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Explore the different stages of star evolution, including star formation, main sequence, red giants, planetary nebulae, white dwarfs, and supernovae. Learn about the properties and behaviors of different types of stars.
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LIFECYCLES OF STARS Option 2601
Stellar Physics • Unit 1 - Observational properties of stars • Unit 2 - Stellar Spectra • Unit 3 - The Sun • Unit 4 - Stellar Structure • Unit 5 - Stellar Evolution • Unit 6 - Stars of particular interest
Unit 5 Stellar Evolution
Stellar Evolution • Star formation • Main sequence • Stellar clusters (open, globular) • Population I & II stars • Red Giants • Planetary Nebulae • White Dwarfs • Supernovae • Neutron Stars
Sequence • Protostar • Pre-main Sequence (PMS) • Main Sequence • Post-main Sequence
Protostars • Stars born by gravitational contraction of interstellar clouds of gas and dust • Gravitation energy 50% thermal & 50% radiative • Cloud is a Protostar before hydrostatic equilibrium is established
Protostars • Collapse starts in “free fall” • Particles do not collide during collapse • i.e. P=0, gravity is only force involved • Collapse is uneven • Core collapses more rapidly forming a small central condensation • Core then accretes material
Protostars • Low mass objects accrete all (most) of material • High mass objects behave similarly, but • Fusion begins before end of accretion • Some material then blown away by radiation pressure
Effect of Rotation • If angular momentum > 0 • Cloud flattens into a disk • In some cases several central blobs form, which can coalesce into fewer… • Multiple star systems
The Main Sequence • Start of nuclear burning zero-age main sequence • As H He composition () changes, structure changes • Rates of evolution depend on two things • Initial mass • Composition
The Main Sequence • High mass stars are hotter & more luminous • Use their energy faster, i.e. evolve faster • Spend less time on the main sequence • O & B stars evolve faster than M stars
Mass-luminosity relation: Giving star lifetime: Quantitatively
Population I Stars • Accreting from the ISM now! (i.e. recent past) • Typical stars are young, in galactic spiral arms where gas and dust found • Typically reside in open star clusters • ~2% of mass elements heavier than H or He (ISM enriched by supernovae) • If M* a little > M energy generation is by CNO cycle • Sun is population I
Evolutionary phases of a solar mass star, post main-sequence
Population II Stars • First stars to be formed in Universe • Have only 0.01% heavy elements • Typically found in galactic bulge and globular clusters • Similar sequence of evolution but occupy different region of H-R diagram during core He burning • Significant temperature changes, heating and then cooling
Evolutionary phases of a solar mass star, post main-sequence
For a degenerate gas (non-relativistic): Constant For a perfect gas: From hydrostatic equilibrium: Greater mass, smaller radius Chandrasekhar Limit White dwarfs form from stars with M 8MSun Degenerate gas pressure prevents further gravitational contraction Chrandrasekhar limit: degeneracy pressure can only support M 1.4MSun. Above this limit a neutron star is formed
White dwarf companions e.g. Sirius – companion Sirius B (Alvan Clark, 1862) Procyon – Procyon B (1882) In binaries we can measure the companion’s mass from Kepler’s laws MSirius B = 1.0MSun TSirius A = 10,000K ; MV = -1.5 TSirius B = 25,000K ; MV = 8 From : R 7 10-3RSun = 3 109kg m-3 3 10-3LSun
Massive Stars • Stars with masses > 7 M • Masses greater than ~ 50 M • Affected by mass loss (i.e. winds) • As mass of star changes so does the structure and luminosity
