Stars and Galaxies

1. Stars and Galaxies

2. Types of Stars

3. Color: • Cooler stars are red. As they get hotter they turn orange, yellow and then blue. • Composition • Stars are made of different forms of gases • Inner layers are dense and hot • Outer layers are made up of cool gases

4. Brightness • Brightest stars: called first-magnitude stars • Dimmest stars: called sixth-magnitude stars • Positive numbers represent dimmest starts • Negative numbers represent brighter stars • Sirius (Dog star) has a magnitude of -1.4

6. Motion of stars: • Because of the Earth’s motion stars appear to rotate around Polaris (North star) • However, stars are moving in space, but it can take thousands of years to see the movement

7. First stage: • enters its life cycle as a ball of gas and dust • Life Cycle of Stars • Gravity pulls the gas and dust together into a sphere • Sphere gets hotter and denser and the hydrogen changes to helium in a process called nuclear fusion

8. Second stage: • Main-sequence star • Longest stage • Energy is made as hydrogen fuses into helium • Third stage: Giants and Supergiants • Red giant: star that expands and cools because it has used all its hydrogen • Red giants can be 10x bigger than the sun • Supergiants can be 100x bigger than the sun

9. Fourth stage: White dwarf • small hot star that is the leftover center of an older star • Can shine for billions of years before they cool completely and the star dies

10. Other Star deaths • Supernova: gigantic explosion in which a large massive star collapses and throws its outer layers into space • Can form a neutron star if the center of the collapsed star contracts to form a new star • if the neutron star is spinning it is called a pulsar • Can form a Black hole if the center of the collapsed star has a mass that is more than 3 times larger than our sun • Gravity pulls so strong that even light can not escape • We can detect black holes X rays to look at nearby light spiraling into the black hole

12. H-R Diagram • Graph that shows the relationship between a star’s surface temperature and how bright it glows (absolute magnitude)

13. Galaxies • large groups of stars, dust, and gas • Edwin Hubble began to classify galaxies by shape in the 1920’s.

14. Types of galaxies • Spiral galaxies: • Has a bulge at the center and spiral arms • Spiral arms are made up of gas, dust, and new stars • Our galaxy, The Milky Way, is thought to be a spiral galaxy

15. Types of galaxies • Elliptical galaxies: • Has a very bright center and very little dust and gas • Contain mostly old stars • Irregular galaxies: • The leftovers which don’t fit into any other class

16. What’s in a galaxy? • Billions of stars and some planetary systems • Nebulas: large clouds of gas and dust • Usually found in spiral galaxies

17. Origin of galaxies • Looking at distance galaxies reveals what early galaxies looked like • Among the most distance objects we can see now are quasars • Quasars: star like sources of light that are extremely far away • A very powerful energy source • May be formed from black holes in the center of some galaxies