A long-time favorite sight for "backyard astronomers," the Milky Way appears in the night sky as a hazy strip of many millions of stars. As large and vast as its scope appears when seen even through a small telescope, the Milky Way is but a part of the Milky Way Galaxy, which contains some two hundred billion stars. Even that mind-boggling number pales in comparison to the estimated twenty trillion stars in the universe.
Photo. The Milky Way, as seen from the NASA Cosmic Background Explorer, a spacecraft launched in 1989. Notice the central bulge, or the nucleus. Courtesy NASA.
The Milky Way Galaxy is a spiral galaxy, nearly 100,000 light-years -- each an incredible 6 trillion miles -- in diameter with two large arms twisting around its central bulge, called the nucleus. A third, smaller arm curves not even half-way around the nucleus, but the two major arms wind around nearly one and one-half times each. The sun, always at the center of our Solar System, is located near the inside of one of the large arms.
As the Galaxy's arms rotate around the nucleus, the sun travels at over 600,000 miles per hour. Though this speed is extraordinary considering that the speed record for a jet plane is just shy of 600 miles per hour, 600,000 miles is a mere ten-millionth of a light-year. At this rate it takes the sun 225 million years to complete one full revolution around the center of the Galaxy.
As do many distances and sizes in space, the Milky Way Galaxy becomes less and less enormous and important as we look farther back. The Galaxy is only one of over two dozen galaxies nearby referred to as the Local Group.
The Andromeda galaxy, M31, resides about 2.2 million light-years away from the Milky Way galaxy, making it the nearest large galaxy. A spiral, Andromeda is very much like the Milky Way in many respects; but, with 300 billion stars, it is half again as large as our galaxy.
The Milky Way galaxy has two satellite galaxies, the Large and Small Magellanic Clouds, named for Ferdinand Magellan, the first to record seeing them. On his famous expedition to sail around the world, Magellan passed through Earth's southern hemisphere, where the two galaxies are visible to the naked eye -- Magellan lived in the 1500's, before Galileo popularized the telescope for astronomy.
Large Magellanic Cloud
About 170,000 light-years away from the Milky Way galaxy, the Large Magellanic Cloud is the nearest galaxy. But with only 15 billion stars, it is only one-quarter the size of our own galaxy. Still, the Large Cloud is quite important because it is the location of Supernova 1987A, an exploded star that for a time shone brightly but that is now dim and dead.
Farther away than the Large Cloud, the Small Magellanic Cloud is 200,000 light-years distant. It is roughly a third the size of the Large Cloud, with only 5 billion stars.
Viewing the Milky Way and its satellite galaxies helps us to understand that there are vast worlds that lie beyond our Earth and provides a basis for understanding the size of the Milky Way Galaxy. But it is also useful and interesting to occasionally put the extent and importance of our galaxy in perspective, as we have tried to do here.