On a clear, dark night, from the darkest of locations, the stars we see seem to stretch on forever. The number of stars seem immense.
From a country sky such as ours, we can make out perhaps 10,000 stars at the most. This is equivelant to the number of grains of sand in one handful of fine beach sand. The total number of stars in the entire milky way galaxy, however, is far greater. There are, perhaps, 400,000,000,000 stars in our galaxy alone. This would be the equivilant of the number of grains used if one were to fill a 20 foot by 20 foot house up to the second floor with sand.
The total number of stars in our visible universe, however, is far greater still. There are more stars in the known universe than there are grains of sand, on all the beaches, on the enitire planet Earth.
Galaxies form in groups, known as clusters, with anywhere from a few to a few dozen different galaxies often being found near one another. Why this is, is unknown. There are different theories, some of which state that the groups formed for the same reason galaxies themselves formed there.
It has been theorized that perhaps a million years after the big bang, when the new universe had cooled enough for matter to form, there were pockets of greater density in some areas then there were in other areas. The newly formed matter then gravitated towards itself, forming first stars, and then these stars fell together to form galaxies.
These galaxies, having formed in the same general region, then fell together to form what we today see as clusters of galaxies. It seems logical, of course, that the same laws we see in action today should have formed these, the largest structures known to the human race. There's only one problem. It should not have happened.
The big bang, it is believed, was perfectly smooth, with no greater amount of energy being produced in any one region than another. This would not allow the first clumping of matter to begin once it had formed. If this were the case, the universe would never have started forming any piece of matter greater than single atoms, life never would have evolved, and the universe today would be a consistent bath of single hydrogen and helium atoms (almost unheard of in the universe we know and love), like so much cosmic tomato soup. The reason for the inconsistency remains one of the great cosmological puzzles of the day.
Galaxies can form into spirals, ellipticals (shaped roughly like an egg), and the unusual irregular galaxies, which have strange, unusual shapes. Our own Milky way galaxy is a run of the mill, humdrum spiral galaxy, about 100,000 light years across, and perhaps 30,000 light years thick.
We are in a group of some 20 odd galaxies, each ranging from a few thousand stars to a few hundred billion stars each. Galaxies move in these groups, orbiting one another in a graceful, complicated pattern which create havoc for anyone attempting to predict the future paths of these families of stars. If the patterns of their orbits are so complicated, you may be inclined to ask, why don't they collide with one another? The answer to this is simple.... they do.
Galaxies collide frequently, and are the largest collisions that occur in the universe, as far as we know. Do these create titanic, star shattering explosions which then rip across the interstellar distances? The answer here is.. no.
The distances between stars are, on average, about 10,000 times the size of the stars themselves. When galaxies collide, there is very little chance of even a significant number of stars colliding. The galaxies pass through one another with little fanfare, the only sign of their passing being a change in the shape of each galaxy. This is possbly the source of the strange, irregular galaxies which we see here and there across the sky.
Clear skies, and good viewing.
