Transcript Stellar Evolution

The Deaths of Stars
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The Final Breaths of Sun-Like Stars:
Planetary Nebulae
Remnants of stars with ~ 1 – a few Msun
Radii: R ~ 0.2 - 3 light years
Expanding at ~10 – 20 km/s (← Doppler shifts)
Less than 10,000 years old
The Helix Nebula
The Formation of Planetary Nebulae
Two-stage process:
Slow wind from a red giant blows
away cool, outer layers of the star
Fast wind from hot, inner layers of the star
overtakes the slow wind and excites it
=> Planetary Nebula
The Dumbbell Nebula in Hydrogen
and Oxygen Line Emission
Planetary Nebulae
Often asymmetric, possibly due to
• Stellar rotation
• Magnetic fields
• Dust disks around the stars
The Butterfly Nebula
A sun-like star is building up
elements through nuclear fusion
up to which element(s)?
1.
2.
3.
4.
5.
Hydrogen
Helium
Carbon and Oxygen
Iron
Uranium
The Remnants of Sun-Like Stars:
White Dwarfs
Carbon-Oxygen (C,O) core
does not ignite Carbon fusion.
He-burning shell keeps dumping C and O onto
the core. C,O core collapses until gravity is
balanced by pressure from a new state of matter:
Degenerate Matter
White Dwarfs
Degenerate stellar remnant (C,O core)
Extremely dense:
1 teaspoon of WD material: mass ≈ 16 tons!!!
Chunk of WD material the size of a beach ball
would outweigh an ocean liner!
White Dwarfs:
Mass ~ Msun
Temp. ~ 25,000 K
Luminosity ~ 0.01 Lsun
1.
2.
3.
4.
5.
Considering the luminosity (0.01 Lsun)
and surface tempmerature (25,000 K)
of a white dwarf, where do you expect
to find white dwarfs in the
Hertzsprung-Russell diagram?
1
In the upper left corner
In the lower left corner.
In the lower right corner.
In the upper right corner.
In the center.
4
5
2
3
White Dwarfs in Binary Systems
T ~ 106 K
Binary consisting of WD + MS or Red Giant star
=> WD accretes matter from the companion
Angular momentum conservation => accreted
matter forms a disk, called accretion disk.
Matter in the accretion disk heats up to ~ 1 million K
Considering that material in the accretion disk
around a white dwarf heats up to ~ 1 million oK,
in which wavelength band do you expect that it
radiates the most strongly?
1.
2.
3.
4.
5.
Infrared
Optical
Ultraviolet
X-rays
Gamma-rays
White Dwarfs in Binary Systems
X-ray
emission
T ~ 106 K
Binary consisting of WD + MS or Red Giant star
=> WD accretes matter from the companion
Angular momentum conservation => accreted
matter forms a disk, called accretion disk.
Matter in the accretion disk heats up to ~ 1 million K
=> X-ray emission => “X-ray binary”.
Nova Explosions
Hydrogen accreted through the
accretion disk accumulates on
the surface of the WD
 Very hot, dense layer of nonfusing hydrogen on the WD surface
 Explosive onset of H fusion
Nova Cygni 1975
 Nova explosion
In many cases: Cycle of
repeating explosions every
few years – decades.
A very massive (> 8 Msun) star is building
up elements through nuclear fusion up to
which element(s)?
1.
2.
3.
4.
5.
Hydrogen
Helium
Carbon and Oxygen
Iron
Uranium
The Deaths of Massive Stars:
Supernovae
Final stages of fusion in highmass stars (> 8 Msun), leading to
the formation of an Iron core,
happen extremely rapidly: Si
burning lasts only for ~ 1 day.
Iron core ultimately collapses,
triggering an explosion that
destroys the star:
A Supernova
The Crab Nebula
The Deaths of Massive Stars:
Supernovae
Do you think a supernova is
bright enough to see it in a
distant galaxy?
1.
2.
3.
4.
5.
No, only out to a few hundred light years, i.e., in the
neighborhood of our Sun, within our Milky Way.
No, not in other galaxies, but throughout our Milky
Way.
Yes, but only nearby galaxies in our Local Group.
Yes, out to about half the distance through the
visible Universe.
Yes essentially throughout the entire visible
Universe.
Supernovae
Energy released in a Supernova:
10,000,000,000,000,000,000,000,000
(1025) times the energy of a
Hydrogen bomb!
Luminosity of a Supernova:
10,000,000,000 (1010) times the
power output of the sun!
Comparable to the power of an
entire galaxy
Supernova in a distant galaxy
The Famous Supernova of 1987:
SN 1987A
Before
At maximum
Unusual Supernova in the Large
Magellanic Cloud in Feb. 1987
The Remnant of SN 1987A
Ring due to SN ejecta catching up with preSN stellar wind; also observable in X-rays.
Supernova Remnants
X-rays
The Crab Nebula:
Remnant of a
supernova observed
in a.d. 1054
Optical
The Cygnus Loop
The VeilANebula
Cassiopeia
A different kind of Supernova:
Type Ia Supernovae
White Dwarf in a binary
system accreting matter
from a companion star.
Untill it becomes too
massive to be a
White Dwarf
 Collapse!
 Supernova
Type Ia Supernovae
Type Ia Supernovae
Alternative Scenario: Merger of two white dwarfs