The Star Garden
Galaxies
Galaxies are massive collections of matter that are bound together by their mutual gravitational attraction, they formed about four hundred million years after the big bang.
There are thought to be hundreds of billions of galaxies in the observable universe, ranging from dwarfs which contain tens of millions of stars, to giants which contain millions of billions. Galaxies have traditionally been categorised according to their appearance. Elliptical galaxies are shaped like ellipses, Spiral galaxies like the Milky Way are disc shaped and other galaxies are classified as irregular. Although most galaxies are separated by distances a thousand times their own diameter, they do sometimes interact and even collide.
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Galaxies are usually gravitationally bound to other galaxies in a cluster and clusters can become bound in superclusters. The largest gravitationally bound structure yet to be found is known as the Sloan Great Wall, which was discovered by American astronomer John Richard Gott III and Croatian astronomer Mario Juric in 2003, using information from the Sloan Digital Sky Survey in New Mexico. This is a 'wall' of galaxy clusters about 1.37 billion light years (over ten thousand billion, billion kilometres) long. It is about one billion light years from Earth.
The Milky Way and Andromeda make up the two largest spiral galaxies in the Local Group cluster, with at least fourteen galaxies orbiting the Milky Way. The Local Group is one of at least a hundred clusters within the Virgo Supercluster, which is part of the Pisces-Cetus Supercluster Complex.
The Milky Way is a barred spiral galaxy containing hundreds of billions of stars. It is over five hundred billion times as massive as the Sun. The galactic disc, composed of the galaxies spiral arms, is about one thousand light years (a million, billion kilometres) thick and about one hundred thousand light years wide. The disc contains many open star clusters, these are regions where many new stars are forming.
The Solar System is about twenty five thousand light years from the centre and is orbiting at over two hundred kilometres a second. It takes about two hundred and fifty million years for the Solar System to complete one orbit around the Galaxy and it has already travelled around at least twenty times.
A spherical halo of globular star clusters orbit beyond the galactic disc, ninety percent of these within a diameter of two hundred thousand light years. There is no active star formation in globular clusters and so the only stars left are very old. Beyond this, a spherical halo of dark matter extends across a diameter of up to six hundred thousand light years.
Globular cluster Open cluster
Like most galaxies, the centre of the Milky Way contains a bulge of stars which conceal a supermassive black hole. The Milky Way's central black hole is at least four million times as massive as the Sun and is only thirty times larger.
References
See NASA's profile of galaxies.