Andromeda Galaxy

Messier 31, NGC 224 - Great Andromeda Galaxy

The Great Andromeda Galaxy, Messier 31, is the nearest spiral galaxy to our own. Visible as a faint smudge on moonless nights, it is one of the farthest objects visible to the naked eye. As a mirror image of the Milky Way, this huge aggregation of stars, gas, and dust allows us to study all the features of our own galaxy that we cannot observe because we are inside it.

Discovery and History

The earliest known record of the Andromeda Galaxy was made in 964 B.C. by the Persian astronomer Abd al-Rahman al-Sufi. He described it as the "Little Cloud" in his Book of Fixed Stars. But it must have been known to Persian astronomers at Isfahan as early as 905 AD. It also appeared on a Dutch star map in 1500.

The first telescopic description of M 31 was given by Simon Marius in 1612, but he did not claim its discovery. Unaware of both Al Sufi's and Marius' observations, Giovanni Batista Hodierna independently rediscovered this object in 1654. Edmond Halley, in his 1716 treatise, credits the discovery of this "nebula" to the French astronomer Ismail Bouillaud, who observed it in 1661. In 1764, Charles Messier catalogued the object as number 31 in his famous list. Unaware of Al Sufi's earlier work, Messier incorrectly credited Marius as its discoverer.

The "Great Andromeda Nebula" was long believed to be one of the nearest gaseous nebulae. In 1785, William Herschel wrote (incorrectly) that, based on its color and magnitude, the distance of the great nebula "would not exceed 2000 times the distance of Sirius" - about 17,000 light years. He viewed M 31 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, with a thickness of 155 times that distance.

William Huggins, the pioneer of spectroscopy, observed the spectrum of M 31 in 1864. The Andromeda "nebula" displayed a star-like, continuous spectrum, unlike the line spectra of gaseous nebulae. From this, Huggins deduced that M 31 had a stellar nature.

The first photographs of M 31 were taken in 1887 by Isaac Roberts from his private observatory in Sussex, England. His long-duration exposures allowed the spiral structure of the "nebula" to be seen for the first time. But Roberts mistakenly believed that it was actually a solar system in formation, with its satellite galaxies as nascent planets.

In 1912, Vesto Slipher of Lowell Observatory measured the radial velocity of the Andromeda "nebula", using spectroscopy. He found it had the highest velocity yet measured, about 300 km/sec, moving toward of the Sun. This also pointed to the extra-galactic nature of the object.

The Great Debate

In 1917, Heber Curtis observed a nova within M 31, and discovered 11 more by searching the photographic record. These novae were, on average, 10 magnitudes fainter than those within the Milky Way. As a result Curtis became a proponent of the so-called "island universes" hypothesis that spiral nebulae were actually independent galaxies. The Great Debate between Harlow Shapley and Heber Curtis took place in 1920, concerning the nature of the Milky Way, spiral nebulae, and the dimensions of the universe.

Edwin Hubble settled the debate in 1925 when he found the first Cepheid variable in the Andromeda galaxy. This proved conclusively that M 31 lies far beyond the Milky Way, and established its true nature as a separate galaxy. Hubble published his epochal study of the Andromeda "nebula" as an extragalactic stellar system in 1929. But because Hubble was not aware of the two classes of Cepheid variables, he underestimated M 31's distance by more than a factor of two. This error was not discovered until the 200-inch Palomar telescope started observing M 31 in 1953.

In 1943, Walter Baade was the first person to resolve stars in the Andromeda Galaxy, and was able to discern two distinct stellar populations. He named 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 other galaxies. Dr. Baade also discovered the two types of Cepheid variables, which doubled the distance estimate of M 31.

Radio emission from the Andromeda Galaxy was first detected by Grote Reber in 1940, and the first radio maps of the galaxy were made in the 1950s at the Cambridge Radio Astronomy Group. The core of the Andromeda Galaxy is called 2C 56 in the 2nd Cambridge radio astronomy catalog.

Observing Andromeda

With an apparent magnitude of 3.4, the Andromeda Galaxy one of the brightest Messier objects. It is visible to the naked eye from areas with moderate light pollution, and can even be seen from urban areas with binoculars.

Although the apparent size of the Andromeda Galaxy is about 3 x 1 degrees - six times the size of the full Moon! - only the bright central region is visible to the naked eye. M 31 harbors a dense and compact nucleus at its center, giving the visual impression of a star embedded in the more diffuse surrounding bulge.

Astrophotographers can gather the fine, faint detail in the spiral arms. M 31 is classified as an "SA(s)b" spiral galaxy, with arms moderately wound up in a clockwise direction. Andromeda's galactic plane is oriented approximately 13° to our line of sight, and is therefore seen nearly edge-on.

The Andromeda galaxy's spiral arms are outlined by a series of H II regions that Baade described as resembling "beads on a string". The brightest star cloud in M 31 has been assigned its own NGC number (NGC 206); after William Herschel's discovery observation in 1786, he catalogued it as H V.36.

Like the Milky Way, the Andromeda Galaxy has satellite galaxies. Charles Messier found the two brightest: M 32 and M 110 are visible in binoculars, and conspicuous in small telescopes. These two are the best-known of a swarm of companions surrounding the Andromeda Galaxy. Others include NGC 147 and NGC 185 in Cassiopeia, as well as the very faint dwarf systems Andromeda I, II, III, IV, V, VI, VII, and VIII. Messier 33, the spiral galaxy in Triangulum, and its probable companion LGS 3 may belong to this subgroup, as well as the more remote Local Group member IC 1613, and possibly UGCA 86 or UGCA 92.

There are approximately 460 globular clusters associated with the Andromeda Galaxy. The most massive is catalogued as Mayall II, and nicknamed G1 ("Globular One"). It outshines the brightest globular in our own Milky Way (Omega Centauri) by a factor of two, and is the most luminous known globular in the Local Group of Galaxies. G1 has an apparent magnitude of 13.72, and can be glimpsed with telescopes of 10" or larger aperture under very favorable conditions. The globular with the greatest apparent brightness is G76, located in the eastern half of the south-west arm.
Properties and Evolution

At least four distinct techniques have been used to measure distances to the Andromeda Galaxy. Averaged together, they give a combined distance estimate of 2.54 million light years (780 kpc). Based on that distance, the 3.2° angular diameter of M 31 is over 140,000 light years. A tenuous sprinkle of stars extending 5.2° x 1.1° outward constitutes evidence of a vast, extended stellar disk more than 220,000 light-years in diameter - twice the size of our galaxy!

Although it is the largest galaxy of the Local Group, Andromeda may not be the most massive. Recent findings suggest that the Milky Way contains more dark matter, implying that is much denser than M 31. The total halo mass of M 31 is estimated at 1.23 trillion Suns, compared to 1.9 trillion for the Milky Way.

Observations by the Spitzer Space Telescope in 2006 revealed that M 31 contains one trillion stars, several times the number in our own galaxy (about 300 to 400 billion). The estimated luminosity of M31, some 26 billion Suns, is about 25% higher than the Milky Way's. However, the rate of star formation in the Milky Way is much higher, with M 31 only producing about 1 solar mass per year, compared with 3 to 5 solar masses for the Milky Way.

The rate of supernovae in M 31 is also half that of the Milky Way. Until now, only one supernova has been recorded in the Andromeda galaxy, in 1885, designated S Andromedae. At the time, M 31 was considered to be a nearby object, so the event was thought to be much less luminous, and accordingly named "Nova 1885".

The nucleus of M 31 has a luminosity in excess of the most luminous globular clusters. The Hubble Space Telescope has revealed that the Andromeda galaxy's is in fact double. The brighter concentration is offset from the center of the galaxy by about 5 light years. The dimmer concentration falls at the true center, and contains a black hole of up to 500 million solar masses.

The Andromeda Galaxy also displays notable interaction with its companion M 32. Computer simulations show that the smaller galaxy passed through the disk of M 31 more than 200 million years ago. This interaction was apparently responsible for a considerable amount of disturbance to the spiral structure of M31, and also stripped more than half the mass from the smaller M 32. M 31 also appears to be interacting with M 110, which contains a dusty lane that may indicate recent star formation.

In 2009, the first planet may have been discovered in the Andromeda Galaxy. This planet candidate was detected using a technique called microlensing, which is caused by the deflection of light by a massive object.

Corrected for the motion of our Solar System around the galactic center, the Andromeda Galaxy is approaching the Milky Way at about 100 km/sec. It is thus expected to collide with our own galaxy in about 2.5 billion years. A likely outcome of the collision is that the galaxies will merge to form a giant elliptical galaxy - a common event in large galaxy groups. The fate of the Solar System in such a collision is currently unknown. If the galaxies do not merge, the Solar System could be ejected from the Milky Way, or even join Andromeda!
From skyportal