Lurking in the Andromeda constellation, south-west of Cassiopeia, the Andromeda Galaxy is a barred spiral galaxy, similar to the one we leave in. Formed roughly 10 billion years ago, Andromeda is the largest galaxy of the Local Group, which includes the Triangulum Galaxy and our Milky Way.
At nearly 2.5 million light years from Earth, Andromeda is the closest large galaxy to the Milky Way, and the farthest celestial object visible to the naked eye. First observations, in fact, date back to the year 964, long before the invention of the telescope.
The record belongs to the Persian astronomer Abd al-Rahman al-Sufi who included Andromeda in his book about “fixed stars” although referring to it as a “small cloud”. For centuries, in fact, Andromeda was considered as a nebula or a gaseous region, and it was only in the 20th century that astronomers were able to resolve the Andromeda spiral nebula into individual stars.
This then led to what is known as “the great debate”: for decades scientists argued on whether spiral nebulas where nearby objects within the Milky Way or large, distant galaxies of their own. It was only in the early 1920s that the debate was finally closed, thanks to Edwin Hubble’s calculations.
Today, after decades of research and studies, we know much more about Andromeda. Scientist believe it formed roughly 10 billion years ago from the collision and merger of several smaller protogalaxies, which formed most of the galaxy’s galactic halo and extended disk. Andromeda today is home to approximately one trillion stars and its diameter is about 200,000 light-years across.
We also know that Andromeda is on a collision course with the Milky Way, with the two galaxies approaching at a speed of around 110km per second. Should this speed remain constant, the merge will happen in about 4.5 billion years and will last around 2 billion years, resulting in a new giant galaxy that astronomers like to call Milkomeda. But don’t worry. Mergers like this are quite common in the universe and besides, humans will be long gone by the time this happens!