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Stellar Evolution. Sam Wilmarth PHYS 43 Younes Ataiiyan SRJC SPRING 2011.

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stellar evolution
Stellar Evolution

Sam Wilmarth


Younes Ataiiyan



Around 400,000 years after the big bang, matter primarily existed in the form of protons, electrons, and lightweight atoms such as 2H floating through space. The particles slowly began to coalesce into nebulous gas clouds.


Gravity pulls the particles together, increasing their kinetic energy and their temperature. As the matter collapses, the core becomes denser and hotter, radiating energy.


FUSION!If the star is massive enough, fusion occurs.Hydrogen is converted to helium through several fusion reactions.1H + 1H ⇒ 2H + e+2H + 1H ⇒ 3He + γ3He + 3He ⇒ 4He + 2 1H


Hydrogen fuses into helium in the core of the star. The star is stable as the pressure of the expanding gas counteracts gravity.

As the helium core grows, hydrogen fusion moves out into the star, and the size increases.

The star eventually runs out of hydrogen it can fuse, and collapses.

If the star is big enough, the collapse produces enough energy and heat to start helium fusion into carbon.


Depending on the mass of the star, it will continue the cycle of expansion and contraction, fusing heavier elements each time.

Each successive cycle is shorter than the last, and even the largest stars stop at iron fusion.


Large Stars

  • The fusion of iron absorbs more energy than it creates.
  • If a star is big enough to reach this stage (about 10 times the mass of the sun), it will lose energy and heat and implode.
  • The implosion produces enough energy to create a supernova.
  • The supernova can last weeks or months, and release as much energy as the sun does in its entire lifespan.
  • The supernova is a main source of heavy elements in the universe.
  • The remaining core will collapse into either a neutron star or a black hole.

Neutron Star

  • Protons and electrons are smashed together to form neutrons.
  • They are on the order of 10 km in diameter.
  • One teaspoon weighs 1 billion tons.
  • Some rotate at very high speeds, and are called pulsars.
  • Pulsars have extreme magnetic fields.
  • Black Hole
  • Matter is collapsed to a single point.
  • Not even light can overcome gravitational pull.
  • Dark sphere where gravity overcomes light is called “event horizon.”

Medium and Small Size Stars

A medium size star that ends hydrogen fusion and begins helium fusion may either continue the cycle onto heavier elements or cease fusion there.

Either way, fusion will stop and the core will cool.

The hydrogen layer expands away from the core and cools, turning red in color. The star becomes a red giant.

As the hydrogen layer expands farther and cools, the star turns into a planetary nebula.

Eventually the gas cloud dissipates into space, leaving a white dwarf that cools over millions of years.

A small star never undergoes helium fusion, and turns directly into a white dwarf.

The composition of elements in a white dwarf depends on the size of the original star.



  • Rex, Andrew, & Thornton, Stephen T. Modern Physics for Scientists and Engineers, Third Edition. 2006. Belmont, CA: Brooks/ Cole.