Wednesday February 10th 1999
Announcements:

Lecture notes:

Class 14.

We talked about how L T are two of the most important properties used to describe stars.
Talked about the H-R diagram L versus T.  Saw that most stars lie along the main sequence.
Here they burn H -> He in core.

While a star is burning H -> He it sits in a stationary place on the main sequence.  Reading the diagram this means that the star
sits there with constant L and T.

Main sequence is not some track along which stars move.  Stars are essentially stationary while on it.  Track is due to many
different masses of stars lying at different places along the main sequence.  The way we talk about the masses of a star is
by comparing it to the mass of the sun = M_(.)

Somewhat suprisingly high mass stars live for less time than low mass stars.  Even though they have more fuel they burn it
faster.

This diagram is very fundamental and we will use it throughout the rest of this class.  On tests you will need to know how to
reason out problems using the HR diagram.

For the rest of the class we will talk about what happens when a star dies.

As the first example we'll talk about our sun as it moves through its life.  We will follow it from birth to death.

Proto Star - star is contracting from a gas cloud down to a star.  It hasn't started fusion yet - gravity powered.  A 1 M_(.) star
takes about 30 million years to reach main sequence.

Main Sequence - has a core that burns H->He.  Over about 10 billion years H in the core is burned to He.  Surface
temperature is about 6000K.  Core temp is about 10 million K.  Sun has used 5 billion and has 5 billion left.  Then the fuel in
the core starts to run out.

Orange Giant Phase - core contracts and heats up but it cannot burn since the H fuel has run out.  The core is He "ashes".
H->He burning continues in a shell around the core.  Causes the envelope of the star to expand and the star becomes 10-15 times the diameter of the sun.

Red Giant Phase - star continues to swell up 50-100 times the diameter of the sun.  Would reach about halfway to the Earth.
Core continues to contract.  Then the core contracts and becomes hotter so that a new type of
fusion can start.  Helium burning starts in the core.  He->Carbon via a complex nuclear reaction chain.  Kind of like proton
proton chain.  Also H burns to He in a shell surrounding the core.  Outer layers still expand.

Planetary Nebula - outer layers expelled by violent burning and pulsations inside.
White Dwarf - C core left over.  Earth-sized but star mass.  Electrons have degeneracy pressure.  No fusion - dead star.