1. Bell Ringer 10/13 Why do we celebrate Columbus Day?

2. Bell Ringer 10/14 Identify three stages of the life cycle of stars from your work yesterday.

3. 29.3 – Stellar Evolution

4. Evolution of Stars Typical star exists for billions of years Astronomers cannot study a star through its entire life –Developed theories about evolution of stars –Different stages of development

5. 1 st Stage of Development Nebula = a cloud of dust and gas –70% hydrogen, 28% helium, 2% other elements Particles in a nebula have weak gravitational attraction

6. Nebula A force causes the particles to connect –Particles continue to increase in size Region of dense matter builds up in nebula cloud and begins to spin Spinning causes matter to shrink into a disk = protostar –May produce more than one star

7. Protostar Protostar = shrinking, spinning region of a nebula Pressure builds up in protostar and it gets hotter –Heats up for several million years –Fusion begins

8. Main-Sequence Stars 2 nd & Longest stage Energy is generated in the core of star –Hydrogen atoms fuse into helium atoms –Releases enormous amount of energy Star does not change in size because of balance of energy and force of gravity

9. Giants & Supergiants Third stage Almost all hydrogen has converted to helium –Without hydrogen, core of star contracts Temperature increases in core –Higher temperature causes helium atoms fuse into carbon atoms –Causes outer shell to expand greatly

10. Giants & Supergiants As star’s shell of gases expands, it cools and star is no longer main-sequence star Becomes red giant or red supergiant

11. Bell Ringer 10/15 What happens after almost all of the hydrogen has been converted into helium in a star? –Hint – we talked about it yesterday in our notes.

12. White Dwarf Stars No energy left for fusion Loses its outer shell, revealing core Core heats and illuminates expanding gases = planetary nebula Gravity causes star to collapse inward Hot, dense core of matter = white dwarf

13. White Dwarf As white dwarf cools, it becomes fainter and fainter When no longer emitting energy, becomes a dead star or a black dwarf

14. Nova Some white dwarfs do not cool and die A large explosion may occur –Releases energy, gas, and dust A nova may appear 1 million times brighter than the sun –Quickly fades back into a white dwarf

15. Supernova Stars with very large masses may cause more intense explosions Star contracts causing high temperatures and high pressure

16. Neutron Stars Neutron stars = after an explosion, the core of a supernova contracts into a small dense ball of neutrons Neutron star has more mass than the sun and rotates very rapidly Some neutron stars emit 2 beams of radiation = pulsar

17. Black Holes Some massive stars are too big to become neutron stars Stars contract with great force and crush the core of the star = black hole Gravity is so great that light cannot escape Astronomers locate black stars by observing effect on neighboring stars

19. In-Class Assignment/Homework 29.3 WKT – just the front side