The Andromeda Galaxy, also known as Messier 31 or M31, is the closest large spiral galaxy to the Milky Way, located about 2.5 million light-years away in the constellation Andromeda.
It is a barred spiral galaxy with a diameter of approximately 200,000 to 260,000 light-years and contains around one trillion stars, making it the most massive galaxy in the Local Group, which also includes the Milky Way, Triangulum Galaxy, and about 50 smaller galaxies.
Under dark skies, Andromeda is visible to the naked eye as a faint, elongated smudge—its apparent size on the sky spans more than six times the diameter of the full Moon. A panoramic Hubble mosaic captured more than 100 million stars across a 40,000-light-year swath of its disk.
Observations from Hubble and other observatories have revealed Andromeda’s rich and turbulent history, including remnants of smaller galaxies it has absorbed and an enormous halo of hot gas that extends far beyond its visible disk.
The Andromeda Galaxy is approaching the Milky Way at about 110 kilometers per second, and while a direct collision is likely in about 4 to 5 billion years, recent studies suggest there is a possibility the two galaxies could instead pass near each other or merge in a more complex interaction.
Due to its proximity and similarities to the Milky Way, Andromeda serves as a vital reference for understanding spiral galaxy formation, structure, and future cosmic evolution—including the eventual fate of our own galaxy.