The Milky Way galaxy is most significant to humans because it is home sweet home. But when it comes down to it, our galaxy is a typical barred spiral, much like billions of other galaxies in the universe. Let's take a look at the Milky Way.
Location, location, location
A glance up at the night sky reveals a broad swath of light. Described by the ancients as a river, as milk, and as a path, among other things, the band has been visible in the heavens since Earth first formed. In reality, this intriguing line of light is the center of our galaxy, as seen from one of its outer arms.
The Milky Way is a barred spiral galaxy, about 100,000 light-years across. If you could look down on it from the top, you would see a central bulge surrounded by four large spiral arms that wrap around it. Spiral galaxies make up about two-third of the galaxies in the universe. [Infographic: Our Milky Way Galaxy: A Traveler's Guide]
Unlike a regular spiral, a barred spiral contains a bar across its center region, and has two major arms. The Milky Way also contains two significant minor arms, as well as two smaller spurs. One of the spurs, known as the Orion Arm, contains the sun and the solar system. The Orion arm is located between two major arms, Perseus and Sagittarius.
The Milky Way does not sit still, but is constantly rotating. As such, the arms are moving through space. The sun and the solar system travel with them. The solar system travels at an average speed of 515,000 miles per hour (828,000 kilometers per hour). Even at this rapid speed, the solar system would take about 230 million years to travel all the way around the Milky Way.
Curled around the center of the galaxy, the spiral arms contain a high amount of dust and gas. New stars are constantly formed within the arms. These arms are contained in what is called the disk of the galaxy. It is only about 1,000 light-years thick. [Photo Gallery: Stunning Photos of Our Milky Way Galaxy]
At the center of the galaxy is the galactic bulge. The heart of the Milky Way is crammed full of gas, dust, and stars. The bulge is the reason that you can only see a small percentage of the total stars in the galaxy. Dust and gas within it are so thick that you can't even peer into the bulge of the Milky Way, much less see out the other side.
Tucked inside the very center of the galaxy is a monstrous black hole, billions of times as massive as the sun. This supermassive black hole may have started off smaller, but the ample supply of dust and gas allowed it to gorge itself and grow into a giant. The greedy glutton also consumes whatever stars it can get a grip on. Although black holes cannot be directly viewed, scientists can see their gravitational effects as they change and distort the paths of the material around it, or as they fire off jets. Most galaxies are thought to have a black hole in their heart. [Photo Gallery: The Milky Way's Core]
The bulge and the arms are the most obvious components of the Milky Way, but they are not the only pieces. The galaxy is surrounded by a spherical halo of hot gas, old stars and globular clusters. Although the halo stretches for hundreds of thousands of light-years, it only contains about two percent as many stars as are found within the disk.
Dust, gas, and stars are the most visible ingredients in the galaxy, but the Milky Way is also made up of dark matter. Scientists can't directly detect the material, but like black holes, they can measure it based on its effect on the objects around it. As such, dark matter is estimated to make up 90 percent of the mass of the galaxy.
Not only is the Milky Way spinning, it is also moving through the universe. Despite how empty space might appear in the movies, it is filled with dust and gas — and other galaxies. The massive collections of stars are constantly crashing into one another, and the Milky Way is not immune.
In about four billion years, the Milky Way will collide with its nearest neighbor, the Andromeda Galaxy. The two are rushing towards each other at about 70 miles per second (112 km per second). When they collide, they will provide a fresh influx of material that will kick of star formation anew.
The Andromeda Galaxy is obviously not the most careful of drivers. It shows signs of having already crashed into another galaxy in the past. Although it is the same age as the Milky Way, it hosts a large ring of dust in its center, and several older stars.
Of course, the imminent collision shouldn't be a problem for inhabitants of Earth. By the time the two galaxies ram headlong, the sun will already have ballooned into a red giant, making our planet uninhabitable.
Milky Way facts
The Milky Way contains over 200 billion stars, and enough dust and gas to make billions more.
The solar system lies about 30,000 light-years from the galactic center, and about 20 light-years above the plane of the galaxy.
More than half the stars found in the Milky Way are older than the 4.5 billion year old sun.
The most common stars in the galaxy are red dwarfs, a cool star about a tenth the mass of the sun. Once thought unsuitable for potential life-bearing planets because such bodies would have to be too close to meet the criteria, red dwarfs are now considered potential suspects.
As late as the 1920s, astronomers thought all of the stars in the universe were contained inside of the Milky Way. It wasn't until Edwin Hubble discovered a special star known as a Cepheid variable, which allowed him to precisely measure distances, that astronomers realized that the fuzzy patches once classified as nebula were actually separate galaxies.
— Nola Taylor Redd, SPACE.com Contributor
Editor's Note: This article was updated Feb. 23 to correct the size of the galaxy.
- Galaxies: Collisions, Types & Other Facts
- The Andromeda Galaxy (M31): Location, Characteristics & Images
- The Expanding Universe: From the Big Bang to Today