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 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

 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

 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

 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

 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

 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

 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

 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

 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