The Milky Way Galaxy

The Milky Way is the galaxy that includes the Solar System, with the name describing the galaxy’s appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye. From Earth, the Milky Way appears as a band because its disk-shaped structure is viewed from within.

The Milky Way is a barred spiral galaxy with a diameter of about 26.8 ± 1.1 kiloparsecs (87,400 ± 3,590 light-years) light-years, but only about 1,000 light-years thick at the spiral arms (more at the bulge). At the center of the galaxy lies a powerful source of radio emissions, believed to be associated with a supermassive black hole. The Milky Way has several satellites, such as the Magellanic Clouds.

It is estimated to contain 100–400 billion stars and at least that number of planets. The Solar System is located at a radius of about 27,000 light-years (8.3 kpc) from the Galactic Center, on the inner edge of the Orion Arm, one of the spiral-shaped concentrations of gas and dust.

The Andromeda Galaxy

The Andromeda Galaxy is a barred spiral galaxy and is the nearest major galaxy to the Milky Way, where the Solar System resides. It was originally named the Andromeda Nebula and is cataloged as Messier 31, M31, and NGC 224. Andromeda has a diameter of about 46.56 kiloparsecs (152,000 light-years) and is approximately 765 kpc (2.5 million light-years) from Earth. The galaxy’s name stems from the area of Earth’s sky in which it appears, the constellation of Andromeda, which itself is named after the princess who was the wife of Perseus in Greek mythology.

Like the Milky Way, the Andromeda Galaxy has satellite galaxies, consisting of over 20 known dwarf galaxies. The Andromeda Galaxy’s dwarf galaxy population is very similar to the Milky Way’s, but the galaxies are much more numerous. The best known and most readily observed satellite galaxies are M32 and M110. Based on current evidence, it appears that M32 underwent a close encounter with the Andromeda Galaxy in the past. M32 may once have been a larger galaxy that had its stellar disk removed by M31 and underwent a sharp increase of star formation in the core region, which lasted until the relatively recent past.

Collision (merger) of galaxies

The Andromeda Galaxy and the Milky Way are currently in the process of approaching each other at a speed of about 110 kilometers per second. They are currently situated at a distance of about 2.5 million light-years from each other. The gravitational interaction between these two vast assemblages of stars and gas is causing them to slowly but steadily draw closer. The Milky Way and Andromeda galaxies are expected to collide in around 4–5 billion years, merging to potentially form a giant elliptical galaxy or a large lenticular galaxy, often referred to as Milkdromeda.