Astronomy 100 -- Death of High-Mass Stars

THE EVOLUTION OF STARS
GREATER THAN 8 SOLAR MASSES

1. As the star leaves the MS, the core goes through the triple-alpha process producing a carbon-oxygen core.
   
2. The core will continue to contract, increasing in density and temperature.
   
3. Many fusion reactions produce heavy elements inside the star and produce an "ONION-SKIN" Structure, with iron in the core.

   Another "Onion-Skin" Diagram

   Click Here to see some of the possible
   fusion reactions that happen in the star.

4. Iron core collapses rapidly!
   • Extremely high temperature and density.
   • Iron fusion absorbs energy!
     Energy ==> Matter
   • Gravitational energy released by violent collapse.
   • Elements heavier than iron are produced.
   • All happens in just a few seconds!!
   • This results in a SUPERNOVA!

AFTER . . . . . BEFORE

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SUPERNOVA = EXPLODING STAR

Two important effects of Supernovae:

1. Many elements are ejected into space.
2. Shock wave will trigger new star formation.

Famous Supernovae:

• SN 1987A in the Large Magallenic Cloud.
  
• Kepler's Supernova in 1604.
  
• Tycho's Supernova in 1572.
  
• Crab Nebula Supernova in 1054.

What's Left Behind?

1. Neutron Star = Pulsar
For stars between 8 and 25 solar masses.
2. Black Hole
For stars greater than 25 solar masses.

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NEUTRON STARS or PULSARS

When the gravitational force becomes so great in a white dwarf (such as when the mass is greater than 1.4 solar masses), gravity forces the electrons and protons together to form neutrons.

Neutron Star Basic Facts:

• Remnants from supernovae of stars between about 8 and 25 solar masses.
• Mass: 1 to 2 solar masses. Largest mass possible is between 2 and 3 solar masses.
• Size: Only 10 to 20 kilometers in radius!
• Just 1 teaspoon of the stuff on Earth would weigh as much as all the people in China!
• Like white dwarfs; the more mass they have, the smaller the radius will be.
  

Neutron stars have also been identified as

Pulsars.

Neutron stars rotate very rapidly, 1 to 1000 times a second! They also have very strong magnetic fields. Electrons in this rapidly rotating magnetic field produce lots of radio waves in a beam coming out the north and south magnetic poles. In a similar fashion to a lighthouse, as the neutron star rotates, this beam is sometimes directed towards us and we receive a pulse of radio emission.

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BLACK HOLES

For stars greater than 25 solar masses, the core has too much mass to become a neutron star after the supernova explosion. In the case, a BLACK HOLE is created.

Some BLACK HOLE preliminary stuff

• The speed of light is the same for ALL observers!
• The speed of light is THE speed limit.
  Nothing can go faster than 300,000 km/sec.
• Escape Velocity:
  Speed required to overcome the gravitational pull and break away from an object.
• E = m c². Mass-Energy Equivalence.
  Gravity pulls Mass.
  Gravity pulls Energy (photons!)
• Moving clocks (Time) appear to run slower.
  The faster the clock moves, the slower it will appear to run.
• Gravity makes clocks (Time) run SLOWER!
  This is NOT a clock problem, but a TIME problem.
  Time is affected by Gravity!

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BLACK HOLES

Black Holes are objects of highly compressed matter from which light cannot escape due to the extremely strong gravitational pull. The mass of the black hole is compressed into a point known as a SINGULARITY.

Black holes are surrounded by an IMAGINARY SURFACE known as the EVENT HORIZON. It is inside this Event Horizon that we cannot observe. The information of any event that occurs inside the Event Horizon cannot reach the outside Universe! Hence the term Black Hole.

The Event Horizon is also known as the
"Schwarzschild Radius"

R_Sch = 3 km x (Mass/M_Sun)

Look for the affects of its gravitational pull. Binary stars where you can only see one star "wobbling" is a good place to start. If gas from the star is falling into the black hole, lots of X-rays will be emitted.

Candidates for being a Black Hole:

• Cygnus X-1
  Unseen companion about 6 solar masses.
• LMC X-3
  Unseen companion about 10 solar masses.

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IMPORTANT POINT Black holes are NOT giant vacuum cleaners in space. Their gravitational force is only strong very close to them. If the Sun were to become a black hole, the orbit of the Earth would not change. Black holes are
not like this!

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FALLING INTO A BLACK HOLE

• If you are falling in, you fall in!

• If you watch someone fall in, they NEVER fall in. They just would seem to get closer and closer to the Event Horizon, never crossing it, and become fainter and fade out of sight. There are two reasons for this being what you see:
  • Fewer photons reach you due to the slowing of Time in the gravity field.
  • Photons that do reach you are redshifted by gravity.

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Summary of Post-Main-Sequence Evolution

• Stars of 0.1 to 8 solar masses end up as WHITE DWARFS.

• Stars of 8 to 25 solar masses end with a SUPERNOVA and leave behind a NEUTRON STAR.

• Stars larger than 25 solar masses end with a SUPERNOVA, but leave behind a BLACK HOLE.

Stellar Structure and Evolution Simulator
Shows the evolution of different mass stars off the MS.
Even includes the supernova explosion for high-mass stars!

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Death of High-Mass Stars

The Milky Way Galaxy.

Back to The Front Page.

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