Milky Way is the galaxy (vast collection of stars) that includes the sun, the earth, and the rest of our solar system. The Milky Way, also known as the Milky Way Galaxy, or simply the Galaxy, contains hundreds of billions of stars. It also includes huge clouds of dust particles and gases that lie throughout it and depths of interstellar space. The term Milky Way also refers to the portion of the Milky Way Galaxy that can be seen from the earth. This is the outer edge of the Galaxy. On clear, dark nights, it appears to the naked eye as a broad, milky-looking band of starlight stretching across the sky. Dark gaps in the band are formed by dust and gas clouds that lie between the earth and the Galaxy's edge and block out light from the stars behind them.
Shape of the Galaxy. The Milky Way Galaxy is shaped like a thin disc with a bulge in the centre. Stars, dust, and gases fan out from the central bulge in long, curving arms that form a spiral (coiled) pattern. For this reason, astronomers classify the Milky Way Galaxy as a spiral galaxy. To an observer far above the Milky Way, the Galaxy would resemble a huge catherine wheel. However, because of our location inside the Galaxy, we see only the hazy light from the strip of stars that are visible as we look out toward the Galaxy's edge.
The flat part of the Milky Way disc contains many young stars and small, irregularly shaped groups of stars called open clusters, also known as galactic clusters. It also has most of the Galaxy's dust and gases. A vast number of older stars are in the central bulge of the disc. The bulge and disc are surrounded by a sphere of stars known as a halo. The halo contains relatively old stars in dense, spherical (ball-like) groups called globular clusters.
Size of the Galaxy. The diameter of the Milky Way Galaxy is about 100,000 light-years. A light-year is the distance that light travels in one year--about 9.46 trillion kilometres. The Milky Way is about 10,000 light-years thick at the central bulge and much flatter toward the edges of the disc. Our solar system is located in the outskirts of the Galaxy, about 25,000 light-years away from the centre. The distance between the stars in our section of the Milky Way averages about 5 light-years. Stars in the galactic centre are about 100 times closer together. Most astronomers estimate that the total mass of the Milky Way is more than 100 billion times that of the sun (see MASS). Much of the mass is concentrated toward the centre of the Galaxy.
The centre of the Galaxy. All stars and star clusters in the Milky Way orbit the centre of the Galaxy, much as the planets in our solar system orbit the sun. For example, the sun completes an almost circular orbit of the centre once about every 250 million years. Almost all the bright stars in the Milky Way orbit in the same direction. For this reason, the entire galactic system appears to rotate about its centre.
The clouds of dust and gases in the Milky Way Galaxy prevent us from seeing very far into its centre. However, astronomers studying radio waves and infrared rays--which can penetrate the clouds--have discovered that the central region gives off enormous amounts of energy. Studies with radio and infrared telescopes have also revealed a powerful gravitational force that seems to come from the exact centre of the Galaxy. Some astronomers believe that the Milky Way Galaxy's centre is a massive black hole, an invisible object whose gravitational pull is so great that not even light can escape from it (see BLACK HOLE). They think the centre's energy is generated when the black hole swallows gas and other matter from the region surrounding it.
