Explanation of Galaxies such as M109 & Andromeda
Billions of galaxies reside in the known Universe, and not any two are completely the same. Galaxies are sizable 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 constructed of 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 assistance of computers these days; we no longer employ folk 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 distance 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 various parts of the Universe. Our own galaxy, the Milky way, will at some particular point collide with Andromeda, are largest partner.
While 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 middle, 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 pretty ordinary compared to a lot of things in the galaxy, always rotating in the same direction around the centralized 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 M58, Ngc 1300���):
This is similar to the Spiral, apart from the arms are more straight reaching from the bulge to edge of the galaxy, rather than curved. Various 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’re in.
Elliptical (examples M87, M110):
These are generally smooth in appearance, and unlike the two galaxy kinds 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 Ic1613,Messier 82):
These have no structure at all, & you can have millions of stars in clumps, then a space, another clump, etc. Some have little star formation; others are full of gas & dust & 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 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.
Astronomy Forum
Ive had an interest in astronomy & cosmology for years, and run a number of sites. Clear? Ive recently taken an interest in microscopy, something to do on those cloudy nights!
http://www.astronomy-forum.co.uk/
Keith Driscoll