Galaxies and the Universe

1. Galaxies and the Universe

2. Galaxies • A large group of stars, gas, and dust held together by gravity. • Each of the galaxies contains the same elements, forces, and types of energy. • Galaxies are grouped into clusters. • The Milky Way galaxy belongs to the cluster called The Local Group. It contains about 45 galaxies of various sizes and types. • Three types: spiral, elliptical, and irregular

3. Spiral Galaxies • Have spiral arms that wind outward from the center. • The arms consist of bright stars, dust and gas. • The Milky Way is a spiral galaxy. • Can be normal or barred • Normal: spiral starts close to center of galaxy • Barred: spiral arms extend from a large bar of stars and gas that passes through the center of the galaxy.

5. Elliptical galaxies • Common Type • Shaped like large, three-dimensional ellipses • Many football shaped, but others are round • Some large ( so large that several galaxies the size of the Milky Way would fit inside) and some small

7. Irregular galaxies • Includes all galaxies that don’t fit into the other categories • Have many different shapes • Smaller than other galaxies • Two irregular galaxies called the Clouds of Magellan orbit the Milky Way

9. Milky Way Galaxy • Contains one trillion stars • Visible disk of stars is about 100,000 light years across. • The Sun is located about 26,000 light years from the center of the galaxy • Classified as a normal spiral, recent evidence suggest that it might be a barred spiral

10. Origin of the Universe • Three theories: steady state, oscillating model, and big bang • Steady State suggests that the universe always has been the same as it is now. The universe always existed an always will • Oscillating Model suggests that the universe began with expansion. Over time the expansion slowed and the universe contracted and the process began again back and forth.

11. Big Bang theory suggests that the universe began with a big bang and has been expanding ever since.

12. Expansion of the Universe • Doppler Shift occurs with light as well as sound • Doppler shift causes changes in the light coming from distant stars and galaxies • Stars wavelengths moving toward Earth become compressed, stars wavelengths moving away are stretched.

13. Red Shift • Galaxies outside the Local groups have a red shift in the spectrograph. • This indicates that they are moving away from the Local group galaxies and therefore the universe must be expanding.

14. Big Bang Theory • According to this theory, approximately 13.7 billion years ago, the universe began with an enormous explosion. • The entire universe began to expand everywhere at the same time. Looking Back in Time: • Whether the universe will continue to expand forever or stop expanding is unknown.

15. If enough matter exists, gravity might halt the expansion and the universe will contract until everything comes to a single point. • Studies of distant supernovae have shown that an energy called a dark energy is causing the universe to expand faster. Scientist are trying to understand how this will effect the fate of the universe