Download Lecture Slides – Stars

Black-body radiation:
Planck distribution
(Rayleigh-Jeans, Wien
distributions)
Wien’s Law
Stefan(-Boltzmann) Law
supergiants (I)
giants (III)
white dwarfs
Observational HRD
may use colour in
place of temperature,
and magnitude
(brightness) in place
of luminosity
The main proton-proton chain
So, what’s SNU?
Mid -1968: Davis, Bahcall, Homestake mine experiment only 1/3 of expected high-energy (PP-II, III) neutrinos found
1989, Kamiokande - only 1/2 of expected high-energy neutrinos
Early 1990s, GALLEX, SAGE confirmed absence of low-energy
neutrinos (important because dominant)
Late 1990s, SuperKamiokande precisely confirmed high-energy deficit
of mainly electron neutrinos but with some senitivity to other flavours
(muon, tau)
2001 June 18 - Sudbury Neutrino Observatory, bigger deficit the
SuperKamiokande, for the same energy electron neutrinos (only)
2002, Davis gets Nobel Prize
The solar neutrino problem:
neutrino oscillations [the Mikheyev-Smirnov-Wolfenstein (MSW)
effect]
In 1967, two years before his epochal paper with Gribov on
solar neutrino oscillations was published, Bruno Pontecorvo wrote:
"Unfortunately, the weight of the various thermonuclear reactions in
the sun, and the central temperature of the sun are insufficiently well
known in order to allow a useful comparison of expected and
observed solar neutrinos..."
In other words, the uncertainties in the solar model are so large that
they prevent a useful interpretation of solar neutrino measurements.
Bruno Pontecorvo's view was echoed more than two decades later
when in 1990 Howard Georgi and Michael Luke wrote as the opening
sentences in a paper on possible particle physics effects in
solar neutrino experiments:
"Most likely, the solar neutrino problem has nothing to do with
particle physics. It is a great triumph that astrophysicists are able to
predict the number of 8B neutrinos to within a factor of 2 or 3..."
C. N. Yang stated on October 11, 2002, a few days after the awarding
of the Nobel Prize in Physics to Ray Davis and Masatoshi Koshiba
for the first cosmic detection of neutrinos, that:
"I did not believe in neutrino oscillations even after Davis' painstaking
work and Bahcall's careful analysis. The oscillations were,
I believed, uncalled for."
Web page:
http :// www.star.ucl.ac.uk/~idh/1B23
Neutrinos: different flavours have different masses
Sum of masses: <1eV
(?)
Differences: O(0.1eV)
(?)
The Hertzsprung-Russell Diagram is a plot of
Temperature (colour, spectral type) vs
Luminosity (brightness)
Most (90%) of stars lie on the Main Sequence, where stars
burning hydrogen to helium (proton-proton or CNO cycles)
are in hydrostatic equilbrium
Sun shines through proton-proton reactions, which emit
electron neutrinos
‘Solar Neutrino Problem’
discovery of ‘neutrino oscillations, neutrino mass
How do stars get on to the main sequence, and what happens
afterwards? – stellar evolution
Giant Molecular Clouds:
Radii 50 pc
Masses 100,000+ solar masses
Temp few 10s of K
Densities of order 10 molecules per cubic cm
(10**20 smaller than the core of a star…)
Collapse to from stars, ca. 0.1-100x solar mass
Main-sequence lifetime can be estimated
For 1 solar mass:
Main Sequence lifetime: 1010 years (ZAMSTAMS)
As 4H  1 He, number of particles falls,
pressure drops
core contracts
core temperature rises  pressure rises
 increased luminosity, increased radius
(‘Mirror law: shrinking core  expanding envelope!)
ZAMS
NOW
Temperature rise = 300K
6% increase in radius
End of core hydrogen burning  core cools, pressure decreases
Cores shrinks  energy deposited in hydrogen burning shell
(Kelvin-Helmholtz contraction; core temperature actually
increases when fusion stops!) – CNO burning (thin)
Luminosity increases,
star expands,
becomes a Red Giant:
burning hydrogen to
helium in a shell around
a helium core (for about
10% of MS lifetime for
a solar-mass star)
supergiants (I)
giants (III)
white dwarfs
Hydrogen “ash” falls onto core, which contracts,
temperature rises; at 108 K core helium burning
(triple alpha) starts. Degenerate core:
temperature increases but pressure does not!
Helium flash (raises degeneracy)
New configuration, core helium burning (+shell hydrogen
burning) on the ‘horizontal branch’ (core expands,
star contracts), for about 1% of the MS lifetime for a solar-mass
star (helium burning goes fast)!
After core helium exhaustion, shell helium burning starts;
the star becomes a second type of ‘red giant’:
Main Sequence
Red Giant Branch
Horizontal Branch
*Asymptotic Giant Branch (AGB)
Helium in shell becomes exhausted
Overlying hydrogen shell falls back & reignites  feeds helium
shell, compressed, heated  helium shell flash (for degenerate
cores) ‘thermal pulse’
Complicated! But result is an unstable star (a pulsating variable)
which loses its outer layers
‘Dredge-up’ – convection brings processed material from core
to surface on red-giant branches
First: during shell hydrogen burning
Second: during shell hydrogen burning
(Further dredge-ups possible)
Of some personal significance…
As the outer layers disperse the carbon-oxygen core (left
from core helium burning) is exposed
Planetary Nebula (lifetime ca. 10,000 years, from expansion)
+ remnant carbon-oxygen white dwarf (electron degenerate)
White dwarf mass-radius relation and the Chandrasekhar Limit
EVOLUTION OF MASSIVE STARS
Initial stages (contraction onto MS, core hydrogen burning
on MS) broadly similar
(Radiation pressure prevents formation of very high masses,
>100 solar masses)
Higher masses  hotter cores; core H burning is through the
CNO cycle
AND later stages of `burning’ (beyond triple-alpha burning
of helium) are possible at later stages of evolution
For stars > 4 solar masses, carbon-oxygen core is
more massive than ‘Chandrasekhar limit’
Electron degeneracy can’t support core, so further
heating & burning occurs:
Carbon burning  O, Ne, Na, Mg
>8 solar masses, neon burning (109K),
then oxygen burning, silicon burning, oxygen burning,
silicon burningvarious products (sulfur—iron)
Faster and faster!! (C: few hundred years; Si, a day)
No further fusion processes possible;
core collapses;
proton & electrons “squeezed” together to form neutrons,
emitting neutrino pulse
“bounceback”  shock wave through overlying layers
(more neutrinos)
 core collapse supernova
SN 1987A (LMC)
SN 1987A (LMC)
Review:
Low & medium-mass stars:
White dwarfs
supported by electron
degeneracy)
7-20 solar masses
Neutron stars (densities
of nuclear matter!)
>20 solar masses
Black holes
Are these products
observable??