Lecture 14
March 27, 2008 Stellar Life Cycle

Key Concepts:

Sunwheel and Observatory reports due Wednesday April 1.

Stars form from interstellar matter -- mostly hydrogen, helium, and some dust. There are many obstacles to forming stars, however:

conservation of angular momentum: as you shrink a gas cloud in size, it rotates faster and faster.

Gas clouds are not uniform, and dense, small clumps collapse under its own gravity.
Solar nebular hypothesis: 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.

Cloud collapse and star formation computer simulation with turbulence (UK Astrophysical Fluids Facility and University of Leicester). Here is the movie that shows evolution of a large diffuse cloud that fragments and forms protostellar systems.
NASA Animation of star/planet formation: this animation graphically illustrates the process of cloud collapse and formation of a protostar and a protostellar disk, and the eventual ignition of the fusion that produces a genuine star.

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.

if the core has too little mass (<0.1 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 (>100 solar mass?), it becomes too hot too quickly, and strong radiation and pressure prevents additional mass from falling onto it.

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)

More massive stars have to 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

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").
This shell fusion causes a widespread heating and increasing pressure. However, there is no additional pressure in the core to counter the gravitational force, despite the ongoing shell fusion, and the core continues to collapse and heats further.
The compression and shell burning lead to increased pressure in and around the core, and this increases the luminosity of the star up to 1000 times the normal rate. This in turn causes the outer parts of the star to expand. This expansion cools the outer layer ( perfect gas law).
Most of the mass is still in the dense core.
Pulsating stars

Helium flash and yellow giant phase
helium burning ("tripple-alpha" reaction) and supergiant phase -- formation of carbon grains in the cool (1000-2000 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.

"Nuclear synthesis" -- formation of heavier atoms like carbon or oxygen by fusion (Carl Sagan: "we are all star stuff")
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")

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.