Friday February 12th 1999
Announcements:

Lecture notes:

Class 15.
 
 

In the last class we talked about the life of a star like our sun - will review and stress new points.

1 M_(.) star.

Proto Star - contracting from gas cloud.  No fusion.  Takes 30 million years.
Main Sequence - has a core that burns H->He.  10 Billion years.  Surface temp is 6000K.  Core is 15 million K.
Orange and Red Giants - core contracts while envelope swells up.  He -> C burning starts in core.  H->He in shell.

But carbon never starts fusion burning.  Needs 600 million K to burn Carbon - not enough gravity so core never gets hot enough.  Carbon core is very dense.  1 grape of it would weigh 1 ton.
Planetary nebula - violent burning inside ejects envelope into space.
White dwarf - Ultra dense carbon core left over - > dead star with no fusion.

Now gravity still squeezes core - always before energy from fusion burning has fought off gravity.  But now no fusion.  How
does a star stay up?  Quantum mechanical effect called electron degeneracy pressure.

White dwarf stars are hot when born but cools to black dwarf.

Now for the rest of the lecture we will focus on the life and death of a massive star.  Consider a 15 M_(.) star.  Many of the
steps are the same as before for 1 M_(.) star so will not go through in detail.

•  Protostar
•  Main Sequence.
•  He core and H shell burning with expansion of outer layers.
•  He ignites in core H burning shell.
•  C core He burning shell H burning shell

Now the life is fundamentally different.  Now carbon burning can start because core temperature is greather than 600 million K due to stronger gravity.  Happens for stars > 8 M_(.) or so.

Then also oxygen fusion neon fusion Mg fusion Si fusion.

Diagram shows "onion skin" - layered core of a 15 M_(.) star near the end of its life.  Heavier elements are deeper in.  Each new type of fusion burning goes faster and faster and supports the star for less time.

H burning - 10 million years
He burning - 1 million years.
C burning - 1000 years
O burning - 1 year
Si burning - 1 week.
Silicon burns to iron and then there is a problem.

Iron catastrophe - iron has a very stable nucleus and does not fusion burn (since iron fusion makes no energy)  Even though the core temperature is 6 billion degrees.

Iron acts as a "fire extinguisher" and the nuclear fires in the core go out.
No iron fusion - > no support against gravity - > star collapses.

The core of the star is squeezed incredibly tightly by gravity.
1x10¹² kg/m³ - 1 grape sized chunk weighs as much as a mountain.

Electrons in atoms combine with protons.
Proton + electron -> neutron + neutrino.  Called neutronization.
Neutrinos fly out of star.  We have detected these neutrinos when a star in a nearby galaxy collapsed (SN 1987A)  Verifies computer models.

Core of a star converted into a huge ball of neutrons - neutron star.
Neutron Star - the very end of certain massive stars.
- A dense ball of neutrons held together by gravity.
- About 10 KM in size but has the mass of the sun - 1Ex10¹⁵ kg/m³
- Held up from collapsing by degeneracy pressure.

Star gets very bright during collapse - can outshine entire galaxy.

The outer layers of the star do not go into the neutron star but are ejected with extremely high velocity - make a supernova
remnant.  Far more violent than a planetary nebula.  1x10⁴ km/s = initial speed for an ejection.

6 recorded supernovae in our galaxy in the past 1000 years.
1006 - Europe - brighter than a quarter moon.  Cast shadows
1054 - Crab - Pueblo Native Americans Chinese
1181
1572 - Tycho
1604 - Kepler.

Supernova remnants last for about 40000 years.  Cygnus Loop is 20000 years old.
Expect one every 100 years.  None in our galaxy since the telescope was invented in 1608.