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

• Stars form from interstellar matter -- mostly hydrogen, helium, and some dust.
• Gas clouds are not uniform, and dense, small clumps collapse under its own gravity.
• Conservation of angular momentum forces the collapsing cloud to form a flattened disk, and a small, hot, dense core called a protostar forms at the center -- as in solar nebular hypothesis.

• a protostar is heated only by the gravitational energy of collapse alone (1500 K), and no nuclear reactions yet.
• Some million years later, the temperature rises high enough (about 7 million K) to ignite nuclear fusion, and a star is born!
• Some of the energy goes into a jet/outflow of material known as a bipolar flow.
• The leftover material continues to form the planetary system and the remnant disk.
• Stars often form in groups or in "clusters". In the same large gas cloud, massive stars and less massive stars form together.
• Stellar Mass Limits: (A Story of Three Bears)
• if the core has too little mass (<0.08 solar mass), not enough temperature and pressure to ignite fusion -- thus a failed star or a "brown dwarf"
• if the core has too much mass (60->100 solar mass?), it becomes too hot too quickly, and strong radiation and pressure prevents additional mass from falling onto it.

Main Sequence

• Nuclear fusion powers the main sequence stars.
• low mass stars (e.g. the Sun): hydrogen to helium (proton-proton chain)
• high mass stars: hydrogen "burning" plus CNO cycle (requires higher temperature) and Helium capture (requires even higher temperature)
• High mass stars must be hotter (thus more luminous) to offset their larger gravity
• more massive stars have stronger gravity
• greater gravitational attraction requires greater pressure to offset it (hydrostatic equilibrium)
• greater pressure means higher temperature (perfect gas law)
• higher temperature means larger luminosity and shorter lifetime


• Differences between upper and lower main sequence stars:


• Mass, luminosity, temperature, way of burning hydrogen to helium, lifetime on the main sequence


• 30 Msun star has 30 times the solar mass. Burns with a luminosity about 300,000 times greater than the Sun. Thus the lifetime on the main sequence is roughly 30/300,000 = 1/10,000 as long as the lifetime of the Sun on the main sequence


• Main Sequence lifetimes


• 10000 Lsun, 10 Msun implies main sequence lifetime is roughly 10 million years


• 1000 Lsun, 6 Msun implies main sequence lifetime is roughly 0.006 times 10000 million years


• 10 Lsun, 3 Msun implies main sequence lifetime is roughly 0.3 times 10000 million years


• 1 Lsun, 1 Msun implies main sequence lifetime is roughly 10000 million years


• 0.01Lsun, 0.3Msun implies main sequence lifetime is roughly 30 times 100000 million years


• Quiz 14a

Low Mass Star Evolution

• When a MS star runs out of fuel (hydrogen) in its core, pressure begins to drop.
• Gravity wins again and the inner region is compressed further.
• As temperature rises, hydrogen outside the core ignites ("hydrogen shell burning").
• More widespread heating causes pressure to rise again, and increased pressure causes the outer parts of the star to expand. This expansion also cools the outer layer ( perfect gas law).
• Most of the mass is still in the dense core.
• Fusion of heavier elements
• electric force stronger for heavier atoms, and higher temperature is needed to overcome Coulomb barrier (100 million K to fuse helium)
• 3 helium atoms form a carbon atom ("triple alpha process")
• Pulsating stars
• giant stars pulsate because their atmosphere trap radiated energy
• this heats outer layers, making them to expand
• expanded gas cools, pressure drops, and gravity pulls the layer back in.
• the cycle starts all over again

Death of Stars like the Sun: Planetary Nebula

• Helium flash and yellow giant phase
• helium burning and supergiant phase -- formation of carbon and silicon grains in the cool (2500 K) photosphere
• planetary nebular: radiation pressure drives the dust grains outwards, and the outer shell becomes transparent
• a white dwarf (the naked hot core) is revealed.

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.

Summary

Quiz 14b: Death of the Solar System

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