2 - A Galaxy with Billions of Stars

Summary

Here, I describe the size and structure of our home galaxy, the Milky Way, and the place within it of our own solar system. The number of stars within the galaxy is somewhere in the range from 100 billion to 500 billion. It now seems likely that the vast majority of these stars have planets. Habitable planets may be commoner in the galaxy’s disc than in its central bulge or outer halo. I use the constellation of Orion as a way to think about how to connect two-dimensional and three-dimensional pictures of the galaxy. Constellations are products of visual astronomy and are patterns in just two dimensions that disappear if we take a 3D perspective. In contrast, the Orion arm of the Milky Way is a real 3D structure whose existence is independent of our vantage point. The stars of the Orion constellation are all within the Orion arm. However, this correspondence is unusual. For example, the stars we see in the Sagittarius constellation are not in the Sagittarius arm. The final section of this chapter deals with the fact that galaxies are not static – they evolve over time. This evolution has consequences for the probability of life originating.