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Stellar Evolution. Andrej Ficnar & Lovro Prepolec. 5 th April 2005. Introduction. A star is any massive gaseous body in space. About 70×10 21 stars in known universe. Lifetime from millions to billions of years. Origins of chemical elements. Introduction.
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Stellar Evolution Andrej Ficnar & Lovro Prepolec 5th April 2005
Introduction • A star is any massive gaseous body in space • About70×1021stars in known universe • Lifetime from millions to billionsof years • Origins of chemical elements
Introduction • Aging and death of stars lead to many interesting astronomical phenomena: • Black holes • Neutron stars • Dwarves • Nebulas • First scientific theories in 19th century (Kelvin and Helmholtz)
Formation of Stars • Form from interstellar gas clouds • Gravitational collapse of gas clumps • Flattening of clumps • Formation of a protostar
Formation of Stars • Protostars - cold and have short lifetime • Characterized by outflow of gas • Mass of most stars between about 0.1 and 30MS
Main Sequence Stars • Characterized by hydrostatic equilibrium • Name from Hertzsprung-Russel diagram • Main - sequence lifetime approx.: where M and L are mass and luminosity in solar units
Leaving the Main Sequence • Main sequence star eventually becomes red giant • Star switches to helium burning by compressing the core
Low - Mass Stars • Mass < 10 MS, colder, less luminous, longer lifetime • To start helium burning, gas degenerates • Helium flashes transform star into a yellow giant • Yellow giants may pulsate • Increased “fuel” consumption leads to red supergiant
Low - Mass Stars • Ejection of outer layers forms planetary nebulas • Core becomes a white dwarf
High - Mass Stars • Mass > 10 MS, hotter, more luminous, shorter lifetime • Begin as blue main sequence stars, and afterwards become yellow supergiants • No degeneration or helium flashes • No planetary nebulas or white dwarves • Different hydrogen fusion process (CNO cycle) • Ability of nucleosynthesis
High - Mass Stars • Core of iron shrinks into a core of neutrons • Gravitationalcollapse of core leads to explosion – supernova • Supernova remnants • Core becomes a neutronstar or a blackhole
Stellar Remnants: White Dwarves • Remnants of low-mass stars • Shell ejected into planetary nebula • Hot (~ 25 000 K), compact stars • Mass comparable to MS, but radius comparable to radiusofEarth • Very dim, nofuel burning • Cool rapidly (black dwarf) • Composed of C and O, surface layer of H and He • Extreme magneticfields (~1000 T)
Stellar Remnants:White Dwarves • Pauli exclusion principle • Very dense packing causes degeneration: added mass causes shrinking • White dwarf may finally collapse • 1931 S.Chandrasekhar calculated limiting mass of a white dwarf • Chandrasekhar limit ~ 1.4 MS
Stellar Remnants: Neutron Star • 1934 W.Baade and F.Zwicky • Theoretical results: • Radius ~10 km • Maximum mass ~2 – 3 MS • 1967 A. Hewish et al. detected radio signal with astonishingly precisepulserate • (period of pulsation) ~(density)-1 • Densities larger then those of white dwarves
Stellar Remnants: Neutron Star • F. Pacini and T. Gold: pulsar is rotating, notpulsating star • TS~1 month Tpulsar from 1 ms to 4 s • Law of conservation of angular momentum • Mechanism of radiation (synchrotron radiation) • X-ray binaries • 1974. J. Taylor and R. Hulse: binary pulsar
Stellar Remnants: Black Holes • Remnants of high-massstars • Escape velocity • 1783. J.Michell • P. S.Laplace • K. Schwartzshild, Schwartzshild radius
Stellar Remnants: Black Holes • Strange properties of black holes • Temperature (J. Bekenstein) (~6×10-8 K) • Radiation (S. Hawking) (lmax~16 RS) • Gravitational waves • Detecting black hole:
