Transcript Stellar Evolutiona
Stellar Evolution
Andrej Ficnar & Lovro Prepolec 5 th April 2005
Introduction
A star is any massive gaseous body in space About 70×10
21
known universe stars in Lifetime from millions to billions of 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
Formation of Stars
Protostars - cold and have short lifetime Characterized by
outflow of gas
Mass of most stars between about 0.1 and 30 M
S
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 M
S
, 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
Low - Mass Stars
High - Mass Stars
Mass > 10 M
S
, 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 Gravitational collapse of core leads to explosion –
supernova
Supernova remnants Core becomes a neutron star or a black hole
High - Mass Stars
Stellar Remnants: White Dwarves
Remnants of low-mass stars Shell ejected into planetary nebula Hot (~ 25 000 K), compact stars Mass comparable to M
S
, but radius comparable to
radius of Earth
Very dim, no fuel burning Cool rapidly (black dwarf) Composed of C and O, surface layer of H and He Extreme magnetic fields (~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 M S
Stellar Remnants: White Dwarves in Binary Systems
Stellar Remnants: Neutron Star
1934 W. Baade and F. Zwicky Theoretical results: Radius ~10 km Maximum mass ~2 – 3 M S 1967 A. Hewish et al. detected radio signal with astonishingly precise pulse rate (period of pulsation) ~(density) -1 Densities larger then those of white dwarves
Stellar Remnants: Neutron Star
F. Pacini and T. Gold: pulsar is rotating, not pulsating star T S ~1 month T pulsar 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-mass stars 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) ( l max ~16 R S ) Gravitational waves Detecting black hole: