Intro to Galaxies
are the largest assemblages in the universe.
They are huge regions of space that contain hundreds of billions
of stars, planets,
gas, dust, and empty space. Some galaxies may also contain a black hole
in their center. They seem to have formed 10 to 20 billion years
ago. It is estimated that there are about 50 billion or so
galaxies in the universe, although we are not absolutely certain
of this number. This does not include dark
matter which may make up as much as 90% of the universe.
Galaxies are separated by huge voids of space. The closest galaxy
that is similar to ours is a total of 2.2 million light-years
away. The light that we are seeing now came left the galaxy when
the first humans were walking on earth.
Our galaxy was thought to be the only galaxy in
the universe until 1924. The astronomer Edwin Hubble (1889-1953)
thought differently. He analyzed some Cepheid variable stars and
proved that some of the fuzzy "nebulas" he was
observing were too far away to be part of our galaxy. He then
concluded that they must be part of another galaxy.
There are three main types of galaxies. These include
elliptical galaxies, spiral galaxies, and irregular galaxies.
When a tally was taken of the 1,000 brightest galaxies, it turned
out that 20% were elliptical, 75% were spiral, and 5% were
irregular. It is uncertain how many of each type of galaxy there
is besides the 1,000 brightest. There are variations of each type
of galaxy, which will be explained in the next few pages.