Bret Betz, Nick Jones, Calvin Schildknecht Stellar Evolution Bret Betz, Nick Jones, Calvin Schildknecht

Formation of a Star Begins with a Nebula- cloud of gas and dust http://ftp.seds.org/pub/images/hst/M16WF2.jpg the density of one region of gas becomes great enough it begins to contract into a ball under the force of gravity. Gravitational energy is converted into thermal energy. As mass increases in one region the ball of gases begins to rotate.

Formation of a Star (cont.) Temperature and pressure increase and as mass accretion begins (mass falling to center) a protostar is formed. Protostar- infant quasi-star which glows in infrared spectrum and is on the fringe of the H.-R. diagram.

Hertzsprung - Russell Diagram Hertzsprung - Russell Diagram – graph of absolute luminosity (J/s) v. surface temperature (K) Main sequence is where 90% of stars fall on the H.-R. diagram.

Luminosity versus Brightness Hertzsprung - Russell Diagram – graph of absolute luminosity (J/s) v. surface temperature (K) Main sequence is where 90% of stars fall on the H.-R. diagram.

Fusion in a Newborn Star (Back to the protostar)… mass accretion stops and a “core” is formed. When kinetic energy is sufficiently high, coulomb repulsion that keeps the hydrogen nuclei apart can be overcome and nuclear fusion can take place. Hydrostatic equilibrium (outward force of fusion balances gravitational inward force) occurs—a star is born.

Fusion in Our Sun http://zebu.uoregon.edu/textbook/images1/pms.gif Proton-proton cycle: four protons fuse to form a 42He nucleus with the release of g rays and neutrinos. Requires temperature of 107 K When most of the H has been fused into He in the core, there is no longer sufficient energy to prevent gravitational forces from causing the core to heat up and contract. The core contracts while outer regions are pushed out and a red giant is formed. http://zebu.uoregon.edu/textbook/images1/pms.gif

Fusion in Our Sun (cont.) When the temperature in a red giant reaches 108 K (due to the core contracting) He nuclei begin to undergo fusion themselves. 3 42He  126C This is known as nucleosynthesis The star moves to the horizontal branch of the H.-R. diagram.

Fusion and Nucleosynthesis Nucleosynthesis – the formation of heavy nuclei from lighter ones by means of fusion. Cannot fuse nuclei heavier than 5626Fe and 5628Ni…yet. The star fuses the remaining elements and cools to become a white dwarf.

What Else Can Happen to a Red Giant? If more massive stars, more elements can be fused (C-Ne-O-Si–Fe) , red giant becomes larger. Fe core is formed and when the temperature gets hot enough, photodisintegration (nuclei break up) occurs, and the core collapses. 1s = core the size of earth to 10km radius of neutrons. Inverse beta decay- P + e- = n (gravity) Outer layers, strike core, rebounds = supernova explosion.

After Supernova Heavier elements are created. Large fraction of star is disbursed, enriching the universe. Most known elements are created.

Remnants of Supernova After supernova, depending on mass: Neutron Star- theoretical object neutrons, which has large B-field, very high mass. Black Hole- (discussed earlier)

Bibliography http://zebu.uoregon.edu/textbook/images1/pms.gif http://ftp.seds.org/pub/images/hst/M16WF2.jpg