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Compare the age of the universe with various other ages. The age of the  Earth is thought to be about $4.6 \times 10^9$ years. The age of the Sun   and the rest of the solar system is probably similar. The age of our  galaxy is estimated as about $10^{10}$ years.\subsection{Appendix 1: Galaxy types}  Galaxies come in two main types, spirals and ellipticals. Nobody knows  why. Ellipticals are fuzzy balls of stars, some spherical, others elongated  like buns (oblate) or AFL footballs (prolate). They don't have any structure,  and no matter what angle you look at them from, they just look like fuzzy  elliptical blobs. Spirals, on the other hand, have much more structure.  They are dominated by a flat disk of gas, stars and dust, looking a bit like  a gramaphone record. If you look at this disk face-on, you can see all sorts  of pretty patterns. Many spirals (but not all) show nice spiral patterns  in this disk. Others show rings, or bars, or combinations of these. Plenty  of spectacular examples of all these types can be found on the Virgo  plate. In the centre of spiral galaxies, in addition to the disk you  have something called the bulge, which is a lot like a small elliptical  galaxy sitting at the centre of the spiral. This bulge is hard to see in  a face-on spiral galaxy, but if you look at an edge-on one it can be pretty  obvious \ -- \ you have the flat disk, looking like a line, with a blob  in the middle, the bulge. Ask your demonstrator to show you some nice  examples.  The main theory for explaining these two types of galaxies these days is a   violent one. The idea is that if you collide two spiral galaxies, all  the stars get so muddled around, you end up with an elliptical. Can you  see any galaxies that look like they are colliding on the plates?   However, several very good astronomers don't believe this theory. The  debate goes on\ldots