Our galaxy is known as the Milky Way, and is sometimes referred to as the Galaxy (with a capital G). It was once believed that the Milky Way constituted the entire universe; but in the 1920s, observations by Edwin Hubble showed that the Milky Way is just one of many galaxies in the universe.
The Milky Way is a barred spiral galaxy. Its name comes from the galaxy's appearance when seen from a dark location on Earth. From our position near the galactic plane, we see the distant stars of the spiral arms as a "milky" band of light stretched across the night sky.
The visible disk containing the spiral arms has a diameter of over 100,000 light-years, but other structures may extend out up to 3 times further. The Milky Way is estimated to contain around 200 billion stars and possibly 100 billion planets. Our solar system is located about 26,500 light-years from the galactic center, on the inner edge of the Orion Spur.
The stars within 10,000 light-years of the galactic core form a bulge with a bar of stars extending out from it. The main spiral arms originate from the ends of the bar. The galactic center contains a strong radio source known as Sagittarius A*, which is believed to be a super-massive black hole.
In summary, the Galaxy can be divided into several overlapping regions based upon the type of stars and stellar motions found there.
• Thin Disk - a 2,000 light-year thick region surrounding the plane of the Galaxy containing the spiral arms and young stars.
• Thick Disk - a 7,000 light-year thick region centered on the galactic plane, extending above and below the thin disk and containing older stars with more eccentric orbits.
• Galactic Bulge - A swelling found in most spiral galaxies, centered on the galactic core and containing old stars with relatively high metallicity.
• Stellar Halo - A spherical region centered on the galactic core that contains the oldest stars in the Galaxy, including the globular clusters.
• Dark Matter Halo - A hypothetical spherical region extending 300,000 light-years from the galactic core. This volume is believed to be filled with dark matter, as inferred from the motion of neighboring galaxies.
The image used for the Milky Way in Our Galaxy is an artist's rendition based upon data from the Spitzer Space Telescope. This image was made in 2008 and at the time was our best guess of the overall look of our galaxy.