Stellar Evolution

1. Stellar Evolution Pg. 43

2. Basic Structure of Stars • Mass governs a star’s properties • Energy is generated by nuclear fusion • Stars that aren’t on main sequence of H-R either have fusion from other elements or don’t undergo fusion at all

3. Stellar Evolution and Life Cycles • Changes as it ages because fusion is continually making new elements • Eventually nuclear fuel runs out • All form from a nebula • As the cloud contracts, particle rotate into a disk with a protostar in the middle. • Eventually temp inside protostar is hot enough to start fusion • Hydrogen then begins converting to helium

4. The Sun’s Life Cycle • Takes about 10 billion years for a star the size of the Sun to convert all the hydrogen into helium • After all H is converted, the core is He surrounded by H gas • The gas expands and cools producing a red giant (luminosity is increasing while temp is decreasing) • Gases escape and eventually helium is converted to carbon • The star shrinks back to normal size and now has a carbon core

5. Cycle cont’d • Carbon never becomes hot enough to react • Outer layers expand again • Gas that leaves is called planetary nebula • The carbon core is left (Earth-size) • This creates a white dwarf

6. White dwarfs • Stable because electrons inside the star resist being forced together • Doesn’t need heat to be maintained • Star that has less mass than the sun has a similar life cycle except the core never gets hot enough to fuse to carbon so it is a white dwarf with a He core • Smaller star has a longer lifetime because it doesn’t use up nuclear fuel as fast

7. Life cycle of Massive stars • Starts higher on the main sequence and uses nuclear fuel very quickly • Undergoes more reactions so has many more elements in the core • Becomes a red giant several times (after each elemental stage) • After it expands, becomes a supergiant • Mass is eventually lost and it becomes a white dwarf

8. Supernovae • Some stars don’t lose enough mass to become a white dwarf • Once reactions have created iron, the core collapses on itself • Protons and electrons merge to form neutrons • Neutrons can’t be squeezed together so collapsing stops and a neutron star is formed • Small but dense • Gas falling to the surface rebounds and explodes outward • Outer portion is blown off in an explosion called a supernova • Creates elements heavier than iron

10. Black holes • Some stars are too massive to even form neutron stars • The core of such a star collapses forever compacting matter into smaller and smaller volumes • The small but dense object left is called a black hole because the gravity is so great that not even light can escape it

11. Star Cycle • On page 44, I want you to create a diagram that illustrates the life cycle of a star. • Include a diagram for the following: • The evolution of a star • Life cycle of a star the size of our sun • Life cycle of a star bigger than our sun • When you are done, read the article on page 828 in the book, there will be a question from it on your next test