THE MILKY WAY GALAXY
The Sun is one star in our Galaxy, the Milky Way Galaxy. The Milky Way gets its name from the fact that it's hard to see stars in the densest parts of the Galaxy, so instead it looks like someone spilled milk across the night sky. The ancient Romans called it the "Via Galactica" or "road of milk". There are about 200 billion stars in the Milky Way, making it one of the largest galaxies in the Universe.
Here is a photograph of the Milky Way, as seen from Earth.
The center of our Galaxy is a bulge of stars, with a black hole at the very center. The black hole contains about a million suns' worth of mass. The dark splotches you see in the picture above are caused by dust particles which block the light from stars behind them. We can't see the black hole directly, but we can watch stars move around it.
The two purplish shapes to the lower right of the bulge are two little galaxies which orbit the Milky Way, much as the Earth orbits the Sun. They are called the Large and Small Magellanic clouds, and are very beautiful features of the night sky in the southern hemisphere.
The Milky Way is a "spiral galaxy", with a disk of stars rotating around the bulge. There are two basic types of galaxies, spiral galaxies and elliptical galaxies. Spiral galaxies have flat rotating disks of stars, whereas ellipticals are more spherical. Spiral galaxies look different depending on how they happen to be oriented in the sky.
Because the Sun is in the disk of the
Milky Way, it's hard for us to see the spiral structure
of the disk. However, radio astronomers have mapped the spiral structure
using spectra. The result is shown below. The Sun
is in the Orion spiral arm, and rotates around
the center of the Milky Way once every 240 million years. New stars
are forming in giant clouds of dust and molecular gas, shown mapped in
the picture below as irregular shapes. The molecular clouds lie along
the spiral arms, traced by the solid lines; dotted lines show arms
which we think exist, but which haven't been completely mapped
If we could fly above the disk of the Milky Way and look back at it, it might look like the galaxy below.
One of the amazing facts about the Milky Way is that most of its mass is not in stars or gas or dust. Astronomers have figured out the mass of the Milky Way by studying the motions of the stars and gas clouds, and calculating how much mass the Milky Way must have in order to keep the stars and gas clouds from flying away into intergalactic space. We find that 90% of the mass of the Milky Way must be something besides the objects like stars that we can see. Astronomers call this unseen mass the "dark matter" and do not know what it could be.
Although galaxies look like dense concentrations of stars they are really very empty. In the Milky Way, the average distance between stars is about 5 light years, or 30 trillion miles.
How can we understand the enormity of these distances in our galaxy? These analogies will help you:
If the Sun were the size of a baseball, the density of the stars in our galaxy would be comparable to scattering fifty baseballs across the United States, so that there would be one star per state.
If the distance from home plate to the pitcher's mound were equal to the distance from Earth to the Sun, the next star would be 800 miles away.