Galaxies are far apart. The Andromeda galaxy, which is also called M31 (Messier object #31), is the closest galaxy to us -- 2.2 million light years away. Astronomers usually measure intergalactic distances in terms of megaparsecs:
one parsec = 3.26 light years
one million parsecs = one megaparsec
one megaparsec (Mpc) = 3.26 million light years
The farthest visible galaxies are approximately 3,000 Mpc away, or about 10 billion light years.
History of Galaxies
Let's look at the history of galaxies in astronomy.
- The Greeks coined the term "galaxies kuklos" for "milky circle" when describing the Milky Way. The Milky Way was a faint band of light, but they had no idea what it was composed of.
- When Galileo looked at the Milky Way with the first telescope, he determined that it was made up of numerous stars.
- We've known for centuries that our solar system was located within the Milky Way because the Milky Way surrounds us. We can see it throughout the year in all parts of the sky, but it's brighter during the summer, when we're looking at the center of the galaxy. However, to astronomers in the 18th century and earlier, it wasn't clear that the Milky Way was a galaxy and not just a distribution of stars.
- In the late 18th century, astronomers William and Caroline Herschel mapped the distances to stars in many directions. They determined that the Milky Way was a disk-like cloud of stars with the sun near the center.
- In 1781, Charles Messier cataloged various nebulae (faint patches of light) throughout the sky and classified several of them as spiral nebulae.
- In the early 20th century, astronomer Harlow Shapely measured the distributions and locations of globular star clusters. He determined that the center of the Milky Way was 28,000 light years from Earth, near the constellations of Sagittarius and Scorpio, and that the center was a bulge, rather than a flat area.
- Shapely later argued that the spiral nebulae discovered by Messier were "island universes" or galaxies (retaining the Greek wording). However, another astronomer named Heber Curtis argued that spiral nebulae were merely part of the Milky Way. The debate raged on for years because astronomers needed larger, more powerful, telescopes to resolve the details.
- In 1924, Edwin Hubble settled the debate. He used a large telescope (100-inch diameter, larger than ones that were available to Shapely and Curtis) at Mount Wilson in California and resolved that the spiral nebulae had structure and stars called Cepheid variables, like those in the Milky Way. (These stars change their brightness regularly, and the luminosity is directly related to the period of their brightness cycle.) Hubble used the light curves of the Cepheid variables to measure their distances from Earth and found that they were much farther away than the known limits of the Milky Way. Therefore, these spiral nebulae were indeed other galaxies outside our own.
There are still many mysteries surrounding galaxy formation, but on the next page we'll explain some of the best theories about it.