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 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 kilometres). 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 around the centre of our galaxy roughly once in 250 million years; that's 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.
Spiral galaxies are flattened disc shapes with a central bulge packed with predominantly old stars. The disc surrounding the bulge is full of stars, but it is only the bright ones that outshine the others that are visible. They appear as arms if stars that wind out from the bulge like spokes on a wheel and give the galaxy it's distinctive shape. Surrounding all this is a halo of old stars too faint to be seen.
Barred spirals have a bar-shaped hub of stars in the centre. The arms spiral out from each end of the bar. The Milky Way is thought to be a barred spiral, although it is difficult to tell from our limited viewpoint. The spirals and barred spirals are further sub-classified according to the size of their central bulge and the tightness of their arms (see the tuning fork below).
Elliptical galaxies appear ball shaped along the lines of a football, rugby ball or flattened ball or somewhere in between. This is just our viewpoint, close up we'd see that most are ellipsoids with three axis of different lengths.
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.
Any galaxy that does not appear to have a defined shape can be considered an irregular galaxy. Small dwarf galaxies are often irregular, as are interacting galaxies where gravitation forces wreck havoc, deforming their shape.
Galaxy Classification "Tuning Fork"
Edwin Hubble introduced a classification scheme for galaxies that are referred to today as the "Hubble Tuning Fork Diagram." This scheme provides for subcategories of both elliptical and spiral galaxies and introduces two new primary types of galaxies, lenticular and irregular. Hubble realised that elliptical galaxies could be classified by how round or flat they look. He classified spiral galaxies by how large and bright their central regions are and how tightly their arms are wound. Hubble also noticed that some spiral galaxies have a bright line, or bar, running through their central regions, and called these barred spiral galaxies. A transition type between the elliptical and spiral galaxies, with a central bulge and a disk but no spiral arms, are known as lenticular galaxies. The final classification, irregular galaxies, are neither spiral nor elliptical and can have any number of shapes.
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.