Galaxy
From the Simple English Wikipedia, the free encyclopedia that anyone can change
A galaxy is a group of many stars including gas, dust, and dark matter. Galaxies are held together by gravity. Everything in a galaxy moves around a center. The name galaxy is taken from the Greek word Galaxia meaning milky, a reference to the our own galaxy, the Milky Way.
There are three main types of galaxies: ellipticals, spirals, and irregulars. All Galaxies exist inside the universe. There are an estimated one hundred billion galaxies within distance we can see or the Observable Universe. Each galaxy contains roughly one hundred thousand to one trillion stars. This makes the number of stars in the Universe more than every grain of sand on every beach on Earth.
In terms of the number of stars, a small dwarf irregular galaxy like the Small Magellanic Cloud, has about one billion stars in it, but there are even smaller systems that are recognized as galaxies such as the Leo I and II dwarf galaxies with about 1 million stars in them, and the Draco System with a few hundred thousand stars in it. The largest star cluster, a globular cluster called Messier 15 has about 6 million stars, so we see that for small galaxies, there is a blurring together of what we mean by a galaxy and a large star cluster. In addition to their mass and numbers of stars, a galaxy is a collection of stars and gas which move through the universe independently of the Milky Way. Globular clusters are roundish swarms of stars that orbit the Milky Way, while the Leo and Draco Systems seem to be independent collections of stars.
Many galaxies also continue to form new generations of stars. The Milky Way, and all spiral shaped galaxies like it (see above image of NGC 2997), produce new stars at a rate of one or two stars per year. These stars are formed in the vast interstellar clouds that account for about 1 to 10 percent of the mass of these galaxies. Globular star clusters, on the other hand, are not currently forming stars because this activity happened billions of years ago and then stopped once all of the gas and dust clouds were used up.
It is hard to say if the galaxies we see are all that there really are. It is even hard to say if "are" means anything if we can never see or measure them. Models of our universe are sometimes finite, and sometimes infinite - if they are infinite we can never see all the galaxies. Often galaxies are also known as island universes.
