About Our Galaxy — The Milky Way
What is the Milky Way?
The Milky Way is a galaxy, a slowly rotating cluster of more than 200 billion stars! It is one of many galaxies in our universe.
What is a galaxy?
Galaxies are large collections of stars, gas and dust. They may have millions to billions of stars, all held into a cluster by gravitational attraction to each other. Most galaxies are flat, but there are different shapes — some are spirals, some are elliptical, and some are irregular.
What does the Milky Way look like?
Our Milky Way galaxy looks a little like a pinwheel. It is a spiral galaxy, about 100,000 light years across, with a bulge in the center (called the nuclear bulge) that contains the nucleus, a wide, flat disk with distinct spiral arms, and a surrounding halo of stars. There are several “spiral arms” in our Milky Way: Sagittarius, Cygnus, Perseus, and Orion. Our solar system is located at the inner edge of the Orion arm. Our galaxy slowly spins and our solar system spins with it. Our solar system takes about 250 million years to make one complete orbit around the center of the galaxy.
The different parts of the galaxy contain different populations of stars. In the center of the galaxy, in the nuclear bulge, old metal-rich stars reside. Hot, young metal-rich stars — like our Sun — exist in the dusty arms of the disk. These tend to be bluish in color, giving the disk a blue tone. Old, metal-poor stars exist in the outer halo of our galaxy. These are the earliest formed stars in our galaxy.
Where are we in the Milky Way galaxy? We are in one of the spiral arms, about 30,000 light years from the center of the Milky Way galaxy — or about two-thirds of the way from the center.
Can you see the Milky Way? Yes! On a very dark night, away from bright lights, you can see a faint, hazy — or milky — band in the sky. This is the Milky Way. The hazy appearance is because there are so many stars that are very distant; your eye cannot distinguish the stars as separate points of light. By using binoculars or a telescope, you can see the individual stars.
Why do we see the Milky Way as a strip in the sky? The Milky Way is like a big, flat disk in space. We are looking into the wide part of the disk when we look at the band across the sky.
Where did our galaxy come from?
Our galaxy is believed to have formed from an immense, churning, slowly rotating, cloud of dust and hydrogen and helium gas that was perhaps 300,000 light years across. Gradually, the cloud began to collapse, becoming denser in the center. Pockets of more dense material formed in swirling portions. These became dense enough to be sites of early star formation, distributed throughout the cloud — the stars and clusters of the galaxy's halo. The early stars formed from the hydrogen and helium. As they aged, they evolved and created new, heavier, more metallic elements. When these stars eventually exploded in huge supernovas, they released the heavier elements back to the cloud.
As the rotating cloud collapsed further, it began to flatten into a slowly spinning disk. More and more stars formed from the dust and gas enriched in the heavier elements. These are the metal-rich stars in our galaxy's center bulge and scattered throughout its disk. Stars still are forming, evolving, and dying, each contributing to the gas and dust within our galaxy — each contributing to the formation of other stars.
Are there other galaxies?
Yes! There are billions of other galaxies in our universe.
Galaxies are organized into clusters. Our Milky Way galaxy is part of a Local Group, a cluster of about 35 galaxies, including the larger Andromeda galaxy, which is about 3 million light years away. There are other clusters of galaxies, and the clusters can be grouped into clusters of clusters — superclusters. The Local Supercluster is about 100 million light years across, and contains our Local Group and the Virgo cluster.