Galaxies are large collections of stars, gas and dust maintained together by the force of gravity. Galaxies contain anywhere from several millions of stars to more than a trillion, and their sizes range from a thousand to several hundred thousand light years in diameter. There are hundreds of billions of galaxies in the universe, and like stars they´re found in pairs and groups known as clusters. There are three basic shapes of galaxies: spirals, ellipticals and irregular galaxies. Andromeda and Milky Way galaxies are of the type of spiral galaxies. The Andromeda galaxy is the closest galaxy to the Milky Way galaxy.
The Milky Way and the Andromeda are both barred spiral galaxies (central bulge of stars with surrounding disc of stars). The Milky Way and the Andromeda galaxy belongs to the local group (group of more than 50 galaxies). Both are composed of similar stellar, dust and gas content. Like the Milky Way, the Andromeda Galaxy has satellite galaxies (orbiting galaxies), and both galaxies are believed to have supermassive black holes at their centers
The Andromeda Galaxy contains more than one trillion stars, while the Milky Way contains between 200-400 billion stars; however, the Milky Way contains more dark matter and could be more massive. The Andromeda Galaxy is approximately 2.5 million light years from Earth, while the Milky Way is the Galaxy containing the Earth and the solar system. The Milky Way is approximately 100,000.120,000 light years in diameter, while the Andromeda Galaxy is larger with 220,000 light years.
The name, Milky Way, derives from the ancient Greek word "galactos” which means milk. According to Greek mythology, while nursing Hercules, milk dripped from Hera´s (queen goddess) breast, spilling milk into the sky, and forming the Milky Way Galaxy. Andromeda takes its name after the constellation (Andromeda) in which it appears in the sky. In Greek mythology, Andromeda, daughter of Cepheus and Cassiopeia, was a princess who was taken in captivity and chained to a rock as an offer of sacrifice to a terrible Sea beast called Cetus. Eventually, she was saved by Perseus whom she married.
The Andromeda Galaxy´s extended halo may be comparable to that of the Milky Way, containing both metal-poor stars that increase in quantity with distance. Studies suggest that the two galaxies may share a common evolutionary origin. Both are likely to have accreted several low mass galaxies during the past 12 billion years; however, the Andromeda galaxy is believed to be wider and possibly brighter than the Milky Way Galaxy.
Halo of stars discovered
The Milky Way´s rotational velocity averages 552-630 km/s (342-395 miles/s), while the Andromeda´s rotational velocity is 225 km/s (139 miles/s). Astronomers discovered in an enormous halo of stars around Andromeda Galaxy, suggesting that Andromeda could be five times larger than previously thought. The halos of stars are so vast that they nearly overlap those of the Milky Way Galaxy.
Andromeda and Milky Way collision
The Andromeda Galaxy, which is two and a half million light years away, is steadily moving in the direction of the Milky Way. The two galaxies are approaching each other at speeds of 120 km/s (74 miles/s). According to eaae-astronomy.org, the Milky Way Galaxy and Andromeda are expected to collide in about 5 billion years. As a result of the collision, millions of new stars will be born due to the giant clouds of dust and gas contained in the two galaxies. From this collision the two galaxies will merge together to form one enormous elliptical galaxy.