Milky Way is the galaxy that contains the sun, Earth, and other objects in our solar system. It also includes hundreds of billions of stars besides the sun. Huge clouds of gas and dust lie throughout the galaxy, and they constantly form new stars. The Milky Way is so massive that about 10 smaller galaxies orbit it like satellites revolving around a planet.
The name Milky Way also refers to the part of the galaxy that can be seen with the unaided eye. On clear, dark nights, this portion of the galaxy appears as a broad, milky-looking band of starlight stretching across the sky. When observed through an entire year, this band appears to circle completely around us in the sky. Dark gaps in the band consist of clouds of gas and dust that block out light from the stars behind them.
Shape of the galaxy.
The Milky Way is shaped like a thin disk with a bulge in the center. The bulge consists of a vast, thick bar of stars. The bar stretches for about 27,000 light-years across the center of the galaxy. A light-year is the distance that light travels in one year—about 5.88 trillion miles (9.46 trillion kilometers).
Stars, dust, and gas fan out from the central bulge in long, curving arms that form a spiral (coiled) pattern. Astronomers believe that the spiral pattern originates at the ends of the bar. Therefore, they classify the Milky Way as a barred spiral galaxy. Barred spirals are a common type of galaxy, perhaps even more common than spiral galaxies without bars.
To someone far above the Milky Way, the galaxy would resemble a huge pinwheel. But our view of the Milky Way is dominated by the hazy light from a strip of nearby stars. There are two reasons why we have this view: (1) we are inside the galaxy, and (2) interstellar dust (dust between the stars) partially blocks the starlight.
The flat part of the Milky Way—the disk—contains many young stars and small, irregularly shaped groups of stars called open clusters or galactic clusters. The disk also has most of the galaxy’s gas and dust. A vast number of older stars are in the central bulge.
The bulge and disk are surrounded by two flattened spheres of stars known as halos. The inner halo rotates in the same direction as does the disk. The outer halo orbits in the opposite direction. The halos include many of the oldest stars in the galaxy. They also contain globular clusters—dense, spherical systems containing a few hundred thousand old stars.
Size of the galaxy.
The diameter of the Milky Way is about 100,000 light-years. The galaxy is about 10,000 light-years thick at the central bulge. Toward the edges of the disk, it is much flatter, with a thickness of only about 1,000 light-years.
Our solar system is in the outskirts of the galaxy, about 25,000 light-years from the center. The distance between the stars in our section of the Milky Way averages about 5 light-years. Stars in the galactic center are about 100 times closer together.
Astronomers estimate that the total mass of the Milky Way measures from hundreds of billions to about one trillion times that of the sun. The mass contained in stars is concentrated toward the center of the galaxy. Most of the mass of the galaxy, however, is spread out in the halo. This mass is not present in the form of stars, but rather as a poorly understood substance known as dark matter. This substance is called dark matter because there is no evidence that it gives off, reflects, or absorbs visible light or any other kind of electromagnetic radiation.
Dark matter interacts with other kinds of matter by means of the gravitational force. It therefore reveals its presence by its gravitational effect on the stars and other objects that we can observe. Because dark matter cannot yet be observed directly, astronomers are not certain about its composition.
The center of the galaxy.
All stars and star clusters in the Milky Way orbit the center of the galaxy. The sun completes an almost circular orbit of the center about every 240 million years. Almost all the bright stars in the Milky Way orbit in the same direction. Thus, the entire galactic system appears to rotate about its center.
The galactic center lies in the direction of the constellation Sagittarius. Because the central bulge of the galaxy is in that direction, this is where we can see one of the most prominent sections of the Milky Way—the Great Star Clouds in Sagittarius. These clouds are the portions of the central bulge that are not hidden by dust clouds in the foreground. They appear as continuous clouds because the vast numbers of stars cannot be viewed individually—except with powerful telescopes. Most of these stars are faint, cool, and as old as the galaxy itself.
Clouds of gas and dust prevent astronomers from using visible light to see into the center of the galaxy. However, radio waves, infrared light, and X rays can penetrate the clouds. By observing these kinds of radiation, astronomers have found unusual objects in the central regions.
In the innermost 1,000 light-years of the galaxy are a massive reservoir of gas and dust, strong magnetic fields, and many of the most massive stars in the galaxy. In addition, studies with radio telescopes and infrared telescopes have shown that a powerful gravitational force comes from the exact center of the galaxy. This force is so strong that the mass responsible for it must be about 4 million times as great as the mass of the sun. Furthermore, this tremendous mass must be packed into a volume of space smaller than the solar system.
Another piece of evidence supports the idea of a central black hole: An unusual source of radio and X-ray energy, called Sagittarius A*, is precisely at the center of the galaxy. Gas falling into a supermassive black hole would give off such energy.
