What is a Galaxy?
Billions of galaxies populate our universe, each one a vast group of stars that exist together in space. We live in one of these galaxies called the Milky Way, named after the path of milky light that it's stars make across the Earth's sky.
It was once thought to be the only galaxy in the universe, but astronomers now estimate that the universe contains about 125 billion galaxies.
Whichever direction we look we find galaxies. A single telescope image can include dozens of them and it's easy to think that the universe is brimming over with them, In reality however, the galaxies are separated by vast expanses of empty space. Multiply the diameter of a galaxy by about 20 and that's the distance between it and another one.
So What is a Galaxy?
By any standard, galaxies are large. Even the dwarf galaxies are measured in thousands of light years (1 light year = 9.46 million million kilometers). By comparison the Milky Way is about 100,000 light years across, and the largest galaxies are over a million light years across. The smallest galaxies contain a few million stars, whereas the Milky Way contains 500 billion and the largest contain a million million stars. each star follows it's own path around the galaxies centre. The sun goes round the centre of our galaxy roughly once in 250 million years; thats one galactic year.
The galaxies are held together by gravity and also contain gas, dust and large amounts of invisible "dark matter". Recent research suggests that at the very centre of many galaxies there exists a black hole.
Types of Galaxy
Galaxies come in four main types and these are classified according to a galaxies shape and structure. The four are: spiral, barred spiral, elliptical and irregular. This last type has no defined shape or structure so each irregular has an individual appearance.
Elliptical galaxies cover a great range of sizes from the dwarf galaxies which are the commonest and contain a few million stars, to the giant ellipticals containing many hundreds of billions of stars.
Galaxy Classification "Tuning Fork"
Our Galactic Neighbours
The Milky Way is one of over 40 galaxies in a cluster known as the Local Group. The closest member is the Canis Major Dwarf is colliding with our galaxy and is about 42,000 light years from the galactic centre. The farthest galaxy is about three million light years away. The two largest galaxies are the Andromeda galaxy and the Milky Way. the large and small Magellanic clouds are two irregular galaxies in the group.
For some stunning images of galaxies taken by the Hubble Space Telescope take a peek at HubbleSite.