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\subsection{Appendix 1: Galaxy types}  Galaxies come in two main types, spirals and ellipticals.   The distinction between the two is likely due to their age (ellipticals tend to  be made of older stars) and environment (ellipticals are often found in denser  environments and near the centres of galaxy clusters).  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, with a central spherical  bulge.   If you look at the disk face-on, you can see all sorts  of pretty patterns.   Many spirals (but not all) have spiral-shaped arms in the disk.   Others show rings, bars, or combinations of these.   Plenty of spectacular examples of all these types can be found on the Virgo  plate.   Spiral galaxies contain a bulge in the centre of the disk, which is a lot   like a small elliptical galaxy sitting at the centre of the spiral.   The bulge can be hard to see when viewing a spiral galaxy face-on, but the bulge  is usually very apparent if the spiral galaxy is aligned edge-on.  One theory for explaining the two types of galaxies is that ellipticals are the   result of a collision between two spiral galaxies.  The idea is that in the collision the orderly structure of the spirals is   disrupted, with the result being a puffy, structureless elliptical galaxy.  Can you see any galaxies that look like they are colliding on the plates?