The Andromeda is the biggest nearer bared spiral galaxy to the Milkyway galaxy. Though it is about 2.5 million light years away from the Milkyway, it is visible by unaided eyes in clear night. The Andromeda galaxy is also known as M31, Messier 31 or NGC 224. It is named after the Ethiopian princess, the wife of Persues in Greek mythology. The Milkyway and Andromeda are assumed to be collided after 4.5 billion years and convert into giant Elliptical Galaxy. The Andromeda is best visible during autumn nights in northern hemisphere when is passes high overhead.
In 964, the Persian astronomer Abd al-Rehman al sufi was the first who described about Andromeda galaxy as “nebulous smear”. Simon Marius gave first telescopic observation while Isaac Roberts took first photograph of Andromeda in 1888. In 1943 Walter Baade was the first person who discovered two different populations of stars in Andromeda, one is young, high velocity stars in disk type I and older, red stars in bulged type II.
Formation of Andromeda Galaxy was done 10 billion years ago by collision and subsequent merger of small galaxies. After the collision for about 100 million years, star formation rate is very high which causes luminous infrared galaxy. Over past two billion years, the star formation rate in disk of Andromeda is decreased to nearly inactive. There is interaction between galaxies like M32, M110 etc. which was already absorbed by Andromeda Galaxy. This interaction creates structure like “Andromeda’s Giant Stellar Stream”.
Andromeda Galaxy is covered by massive halo of hot gases which contain about half of the stars of galaxy. This halo is believed to be formed at the time of the formation of galaxy which is enriched by heavier elements than hydrogen and helium.
The Andromeda galaxy is classifies as SAb galaxy. Like Milkyway galaxy, Andromeda galaxy is also barred spiral galaxy. The Andromeda galaxy is about 2,20,000 light years in diameter which is inclined at 77o relative to the earth. The maximum rotational velocity of the galaxy is 225km/s at 1300 light years from the centre while reaches at minimum at 50km/s at 7000 light years from centre and further at 33000 light years from centre, it reaches to 250km/s. Again at 80000 light years from the centre, velocity decreases to 200 km/s.
The mass of any galaxy is difficult to measure accurately but total mass of Andromeda galaxy is estimated about 8X1011 to 1.1X1012 solar mass. The stellar mass of Andromeda is 10-15 X 1010 solar mass. Among it 30% mass is in central bulge, 56% in disk and 14% in the stellar halo. Along with the stars Andromeda’s interstellar medium contains 7.2X109 solar mass in the form of neutral mass and 3.4X108 solar mass in the form of molecular hydrogen and 5.4X 107 solar mass of dust.
The gas and dust from the Andromeda galaxy form number of overlapping rings. One of the rings formed at 32000 ly from the core which is called “Ring of Fire”. This ring is not visible due to cold dust structure. Latest study shows that the Andromeda galaxy has two spiral arms which emerge from central bar and continue beyond the ring of fire. These arms are not continues but segment.
In 1991, the astronomers find the image of nucleus of Andromeda galaxy which shows two different concentrations in it at about 4.9 light years away from each other. One concentration is P1 which is brighter and offset from the core of galaxy. Other is P2 which is dimmer and located at the centre and contain black hole.
The Andromeda galaxy contains approximately 460 globular clusters. Globular One or Mayall II is the most luminosity cluster in the galaxy. It has millions of stars and brighter than Omega century-a brightest clusters in Milkyway galaxy. The Andromeda galaxy has about 20 known dwarf galaxies. M32 and M110 are main galaxies.
Collision with the Milkyway
The Andromeda galaxy is coming toward the Milkyway at the rate of 110km/s. Collision between Milkyway and Andromeda is precicted to be taken place in 4.5 million years and after the collision two galaxies will form giant elliptical galaxy or large disk galaxy. Existence of the Earth and Solar system after this collision is unknown. Before merging of two galaxies, solar system could be ejected from Milkyway or join to Andromeda.
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Authored by BK Suthar.
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