Andromeda Galaxy

Two-and-a-half million years ago, Earth was going through some big changes. It was the start of the Pleistocene epoch – a period of major ice ages. And the ancestors of modern humans were just appearing. That timescale puts a little perspective on the distance to M31, the Andromeda Galaxy – the closest major galaxy to the Milky Way. It’s two-and-a-half million light-years away. So when we look at the galaxy, we’re seeing it as it looked two-and-a-half million years ago. Andromeda and the Milky Way are much alike. Both of them are barred spirals – they have a long bar of stars in the center, with spiral arms wrapping off the ends of the bar. They’re about the same mass – roughly one trillion times the mass of the Sun. Andromeda appears to be wider, although it’s hard to measure the size of the Milky Way from our location inside it. Andromeda isn’t giving birth to as many stars as the Milky Way is. That may be because it collided with a smaller galaxy in the distant past. The collision triggered a huge wave of starbirth that used up most of the star-making material – limiting the future population of the Andromeda Galaxy. M31 is well up in the east-northeast at nightfall. Under fairly dark skies, it looks like a faint smudge of light wider than the Moon – the most-distant object that’s easily visible to the eye alone. We’ll have more about the Andromeda Galaxy tomorrow. Script by Damond Benningfield