Stars and Galaxies

1. Stars and Galaxies Galaxies and the Universe

2. Galaxies • We are on Earth, Earth orbits the Sun. Does the Sun orbit anything? Where is the sun in the galaxy? • The Sun is just one star among many in the galaxy (a large group of stars, gas, and dust held together by gravity).

3. Galaxies • Galaxies are separated by huge distances, often million of light-years. • In the same way that stars are grouped together in a galaxy, galaxies are grouped together in clusters. • The Milky Way (galaxy our solar system belongs to) belongs to a cluster called the Local Group. • The Local Group contains over 30 galaxies.

4. Types of Galaxies • There are three types of galaxies: • Spiral • Elliptical • Irregular

5. Spiral Galaxies • Spiral galaxies have spiral arms that wind outward from inner regions. • Arms are made up of stars and dust, in a normal spiral the arms start close to the center. • Barred spirals have arms extending from a large bar of stars and gas that passes through the center of the galaxies.

6. Elliptical Galaxies • Galaxies that are shaped like large, three-dimensional ellipses. • Many are footballed shaped while others are round.

7. Irregular Galaxies • Irregular galaxies have many different shapes and sizes, galaxies that do no fit into the other classifications. • Two irregular galaxies called the Clouds of Magellan orbit the Milky Way

8. Milky Way • Contains one trillion stars. • The visible disk of stars is about 100,000 light-years across.

9. Find the Sun, notice that it is about 30,000 light-years from the center in one of the spiracle arms. • All stars orbit around a central region or core. Based on the 30,000 light-years and a speed of 235 km/s the Sun orbits the center of the Milky Way once ever 240 million years.

10. Normal Spiral or Barred? • The Milky Way is normally classified as a normal spiral galaxy, however there is some evidence that suggest that it is barred. It’s hard to tell with the limited information we have of the it. • So what can we see of the Milky Way? Look out at the night sky – all the stars that you see are part of the Milky Way.

11. Origin of the Universe • There have been many different models proposed for how the Universe formed: • Steady State Theory – proposed that the universe has always been the way it is now. It has always existed and always will. As the universe expands new matter is created to keep the overall density of the universe the same or in a steady state. • Oscillating Model – the universe began with expansion occurring in all areas of the universe. Over time the expansion slowed and the matter in the universe contracted. This process happens again and again.

12. Steady state Theory: • Oscillating Model Theory:

13. Expansion of the Universe • When light or sound seems to shift in color or pitch depending on if they are moving toward or away from you it is known as a Doppler Shift. • If a star is moving toward Earth light waves are compressed and are shifted to the blue end of the spectrum • If a star is moving away from the Earth light waves are elongated and are shifted to the red end of the spectrum.

15. Red Shift • In 1929, Edwin Hubble published a fact about light coming from most galaxies. • When a spectrograph is used to study light from galaxies beyond the Local Group a red shift of light occurs. This tells us that galaxies outside of the Local Group are moving away from Earth, therefore the universe must be expanding.

16. Big Bang • The third theory, though neither of the other two can be proven or disproven, seems to be the most likely theory. It states that the universe started with a big bang and has been expanding ever since. • When scientists determined that the universe was expanding they realized that the galaxy clusters must have been closer together in the past. This theory is known as the Big Bang Theory

17. According to this theory 12 billion to 15 billion years ago, the universe began with an enormous explosion. Then entire universe began to expand at the exact same time.