The name of our galaxy is the Milky Way. All of the stars that you see at night and our Sun belong to the Milky Way. When you go outside in the country on a dark night and look up, you will see a milky, misty-looking band stretching across the sky. When you look at this band, you are looking into the densest parts of the Milky Way: the disk and the bulge.

Our solar system is in a spiral arm called the Orion Arm, and is about two-thirds of the way from the center of our galaxy to the edge of the starlight. Earth is the third planet from the Sun in our solar system of nine planets.

The closest spiral galaxy is Andromeda, a galaxy much like our own Milky Way. It is 2.2 million light-years away from us. Andromeda is approaching our galaxy at a rate of 670,000 miles per hour. Five billion years from now it may even collide with our Milky Way galaxy.

By studying other galaxies, astronomers learn more about the Milky Way, the galaxy that contains our solar system. Answers to such questions as "Do all galaxies have the same shape?," "Are all galaxies the same size?," "Do they all have the same number of stars?," and "How and when did galaxies form?" help astronomers learn about the history of the universe. Galaxies are visible to vast distances, and trace the structure of the visible universe with their collections of billions of stars, gas, and dust.

A galaxy contains stars, gas, and dust. In a spiral galaxy like the Milky Way the stars, gas, and dust are organized into a bulge, a disk containing spiral arms, and a halo. Elliptical galaxies have a bulge-shape and a halo, but do not have a disk.

Bulge — A round structure made primarily of old stars, gas, and dust. The bulge of the Milky Way is roughly 10,000 light-years across. The outer parts of the bulge are difficult to distinguish from the halo.

Disk — A flattened region that surrounds the bulge in a spiral galaxy. The disk is shaped like a pancake. The disk of the Milky Way is 100,000 light-years across and 2000 light-years thick. It contains mostly young stars, gas, and dust, which are concentrated in spiral arms. Some old stars are also present.

Spiral arms — Curved extensions beginning at the bulge of a spiral galaxy, giving it a "pinwheel" appearance. Spiral arms contain a lot of gas and dust as well as young blue stars. Spiral arms are found only in spiral galaxies.

Halo — The halo primarily contains individual old stars and clusters of old stars (globular clusters). It may be over 130,000 light-years across. The halo also contains dark matter, which is material that we cannot see but whose gravitational force can be measured.

Stars, gas, and dust — Stars come in a variety of types. Blue stars, which are very hot, tend to have shorter lifetimes than red stars, which are cooler. Regions of galaxies where stars are currently forming are therefore bluer than regions where there has been no recent star formation. Spiral galaxies seem to have a lot of gas and dust, while elliptical galaxies have very little gas or dust.

Edwin Hubble classified galaxies into four major types: spiral, barred spiral, elliptical, and irregular (see also Question 8 and Question 10). Most galaxies are spirals, barred spirals, or ellipticals.

A spiral galaxy consists of a flattened disk containing spiral (pinwheel-shaped) arms, a bulge at its center, and a halo. Spiral galaxies have a variety of shapes, and they are classified according to the size of the bulge and the tightness and appearance of the arms. The spiral arms, which wrap around the bulge, contain many young blue stars and lots of gas and dust. Stars in the bulge tend to be older and redder. Yellow stars like our Sun are found throughout the disk of a spiral galaxy. These galaxies rotate somewhat like a hurricane or a whirlpool. (See a side view of a spiral galaxy, below.)

A barred spiral galaxy is a spiral that has a bar-shaped collection of stars running across its center.

An elliptical galaxy does not have a disk or arms; rather, it is characterized by a smooth, ball-shaped appearance. Ellipticals contain old stars and possess little gas or dust. They are classified by the shape of the ball, which can range from round to oval (baseball-shaped to football-shaped). The smallest elliptical galaxies (called dwarf ellipticals) are probably the most common type of galaxy in the nearby universe. In contrast to spirals, the stars in ellipticals do not revolve around the center in an organized way. The stars move on randomly-oriented orbits within the galaxy like a swarm of bees.

An irregular galaxy is neither spiral nor elliptical. Irregular galaxies tend to be smaller objects without definite shape, and they typically have very hot newer stars mixed in with lots of gas and dust. These galaxies often have active regions of star formation. Sometimes their irregular shape is the result of interactions or collisions between galaxies. Observations such as the Hubble Deep Fields show that irregular galaxies were more common in the distant (early) universe.