1. Review: 1. How is the mass of stars determined?
2. How is the distance between stars determined?
3. How can astronomers tell what a star's temperature is?
4. What are four things that the H-R diagram tells you about stars?

2. Types of Stars & Stellar Evolution
I. Stars are not uniformly distributed and identical. There are many types of stars.
A. Main Sequence Stars make up of 90% of all of the stars.

3. 1. They range from being bright/hot to dim/cool.
2. Their brightness is related to their mass.
3. The sun is a main sequence star. The brightest main sequence star has 50x more mass than the sun. (You can fit 1,000,000 earths in the sun)

4. B. Red Giants are large, cool stars, with high luminosity.
C. Supergiants are very large, very bright red giant stars.
Ex: Betelguese has a radius that is 800 times of the sun.

5. D. A nova is a star that suddenly gets bright.
E. A nebula is a cloud of gas and/or dust in space.
1. Nebula are composed of Hydrogen gas that that glows when a hot star is nearby.
Eagle Nebula

6. II. Stellar Evolution
A. It is difficult to determine how stars are born, their actual ages and when they die because stars lifespan can be billions of years long.

7. B. The Lifecycle of a Star
1. Star Birth: Stars begin as cool, interstellar clouds, called nebulae.
(92% H, 7% He, plus other elements)
2. The nebulae become dense enough to contract. Gravity pulls every particle towards the center.
3. As it shrinks, gravitational energy is converted into heat energy. (This is estimated to take a million years or so.)

8. 2. Protostar Stage
1. The temperature gets hot enough to radiate red light.
2. It isn't hot enough to engage nuclear fusion.
3. When the core of the protostar reaches 10 million K, fusion begins and a star is born.

9. 3. Main Sequence Stage
a. The inward force of gravity is equal to the outward force of the expanding gases (stable).
(Hydrogen fusion is occuring, where two hydrogen atoms collide to form Helium=LOTS of energy)
b. This lasts for up to 10 billion years.
c. 90% of a star's life is as a main sequence star.

10. 4. Red Giant Stage
a. This occurs when the hydrogen fusion moves outward and the remaining bit of Hydrogen is consumed.
b. The force of gravity overtakes the star.
c. It grows hotter and expands outward up to 1000x bigger than the main sequence star.
d. The more massive the star, the faster this stage.

11. 5. Burnout and Death
Stages 1-4 are well documented; however, burnout and death is all based on theory.
We do know that all stars will eventually run out of fuel and collapse due to gravity.

12. a. Death of a low-mass star
i. These small cool red stars may remain as main sequence stars for 100 billion years.
ii. They don't get hot enough to fuse helium and become red giants. They collapse into white dwarfs.

13. b. Death of a medium-mass star
i. These Red Giants collapse into white dwarfs also, but they cast off an expanding round cloud of gas, called planetary nebulae (not related to planets).

14. c. Death of a Massive Star
i. These stars end with a brilliant explosion called a supernova.
ii. The star becomes a million times brighter.
Iii. None have been seen by a telescope but Tycho Brahe and Galileo each recorded one 30 yrs apart.

15. vi. Supernova are triggered when the star's gravity causes it to implode, which results in a large shock wave.

16. d. After Death
i. Supernova become neutron stars--which are high density stars composed of neutrons.
ii. Some supernova rotate giving off pulses of energy. These are called pulsars.

17. Iii. Black Holes
Sometimes, during a supernova event, the star has so much mass, that it collapses inward.
These are very hot and have so much gravity that light or matter cannot escape from it.