Life story of a star
Micro-world Macro-world
Lecture 20
Life Cycle of Stars
Recycling
Supernovae produce
- heavy elements
- neutron stars
- black holes
Martin Rees - Our Cosmic
Habitat
Our favorite star: The Sun
R๏
= 696,000 km
(109 x Rearth)
M= 2x1030kg
( 3x105 x Mearth)
•Rotation period:
25 days(equator)
30 days (poles)
Composition:
70% Hydrogen
28% Helium
Stars have different colors
Stars have different colors
• B: blue – hottest
• A: green – warm
• C: red - cool
Infer temperature of a star
from the peak wavelength
of its black body radiation
Color, Brightness + Count them
Sun
May 2006April 2004
Belinda Wilkes
Solar
fusion
processes
+ 1.4 MeV
+ 5.5 MeV
+ 12.9 MeV
Neutrinos come directly from solar core
Superkamiokande
Sun as seen by a neutrino detector
What happens when the Sun’s
Hydrogen is all used up?
Evolution of a Star
Red Giant
(Sun)
Main Sequence Evolution
• Core starts with same
fraction of hydrogen as
whole star
• Fusion changes H  He
• Core gradually shrinks and
Sun gets hotter and more
luminous
Evolution of the Sun
• Fusion changes H  He
• Core depletes of H
• Eventually there is not
enough H to maintain
energy generation in the
core
• Core starts to collapse
The Sun will become a Red Giant
The Sun 5 Billion years from now
Earth
The Sun Engulfs the Inner Planets
Red Giant Phase
• He core
– No nuclear fusion
– Gravitational contraction
produces energy
• H layer
– Nuclear fusion
• Envelope
– Expands because of
increased energy production
– Cools because of increased
surface area
Helium fusion
Helium fusion does not begin right away because it
requires higher temperatures than hydrogen fusion—larger
charge leads to greater repulsion
Fusion of two helium nuclei doesn’t work, so helium fusion
must combine three He nuclei to make carbon
Helium Flash
• He core
– Eventually the core gets hot
enough to fuse Helium into
Carbon.
– This causes the temperature to
increase rapidly to 300 million K
and there’s a sudden flash when
a large part of the Helium gets
burned all at once.
– We don’t see this flash because
it’s buried inside the Sun.
• H layer
• Envelope
Red Giant after Helium Ignition
• He burning core
– Fusion burns He into C, O
• He rich core
– No fusion
• H burning shell
– Fusion burns H into He
• Envelope
– Expands because of
increased energy
production
What happens when the star’s core runs
out of helium?
–
–
–
–
The star explodes
Carbon fusion begins
The core starts cooling off
Helium fuses in a shell around the core
Helium burning in the core stops
H burning is continuous
He burning happens in “thermal
pulses”
Core is degenerate
Sun looses mass via winds
• Creates a “planetary nebula”
• Leaves behind core of carbon and oxygen
surrounded by thin shell of hydrogen
a “white dwarf star”
Planetary nebula
Planetary nebula
Planetary nebula
Hourglass
nebula
White dwarf
• Star burns up rest of hydrogen
• Nothing remains but degenerate core of
Oxygen and Carbon
• “White dwarf” cools
• No energy from fusion, no energy from
gravitational contraction
• White dwarf slowly fades away…
Time line for Sun’s evolution
Brightest Star – Sirius A – (Sirius B is a white dwarf)
Sirius
Orion
Constellation
( Nebula)
Betelgeuse
(Red Giant)
Sirius B
Comet HaleBop
Betelgeuse
is a red supergiant star
about 600 light years distant
1. This is a Hubble Space Telescope
image - the first direct picture of the
surface of a star other than the Sun.
2.
While Betelgeuse is cooler than the
Sun, it is more massive and over
1000 times larger. If placed at the
center of our Solar System, it would
extend past the orbit of Jupiter.
3. Betelgeuse is also known as Alpha
Orionis, one of the brightest stars in
the familiar constellation of Orion, the
Hunter.
4. The name Betelgeuse is Arabic in
origin. As a massive red supergiant, it
is nearing the end of its life and will
soon become a supernova. In this
historic image, a bright hotspot is
revealed on the star's surface.
The Sun Engulfs the Inner Planets
The Sun becomes a White Dwarf
Composition:
Carbon & Oxygen
What about M>1.4 M๏ stars?
Nuclear burning
continues past
Helium
1. Hydrogen burning: 10 Myr
2. Helium burning: 1 Myr
3. Carbon burning: 1000 years
4. Neon burning: ~10 years
5. Oxygen burning: ~1 year
6. Silicon burning: ~1 day
Finally builds up an inert Iron core
Multiple Shell Burning
• Advanced nuclear
burning proceeds in
a series of nested
shells
Fusion stops at Iron
Fusion versus Fission
Advanced reactions in stars make elements like Si, S, Ca, Fe
Atomic collapse 
Supernova Explosion
• Core pressure goes
away because atoms
collapse: electrons
combine with
protons, making
neutrons and
neutrinos
• Neutrons collapse to
the center, forming a
neutron star
Atomic Collapse
Ordinary matter
~few grams/cm3
White Dwarfs
~1 ton/cm3
Neutron star
~108 ton/cm3
Core collapse
• Iron core grows until it is too heavy to support
itself
• Atoms in the core collapse, density increases,
normal iron nuclei are converted into neutrons
with the emission of neutrinos
• Core collapse stops, neutron star is formed
• Rest of the star collapses in on the core, but
bounces off the new neutron star (also pushed
outwards by the neutrinos)
Supernova explosion
SN1987A
Tarantula Nebula in LMC
Neutrinos are detected
Feb 23, 1987
Feb 22, 1987
Previously observed Supernova
“Kepler’s Supernova” Oct 8, 1604
Chosun Silok
Kepler’s Supernova today
Light curve from Kepler’s Supernova
Where do the elements in your body
come from?
• Solar mass star produce elements up to Carbon
and Oxygen – these are ejected into planetary
nebula and then recycled into new stars and
planets
• Supernova produce all of the heavier elements
– Elements up to Iron can be produced by fusion
– Elements heavier than Iron are produced by the
neutrons and neutrinos interacting with nuclei
during the supernova explosion
How do high-mass stars make the
elements necessary for life?
http://en.wikipedia.org/wiki/Triple-alpha_process
http://en.wikipedia.org/wiki/Neon_burning_process
http://en.wikipedia.org/wiki/Silicon_burning_process
Advanced Nuclear Burning
•
Core temperatures in stars with >8MSun
allow fusion of elements as heavy as iron
We
We are
are made
made of
of stardust!
stardust
May 2006April 2004
Belinda Wilkes
What about M>8 M๏ stars?
Gravity deforms space-time
Light follows curved paths
Gravity bends the path of light
Curved Space
• Einstein related gravity
forces to space
curvature.
• Black holes deeply warp
space.
• Everything falls in,
nothing can climb out.
• How does this work?
The Event Horizon
• Event Horizon = black hole “surface”
Object
Mass
Radius
Earth
6 x 1024 kg 1 cm
Jupiter
300 x Earth 3 m
Sun
300,000 x
Earth
3 km
Mearth = 6x1024 kg
Normal density
R=6400km
If the Earth was
the density of a
white dwarf
If the Earth was
the density of a
neutron star
R≈10km
R≈2.5m
If the Earth was
Compressed into
A Black Hole
Rhoriz≈1cm
A nonrotating black hole has only a “center”
and a “surface”
• The black hole is surrounded
by an event horizon which is
the sphere from which light
cannot escape
• The distance between the
black hole and its event
horizon is the Schwarzschild
radius (RSch= 2GM/c2)
• The center of the black hole
is a point of infinite density
and zero volume, called a
singularity
Black Holes
• Light is bent by the
gravity of a black
hole.
• The event horizon is
the boundary inside
which light is bent
into the black hole.
• Approaching the
event horizon time
slows down relative
to distant observers.
• Time stops at the event horizon.
Binaries
• Gravitational tides pull matter off big low
density objects towards small high density
objects.
Cygnus X-1
“Seeing” Black Holes
The First “First” Black Hole
• Cygnus X-1 binary
system
• Most likely mass
is 16 (+/- 5) Mo
• Mass determined
by Doppler shift
measurements of
optical lines
Galaxy M84 core =
“Super-massive”Black Hole?
Gas, stars
moving toward
Gas, stars
us
Image of
M84
Gas, stars
moving away
from us
moving across
Area STIS
observes
Spectrogram of gas and stars moving around the
core
Space Telescope Imaging Spectrograph
spectrogram
The core of Galaxy M84 contains a total mass = 300 million x M๏ in R<26 cyr!
http://www.youtube.com/watch?v=dipFMJckZOM