What you will learn in Galaxies

1. 1. Our own galaxy

   Every face-on picture of the Milky Way is a model. To photograph the disc whole you would have to be 43,500 light-years above it, and nothing we have built will get there in under 770 million years.

2. 2. Why it looks like a band

   A thin disc seen from inside piles up along a great circle. In galactic coordinates that circle is the equator by definition; in equatorial coordinates it crosses the celestial equator at 62.9 degrees and looks arbitrary.

3. 3. Kinds of galaxy

   Hubble sorted galaxies by how they look, and the sorting turned out to track how they live: ellipticals are red and gas-poor, late spirals blue and gas-rich. The tuning fork is a classification, not a family tree.

4. 4. The rotation problem

   Stars at the edge of a galaxy orbit as fast as stars halfway in. No amount of visible matter does that, and the learner finds out by trying.

5. 5. Groups, clusters, the cosmic web

   The Milky Way is one of about 80 known galaxies in a Local Group 10 million light-years across, which is bound and collapsing while the universe around it expands.

6. 6. When galaxies meet

   Andromeda and the Milky Way will merge in about 4.5 billion years. The galaxies will pass through each other, the gas clouds will collide, and the chance that any given star hits another is about one in 27 billion.