPC, then on board the w:Hubble Space Telescope, to image Andromeda’s inner nucleus. The nucleus consists of two concentrations separated by 1.5 w:parsecs. The brighter concentration, designated as P1, is offset from the center of the galaxy. The dimmer concentration, P2, falls at the true center of the galaxy and contains a 108M w:black hole.[44]

w:Scott Tremaine has proposed that the observed double nucleus could be explained if P1 is the projection of a disk of stars in an eccentric orbit around the central black hole.[45] The eccentricity is such that stars linger at the orbital apocenter, creating a concentration of stars. P2 also contains a compact disk of hot, spectral class A stars. The A stars are not evident in redder filters, but in blue and ultraviolet light they dominate the nucleus, causing P2 to appear more prominent than P1.[46]

While at the initial time of its discovery it was hypothesized that the brighter portion of the double nucleus was the remnant of a small galaxy “cannibalized” by Andromeda,[47] this is no longer considered to be a viable explanation. The primary reason is that such a nucleus would have an exceedingly short lifetime due to tidal disruption by the central black hole. While this could be partially resolved if P1 had its own black hole to stabilize it, the distribution of stars in P1 does not suggest that there is a black hole at its center.[45]

Discrete sources

Multiple X-ray sources have been detected in the Andromeda Galaxy, using observations from the ESA’s w:XMM-Newton orbiting observatory. w:Robin Barnard et al. hypothesized that these are candidate black holes or w:neutron stars, which are heating incoming gas to millions of kelvins and emitting X-rays. The spectrum of the neutron stars is the same as the hypothesized black holes, but can be distinguished by their masses.[48]

There are approximately 460 w:globular clusters associated with the Andromeda galaxy.[49] The most massive of these clusters, identified as w:Mayall II, nicknamed Globular One, has a greater luminosity than any other known globular cluster in the w:local group of galaxies.[50] It contains several million stars, and is about twice as luminous as w:Omega Centauri, the brightest known globular cluster in the w:Milky Way.
Globular One (or G1) has several stellar populations and a structure too massive for an ordinary globular. As a result, some consider G1 to be the remnant core of a w:dwarf galaxy that was consumed by M31 in the distant past.[51]
The globular with the greatest apparent brightness is w:G76 which is located in the south-west arm’s eastern half.[11]

In 2005, astronomers discovered a completely new type of star cluster in M31. The new-found clusters contain hundreds of thousands of stars, a similar number of stars that can be found in globular clusters. What distinguishes them from the globular clusters is that they are much larger – several hundred light-years across – and hundreds of times less dense. The distances between the stars are, therefore, much greater within the newly discovered extended clusters.[52]


Main page: Andromeda’s satellite galaxies

Like the Milky Way, Andromeda Galaxy has satellite galaxies, consisting of 14 known dwarf galaxies. The best known and most readily observed satellite galaxies are M32 and M110. Based on current evidence, it appears that M32 underwent a close encounter with M31 (Andromeda) in the past. M32 may once have been a larger galaxy that had its stellar disk removed by M31, and underwent a sharp increase of w:star formation in the core region, which lasted until the relatively recent past.[53]

M110 also appears to be interacting with M31, and astronomers have found a stream of metal-rich stars in the halo of M31 that appears to have been stripped from these satellite galaxies.[54] M110 does contain a dusty lane, which may indicate recent or ongoing star formation.[55]

In 2006 it was discovered that nine of these galaxies lay along a plane that intersects the core of the Andromeda Galaxy, rather than being randomly arranged as would be expected from independent interactions. This may indicate a common tidal origin for the satellites.[56]


^a average(787 ± 18, 770 ± 40, 772 ± 44, 783 ± 25) = ((787 + 770 + 772 + 783) / 4) ± ((182 + 402 + 442 + 252)0.5 / 4) = 778 ± 17
^b Apparent Magnitude of 4.36 – w:distance modulus of 24.4 = −20.0
^c J00443799+4129236 is at celestial coordinates R.A. 00h 44m 37.99s, Dec. +41° 29′ 23.6″.
^d distance × tan( diameter_angle = 190′ ) = 141 ± 3 kly diameter


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  2. abcd Karachentsev, I. D.; Kashibadze, O. G. (2006). “Masses of the local group and of the M81 group estimated from distortions in the local velocity field”. Astrophysics 49 (1): 3–18. doi:10.1007/s10511-006-0002-6.…..49….3K. 
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  22. van der Kruit, P. C.; Allen, R. J. (1976). “The Radio Continuum Morphology of Spiral Galaxies”. Annual Review of Astronomy and Astrophysics 14: 417-445. doi:10.1146/annurev.aa.14.090176.002221. 
  23. Ingrosso, G.; Calchi Novati, S.; De Paolis, F.; Jetzer, Ph.; Nucita, A. A.; Zakharov, A. F.. “Pixel-lensing as a way to detect extrasolar planets in M31”. arXiv. Retrieved 2009-07-10. 
  24. Malik, Tariq (2002-05-07). “Crash Course: Simulating the Fate of Our Milky Way”. Archived from the original on 2002-06-06. Retrieved 2006-09-18. 
  25. Cox, T.J., Loeb, A. (2008). “The collision between the Milky Way and Andromeda”. Monthly Notices of the Royal Astronomical Society 386 (1): 461–474. doi:10.1111/j.1365-2966.2008.13048.x. 
  26. “The Grand Collision”. The Sky At Night. November 5, 2007.
  27. Cain, Fraser (2007). “When Our Galaxy Smashes Into Andromeda, What Happens to the Sun?”. Universe Today. Retrieved 2007-05-16. 
  28. Holland, Stephen (1998). “The Distance to the M31 Globular Cluster System”. The Astronomical Journal 115 (5): 1916–1920. doi:10.1086/300348. 
  29. Stanek, K.Z., Garnavich, P.M. (1998). “Distance to M31 With the HST and Hipparcos Red Clump Stars”. Astrophysical Journal Letters 503: 131–141. 
  30. N. W. Evans & M. I. Wilkinson (2000). “The mass of the Andromeda galaxy”. Monthly Notices of the Royal Astronomical Society 316 (4): 929–942. doi:10.1046/j.1365-8711.2000.03645.x. 
  31. Kalirai, J.S. et al. (2006). “The Metal-Poor Halo of the Andromeda Spiral Galaxy (M31)”. Astrophysical Journal 648: 389–404. doi:10.1086/505697. 
  32. ab van den Bergh, Sidney (1999). “The local group of galaxies”. The Astronomy and Astrophysics Review 9 (3–4): 273–318. doi:10.1007/s001590050019. 
  33. W. Liller, B. Mayer (July 1987). “The Rate of Nova Production in the Galaxy”. Publications Astronomical Society of the Pacific 99: 606–609. doi:10.1086/132021.…99..606L. 
  34. R.L. Beaton, E. Athanassoula, S.R. Majewski, P. Guhathakurta, M.F. Skrutskie, R.J. Patterson, M. Bureau (2006). “Unveiling the Boxy Bulge and Bar of the Andromeda Spiral Galaxy”. Astrophysical Journal Letters 658: L91. doi:10.1086/514333. 
  35. S. C. Chapman, R. Ibata, G. F. Lewis, A. M. N. Ferguson, M. Irwin, A. McConnachie, N. Tanvir (2006). “A kinematically selected, metal-poor spheroid in the outskirts of M31”. Astrophysical Journal 653: 255. doi:10.1086/508599.  Also see the press release, CalTech Media Relations (February 27, 2006). “Andromeda’s Stellar Halo Shows Galaxy’s Origin to Be Similar to That of Milky Way”. Press release. Retrieved 2006-05-24. 
  36. UC Santa Cruz (January 9, 2001). “Astronomers Find Evidence of an Extreme Warp in the Stellar Disk of the Andromeda Galaxy”. Press release. Retrieved 2006-05-24. 
  37. V. C. Rubin, W. K. J. Ford (1970). “Rotation of the Andromeda Nebula from a Spectroscopic Survey of Emission”. Astrophysical Journal 159: 379. doi:10.1086/150317.…159..379R. 
  38. H. Arp (1964). “Andromeda Nebula from a Spectroscopic Survey of Emission”. Astrophysical Journal 139: 1045. doi:10.1086/147844.…139.1045A. 
  39. R. Braun (1991). “The distribution and kinematics of neutral gas, w:HI region in M31”. Astrophysical Journal 372, part 1: 54–66. doi:10.1086/169954.…372…54B. 
  40. Esa Science News (October 14, 1998). “ISO unveils the hidden rings of Andromeda”. Press release. Retrieved 2006-05-24. 
  41. “Busted! Astronomers Nab Culprit in Galactic Hit-and-Run”. Harvard-Smithsonian Center for Astrophysics. October 18, 2006. Retrieved 2006-10-18. 
  42. J. S. Kalirai, K. M. Gilbert, P. Guhathakurta, S. R. Majewski, J. C. Ostheimer, R. M. Rich, M. C. Cooper, D. B. Reitzel, R. J. Patterson (2006). “The Metal-Poor Halo of the Andromeda Spiral Galaxy (M31)”. Astrophysical Journal 648: 389. doi:10.1086/505697. 
  43. J.S. Bullock and K.V. Johnston (2005). “Tracing Galaxy Formation with Stellar Halos I: Methods”. Astrophysical Journal 635 (2): 931–949. doi:10.1086/497422.…635..931B. 
  44. Lauer, T. R. et al. (1993). “Planetary camera observations of the double nucleus of M31”. Astronomical Journal 106 (4): 1436–1447, 1710–1712. doi:10.1086/116737. 
  45. ab
    Tremaine, Scott (1995). “An Eccentric-Disk Model for the Nucleus of M31”. Astronomical Journal 110: 628–633. doi:10.1086/117548.….110..628T. 
  46. Hubble news desk STScI-1993-18 (July 20, 1993). “Hubble Space Telescope Finds a Double Nucleus in the Andromeda Galaxy”. Press release. Retrieved 2006-05-26. 
  47. Schewe, Phillip F.; Stein, Ben. “The Andromeda Galaxy has a Double Nucleus”. Physics News Update (American Institute of Physics). Retrieved 2009-07-10. 
  48. R., Barnard; U. Kolb; J.P. Osborne (August 2005). “Timing the bright X-ray population of the core of M31 with XMM-Newton”. A&A. 
  49. P. Barmby, J.P. Huchra (2001). “M31 Globular Clusters in the Hubble Space Telescope Archive. I. Cluster Detection and Completeness”. Astronomical Journal 122: 2458–2468. doi:10.1086/323457. 
  50. Hubble news desk STSci-1996-11 (April 24, 1996). “Hubble Spies Globular Cluster in Neighboring Galaxy”. Press release. Retrieved 2006-05-26. 
  51. G. Meylan, A. Sarajedini, P. Jablonka, S.G. Djorgovski, T. Bridges, R.M. Rich (2001). “G1 in M31 – Giant Globular Cluster or Core of a Dwarf Elliptical Galaxy?”. Astronomical Journal 122: 830–841. doi:10.1086/321166. 
  52. A.P. Huxor, N.R. Tanvir, M.J. Irwin, R. Ibata (2005). “A new population of extended, luminous, star clusters in the halo of M31”. Monthly Notices of the Royal Astronomical Society 360: 993–1006. doi:10.1111/j.1365-2966.2005.09086.x. 
  53. K. Bekki, W.J. Couch, M.J. Drinkwater, M.D. Gregg (2001). “A New Formation Model for M32: A Threshed Early-type Spiral?”. Astrophysical Journal 557 (1): L39–L42. doi:10.1086/323075.…557L..39B. 
  54. R. Ibata, M. Irwin, G. Lewis, A.M. Ferguson, N. Tanvir (July 5, 2001). “A giant stream of metal-rich stars in the halo of the galaxy M31”. Nature 412 (6842): 49–52. doi:10.1038/35083506. 
  55. Young, L. M. (November 2000). “Properties of the Molecular Clouds in NGC 205”. The Astronomical Journal (5): 2460–2470. doi:10.1086/316806. 
  56. A. Koch and E.K. Grebel (2006). “The Anisotropic Distribution of M 31 Satellite Galaxies: A Polar Great Plane of Early-Type Companions”. Astronomical Journal 131 (3): 1405–1415. doi:10.1086/499534. 

External links

Messier 32 (also known as NGC 221 and w:Le Gentil) is a w:dwarf elliptical galaxy about 2.65 million w:light-years away in the w:constellation Andromeda. M32 is a w:satellite galaxy of the famous w:Andromeda Galaxy (M31) and was discovered by w:Le Gentil in w:1749. M32 measures only 6.5 ± 0.2 kly[5] in diameter at the widest point. Like most w:elliptical galaxies, M32 contains mostly older faint red and yellow stars with practically no dust or gas and consequently no current w:star formation.[6] It does, however, show hints of star formation in the relatively recent past.

The structure and stellar content of M32 is difficult to explain by traditional w:galaxy formation models. Recent simulations suggest a new scenario in which the strong tidal field of M31 can transform a w:spiral galaxy into a compact elliptical. As a small spiral galaxy falls into the central parts of M31, most of the outer layers of the smaller spiral are stripped away. The central bulge of the galaxy is much less affected and retains its morphology. Tidal effects trigger a massive star burst in the core, resulting in the high density of M32 we observe today.[7]
There is also evidence that M32 has an outer disk.[8]

Distance measurements

At least two techniques have been used to measure distances to M32. The infrared w:surface brightness fluctuations distance measurement technique estimates distances to spiral galaxies based on the graininess of the appearance of their bulges. The distance measured to M32 using this technique is 2.46 ± 0.09 Mly (755 ± 28 kpc).[2] However, M32 is close enough that the w:tip of the red giant branch (TRGB) method may be used to estimate its distance. The estimated distance to M32 using this technique is 2.51 ± 0.13 Mly (770 ± 40 kpc).[3][4] Averaged together, these distance measurements give a distance estimate of 2.49 ± 0.08 Mly (763 ± 24 kpc).b

A recent paper argues that M32 may actually be a normal (non-dwarf) galaxy that is actually three times farther away, outside the w:Local Group.[9]


^ ^ average(755 ± 28, 770 ± 40) = ((755 + 770) / 2) ± ((282 + 402)0.5 / 2) = 763 ± 24


  1. abcdefghij “NASA/IPAC Extragalactic Database”. Results for NGC 221. Retrieved 2006-11-29. 
  2. ab Jensen, Joseph B.; Tonry, John L.; Barris, Brian J.; Thompson, Rodger I.; Liu, Michael C.; Rieke, Marcia J.; Ajhar, Edward A.; Blakeslee, John P. (February 2003). “Measuring Distances and Probing the Unresolved Stellar Populations of Galaxies Using Infrared Surface Brightness Fluctuations”. Astrophysical Journal 583 (2): 712–726. doi:10.1086/345430.…583..712J. 
  3. ab I. D. Karachentsev, V. E. Karachentseva, W. K. Hutchmeier, D. I. Makarov (2004). “A Catalog of Neighboring Galaxies”. Astronomical Journal 127: 2031–2068. doi:10.1086/382905.….127.2031K. 
  4. ab Karachentsev, I. D.; Kashibadze, O. G. (2006). “Masses of the local group and of the M81 group estimated from distortions in the local velocity field”. Astrophysics 49 (1): 3–18. doi:10.1007/s10511-006-0002-6.…..49….3K. 
  5. Diameter = distance × sin(diameter_angle) = 6.5 ± 0.2 kly. diameter
  6. Kepple, George Robert; Glen W. Sanner (1998). The Night Sky Observer’s Guide, Volume 1. Willmann-Bell, Inc.. p. 17. ISBN 0-943396-58-1. 
  7. Bekki, Kenji; Couch, Warrick J.; Drinkwater, Michael J.; Gregg, Michael D. (2001). “A New Formation Model for M32: A Threshed Early-Type Spiral Galaxy?”. The Astrophysical Journal 557: L39. doi:10.1086/323075.…557L..39B. 
  8. Graham, A. W. (2002). “Evidence for an Outer Disk in the Prototype Compact Elliptical Galaxy M32”. The Astrophysical Journal Letters 568: L13. doi:10.1086/340274.…568L..13G. 
  9. Young, K. S. et al. (2008), A critical review of the evidence for M32 being a compact dwarf satellite of M31 rather than a more distant normal galaxy

External links

The Triangulum Galaxy (also known as Messier 33 or NGC 598) is a w:spiral galaxy approximately 3 million light-years away in the w:constellation w:Triangulum. The galaxy is also sometimes informally referred to as the Pinwheel Galaxy by some amateur astronomy references[2]
and in some public outreach websites.[3]
However, the w:SIMBAD Astronomical Database, a professional astronomy database that contains formal designations for astronomical objects, indicates that the name “Pinwheel Galaxy” is used to refer to w:Messier 101,[4]
and several other amateur astronomy resources and other public outreach websites also identify Messier 101 by that name.[5][6]
It is the third largest galaxy in the w:Local Group, a w:group of galaxies that also contains the w:Milky Way Galaxy and the w:Andromeda Galaxy, and it may be a gravitationally bound companion of the Andromeda Galaxy. The w:Pisces Dwarf (LGS 3), one of the small Local Group member galaxies, is possibly a w:satellite of Triangulum.

General information

Triangulum (M33) and Andromeda (M31)

The Triangulum Galaxy can be seen with the naked eye under exceptionally good conditions. While the fainter and more distant galaxy w:Messier 81 has also been seen with the naked eye by very experienced observers. However, some amateur astronomers may confuse the object with the nearby w:NGC 752, an w:open cluster that is brighter than the Triangulum Galaxy.[citation needed] No known pre-telescopic observer notes it, which is not surprising: given its indistinctness, it is not likely to be noticed as an object unless one already knows of its existence.

The Triangulum Galaxy was probably discovered by w:Giovanni Batista Hodierna before 1654, who may have grouped it together with w:open cluster NGC 752. It was independently discovered by w:Charles Messier in 1764, who catalogued it as M33 on August 25. M33 was also catalogued independently by w:William Herschel on September 11, 1784 number H V.17. It was among the first “w:spiral nebulae” identified as such by Lord Rosse.

Herschel also cataloged The Triangulum Galaxy’s brightest and largest w:H II region (diffuse w:emission nebula containing w:ionized w:hydrogen) as H III.150 separately from the galaxy itself, which eventually obtained NGC number 604. As seen from Earth NGC 604 is located northeast of the galaxy’s central core, and is one of the largest H II regions known with a diameter of nearly 1500 w:light-years and a spectrum similar to the w:Orion Nebula. Herschel also noted 3 other smaller H II regions (NGC 588, 592 and 595).

In 2005, using observations of two water masers on opposite sides of Triangulum via the w:VLBA, researchers were, for the first time, able to estimate the angular rotation and proper motion of Triangulum. A velocity of 190 ± 60 km/s relative to the Milky Way is computed which means Triangulum is moving towards Andromeda.[7]

In 2007, a black hole about 15.7 times the mass of the Sun was detected in the galaxy using data from the w:Chandra X-ray Observatory. The black hole, named M33 X-7, orbits a companion star which it eclipses every 3.5 days.[8]

The galaxy has an H II nucleus.[9]


At least three techniques have been used to measure distances to M 33. Using the w:Cepheid variable method, an estimate of 2.77 ± 0.13 Mly (850 ± 40 kpc) was achieved in 2004.[10][11]

Also 2004, the w:Tip of the Red Giant Branch (TRGB) method was used to derive a distance estimate of 2.59 ± 0.08 Mly (794 ± 23 kpc).[12]

In 2006, a group of astronomers announced the discovery of an eclipsing binary star in the Triangulum Galaxy. By studying the eclipses of the stars, the astronomers were able to measure their sizes. Knowing the sizes and temperatures of the stars they were able to measure the w:absolute magnitude of the stars. When the visual and absolute magnitudes are known, the distance to the star can be measured. The stars lie at the distance of 3.1 ± 0.2 Mly (940 ± 70 kpc).[13][14]

Averaged together, all these distance measurements give a combined distance estimate of 2.81 ± 0.09 Mly (861 ± 28 kpc).[14]

References in fiction

Main page: Galaxies in fiction

In the episode “w:Where No One Has Gone Before” of the first season of w:Star Trek: The Next Generation, the Enterprise is hurled into the far side of M33 by an alien visitor known only as “The Traveler”.

In the TV series Andromeda, Triangulum Galaxy is one of the three galaxies forming the Systems Commonwealth, along with The Milky Way and Andromeda.

In the 2007 computer game w:Crysis, the alien antagonists of the game are said to have originated from the galaxy M33.

In the comic-book series w:Yoko Tsuno, the Vineans originated from M33, with the first mention of it being in the sixth album w:Les Trois soleils de Vinéa (The Three Suns of Vinea).


  1. abcdefgh “NASA/IPAC Extragalactic Database”. Results for NGC 598. Retrieved 2006-12-01. 
  2. S. J. O’Meara (1998). The Messier Objects. Cambridge: Cambridge University. ISBN 0-521-55332-6. 
  3. “NASA Spitzer Telescope Reveals Pinwheel Galaxy’s Hidden Wonders”. Retrieved 2007-04-07. 
  4. “SIMBAD Astronomical Database”. Results for Messier 101.. Retrieved 2007-04-07. 
  5. “Messier Object 101”. Retrieved 2007-04-07. 
  6. “Best of AOP: M101: Pinwheel Galaxy”. Retrieved 2007-04-07. 
  7. Brunthaler, Andreas; Reid, Mark J.; Falcke, Heino; Greenhill, Lincoln J.; Henkel, Christian (2005). “The Geometric Distance and Proper Motion of the Triangulum Galaxy (M33)”. Science 307 (5714): 1440–1443. doi:10.1126/science.1108342. PMID 15746420.…307.1440B. 
  8. Morcone, Jennifer, Heaviest Stellar Black Hole Discovered in Nearby Galaxy, w:Chandra X-ray Observatory press release, Template:Date
  9. Ho, Luis C.; Filippenko, Alexei V.; Sargent, Wallace L. W. (October 1997), “A Search for “Dwarf” Seyfert Nuclei. III. Spectroscopic Parameters and Properties of the Host Galaxies”, The Astrophysical Journal Supplement Series 112: 315–390, doi:10.1086/313041, 
  10. I. D. Karachentsev, V. E. Karachentseva, W. K. Hutchmeier, D. I. Makarov (2004). “A Catalog of Neighboring Galaxies”. Astronomical Journal 127: 2031–2068. doi:10.1086/382905.….127.2031K. 
  11. Karachentsev, I. D.; Kashibadze, O. G. (2006). “Masses of the local group and of the M81 group estimated from distortions in the local velocity field”. Astrophysics 49 (1): 3–18. doi:10.1007/s10511-006-0002-6.…..49….3K. 
  12. McConnachie, A. W.; Irwin, M. J.; Ferguson, A. M. N.; Ibata, R. A.; Lewis, G. F.; Tanvir, N. (May 2004), “Determining the location of the tip of the red giant branch in old stellar populations: M33, Andromeda I and II”, Monthly Notices of the Royal Astronomical Society 350 (1): 250, doi:10.1111/j.1365-2966.2004.07637.x, 
  13. Bonanos, A. Z.; Stanek, K. Z.; Kudritzki, R. P.; Macri, L.; Sasselov, D. D.; Kaluzny, J.; Bersier, D.; Bresolin, F.; Matheson, T.; Mochejska, B. J.; Przybilla, N.; Szentgyorgyi, A. H.; Tonry, J.; Torres, G. (2006). “The First DIRECT Distance to a Detached Eclipsing Binary in M33”. Astrophysics and Space Science Online First: 207. doi:10.1007/s10509-006-9112-1. 
  14. ab average(850 ± 40, 794 ± 23, 940 ± 70) = ((850 + 794 + 940) / 3) ± ((40² + 23² + 70²)0.5 / 3) = 861 ± 28

External links

Messier 34 (also known as M 34 or NGC 1039) is an w:open cluster in the w:constellation Perseus. It was discovered by w:Giovanni Batista Hodierna before w:1654 and included by w:Charles Messier in his catalog of w:comet-like objects in w:1764.

M34 is at a distance of about 1,400 w:light-years away from w:Earth and consists of some 100 w:stars. It spans about 35′ on the sky which translates to a true radius of 7 w:light years. The cluster is just visible to the naked eye in very dark conditions, well away from city lights. It is well seen in w:binoculars.

External links


Messier 35 (also known as M 35, or NGC 2168) is an w:open cluster in the constellation Gemini. It was discovered by w:Philippe Loys de Chéseaux in w:1745 and independently discovered by w:John Bevis before w:1750. The cluster is scattered over an area of the sky almost the size of the full moon and is located 2,800 w:parsecs from Earth.


External links

Open Cluster M36 (also known as Messier Object 36, Messier 36, M36, or NGC 1960) is an w:open cluster in the w:Auriga constellation. It was discovered by w:Giovanni Batista Hodierna before w:1654. M36 is at a distance of about 4,100 w:light years away from w:Earth and is about 14 light years across. There are at least sixty members in the cluster. The cluster is very similar to the Pleiades cluster (M45), and if it were the same distance from Earth it would be of similar magnitude.

External links

Messier 37 (also known as M37 or NGC 2099) is the richest w:open cluster in the w:constellation Auriga. It was discovered by w:Giovanni Batista Hodierna before w:1654.

Messier 37 is the brightest of the three open clusters in Auriga. M37 was missed by w:Le Gentil when he rediscovered M36 and M38 in 1749. w:Charles Messier independently rediscovered M37 in September of w:1764 but all three clusters were recorded by Hodierna before 1654. All three clusters were generally unknown until w:1984.

M37 is roughly 300 million years old and contains over 500 stars with roughly 150 stars brighter than magnitude 12.5. M37 also contains at least a dozen w:red giants with the hottest w:main sequence star of w:spectral type B9V. Its distance is between 3,600 to 4,700 w:light years and the apparent diameter of 24′ corresponds to a w:linear extension of about 20 to 25 light years. It is classified as Trumpler type I,1,r or I,2,r.

External links

Messier 38 (also known as M38 or NGC 1912) is an w:open cluster in the w:Auriga constellation.

It was discovered by w:Giovanni Batista Hodierna before w:1654 and independently found by w:Le Gentil in w:1749. w:M36 and w:M37, also discovered by Hodierna, are grouped together with w:M38 at a distance of about 3,420 w:light years away from w:Earth.[1]

The cluster’s brightest stars form a pattern resembling the Greek letter Pi or, according to Webb, an “oblique cross.” At its distance of 4,200 light years, its angular diameter of about 20′ corresponds to about 25 light years, similar to that of its more distant neighbor M37. It is of intermediate age (about 220 million years, according to Sky Catalog 2000) and features a yellow giant of w:apparent magnitude +7.9 and w:spectral type G0 as its brightest member. This corresponds to an w:absolute magnitude of -1.5, or a luminosity of 900 suns. For comparison, the w:Sun would appear as a faint magnitude +15.3 star from the distance of M38.


NAME w:Right ascension w:Declination w:Spectral type
w:HD 35519 05h 26m 54.3176s +35° 27′ 26.181 K2
NGC 1912 HOAG 3
NGC 1912 HOAG 4 05h 28m 35.39s +35° 52′ 51.2 A0V
NGC 1912 HOAG 5 05h 28m 50.73s +35° 46′ 47.2 A0Vn
NGC 1912 HOAG 6 05h 28m 10.46s +35° 55′ 26.0 A0:V
NGC 1912 HOAG 7 05h 28m 34.25s +35° 53′ 29.7 A2V
NGC 1912 HOAG 11
NGC 1912 HOAG 19 K2IIIb
NGC 1912 HOAG 104 G5III
NGC 1912 SS G2
NGC 1912 HOAG 128 K0III
NGC 1912 SS G4 A5:V
NGC 1912 HOAG 153 K0V
NGC 1912 SS G3 A3V
NGC 1912 HOAG 160 K1IV
NGC 1912 HOAG 161 G5V
NGC 1912 HOAG 171 G7IV
NGC 1912 HOAG 172


  1. Majaess D. J., Turner D., Lane D. (2007).
    In Search of Possible Associations between Planetary Nebulae and Open Clusters, PASP, 119, 1349

External links

Open Cluster M39 (also known as Messier Object 39, Messier 39, M39, or NGC 7092) is an w:open cluster in the w:Cygnus constellation. It was discovered by w:Charles Messier in w:1764. M39 is at a distance of about 800 w:light years away from w:Earth. Its age is estimated to be from 200 to 300 million years.

It is located at Right Ascension 21hours, 32.2 minutes, and Declination +48 degrees 26′.

External links

Winnecke 4 (also known as Messier 40 or WNC 4) is a w:double star in the constellation w:Ursa Major. It was discovered by w:Charles Messier in w:1764 while he was searching for a nebula that had been reported in the area by Johann Hevelius. Not seeing any nebulae, Messier catalogued this double star instead. It was subsequently rediscovered by w:Friedrich August Theodor Winnecke in 1863.

In 1991 the separation between the components was measured at 51″.7, an increase since Messier’s time. The general consensus is that this is merely an optical double star rather than a physically connected system.

left|thumb|Winnecke 4 double star

External links

Messier 41 (also known as M41 or NGC 2287) is an w:open cluster in the Canis Major constellation. It was discovered by w:Giovanni Batista Hodierna before w:1654 and was perhaps known to w:Aristotle about w:325 BC.[2] M41 lies about four degrees almost exactly south of w:Sirius. It contains about 100 stars including several w:red giants, the brightest being a w:spectral type K3 giant near the cluster’s center. The cluster is estimated to be moving away from us at 23.3 km/s.[1] The w:diameter of the cluster is between 25 and 26 w:light years. Its age is estimated at between 190 and 240 million years old. M41 is also referred to as NGC 2287.

External links


The Orion Nebula (also known as Messier 42, M42, or NGC 1976) is a w:diffuse nebula situated southb of Orion’s Belt. It is one of the brightest w:nebulae, and is visible to the w:naked eye in the night sky. M42 is located at a distance of 1,344±20 w:light years[2][5] and is the closest region of massive w:star formation to w:Earth. The M42 nebula is estimated to be 24 light years across. Older texts frequently referred to the Orion Nebula as the Great Nebula in Orion or the Great Orion Nebula. Yet older, astrological texts refer to it as Ensis (w:Latin for “sword”), which was also the name given to the star w:Eta Orionis, which can be seen close to the nebula from Earth.[6]

The Orion Nebula is one of the most scrutinized and photographed objects in the night sky, and is among the most intensely studied celestial features.[7] The nebula has revealed much about the process of how w:stars and w:planetary systems are formed from collapsing clouds of gas and dust. Astronomers have directly observed w:protoplanetary disks, w:brown dwarfs, intense and turbulent motions of the gas, and the photo-ionizing effects of massive nearby stars in the nebula. There are also w:supersonic “bullets” of gas piercing the dense hydrogen clouds of the Orion Nebula. Each bullet is ten times the diameter of w:Pluto’s orbit and tipped with iron atoms glowing bright blue. They were probably formed one thousand years ago from an unknown violent event.

General information

The Nebula is in fact part of a much larger nebula that is known as the w:Orion Molecular Cloud Complex. The Orion Molecular Cloud Complex extends throughout the w:constellation of Orion and includes w:Barnard’s Loop, the w:Horsehead Nebula, M43, w:M78 and the w:Flame Nebula. Stars are forming throughout the Orion Nebula, and due to this heat-intensive process the region is particularly prominent in the w:infrared.

The nebula is visible with the naked eye even from areas affected by some w:light pollution. It is seen as the middle “star” in the sword of Orion, which are the three stars located south of Orion’s Belt. The star appears fuzzy to sharp-eyed observers, and the nebulosity is obvious through w:binoculars or a small w:telescope.

The Orion Nebula contains a very young w:open cluster, known as the Trapezium due to the asterism of its primary four stars. Two of these can be resolved into their component binary systems on nights with good seeing, giving a total of six stars. The stars of the Trapezium, along with many other stars, are still in their early years. The Trapezium may be a component of the much-larger Orion Nebula Cluster, an association of about 2,000 stars within a diameter of 20 light years. Two million years ago this cluster may have been the home of the w:runaway stars w:AE Aurigae, w:53 Arietis, and w:Mu Columbae, which are currently moving away from the nebula at velocities greater than 100 km/s.[8]

Observers have long noted a distinctive greenish tint to the nebula, in addition to regions of red and areas of blue-violet. The red hue is well-understood to be caused by Hα recombination line w:radiation at a w:wavelength of 656.3 nm. The blue-violet coloration is the reflected radiation from the massive O-class stars at the core of the nebula.

The green hue was a puzzle for astronomers in the early part of the 20th century because none of the known w:spectral lines at that time could explain it. There was some speculation that the lines were caused by a new element, and the name “nebulium” was coined for this mysterious material. With better understanding of atomic physics, however, it was later determined that the green spectra was caused by a low-probability w:electron transition in doubly w:ionized w:oxygen, a so-called “forbidden transition”. This radiation was all but impossible to reproduce in the laboratory because it depended on the quiescent and nearly collision-free environment found in deep space.[9]


Messier’s drawing of the Orion Nebula in his 1771 memoir, Mémoires de l’Académie Royale.

The Maya of w:Central America had a folk tale which dealt with Orion’s part of the sky, known as w:Xibalba.[10] Their traditional w:hearths included in their middle a smudge of glowing fire that corresponded with the Orion nebula. This is clear pre-telescope evidence that the Maya detected a diffuse area of the sky contrary to the pin points of stars.[11]

This nebula is currently visible to the unaided eye, yet oddly there is no mention of the nebulosity in the written astronomical records prior to the 17th century. In particular, neither w:Ptolemy in the w:Almagest nor Al Sufi in his w:Book of Fixed Stars noted this nebula, even though they both listed patches of nebulosity elsewhere in the night sky. Curiously this nebula was also not mentioned by Galileo, even though he made telescope observations of this part of the constellation in 1610 and 1617.[12] This has led to some speculation that a flare-up of the illuminating stars may have increased the brightness of the nebula.[13]

The Orion Nebula is generally credited as being first discovered in w:1610 by w:Nicolas-Claude Fabri de Peiresc as noted in Peiresc’s own records. Cysatus of w:Lucerne, a w:Jesuit astronomer, was the first to publish note of it (albeit somewhat ambiguously) in a book about a bright w:comet in 1618. It was independently discovered by several prominent astronomers in the following years, including w:Christiaan Huygens in 1656 (whose sketch was the first published in 1659). w:Charles Messier first noted the nebula on March 4, w:1769, and he also noted three of the stars in Trapezium. (The first detection of these three stars is now credited to Galileo in 1617, but he did not notice the surrounding nebula—possibly due to the narrow field of vision of his early w:telescope.) Charles Messier published the first edition of his catalog of deep sky objects in 1774 (completed in 1771).[14] As the Orion Nebula was the 42nd object in his list, it became identified as M42.

w:Spectroscopy done by w:William Huggins showed the gaseous nature of the nebula in 1865. w:Henry Draper took the first astrophoto of the Orion Nebula on w:September 30, w:1880, which is credited with being the first instance of deep-sky astrophotography in history.

In 1902, Vogel and Eberhard discovered differing velocities within the nebula and by 1914 astronomers at w:Marseilles had used the interferometer to detect rotation and irregular motions. Campbell and Moore confirmed these results using the spectrograph, demonstrating turbulence within the nebula.[15]

In 1931, Robert J. Trumpler noted that the fainter stars near the Trapezium formed a cluster, and he was the first to name them the Trapezium cluster. Based on their magnitudes and spectral types, he derived a distance estimate of 1,800 light years. This was three times further than the commonly accepted distance estimate of the period but was much closer to the modern value. [16]

In 1993, the w:Hubble Space Telescope first observed the Orion Nebula. Since then, the nebula has been a frequent target for HST studies. The images have been used to build a detailed model of the nebula in three dimensions. w:Protoplanetary disks have been observed around most of the newly formed stars in the nebula, and the destructive effects of high levels of w:ultraviolet energy from the most massive stars have been studied.[17]

In 2005, the Advanced Camera for Surveys instrument of the Hubble Space Telescope finished capturing the most detailed image of the nebula yet taken. The image was taken through 104 orbits of the telescope, capturing over 3,000 stars down to the 23rd magnitude, including infant w:brown dwarfs and possible brown dwarf w:binary stars.[18] A year later, scientists working with the HST announced the first ever masses of a pair of eclipsing binary brown dwarfs, 2MASS J05352184–0546085. The pair are located in the Orion Nebula and have approximate masses of 0.054 M and 0.034 M respectively, with an orbital period of 9.8 days. Surprisingly, the more massive of the two also turned out to be the less luminous.[19]


Optical images reveal clouds of gas and dust in the Orion Nebula; an infrared image (right) reveals the new stars shining within. Credit: C. R. O’Dell-Vanderbilt University, NASA, and ESA.

The entirety of the Orion Nebula extends across a 10° region of the sky, and includes neutral clouds of gas and dust, associations of stars, ionized volumes of gas and w:reflection nebulae.

The nebula forms a roughly spherical cloud that peaks in density near the core.[20] The cloud has a temperature ranging up to 10,000 K, but this temperature falls dramatically near the edge of the nebula.[21] Unlike the density distribution, the cloud displays a range of velocities and turbulence, particularly around the core region. Relative movements are up to 10 km/s (22,000 mi/h), with local variations of up to 50 km/s and possibly higher.

The current astronomical model for the nebula consists of an ionized region roughly centered on Theta1 Orionis C, the star responsible for most of the w:ultraviolet ionizing radiation. (It emits 3-4 times as much photoionizing light as the next brightest star, Theta2 Orionis A.[22]) This is surrounded by an irregular, concave bay of more neutral, high-density cloud, with clumps of neutral gas lying outside the bay area. This in turn lies on the perimeter of the Orion Molecular Cloud.

Observers have given names to various features in the Orion Nebula. The dark lane that extends from the north toward the bright region is called the “Fish’s Mouth”. The illuminated regions to both sides are called the “Wings”. Other features include “The Sword”, “The Thrust” and “The Sail”.[23]

Stellar formation

The Orion Nebula is an example of a w:stellar nursery where new stars are being born. Observations of the nebula have revealed approximately 700 stars in various stages of formation within the nebula.

Recent observations with the w:Hubble Space Telescope have yielded the major discovery of w:protoplanetary disks within the Orion Nebula, which have been dubbed proplyds.[24] HST has revealed more than 150 of these within the nebula, and they are considered to be systems in the earliest stages of w:solar system formation. The sheer numbers of them have been used as evidence that the formation of star systems is fairly common in our w:universe.

Stars form when clumps of w:hydrogen and other gases in an w:H II region contract under their own gravity. As the gas collapses, the central clump grows stronger and the gas heats to extreme temperatures by converting w:gravitational potential energy to w:thermal energy. If the temperature gets high enough, w:nuclear fusion will ignite and form a w:protostar. The protostar is ‘born’ when it begins to emit enough radiative energy to balance out its gravity and halt w:gravitational collapse.

Typically, a cloud of material remains a substantial distance from the star before the fusion reaction ignites. This remnant cloud is the protostar’s protoplanetary disk, where planets may form. Recent w:infrared observations show that dust grains in these protoplanetary disks are growing, beginning on the path towards forming w:planetesimals.[25]

Once the protostar enters into its w:main sequence phase, it is classified as a star. Even though most planetary disks can form planets, observations show that intense stellar radiation should have destroyed any proplyds that formed near the Trapezium group, if the group is as old as the low mass stars in the cluster.[17] Since proplyds are found very close to the Trapezium group, it can be argued that those stars are much younger than the rest of the cluster members.c

Stellar wind and effects

Once formed, the stars within the nebula emit a stream of charged particles known as a w:stellar wind. Massive stars and young stars have much stronger stellar winds than the w:Sun.[26] The wind forms shock waves when it encounters the gas in the nebula, which then shapes the gas clouds. The shock waves from stellar wind also play a large part in stellar formation by compacting the gas clouds, creating density inhomogeneities that lead to gravitational collapse of the cloud.

There are three different kinds of shocks in the Orion Nebula. Many are featured in w:Herbig-Haro objects:[27]

  • w:Bow shocks are stationary and are formed when two particle streams collide with each other. They are present near the hottest stars in the nebula where the stellar wind speed is estimated to be thousands of kilometers per second and in the outer parts of the nebula where the speeds are tens of kilometers per second. Bow shocks can also form at the front end of stellar jets when the jet hits interstellar particles.
  • Jet-driven shocks are formed from jets of material sprouting off newborn w:T Tauri stars. These narrow streams are traveling at hundreds of kilometers per second, and become shocks when they encounter relatively stationary gases.
  • Warped shocks appear bow-like to an observer. They are produced when a jet-driven shock encounters gas moving in a cross-current.

The dynamic gas motions in M42 are complex, but are trending out through the opening in the bay and toward the Earth.[28] The large neutral area behind the ionized region is currently contracting under its own gravity.


Panoramic image of the center of the nebula, taken by the Hubble Telescope. This view is about 2.5 light years across. The Trapezium is at center left. Credit:NASA/ESA.

w:Interstellar clouds like the Orion Nebula are found throughout galaxies such as the w:Milky Way. They begin as gravitationally bound blobs of cold, neutral hydrogen, intermixed with traces of other elements. The cloud can contain hundreds of thousands of w:solar masses and extend for hundreds of light years. The tiny force of gravity that could compel the cloud to collapse is counter-balanced by the very faint pressure of the gas in the cloud.

Whether due to collisions with a spiral arm, or through the shock wave emitted from w:supernovae, the atoms are precipitated into heavier molecules and the result is a molecular cloud. This presages the formation of stars within the cloud, usually thought to be within a period of 10-30 million years, as regions pass the w:Jeans mass and the destabilized volumes collapse into disks. The disk concentrates at the core to form a star, which may be surrounded by a protoplanetary disk. This is the current stage of evolution of the nebula, with additional stars still forming from the collapsing molecular cloud. The youngest and brightest stars we now see in the Orion Nebula are thought to be less than 300,000 years old,[29] and the brightest may be only 10,000 years in age.

Some of these collapsing stars can be particularly massive, and can emit large quantities of ionizing w:ultraviolet radiation. An example of this is seen with the Trapezium cluster. Over time the ultraviolet light from the massive stars at the center of the nebula will push away the surrounding gas and dust in a process called w:photo evaporation. This process is responsible for creating the interior cavity of the nebula, allowing the stars at the core to be viewed from Earth.[7] The largest of these stars have short life spans and will evolve to become supernovae.

Within about 100,000 years, most of the gas and dust will be ejected. The remains will form a young open cluster, a cluster of bright, young stars surrounded by wispy filaments from the former cloud. The Pleiades is a famous example of such a cluster.



^a 1,270 × tan( 66′ / 2 ) = 12 ly. radius
^b From temperate zones in the Northern Hemisphere, the nebula appears below the Belt of Orion; from temperate zones in the Southern Hemisphere the nebula appears above the Belt.
^c C. Robert O’Dell commented about this Wikipedia article, “The only egregious error is the last sentence in the Stellar Formation section. It should actually read

‘Even though most planetary disks can form planets, observations show that intense stellar radiation should have destroyed any proplyds that formed near the Trapezium group, if the group is as old as the low mass stars in the cluster. Since proplyds are found very close to the Trapezium group, it can be argued that those stars are much younger than the rest of the cluster members.'”


  1. ab “SIMBAD Astronomical Database”. Results for NGC 7538. Retrieved 2006-10-20. 
  2. ab Reid, M. J.; Menten, K. M.; Zheng, X. W.; Brunthaler, A.; Moscadelli, L.; Xu, Y.; Zhang, B.; Sato, M.; Honma, M.; Hirota, T.; Hachisuka, K.; Choi, Y. K.; Moellenbrock, G. A.; Bartkiewicz, A. (2009). “Trigonometric Parallaxes of Massive Star Forming Regions: VI. Galactic Structure, Fundamental Parameters and Non-Circular Motions”. The Astrophysical Journal, in press. Retrieved 2009-05-13. 
  3. “Nasa/Ipac Extragalactic Database”. Results for NGC 1976. Retrieved 2006-10-14. 
  4. Revised NGC Data for NGC 1976 per Wolfgang Steinicke’s NGC/IC Database Files.
  5. Hirota, Tomoya; Bushimata, Takeshi; Choi, Yoon Kyung; Honma, Mareki; Imai, Hiroshi; Iwadate, Kenzaburo; Jike, Takaaki; Kameno, Seiji; Kameya, Osamu; Kamohara, Ryuichi; Kan-Ya, Yukitoshi; Kawaguchi, Noriyuki; Kijima, Masachika; Kim, Mi Kyoung; Kobayashi, Hideyuki; Kuji, Seisuke; Kurayama, Tomoharu; Manabe, Seiji; Maruyama, Kenta; Matsui, Makoto; Matsumoto, Naoko; Miyaji, Takeshi; Nagayama, Takumi; Nakagawa, Akiharu; Nakamura, Kayoko; Oh, Chung Sik; Omodaka, Toshihiro; Oyama, Tomoaki; Sakai, Satoshi; Sasao, Tetsuo; Sato, Katsuhisa; Sato, Mayumi; Shibata, Katsunori M.; Shintani, Motonobu; Tamura, Yoshiaki; Tsushima, Miyuki; Yamashita, Kazuyoshi (2007). “Distance to Orion KL Measured with VERA”. Publications of the Astronomical Society of Japan 59 (5): 897-903.…59..897H. Retrieved 2009-05-13. 
  6. Allen, Richard Hinchley; Starnames, Their Lore and Meaning, 1889
  7. ab Press release, “Astronomers Spot The Great Orion Nebula’s Successor”, Harvard-Smithsonian Center for Astrophysics, 2006.
  8. Blaauw, A.; Morgan, W. W. (May 1954), “The Space Motions of AE Aurigae and μ Columbae with Respect to the Orion Nebula”, Astrophysical Journal 119: 625, doi:10.1086/145866,…119..625B 
  9. Bowen, Ira Sprague (w:October 1 w:1927), “The Origin of the Nebulium Spectrum” (Scholar search), Nature 120: 473, doi:10.1038/120473a0, 
  10. Kaufman, Anthony (November 2006). “Transcending Death: An interview with Darren Aronofsky, director of The Fountain”. Seed (November). Retrieved 2007-05-22. 
  11. Krupp, Edward C. (February 1999). “Igniting the Hearth”. Sky & Telescope (February): 94. Retrieved 2006-10-19. 
  12. James, Andrew (October 29, 2005). “The Great Orion Nebula: M42 and M43”. Southern Astronomical Delights. Archived from the original on 2012-06-28. Retrieved 2006-10-27. 
  13. Tibor Herczeg, Norman (January 22, 1999). “The Orion Nebula: A chapter of early nebular studies”. History of Astronomy. Retrieved 2006-10-27. 
  14. Messier, Charles (1774), “Catalogue des Nébuleuses & des amas d’Étoiles, que l’on découvre parmi les Étoiles fixes sur l’horizon de Paris; observées à l’Observatoire de la Marine, avec differens instruments.”, Mémoires de l’Académie Royale des Sciences (Paris), archived from the original on 2003-10-21, 
  15. Campbell, W. W.; Moore, J. H. (June 1917), “On the Radial Velocities of the Orion Nebula”, Publications of the Astronomical Society of the Pacific 29 (169): 143, doi:10.1086/122612,…29..143C 
  16. Trumpler, Robert Julius (August 1931), “The Distance of the Orion Nebula”, Publications of the Astronomical Society of the Pacific 43 (254): 255, doi:10.1086/124134,…43..255T 
  17. ab David F. Salisbury, 2001, “Latest investigations of Orion Nebula reduce odds of planet formation”.
  18. Robberto, M.; O’Dell, Robert C.; Hillenbrand, L. A.; Simon, M.; Soderblom, D.; Feigelson, E.; Krist, J.; McCullough, P. et al. (December 2005), “An overview of the HST Treasury Program on the Orion Nebula”, Bulletin of the American Astronomical Society (American Astronomical Society Meeting 207) 37: 1404, Also see the NASA Press Release.,…20714601R 
  19. K.G. Stassun, R.D. Mathieu and J.A. Valenti, “Discovery of two young brown dwarfs in an eclipsing binary system”, Nature, 440, 311-314, 16 March 2006.
  20. B. Balick et al., 1974, “The structure of the Orion nebula”, 1974, Astronomical Society of the Pacific, Vol. 86, Oct., p. 616.
  21. ibid, Balick, pg. 621.
  22. C. R. O’Dell, 2000, “Structure of the Orion Nebula”, Publications of the Astronomical Society of the Pacific, 113:29-40.
  23. “M-42”, Students for the Exploration and Development of Space, April 12, 2006.
  24. McCaughrean, Mark J.; O’dell, C. Robert. (May 1996), “Direct Imaging of Circumstellar Disks in the Orion Nebula”, Astronomical Journal 111: 1977, doi:10.1086/117934,….111.1977M 
  25. Kassis, Marc; Adams, Joseph D.; Campbell, Murray F.; Deutsch, Lynne K.; Hora, Joseph L.; Jackson, James M.; Tollestrup, Eric V. (February 2006), “Mid-Infrared Emission at Photodissociation Regions in the Orion Nebula”, The Astrophysical Journal 637 (2): 823–837, doi:10.1086/498404, Also see the press release,…637..823K 
  26. Ker Than, 11 January 2006, “The Splendor of Orion: A Star Factory Unveiled”,
  27. “Mapping Orion’s Winds”, January 16, 2006, Vanderbilt News Service
  28. ibid, Balick, pp. 623 624.
  29. “Detail of the Orion Nebula”, HST image and text.

External links

Messier 43 (also known as M43, De Mairan’s Nebula, and NGC 1982) is an w:H II region in the w:Orion constellation. It was discovered by w:Jean-Jacques Dortous de Mairan before w:1731. The De Mairan’s Nebula is part of the w:Orion Nebula, separated from the main nebula by a lane of w:dust. It is part of the much larger w:Orion Molecular Cloud Complex.

External links

The Beehive Cluster (also known as Praesepe (Latin for
“manger”), M44, NGC 2632, or Cr 189) is an w:open cluster in the
constellation Cancer. It is one of the nearest open clusters to the w:Solar System, and it contains a larger star population than most other nearby clusters. Under dark skies the Beehive Cluster looks like a nebulous object to the
naked eye; thus it has been known since ancient
times. The classical astronomer w:Ptolemy called it “the nebulous mass in the breast of Cancer,” and it was among the first objects that w:Galileo studied with his

The cluster’s age and proper motion coincide with those of the Hyades open cluster, suggesting that both share a similar origin.[4][5] Both clusters also contain w:red giants and w:white dwarfs, which represent later stages of stellar evolution, along with w:main sequence stars of w:spectral classes A, F, G, K, and M.

Currently there is no consensus on the cluster’s distance, with recent sources suggesting 160 to 187 w:parsecs (520-610 w:light years).[6][7][8] There is better agreement on its age, at about 600 million years.[9][7][5] This is equivalent to the age of the Hyades (~625 million years).[10] The bright central core of the cluster has a diameter of about 7 parsecs (22.8 light years).[9]

The Beehive is most easily observed when Cancer is high in the sky; in northern latitudes this occurs during the evening from February to May. At 95 arcminutes across, the cluster fits well in the field of view of a pair of binoculars or a telescope of low power.


Galileo was the first to observe the Beehive in a telescope, in 1609, and was able to resolve it into 40 stars. w:Charles Messier added it to his famous catalog in 1769 after precisely measuring its position in the sky. Along with the w:Orion Nebula and the Pleiades cluster, Messier’s inclusion of the Beehive has been noted as curious, as most of Messier’s objects were much fainter and more easily confused with comets. One possibility is that Messier simply wanted to have a larger catalog than his scientific rival Lacaille, whose 1755 catalog contained 42 objects, and so he added some bright, well-known objects to boost his list.[11]

Ancient Greeks and Romans saw this object as a manger from which two donkeys, the adjacent stars Asellus Borealis and Asellus Australis, are eating; these are the donkeys that w:Dionysos and w:Silenus rode into battle against the Titans.[12]

This perceived nebulous object is the main celestial object in the 23rd lunar mansion (Hsiu Kuei or Xiu Gui) of ancient Chinese astrology. Ancient Chinese skywatchers saw this as a ghost or demon riding in a carriage and likened its appearance to a “cloud of pollen blown from willow catkins.”

Morphology and Composition

Like many w:star clusters of all kinds, Praesepe has experienced w:mass segregation.[13][9][7] This means that bright, massive stars are concentrated in the cluster’s core, while dimmer, less massive stars populate its halo (sometimes called the “corona”). The cluster’s core radius is estimated at 3.5 parsecs (11.4 light years); its half-mass radius is about 3.9 parsecs (12.7 light years); and its tidal radius is about 12 parsecs (39 light years).[9][7] However, the tidal radius also includes many stars that are merely “passing through” and not bona fide cluster members.

Altogether, the Praesepe cluster contains at least 1000 gravitationally bound stars, for a total mass of about 500-600 Solar masses.[9][7] A recent survey counts 1010 high-probability members, of which 68% are M dwarfs, 30% are Sun-like stars of spectral classes F, G, and K, and about 2% are bright stars of spectral class A.[7] Also present are five giant stars, four of which have spectral class K0 III and the fifth G0 III.[4][14][7]

So far, eleven w:white dwarfs have been identified, representing the final evolutionary phase of the cluster’s most massive stars, which originally belonged to spectral type B.[5]w:Brown dwarfs, however, are extremely rare in this cluster,[15] probably because they have been lost by tidal stripping from the halo.[7]

The cluster has a visual brightness of magnitude 3.1. Its brightest stars are blue-white and of magnitude 6 to 6.5. w:42 Cancri is a confirmed member.


  1. (IAAC) OBJECT: M44 (Beehive cluster)
  2. Beehive Cluster – Encharta
  3. Messier 44: Observations and Descriptions, at
  4. ab Klein-Wassink WJ. (1927) The proper motion and the distance of the Praesepe cluster. Publications of the Kapteyn Astronomical Laboratory Groningen, 41: 1-48.
  5. abc Dobbie PD, Napiwotzki R, Burleigh MR, et al. (2006) New Praesepe white dwarfs and the initial mass-final mass relation. Monthly Notices of the Royal Astronomical Society, 369: 383-389.
  6. Pinfield DJ, Dobbie PD, Jameson F, Steele IA, Jones HRA, Katsiyannis AC. (2003) Brown dwarfs and low-mass stars in the Pleiades and Praesepe: Membership and binarity. Monthly Notices of the Royal Astronomical Society, 342: 1241-1259.
  7. abcdefgh Kraus AL, Hillenbrand LA. (2007) The stellar populations of Praesepe and Coma Berenices. Astronomical Journal, 134: 2340-2352.
  8. WEBDA at
  9. abcde Adams JD, Stauffer JR, Skrutskie MF, et al. (2002) Structure of the Praesepe Star Cluster. Astronomical Journal, 124: 1570-1584.
  10. Perryman M, Brown A, Lebreton Y, Gomez A, Turon C, Cayrel de Strobel G, Mermilliod J, Robichon N, Kovalevsky J, Crifo F. (1998) The Hyades: Distance, structure, dynamics, and age. Astronomy & Astrophysics, 331: 81-120.
  11. Frommert, Hartmut (1998). “Messier Questions & Answers”. Retrieved March 1, 2005.
  12. M44, Students for the Exploration and Development of Space, February 6, 2005.
  13. Portegies Zwart SF, McMillan SL, Hut P, Makino J. (2001) Star cluster ecology IV. Dissection of an open star cluster: Photometry. Monthly Notices of the Royal Astronomical Society, 321: 199-226.
  14. Abt HA, Willmarth DW. (1999) Binaries in the Praesepe and Coma star clusters and their implications for binary evolution. Astrophysical Journal, 521: 682-690.
  15. Gonzalez-Garcia BM, Zapatero Osorio MR, Bejar VJS, Bihain G, Barrado y Navascues D, Caballero JA, Morales-Calderon M. (2006) A search for substellar members in the Praesepe and Sigma Orionis clusters. Astronomy & Astrophysics, 460: 799-810.

External links

In astronomy, the Pleiades, or seven sisters, (Messier object 45) are an open star cluster in the constellation of Taurus. It is among the nearest star clusters to Earth and is the cluster most obvious to the naked eye in the night sky. Pleiades has several meanings in different cultures and traditions.

The cluster is dominated by hot blue stars that have formed within the last 100 million years. Dust that forms a faint reflection nebulosity around the brightest stars was thought at first to be left over from the formation of the cluster (hence the alternate name Maia Nebula after the star Maia), but is now known to be an unrelated dust cloud in the interstellar medium that the stars are currently passing through. Astronomers estimate that the cluster will survive for about another 250 million years, after which it will disperse due to gravitational interactions with its galactic neighborhood.

Other notable names of Pleiades include:

  • الثريا (al-Thurayya) in Arabic
  • כִּימָה in Biblical Hebrew
  • ثريا (Sorayya) in Persian and Urdu
  • 좀생이 (Jomsaeng-i) in Korean (usually suffixed with 별 byeol “star” or 성단 seongdan “star cluster”)
  • Subaru in Japanese
  • Matariki in Maori
  • Kṛttikā in Sanskrit
  • Parveen (پروین) in Persian, Urdu and Indian

Observational history

Comet Machholz appears to pass near the Pleiades in early 2005

The Pleiades are a prominent sight in winter in the Northern Hemisphere and in summer in the Southern Hemisphere, and have been known since antiquity to cultures all around the world, including the Māori (who call them Matariki) and Australian Aborigines, the Persians (who called them Parveen/parvin and Sorayya), the Chinese, the Maya (who called them Tzab-ek), the Aztec (Tianquiztli), and the Sioux of North America.

The Babylonian star catalogues name them MUL.MUL or “star of stars”, and they head the list of stars along the ecliptic, reflecting the fact that they were close to the point of vernal equinox around the 23rd century BC.
Some Greek astronomers considered them to be a distinct constellation, and they are mentioned by Hesiod, and in Homer’s Iliad and Odyssey. They are also mentioned three times in the Bible (Job 9:9 and 38:31, as well as Amos 5:8). The Pleiades (Krittika) are particularly revered in Hindu mythology as the six mothers of the war god Skanda, who developed six faces, one for each of them. Some scholars of Islam suggested that the Pleiades (Al thuraiya) are the Star in Najm which is mentioned in the Quran.

A Spitzer image of the Pleiades in infrared light, showing the associated dust. Credit: NASA/JPL-Caltech

They have long been known to be a physically related group of stars rather than any chance alignment. The Reverend John Michell calculated in 1767 that the probability of a chance alignment of so many bright stars was only 1 in 500,000, and so correctly surmised that the Pleiades and many other clusters of stars must be physically related.[4] When studies were first made of the stars’ proper motions, it was found that they are all moving in the same direction across the sky, at the same rate, further demonstrating that they were related.

Charles Messier measured the position of the cluster and included it as M45 in his catalogue of comet-like objects, published in 1771. Along with the Orion Nebula and the Praesepe cluster, Messier’s inclusion of the Pleiades has been noted as curious, as most of Messier’s objects were much fainter and more easily confused with comets—something which seems scarcely possible for the Pleiades. One possibility is that Messier simply wanted to have a larger catalogue than his scientific rival Lacaille, whose 1755 catalogue contained 42 objects, and so he added some bright, well-known objects to boost his list.[5]


The distance to the Pleiades is an important first step in the so-called cosmic distance ladder, a sequence of distance scales for the whole universe. The size of this first step calibrates the whole ladder, and the scale of this first step has been estimated by many methods. As the cluster is so close to the Earth, its distance is relatively easy to measure. Accurate knowledge of the distance allows astronomers to plot a Hertzsprung-Russell diagram for the cluster which, when compared to those plotted for clusters whose distance is not known, allows their distances to be estimated. Other methods can then extend the distance scale from open clusters to galaxies and clusters of galaxies, and a cosmic distance ladder can be constructed. Ultimately astronomers’ understanding of the age and future evolution of the universe is influenced by their knowledge of the distance to the Pleiades.

Results prior to the launch of the Hipparcos satellite generally found that the Pleiades were about 135 parsecs away from Earth. Hipparcos caused consternation among astronomers by finding a distance of only 118 parsecs by measuring the parallax of stars in the cluster—a technique which should yield the most direct and accurate results. Later work has consistently found that the Hipparcos distance measurement for the Pleiades was in error, but it is not yet known why the error occurred.[6] The distance to the Pleiades is currently thought to be the higher value of about 135 parsecs (roughly 440 light years).[2][3][7]


X-ray images of the Pleiades reveal the stars with the hottest atmospheres. Green squares indicate the seven optically brightest stars.

The cluster core radius is about eight light-years and tidal radius is about 43 light years. The cluster contains over 1,000 statistically confirmed members, although this figure excludes unresolved binary stars.[8] It is dominated by young, hot blue stars, up to 14 of which can be seen with the naked eye depending on local observing conditions. The arrangement of the brightest stars is somewhat similar to Ursa Major and Ursa Minor. The total mass contained in the cluster is estimated to be about 800 solar masses.[8]

The cluster contains many brown dwarfs, which are objects with less than about 8% of the Sun’s mass, not heavy enough for nuclear fusion reactions to start in their cores and become proper stars. They may constitute up to 25% of the total population of the cluster, although they contribute less than 2% of the total mass.[9] Astronomers have made great efforts to find and analyse brown dwarfs in the Pleiades and other young clusters, because they are still relatively bright and observable, while brown dwarfs in older clusters have faded and are much more difficult to study.

Age and future evolution

Ages for star clusters can be estimated by comparing the Hertzsprung-Russell diagram for the cluster with theoretical models of stellar evolution, and using this technique, ages for the Pleiades of between 75 and 150 million years have been estimated. The spread in estimated ages is a result of uncertainties in stellar evolution models. In particular, models including a phenomenon known as convective overshoot, in which a convective zone within a star penetrates an otherwise non-convective zone, result in higher apparent ages.

Another way of estimating the age of the cluster is by looking at the lowest-mass objects. In normal main sequence stars, lithium is rapidly destroyed in nuclear fusion reactions, but brown dwarfs can retain their lithium. Due to lithium’s very low ignition temperature of 2.5 million kelvins, the highest-mass brown dwarfs will burn it eventually, and so determining the highest mass of brown dwarfs still containing lithium in the cluster can give an idea of its age. Applying this technique to the Pleiades gives an age of about 115 million years.[10][11]

The cluster’s relative motion will eventually lead it to be located, as seen from Earth many millennia in the future, passing below the feet of what is currently the constellation of Orion. Also, like most open clusters, the Pleiades will not stay gravitationally bound forever, as some component stars will be ejected after close encounters and others will be stripped by tidal gravitational fields. Calculations suggest that the cluster will take about 250 million years to disperse, with gravitational interactions with giant molecular clouds and the spiral arms of our galaxy also hastening its demise.

Reflection nebulosity

Hubble Space Telescope image of reflection nebulosity near Merope

Under ideal observing conditions, some hint of nebulosity may be seen around the cluster, and this shows up in long-exposure photographs. It is a reflection nebula, caused by dust reflecting the blue light of the hot, young stars.

It was formerly thought that the dust was left over from the formation of the cluster, but at the age of about 100 million years generally accepted for the cluster, almost all the dust originally present would have been dispersed by radiation pressure. Instead, it seems that the cluster is simply passing through a particularly dusty region of the interstellar medium.

Studies show that the dust responsible for the nebulosity is not uniformly distributed, but is concentrated mainly in two layers along the line of sight to the cluster. These layers may have been formed by deceleration due to radiation pressure as the dust has moved towards the stars.[12]

Brightest stars in Pleiades

The nine brightest stars of the Pleiades are named for the Seven Sisters of Greek mythology: Sterope, Merope, Electra, Maia, Taygete, Celaeno, and Alcyone, along with their parents Atlas and Pleione. As daughters of Atlas, the Hyades were sisters of the Pleiades. The English name of the cluster itself is of Greek origin, though of uncertain etymology. Suggested derivations include: from πλεîν plein, to sail, making the Pleiades the “sailing ones”; from pleos, full or many; or from peleiades, flock of doves. The following table gives details of the brightest stars in the cluster:

Pleiades Bright Stars
Name Pronunciation (IPA & respelling) Designation Apparent magnitude Stellar classification
Alcyone /ælˈsaɪ.əni:/ Eta (25) Tauri 2.86 B7IIIe
Atlas /ˈætləs/ 27 Tauri 3.62 B8III
Electra /ɪˈlɛktrə/ 17 Tauri 3.70 B6IIIe
Maia /ˈmeɪə, ˈmaɪə/ 20 Tauri 3.86 B7III
Merope /ˈmɛrəpi:/ 23 Tauri 4.17 B6IVev
Taygeta /teɪˈɪdʒɪtə/ 19 Tauri 4.29 B6V
Pleione /ˈplaɪ.əni:/ 28 (BU) Tauri 5.09 (var.) B8IVep
Celaeno /sɪˈliːnoʊ/ 16 Tauri 5.44 B7IV
Sterope, Asterope /ˈstɛrɵpi:, əˈstɛrɵpi:/ 21 and 22 Tauri 5.64;6.41 B8Ve/B9V
18 Tauri 5.65 B8V


  1. abcd “SIMBAD Astronomical Database”. Results for M45. Retrieved 2007-04-20. 
  2. ab Percival, S. M.; Salaris, M.; Groenewegen, M. A. T. (2005), The distance to the Pleiades. Main sequence fitting in the near infrared, Astronomy and Astrophysics, v.429, p.887.
  3. ab Zwahlen, N.; North, P.; Debernardi, Y.; Eyer, L.; Galland, F.; Groenewegen, M. A. T.; Hummel, C. A. (2004), A purely geometric distance to the binary star Atlas, a member of the Pleiades, Astronomy and Astrophysics, v.425, p.L45.
  4. Michell J. (1767), An Inquiry into the probable Parallax, and Magnitude, of the Fixed Stars, from the Quantity of Light which they afford us, and the particular Circumstances of their Situation, Philosophical Transactions, v. 57, p. 234-264
  5. Frommert, Hartmut (1998) “Messier Questions & Answers”. Retrieved March 1, 2005.
  6. Soderblom D.R., Nelan E., Benedict G.F., McArthur B., Ramirez I., Spiesman W., Jones B.F. (2005), Confirmation of Errors in Hipparcos Parallaxes from Hubble Space Telescope Fine Guidance Sensor Astrometry of the Pleiades, The Astronomical Journal, v. 129, pp. 1616-1624.
  7. Turner, D. G. (1979),[2], Publications of the Astronomical Society of the Pacific, v. 91, pp. 642-647.
  8. ab Adams, Joseph D.; Stauffer, John R.; Monet, David G.; Skrutskie, Michael F.; Beichman, Charles A. (2001), The Mass and Structure of the Pleiades Star Cluster from 2MASS, The Astronomical Journal, v.121, p.2053.
  9. Moraux, E.; Bouvier, J.; Stauffer, J. R.; Cuillandre, J.-C. (2003), […400..891M Brown
    in the Pleiades cluster: Clues to the substellar mass function], Astronomy and Astrophysics, v.400, p.891.
  10. Basri G., Marcy G. W., Graham J. R. (1996), Lithium in Brown Dwarf Candidates: The Mass and Age of the Faintest Pleiades Stars, Astrophysical Journal v.458, p.600
  11. Ushomirsky, G., Matzner, C., Brown, E., Bildsten, L., Hilliard, V., Schroeder, P. (1998), Light-Element Depletion in Contracting Brown Dwarfs and Pre-Main-Sequence Stars, Astrophysical Journal v.497, p.253
  12. Gibson, Steven J.; Nordsieck, Kenneth H. (2003), The Pleiades Reflection Nebula. II. Simple Model Constraints on Dust Properties and Scattering Geometry, The Astrophysical Journal, v.589, p. 362

External links

Messier 46 (also known as M 46 or NGC 2437) is an
w:open cluster in the w:constellation of w:Puppis. It was discovered by
w:Charles Messier in w:1771. Dreyer described it as “very bright,
very rich, very large.” M46 is about 5,500 w:light-years away with an estimated age on the order of several 100 million years.[1]

The w:planetary nebula NGC 2438 appears to lie within the cluster near its northern edge (the faint smudge at the top center of the image), but it is most likely unrelated since it does not share the cluster’s radial velocity.[1][2] The case is yet another example of a superposed pair, joining the famed case of w:NGC 2818.

M46 is about a degree east of M47 in the sky, so the
two fit well in a binocular or wide-angle telescope field.


  1. abc Majaess D. J., Turner D., Lane D. (2007). In Search of Possible Associations between Planetary Nebulae and Open Clusters, PASP, 119, 1349
  2. Kiss, L. L., Szabó, Gy. M., Balog, Z., Parker, Q. A., Frew, D. J. (2008). AAOmega radial velocities rule out current membership of the planetary nebula NGC 2438 in the open cluster M46, MNRAS
  3. Mermilliod, J.-C., Clariá, J. J., Andersen, J., Piatti, A. E., Mayor, M. (2001). Red giants in open clusters. IX. NGC 2324, 2818, 3960 and 6259, A&A

External links

Open Cluster M47 (also known as Messier Object 47 or NGC 2422) is an w:open cluster in the w:constellation w:Puppis. It was discovered by w:Giovanni Batista Hodierna before w:1654 and independently discovered by w:Charles Messier on w:February 19, w:1771.

M47 is at a distance of about 1,600 w:light-years from w:Earth with an estimated age of about 78 million years. There are about 50 w:stars in this cluster, the brightest one being of magnitude +5.7.

External links

Messier 48 (also known as M 48 or NGC 2548) is an w:open cluster in the w:Hydra constellation. It was discovered by w:Charles Messier in w:1771. M48 is visible to the w:naked eye under good atmospheric conditions. Its age is estimated to amount 300 million years.

External links

Messier 49 (also known as M 49 or NGC 4472) is an elliptical / w:lenticular galaxy about 49 million w:light-years away in the w:constellation Virgo. The galaxy was discovered by w:Charles Messier in w:1771.[3]


The only w:supernova observed within this galaxy is w:SN 1969Q.[4] The supernova was discovered in June 1969.[5]

Companion galaxies

w:NGC 4467 forms a visual pair with Messier 49[citation needed].

Virgo Cluster membership

Messier 49 is the brightest member of the w:Virgo Cluster.[6] The galaxy is located at the center of one of the subclusters within the Virgo Cluster.[6]

External links


  1. Jensen, Joseph B.; Tonry, John L.; Barris, Brian J.; Thompson, Rodger I.; Liu, Michael C.; Rieke, Marcia J.; Ajhar, Edward A.; Blakeslee, John P. (February 2003). “Measuring Distances and Probing the Unresolved Stellar Populations of Galaxies Using Infrared Surface Brightness Fluctuations”. Astrophysical Journal 583 (2): 712–726. doi:10.1086/345430.…583..712J. 
  2. abcdefghi “NASA/IPAC Extragalactic Database”. Results for NGC 4472. Retrieved 2006-09-26. 
  3. K. G. Jones (1991). Messier’s Nebulae and Star Clusters (2nd ed.). Cambridge: Cambridge University Press. ISBN 0-521-37079-5. 
  4. “NASA/IPAC Extragalactic Database”. Results for supernova search near name “NGC 4472”. Retrieved 2007-02-12. 
  5. R. Barbon, E. Cappellaro, F. Ciatti, M. Turatto, C. T. Kowal (1984). “A revised supernova catalogue”. Astronomy & Astrophysics Supplement Series 58: 735–750.…58..735B. 
  6. ab A. Sandage, J. Bedke (1994). Carnegie Atlas of Galaxies. Washington, D.C.: Carnegie Institution of Washington. ISBN 0-87279-667-1. 

Messier 50 (also known as M 50 or NGC 2323) is an w:open cluster in the w:constellation w:Monoceros. It was perhaps discovered by G. D. Cassini before w:1711 and independently discovered by w:Charles Messier in w:1772. M50 is at a distance of about 3,000 w:light-years away from w:Earth. It is described as a ‘heart-shaped’ figure.

External links

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