Some Astronomy: Galaxies
There are about 100 billion galaxies in the Universe. They contain nearly all ordinary matter, and some dark matter. Most contain billions of stars, and clouds of dust & gas. They can be 10,000 - 200,000 light years in diameter. Most galaxies are named with a combination of letters & numbers.
Galaxies are usually grouped into clusters of 20 - several thousand galaxies.
Clusters are grouped into superclusters.
Superclusters form filaments (broad bands), with large voids between them.
Galaxy Shapes
There are five basic galaxy shapes:
Elliptical galaxies are roughly oval-shaped, with a smooth profile. Their brightness profile is nearly featureless. They are more three-dimensional than spiral galaxies, and their stars are in random orbits around the centre
Elliptical galaxy.
Lenticular galaxies are disc-shaped. They have lost or used up most of their interstellar matter, and therefore have very little star formation occurring.
Two lenticular galaxies.
Spiral galaxies are also disc-shaped. They are a flat, rotating disc which contains stars, dust and gas. The central concentration stars is called the “bulge”.
They have spiral arms extending outwards, and star formation occurs here.. They are brighter than the surrounding disc, because they have young, hot stars in them.
Spiral galaxy.
Barred spiral galaxies are the same as spiral galaxies, but with a central bar-shaped structure extending from the central bulge. The spiral arms begin at the ends of the bar. There are about twice as many barred spiral galaxies as regular spiral galaxies.
Barred spiral galaxy.
Irregular galaxies are exactly that - irregular.
One example of an irregular galaxy - but there are many, many different shapes!
Black Holes & Quasars
Quasars are remote, highly energetic objects, and they have puzzled astronomers since the 1960′s. It is now believed that they were caused by the formation of black holes in the centres of early galaxies.
Black holes are areas of extremely compressed matter. It is believed that most large galaxies have black holes.