Explanation of Galaxies Such as M49 and M91

Billions of galaxies reside in the known Universe, and not any two are exactly the same. Galaxies are large compared to what we can comprehend, most 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 and dust to build another galaxy.

Most galaxy counts are done with the benefit of computers these days; we no longer employ people to sit up all night counting them by hand!

Star counts for galaxies can be worked out 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, and interactions as they pass each other on their continuous journey in various parts of the Universe. Our own galaxy, the Triangulum, will at some point collide with Andromeda, are largest neighbour.

While there are a number of different kinds of galaxies, they can be classified into four areas;

Spiral (examples Sunflower, Milky way):
These are flattened disks with a spiral pattern within the disk. They have a large bulge in the middle, with what is believed to be a black hole in the middle, and from this bulge come the spiral arms which reach out to the border of the galaxy. It can usually be clearly seen in pictures that around the outside are huge gas and dust clouds.

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

Barred (examples M109, Ngc 1365):
This is comparable to the Spiral, apart from the arms are more straight reaching from the bulge to border of the galaxy, rather than curved. Most astronomers now believe that the Triangulum 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 are in.

Elliptical (examples M87, M49):
These are generally smooth in appearance, and unlike the two galaxy types above, the stars in them can be moving in any direction, and they have very little gas and 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,Wolf-lundmark-melotte):
These have no structure at all, and you get millions of stars in clumps, then a space, another clump, etc. Most have little star formation; others are full of gas and dust and have very high star birth rates.

Out of all the galaxy kinds, 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 kinds, with most being called dwarf.

It is more than likely there are 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 able to see them all due to the lack of luminosity from them.

Astronomy Forum Ive had an interest in astronomy and cosmology for years, and run a number of sites. Ive recently taken an interest in microscopy, something to do on those cloudy nights!

http://www.astronomy-forum.co.uk

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