Astronomy Picture of the Day

Andromeda Galaxy

The most distant object that most folks can see with the unaided eye is also the closest major galaxy to our own Milky Way. Although the Andromeda Galaxy lies a mind-bending 2.5 million light-years from Earth, its trillion stars are enough to emit a strong glow across the vast reaches of space. Despite the wide gap, the Milky Way and Andromeda are on a collision course and predicted to begin merging in roughly 4.5 billion years. (You’ve been warned.)

The Andromeda Galaxy was once thought to be a nebula. Early in the development of modern astronomy, any diffuse, undefined celestial body was called a ‘nebula,’ including what we now know to be a neighboring galaxy. Our modern definition of nebula refers to galaxies-in-training that will coalesce over cosmic time into something less…nebulous.