The Andromeda Galaxy is 2.48 million light years from earth. It is the nearest major galaxy to the Milky Way. It may be similar in estimated mass to the Milky way, but is larger by extension.
It was determined to be a separate galaxy when detected super nova in Andromeda were 10 magnitudes fainter, which led to the idea that they were in a more distant separate galaxy. This was verified later by Edwin Hubble and measurements of Cephid variable stars.
Andromeda is also known as Messier 31, M31 or NGC 224. It has an approximate diameter 200,000 light years, contains about 1 trillion stars, [1 x 1012], About twice those in the Milky Way. It is one of few galaxies visible to the naked eye, i.e. the most distant thing you can see with your eyes alone.
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 Ethiopian (or Phoenician) princess who was the wife of Perseus in Greek mythology. If you follow the left V in the Bright stars of Cassiopeia’s W, about 3 lengths of the height of the W, you should find the smudge easily in binoculars and by unaided eye in darker skies.
The term Milky Way is a translation of the Latin via lactea, from the Greek γαλακτικός κύκλος (galaktikos kýklos, “milky circle”). A hazy band of light in which stars can’t be individually distinguished by the naked eye.
Some Additional Trivia:
Milky Way – 100,000 to 120,000 light years wide and 1000 light years thick
Milky Way – 100 to 400 billion stars
An estimated 100 million black holes are part of that count.