Astronomy

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M31 - The Great Andromeda Galaxy. M32 and M110 are also visible in this image.

Orion ED80 f/7.5, canon 20D, 800 iso. 10 x 5min exposures stacked in Images Plus 2.83. Prettied up in photoshop.

M31 is about 2.4 million light years away and it is the most distant object viewable with the naked eye. Our Milky Way galaxy is falling toward M31 at nearly 1 million miles per hour.



Messier 31
Spiral Galaxy M31 (NGC 224), type Sb, in Andromeda
Andromeda Galaxy

Right Ascension 00 : 42.7 (h:m)
Declination +41 : 16 (deg:m)
Distance 2900 (kly)
Visual Brightness 3.4 (mag)

Apparent Dimension 178x63 (arc min)

Known to Al-Sufi about AD 905.


Excerpt from seds.org:

Messier 31 (M31, NGC 224) is the famous Andromeda galaxy, our nearest large neighbor galaxy, forming the Local Group of galaxies together with its companions (including M32 and M110, two bright dwarf elliptical galaxies), our Milky Way and its companions, M33, and others.

Visible to the naked eye even under moderate conditions, this object was known as the "little cloud" to the Persian astronomer Abd-al-Rahman Al-Sufi, who described and depicted it in 964 AD in his Book of Fixed Stars: It must have been observed by and commonly known to Persian astronomers at Isfahan as early as 905 AD, or earlier. R.H. Allen (1899/1963) reports that it was also appeared on a Dutch starmap of 1500. Charles Messier, who cataloged it on August 3, 1764, was obviously unaware of this early reports, and ascribed its discovery to Simon Marius, who was the first to give a telescopic description in 1612, but (according to R.H. Allen) didn't claim its discovery. Unaware of both Al Sufi's and Marius' discovery, Giovanni Batista Hodierna independently rediscovered this object before 1654. Edmond Halley, however, in his 1716 treat of "Nebulae", accounts the discovery of this "nebula" to the French astronomer Bullialdus (Ismail Bouillaud), who observed it in 1661; but Bullialdus mentions that it had been seen 150 years earlier (in the early 1500s) by some anonymous astronomer (R.H. Allen, 1899/1963).

It was longly believed that the "Great Andromeda Nebula" was one of the nearest nebulae. William Herschel believed, wrongly of course, that its distance would "not exceed 2000 times the distance of Sirius" (17,000 light years); nevertheless, he viewed it at the nearest "island universe" like our Milky Way which he assumed to be a disk of 850 times the distance of Sirius in diameter, and of a thickness of 155 times that distance.

It was William Huggins, the pioneer of spectroscopy, who noted in 1864 the difference between gaseous nebula with their line spectra and those "nebulae" with star-like, continuous spectra, which we now know as galaxies, and found a continuous spectrum for M31 (Huggins and Miller 1864).

In 1887, Isaac Roberts obtained the first photographs of the Andromeda "Nebula," which showed the basic features of its spiral structure for the first time.

In 1912, V.M. Slipher of Lowell Observatory measured the radial velocity of the Andromeda "nebula" and found it the highest velocity ever measured, about 300 km/sec in approach. This already pointed to the extra-galactic nature of this object. According to Burnham, a better value is about 266 km/sec, but R. Brent Tully gives 298 km/sec, and NED has again 300 +/- 4 km/s as the modern value. Note that all the previous values describe the motion with respect to our Solar System, i.e. heliocentric motion, not that w.r.t. the Milky Way's Galactic Center. The latter value can be obtained by correcting for the motion of our Solar System around that center. The modern values for Galactic rotation and heliocentric radial velocity yield that the Andromeda Galaxy and the Milky Way are approaching each other at about 100 km/sec.

In 1923, Edwin Hubble found the first Cepheid variable in the Andromeda galaxy and thus established the intergalactic distance and the true nature of M31 as a galaxy. Because he was not aware of the two Cepheid classes, his distance was incorrect by a factor of more than two, though. This error was not discovered until 1953, when the 200-inch Palomar telescope was completed and had started observing. Hubble published his epochal study of the Andromeda "nebula" as an extragalactic stellar system (galaxy) in 1929 (Hubble 1929).

At modern times, the Andromeda galaxy is certainly the most studied "external" galaxy. It is of particular interest because it allows studies of all the features of a galaxy from outside which we also find in Milky Way, but cannot observe as the greatest part of our Galaxy is hidden by interstellar dust. Thus there are continuous studies of the spiral structure, globular and open clusters, interstellar matter, planetary nebulae, supernova remnants (see e.g. Jeff Kanipe's article in Astronomy, November 1995, p. 46), galactic nucleus, companion galaxies, and more.

Some of the features mentioned above are also of interest for the amateur: Even Charles Messier found its two brightest companions, M32 and M110 which are visible in binoculars and conspicuous in small telescopes, and created a drawing of all three. These two relatively bright and relatively close companions are visible in many photos of M31, including the one in this page. They are only the brightest of a "swarm" of smaller companions which surround the Andromeda Galaxy, and form a subgroup of the Local Group. At the time of this writing (September 2003), at least 11 of them are known: Besides M32 and M110 these are NGC 185, which was discovered by William Herschel, and NGC 147 (discovered by d'Arrest) as well as the very faint dwarf systems And I, And II, And III, possibly And IV (which may however be a cluster or a remote background galaxy), And V, And VI (also called the Pegasus dwarf), And VII (also Cassiopeia dwarf), and And VIII. It is still not clear if M33, the smaller spiral galaxy in Triangulum, and its probable companion LGS 3 belong to this subgroup, or the more remote Local Group member IC 1613, or one of the possible member candidates UGCA 86 or UGCA 92.

The Andromeda Galaxy is in notable interaction with its companion M32, which is apparently responsible for a considerable amount of disturbance in the spiral structure of M31. The arms of neutral hydrogen are displaced from those consisted of stars by 4000 light years, and cannot be continuously followed in the area closest to its smaller neighbor. Computer simulations have shown that the disturbances can be modelled by a recent close encounter with a small companion of the mass of M32. Very probably, M32 has also suffered from this encounter by losing many stars which are now spread in Andromeda's halo.

The brightest globular cluster of the Andromeda Galaxy M31, G1, is also the most luminous globular in the Local Group of Galaxies; its apparent visual brightness from Earth is still about 13.72 magnitudes. It outshines even the brightest globular in our Milky Way, Omega Centauri, and can be glimpsed even by better equipped amateurs under very favorable conditions, with telescopes starting at 10-inch aperture (see Leos Ondra's article in Sky & Telescope, November 1995, p. 68-69). The Hubble Space Telescope was used to investigate globular cluster G1 in mid-1994 (published April 1996). While the easiest, G1 is not the only M31 globular cluster which is in the reach of large amateur telescopes: Amateur Steve Gottlieb has observed 18 globular clusters of M31 with a 44cm telescope. With their 14-inch Newton and CB245 CCD camera, observers of the Ferguson Observatory near Kenwood, CA have photographed G1 and four fainter M31 globulars. Barmby et.al (2000) have found 435 globular cluster candidates in M31, and estimate the total number at 450 +/- 100.

The astrophotographer is even better off, as he can gather the fainter light of the fine detail in the spiral arms, as in our image: Amateurs can obtain most striking pictures even with inexpensive equipment, from wide-field exposures to detailed close-ups. Also in photography, better equipment pays off, as is demonstrated by our image, which was obtained by (and is courtesy of) Texas amateur Jason Ware, with a 6-inch refractor. More information on this image is available.

The brightest star cloud in the Andromeda galaxy M31 has been assigned an own NGC number: NGC 206, because William Herschel had taken it into his catalog as H V.36 on the grounds of his discovery observation of October 17, 1786. It is the bright star cloud at the upper left, just below a conspicuous dark nebula, in our photograph (very conspicuous in the larger photo).

Despite the large amount of knowledge we now have about the Andromeda Galaxy, its distance, though among the best known intergalactic distances, is not really well-known. While it is well established that M31 is about 15-16 times further away than the Large Magellanic Cloud (LMC), the absolute value of this measure is still uncertain, and in current sources, usually given between 2.4 and 2.9 million light-years - a consequence of the uncertainty in the LMC distance and thus the overall intergalactic distance scale. E.g., the semi-recent correction from data by ESA's astrometrical satellite Hipparcos has pushed this value up by more than 10 percent, from about 2.4-2.5 to the about 2.9 million light-years we use here.

Under "normal" viewing conditions, the apparent size of the visible Andromeda Galaxy is about 3 x 1 degrees (our acurate value, given above, is 178x63 arc minutes, while NED gives 190x60'). Careful estimates of its angular diameter, performed with 2-inch binoculars, by the French astronomer Robert Jonckhere in 1952-1953, revealed an extension of 5.2 times 1.1 degrees (reported by Mallas), corresponding to a disk diameter of over 250,000 light years at its distance of 2.9 million light years, so that this galaxy is more than double as large as our own Milky Way galaxy ! Its mass was estimated at 300 to 400 billion times that of the sun. Compared to the newer estimates for our Milky Way galaxy, this is considerably less than the mass of our galaxy, implying that the Milky Way may be much denser than M31. These results are confirmed by new estimates of the total halo masses, which turn out to be about 1.23 trillion solar masses for M31, compared to 1.9 trillion for the Milky Way (Evans and Wilkinson, 2000).

The Hubble Space Telescope has revealed that the Andromeda galaxy M31 has a double nucleus. This suggests that either it has actually two bright nuclei, probably because it has "eaten" a smaller galaxy which once intruded its core, or parts of its only one core are obscured by dark material, probably dust. In the first case, this second nucleus may be a remainder of a possibly violent dynamical encountering event in the earlier history of the Local Group. In the second case, the duplicity of Andromeda's nucleus would be an illusion causes by a dark dust cloud obstructing parts of a single nucleus in the center of M31.

Up to now, only one supernova has been recorded in the Andromeda galaxy, the Supernova 1885, also designated S Andromedae. This was the first supernova discovered beyond our Milky Way galaxy, on August 20, 1885, by Ernst Hartwig (1851-1923) at Dorpat Observatory in Estonia. It reached mag 6 between August 17 and 20, and it was independently found by several observers. However, only Hartwig realized its significance. It faded to mag 16 in February 1890.

Messier 32
Elliptical Galaxy M32 (NGC 221), type E2, in Andromeda
A Satellite of the Andromeda Galaxy, M31

Right Ascension 00 : 42.7 (h:m)
Declination +40 : 52 (deg:m)
Distance 2900 (kly)
Visual Brightness 8.1 (mag)

Apparent Dimension 8x6 (arc min)

Discovered 1749 by Guillaume-Joseph-Hyacinthe-Jean-Baptiste Le Gentil de la Galaziere (Le Gentil).

Messier 32 (M32, NGC 221) is the small yet bright companion of the Great Andromeda Galaxy, M31, and as such a member of the Local Group of galaxies. It can be easily found when observing the Andromeda Galaxy, as it is situated 22 arc minutes exactly south of M31's central region, overlaid over the outskirts of the spiral arms. It appears as a remarkably bright round patch, slightly elongated at position angle 150-330 deg, and is easily visible in small telescopes. Its ellipticity is about E2, i.e. the smaller diameter, or axis, or its elliptically shaped image, projected along our line of sight, is about a fraction of 0.2, or 20 percent, shorter than its larger axis.

M32 is an elliptical dwarf of only about 3 billion solar masses, and a linear diameter of some 8,000 light years, very small compared to its giant spiral-shaped neighbor. Nevertheless and surprising for such a small galaxy, its nucleus is of comparable properties as that of M31: About 100 million solar masses, 5000 suns per cubic parsecs, are in rapid motion around a central supermassive object. Because of this nucleus, M32 is sometimes classified as cE2 instead of simply E2, e.g. by NED.

Near the center of this galaxy, the sky would be dominated by this object, and full with the members of this galaxy, while at the edges, only one hemisphere would be filled with them, the other showing only few outlying stars and the intergalactic space. Toward M31, this galaxy would give a fascinating view in the night sky of a virtual astronomer in the outskirts of M32.

M32 appears to us superimposed over the spiral arms of greater M31. Therefore, it is of interest if it lies before or behind the great galaxy's disk. Spectroscopic investigations have not shown any absorption which would be expected if its light had passed the interstellar matter in M31's disk, which suggests that M32 is closer to us than that portion of M31.

The radial velocity of M32 has been measured at 203 km/s (R. Brent Tully) or 205 +/- 8 km/s (NED) in approach in the heliocentric system, i.e., toward our Solar System; corrected for galactic rotation, M32 is currently about at rest (RV=0) w.r.t. the Milky Way's Galactic Center. Compared to M31, it is approaching about 100 km/s slower, and considering its closer distance, it is apporaching M31 at this velocity in the radial component.

M32 and the other bright companion of M31, M110, are the closest bright elliptical galaxies to us, therefore also the among best investigated. They were both first resolved into stars by Walter Baade in 1944 with the 100-inch Hooker telescope on Mt. Wilson when he also resolved the nucleus of M31 (Baade 1944). Baade recognized that their stars were mostly old population II stars, and about as bright (and thus at roughly the same distance) as M31, thus confirming their proximity to the large spiral galaxy. There are remarkable differences between these dwarf galaxies: While M32 is a typical generic elliptical, compact and of high surface brightness, M110 is much more loose, of lower surface brightness, and exposes peculiar structures; now, M110 is often classified as a dwarf spheroidal galaxy instead of elliptical. Remarkably, M32 has no globular clusters (again, in difference to M110 which has 8).

M32, like typical elliptical galaxies, is mostly made up of old stars, of which only the lower-mass, intrinsically fainter ones have survived to now; as usual in such old populations (e.g., also in globular clusters), the more massive stars have presumably ended their active, nuclear-burning lives long ago - they are now white dwarfs or neutron stars. However, spectra and color of this galaxy (M32 has an overall spectral type of G3 and color index B-V = +0.75) indicate that its stars have chemical abundances different from those in old globulars which are poor in heavy elements. Instead, there seems to be a population of stars richer in heavy elements, which are apparently much younger, only 2 or 3 billion years old, mixed between the old stars as minor contamination.

Between the stars of M32, some planetary nebulae have been found, but no clouds of interstellar matter, neither gas clouds nor dust lanes nor neutral hydrogen, nor any open clusters. Apparently, M32 is no more able to form any new stars, but consists of old stars, mixed up with some of intermediate age. According to investigations of multicolor data, this stellar population is much more similar to that of much larger elliptical than that of typical dwarfs of its size, which are typically of dwarf spheroidal type.

Novae occur in M32 occasionally. One recent nova was discovered in M32 on August 31, 1998 within the Lick Observatory Supernova Search Program by a team of astronomers from the University of California at Berkeley headed by E. Halderson (1998). This nova occurred about 28.5 arc seconds west and 44.7" south of the galaxy's nucleus and reached mag 16.5. Supernovae have not yet been observed in this galaxy.

As its stellar population, size of nucleus, and compactness indicate, M32 looks more like a much larger elliptical galaxy. Therefore, it seems possible that M32 was once much larger, but lost its outer stars, and also all globular clusters it may have had, in one or more past close encounters with the Andromeda Galaxy M31. These stars and clusters were absorbed by, or integrated in, and are now part of the halo of M31. That M32 has recently undergone a closer encounter with its larger neighbor is suggested because it apparently caused and left disturbances in the big galaxy's spiral pattern.

M32 was the first elliptical galaxy ever discovered, by Le Gentil on October 29, 1749. Charles Messier remarked in his description that he had first seen this object in 1757 (his first record of an observation of one of "his" objects), cataloged it on August 3, 1764, and included M32, together with M110, in his drawing of Andromeda's "Great Nebula". Halton Arp has included it as No. 168 in his Catalogue of Peculiar Galaxies.


Messier 110
Elliptical Galaxy M110 (NGC 205), type E6p, in Andromeda
A Satellite of the Andromeda Galaxy, M31

Right Ascension 00 : 40.4 (h:m)
Declination +41 : 41 (deg:m)
Distance 2900 (kly)
Visual Brightness 8.5 (mag)

Apparent Dimension 17x10 (arc min)

Discovered by Charles Messier in 1773.

Messier 110 (M110, NGC 205) is the second brighter satellite galaxy of the Andromeda galaxy M31, together with M32, and thus a member of the Local Group.

Curiously, this galaxy was discovered by Charles Messier on August 10, 1773, as described in the Connaissance des Tems for 1801, and depicted on his fine drawing of the "Great Andromeda Nebula" and its companions published in 1807. However, Messier did never himself include this object in his catalog, due to unknown reasons, perhaps a certain sloppiness in recording. It was the last additional object, added finally by Kenneth Glyn Jones in 1966. Independent of Messier's discovery, Caroline Herschel independently discovered M110 on August 27, 1783, little more than 10 years after Messier, and William Herschel numbered it H V.18 when he cataloged it on October 5, 1784.

The small elliptical galaxy M110 is at about the same distance as the Andromeda galaxy M31, about 2.9 million light years, as confirmed by Walter Baade in 1944, when he resolved it into stars (Baade 1944). It is of Hubble type E5 or E6 and is designated "peculiar" because it shows some unusual dark structure (probably dust clouds). M110 is now often classified as a dwarf spheroidal galaxy, not a generic elliptical one (this would make it the first ever known dwarf spheroid, of course). However, as it is much brighter than typical dwarf spheroids, Sidney van dan Bergh has recently introduced the term "Spheroidal Galaxy" for this and similar galaxies, including Local Group members NGC 147 and NGC 185. M110's mass was estimated to be between 3.6 and 15 billion solar masses.

Apparently, despite its comparatively small size, this dwarf elliptical galaxy has also a remarkable system of 8 globular clusters in a halo around it. The brightest of them, G73, is of about 15th magnitude and thus within the reach of large amateur telescopes; Steve Gottlieb has observed it with a 44-cm telescope together with M31 globulars, and amateurs at the Ferguson Observatory near Kenwood, CA obtained a CCD image showing 7 of them with their 14-inch Newtonian and CB245 CCD camera (via the M31 GC images page).