Image Credit & Copyright: Dieter Beer (http://www.starhopper.at/) & Patrick Hochleitner (http://www.photonhunter.at)
The Andromeda Galaxy (Messier 31 or NGC 224) is a barred spiral galaxy of more than 220,000 light-years across, located approximately 2.54 million light-years away in the Andromeda constellation. It is the largest galaxy of the Local Group, which also contains the Milky Way, the Triangulum Galaxy (M33), and about 30 other smaller galaxies.
Although the largest, Andromeda may not be the most massive, as recent findings suggest that the Milky Way contains more dark matter and could be the most massive of the two. Mass estimates for Andromeda (including dark matter) give a value of approximately 1.2 trillion solar masses compared to 1.9 trillion solar masses for the Milky Way. But, Andromeda has a higher stellar density than that of the Milky Way. Andromeda contains one trillion stars: at least twice the number of stars in our Milky Way, which is estimated to be 200–400 billion. There may be several stellar black holes or neutron stars in Andromeda, which are heating incoming gas to millions of kelvins and emitting X-rays.
Andromeda appears to have also significantly more common stars than the Milky Way, and the estimated luminosity of Andromeda is about 25% higher than that of our own galaxy. However the rate of star formation in the Milky Way is much higher, with Andromeda producing only about one solar mass per year compared to 3–5 solar masses for the Milky Way. Andromeda contains two distinct populations of stars based on their metallicity, young, high velocity stars in the disk and older, red stars in the bulge.
The rate of supernovae in the Milky Way is also double that of Andromeda. This suggests that Andromeda once experienced a great star formation phase, but is now in a relative state of quiescence, whereas the Milky Way is experiencing more active star formation. Should this continue, the luminosity in the Milky Way may eventually overtake that of Andromeda.
Andromeda has a bright yellow nucleus, dark winding dust lanes and young star clusters, while reddish star-forming regions are dotting the clockwise wound-up spiral arms. These arms appear to be tightly wound, although they are more widely spaced than in our galaxy. 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.
Andromeda’s nucleus consists of two concentrations separated by 4.9 light-years. The brighter concentration, designated as P1, is offset from the center of the galaxy and has no black hole at its center. The dimmer concentration, P2, falls at the true center of the galaxy and does contain a supermassive black hole. The velocity of material spiralling around it is about 160 km/s.
There are approximately 460 globular clusters in Andromeda, including a dense and compact star cluster at its very center. The most massive of these clusters, Mayall II, has a greater luminosity than any other known globular cluster in the Local Group of galaxies. It contains several million stars, and is about twice as luminous as Omega Centauri, the brightest known globular cluster in the Milky Way. Mayall II (G1) has several stellar populations and a structure too massive for an ordinary globular. As a result, some consider Mayall II to be the remnant core of a dwarf galaxy that was consumed by Andromeda in the distant past.
Unlike the globular clusters of the Milky Way, that show a relatively low age dispersion, Andromeda’s globular clusters have a much larger range of ages: from systems as old as the galaxy itself to much younger systems, with ages between a few hundred million years to five billion years.
In 2005, astronomers discovered a completely new type of star cluster in Andromeda. 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 these stars 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.
In 2009, the first planet has been discovered in the Andromeda Galaxy. This exoplanet was detected using a technique called microlensing, which is caused by the deflection of light by a massive object.
Both Andromeda and the Milky Way are likely to have accreted and assimilated about 1–200 small galaxies during the past 12 billion years. The most important event in Andromeda’s past history was the merger with another galaxy that took place about 8 billion years ago. This violent collision formed most of its (metal-rich) galactic halo and extended disk and during that epoch Andromeda’s star formation would be very high, to the point of becoming a Luminous infrared galaxy during roughly 100 millon years.
Andromeda and the Triangulum Galaxy had a very close passage 2-4 billion years ago. This event produced high levels of star formation across the Andromeda Galaxy’s disk -even some globular clusters- and disturbed Andromeda’s outer disk.
While there has been activity during the last 2 billion years, this has been much lower than during the past. There have been interactions with satellite galaxies like M32, M110, or others that have already disappeared, absorbed by Andromeda that have formed structures like Andromeda’s Giant Stellar Stream and a merger roughly 100 million years ago that is behind a counter-rotating disk of gas found in the center of M31 as well as the presence there of a relatively young (100 million years) stellar population. During this epoch, star formation through Andromeda’s disk decreased to the point of nearly shutting down to increasing again relatively recently.
The Andromeda Galaxy is approaching the Milky Way at about 100 to 140 kilometres per second (400 light-years every million years). The Andromeda Galaxy and the Milky Way are thus expected to collide in about 3.75 to 4.5 billion years. A likely outcome of the collision is that the galaxies will merge to form a giant elliptical galaxy. If the galaxies do not merge, there is a small chance that the Solar System could be ejected from the Milky Way or join Andromeda.
Like the Milky Way, the Andromeda Galaxy has satellite galaxies, consisting of 14 known dwarf galaxies. The best known and most observed satellites are M32 and M110. M32 may once have been a larger galaxy that had its stellar disk removed by Andromeda, when they had a close encounter more than 200 million years ago, and underwent a sharp increase of star formation in the core region, which lasted until the relatively recent past.
M110 also appears to be interacting with M31, and astronomers have found in the halo of Andromeda a stream of metal-rich stars that appear to have been stripped from both of these satellite galaxies. M110 contains a dusty lane, which may indicate recent or ongoing star formation.
Because Andromeda is one of the brightest Messier objects, it is visible to the naked eye on moonless nights even when viewed from areas with moderate light pollution. However, only the brighter central region is visible to the naked eye or when viewed using binoculars or a small telescope.