1. It is the nearest major galaxy to the Milky Way

The Andromeda Galaxy is located 2.5 million light-years from Earth. It is the closest large galaxy to the Milky Way and is one of a few galaxies that can be seen unaided from the Earth. Although it appears more than six times as wide as the full moon when photographed through a larger telescope, only the brighter central region is visible to the naked eye or when viewed using binoculars or a small telescope, making it appear similar to a star.

2. The king of our local group

The Andromeda Galaxy is large. At approximately 220,000 light years across, it is the largest galaxy of the Local Group, which also contains the Milky Way, the Triangulum Galaxy, and about 44 other smaller galaxies.

3. It is on a crash course with our Milky Way

In approximately 4.5 billion years the Andromeda Galaxy and the Milky Way are expected to collide. Fueled by gravity, the two galaxies are hurtling toward one another at 402,000 kilometers (249,791 miles) per hour. The two galaxies will collide head-on and fly through one another. For eons, the pair will continue to come together scrambling stars and redrawing constellations until eventually the two galaxies merge.

4. Andromeda contains twice the number of stars than the Milky Way

Andromeda contains one trillion stars, at least twice the number of stars in the Milky Way, which is estimated to be 200–400 billion. Moreover, the Andromeda Galaxy is estimated to be 1.5×1012  solar masses while the mass of the Milky Way is estimated to be 8.5×1011 solar masses (solar mass is a standard unit in astronomy which is used to indicate the masses of stars, clusters, galaxies and so on. It does not take into account the dark matter. Also 1 solar mass is approximately equal to the mass of our sun).

5. The first supernova to be detected outside the Milky Way

Known as Supernova 1885 for the year of its appearance, it was the first to be detected outside the Milky Way and the only supernova ever recorded in the Andromeda Galaxy. Ernst Albrecht Hartwig (1851-1923) observed it on August 20, 1885 at Dorpat Observatory in Estonia. The supernova reached a magnitude of six between August 17th and 20th but then faded to magnitude 16 by February 1890.

6. The Galaxy’s bright center

In 2005, astronomers using the Hubble Space Telescope discovered that the center of the Andromeda Galaxy is composed of an elliptical ring of older red stars and a smaller, brighter, and denser disk of young blue stars of around 200 million years old around the galaxy’s central super-massive black hole of around 140 million Solar-masses.

7. Satellites

Like the Milky Way, the Andromeda Galaxy has satellite galaxies, consisting of 14 known dwarf galaxies. Nine out of 14 low-mass satellites lying with 1.3 million light-years from Andromeda are found within a thin sheet running perpendicular through the galaxy’s disk, whose typical width is only 52,000 light years. The best known and most readily observed satellite galaxies are M32 and M110.