There was one class of nebula that was particularly interesting to many scientists. They were known as "spiral nebulas." One reason that these were of particular interest was that they were seen by some as new solar systems in the act of creation. Scientists such as LaPlace and Gould had developed ideas about how systems of planets like ours could come into existance. If the spiral nebulas were new planetary systems being formed, studying them would give the scientists more information to choose from the available theories and enough information to refine them or develop new theories.
M-51, known as The Great Spiral Nebula until about 80 years ago.
In ancient times the Milky Way was thought to be a cloudy mass. When Galileo turned his telescope on the Milky Way about 400 years ago, he saw that it was individual stars in such great numbers and so faint that they blend into a cloudy mass when viewed without a telescope. The spiral nebulas did not look like stars, though. They looked like clouds, even through telescopes.
What Is It?
In the early part of the 20th century, some scientists felt that the spiral nebulas were not as close to us as other stars. They felt that they could not be new solar systems forming, because they were so far away that they would have to be many light years across, far larger than out solar system. Finally, it became possible to measure distances to the spiral nebulas. The distances were shocking to those who saw our universe as being a few thousand light years across. The distances to the spiral nebulas were in the millions of light years.
Further study determined that the spiral nebulas are made out of stars. They are so far away that they don't resolve into stars as easily as the other known star clusters that are only hundreds or thousands of light years from us. At first, these other bodies of stars so far from us were called "island universes." But this was a mis-use of the word "universe." The word universe implies everything. There can't be more than one "everything." If they were there, they were part of "everything", too.
Up until that time the word Galaxy had been the name for the Milky Way (which is what the word means.) It got generalized into the common noun galaxy as a word for these other groups of stars millions of light years away, and the word universe went back to meaning absolutely everything. Though today the word universe gets misused as a way of describing other sorts of places that you can get to in mystical or science fictional ways. The rule is, though, that if you can get there or interact with it in any way, it's still part of the same universe. The only way something can be another universe is if there's no possible way to have anything at all to do with it.
How Far Can You See?
One galaxy outside our own is commonly known as being visible to the eye. This is the Andromeda Galaxy. While all the other stars, planets, and cloudy patches you can see in the sky with your eyes are from a few light seconds (the Moon) to a few thousand light years away, the Andromeda Galaxy is about 3 million light years away. It's a fuzzy patch in the constellation of Andromeda. It doesn't look like much, in fact it's hard to see if you look directly at it unless you're in a really dark area with good skies for astronomy overhead. It shows up a little better if you look a little bit away from where it is because our eyes are more sensitive to slight differences of light a little off center of our field of vision.
A photograph of the Andromeda Galaxy from 1888. At the time Andromeda was thought to be a nebula within our galaxy.
Both these galaxies are close to the Milky Way, relative to many other galaxies. They're both part of what's called the Local Group of galaxies. A small telescope can see many more, also quite close to us, but also many that are outside the Local Group.
How Many Galaxies Are There?
Look at all the stars you can see in the sky. Now look through a telescope. There are about 100 million stars in our galaxy, scattered like glowing sand across a black sheet of paper.
There are more galaxies that we have seen than there are stars in our galaxy, however. There are well over a billion galaxies. Everywhere that we look where our view of deep space is not blocked by something in our own galaxy, we see countless other galaxies.
Everything in this photo that doesn't have four little 'diffraction spikes' of light coming from it is a galaxy.
What Are Galaxies Made Of?
There's nothing mystical about the other galaxies in our universe. They are made of the same stuff as our own galaxy and the same stuff we see here on the Earth. They have the same elements in them, and they are made up the same way. Just because they are far away doesn't mean that they have different rules of physics than us. We see the same physics there as we see here on Earth.
They are very interesting, though. Just as you have to go to special places on the Earth to see a tall waterfall, there are special places in the universe that are unusual and beautiful.
To see more interesting galaxies, and take part in doing some galaxy science yourself, visit the Galaxy Zoo. You can see lots of interesting and beautiful galaxies there, learn something about how they are classified, and help with classifying galaxies yourself. There are so many galaxies that there aren't enough astrophysicists to study each and every one. So members of the public have been called upon to help! Head on over, and give it a look.