Galaxies

• Galaxies contain millions or billions of stars.

• Galaxy a collection of stars, dust, and gas bound together by gravity

• Because stars age at different rates, a galaxy may contain many types of stars.

• Gravity holds galaxies together in clusters.

• Galaxies are not spread evenly throughout space.

• Cluster a group of stars or galaxies bound by gravity

• The Milky Way galaxy and the Andromeda galaxy are two of the largest members of the Local Group, a cluster of more than 30 galaxies.

• Clusters of galaxies can form even larger groups, called superclusters.
• We live in the Milky Way galaxy.

• Edwin Hubble divided all galaxies into three major types: spiral, elliptical, and irregular.

• Most of the objects visible in the night sky are part of the Milky Way galaxy.

• Scientists use astronomical data to piece together a picture of the Milky Way galaxy.
• The Milky Way is a spiral galaxy.

• Our galaxy is a huge spiraling disk of stars, gas, and dust.

• Our solar system is located within a spiral arm.

• The nucleus of the galaxy is dense and has many old stars.

• The gas and dust is called interstallar matter.

• Interstellar matter the gas and dust located between the stars in a galaxy.
• Eliptical galaxies have no spiral arms.

• Elliptical galaxies are spherical or egg shaped.

• They contain mostly older stars and have little interstellar matter.

• Because older stars are red, elliptical galaxies often have a reddish color.
• All other galaxies are irregular galaxies.

• Irregular galaxies lack regular shapes and do not have a well-defined structure.

• Some irregular galaxies may be oddly shaped because the gravitational influence of nearby galaxies distorts their spiral arms.
• Quasars may be infant galaxies.

• In 1960, a faint object was matched with a strong radio signal. This object was called a quasar.

• quasar quasi-stellar radio sources; very luminous objects that produce energy at a high rate and that are thought to be the most distant objects in the universe

• Each quasar has a huge central black hole and a large disk of gas and dust around it.
• Galaxies change over time.

• Galaxies change as they use up their stores of gas and dust

• Galaxies also change as a result of collisions.

• As galaxies approach each other, mutual gravitational attraction changes their shape.

• Collisions of gas and dust may cause new stars to begin forming.