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Billions of galaxies exist in the known Universe, and not any two are explicitly the same. Galaxies are large compared to what we can comprehend, various thousands of light years from end to end (a light year is approx 300,000kms a second!), others are hundreds of thousands of light years from one side to the other.

Each galaxy is made up of tens of millions of stars, sometimes trillions of stars, and includes enough gas & dust to build another galaxy.

Most galaxy counts are done with the help of pc’s these days; we no longer employ folk to sit up all night counting them by hand!

Star counts for galaxies can be calculated by a a good number of ways, one of which is by dividing the total luminosity of a galaxy, by an average stars mass.

The measure between galaxies is usually millions of light years, but we can see collisions between them, & interactions as they pass each other on their infinite journey in some parts of the Universe. Our own galaxy, the Milky way, will eventually collide with Andromeda, are largest neighbour.

Even though there’re a number of different types of galaxies, they can be classified into four areas;

Spiral (examples Andromeda, Triangulum):

These are flattened disks with a spiral pattern within the disk. They have a large bulge in the center, with what’s believed to be a black hole in the middle, and from this bulge come the spiral arms which reach out to the extremity of the galaxy. It can usually be clearly seen in pictures that around the outside are enormous gas & dust clouds.

Their rotation is quite ordinary compared to a lot of things in the galaxy, always rotating in the same direction around the fundamental bulge. Generally you find the older stars in the center, while new young blue stars are forming amongst the gas & dust around the edges.

Barred (examples M95, Ngc 1365):

This is comparable to the Spiral, apart from the arms are more straight reaching from the bulge to extremity of the galaxy, rather than curved. Most astronomers now believe that the Milky way we live in is a barred galaxy.

Various of these can look irregular, but closer inspection, such as the Magellanic Cloud show bars within it, distorted by interactions with other galaxies in the local cluster we’re in.

Elliptical (examples M32, M49):

These are generally smooth in appearance, and unlike the two galaxy types above, the stars in them can be moving in any direction, & they have very little gas & dust within them, so this means very little star formation. Something that will result in a quicker end than spirals, although ‘quicker’ as in billions of years!

Irregular (examples Sextans a,M82):

These have no structure at all, & you get millions of stars in clumps, then a space, another clump, etc. Various have little star formation; others are full of gas & dust & have very high star birth rates.

Out of all the galaxy types, these are usually the hardest for the amateur astronomer to pick up due to the overall lack of brightness. Also these galaxies are usually nowhere near as big as the other types, with most being called dwarf.

It’s more than likely there’re a lot more of these galaxies than the others, but until full computerised sky surveys have taken place we do not know at the minute, and we may never be easily able to see them all due to the lack of luminosity from them.

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I now keep a blog of my observations, news, and any other waffle I care to tell the world about!! blogz4u.co.uk/telescope_microscope_astronomy/>Astronomy & Telescope Blog

Kevin Phillips

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