Star's Life Cycle

Key Concepts:

  1. How are stars formed?
  2. How do stars evolve during their "main sequence" phase?
  3. How do stars meet their deaths?
  4. What is the ultimate fate of our Sun and the solar system?

Latest Pictures from the Cassini-Huygens Probe to Saturn

Astronomy Picture of the Day (2003 October 19)

Stellar Birth

  • Protostellar track: gas cloud to main sequence - Times to get to the main sequence:

  • 15 Msun about 60,000 years

  • 9 Msun about 150,000 years

  • 3 Msun about 3 million years

  • 1 Msun about 50 million years

  • 0.5 Msun about 150 million years

  • So the larger the birth mass, the shorter the time to get to the main sequence

  • Main Sequence

  • Stellar evolution applet is here

  • Low Mass Star Evolution

    (watch the animation from the U. Oregon Electronic Universe Project)

    Death of Stars like the Sun: Planetary Nebula

    Evolution of a Massive (>8 solar mass) Star: Supernova

    • "Nuclear synthesis" -- formation of heavier atoms like carbon or oxigen by fusion (Carl Sagan: "we are all star stuff")
    • fusing heavier atoms require higher T but less energy comes out
    • core collapse: iron cannot burn and release energy, no more source of heat and pressure to counter gravity --> supernova
    • either a neutron star or a black hole is formed.


    Quiz 14b: Death of the Solar System

  • Questions you should be able to answer on stellar evolution/structure

  • Why do upper main sequence stars spend less time on the main sequence>

  • What is left in the core of a star after it has finished hydrogen burning in the core?

  • What is left in the core of a star after it has finished helium burning in the core>

  • Why cant a low mass star burn carbon in its core?

  • Why can a high mass star burn carbon in its core?

  • Where does the carbon in your body come from>

  • How does a white dwarf get its energy?

  • What is a white dwarf made of?

  • When a low mass star runs out of hydrogen in its core, describe what happens next?

  • When a low mass star runs out of hydrogen in a shell, describe what happens next?

  • How are upper and low mass stars different in the way they start helium burning in the core?

  • What can a star use to overcome the gravitational force that is always trying to make it smaller?

  • Whats the physical law that connects pressure density and temperature?

  • So why is the surface of a star cooler than its center?

  • Reading assignment for next lecture: Chapter S3 (p.454-p.472)