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Stellar Evolution – the Life and Death of a
Star…Here’s the story we’ll tell…• Lowest mass stars and their evolution• Low mass star evolution• High mass star evolution• Stellar death, and stellar corpses• Origin of the chemical elements – stars do
it!
Stars: always born in star clusters!
• Low temperature requires shielding from the radiation of other stars; requires dust which requires a lot of mass, since dust is a relatively rare component of interstellar clouds
• Star clusters forming in today’s environment are called “open star clusters”, dozens to hundreds of stars
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Size vs mass for planets, bd’s,stars
Stellar Evolution: How stars live and die
• Visualize stellar evolution as a path on the H-R Diagram
• Remember, it’s a plot of Surface Temperature vs. Luminosity
• Where do you suppose stars first appear on the diagram? Ponder……
HR pre main sequence sun
Another Quick Overview First…
• Stars burn through their hydrogen, evolve off Main Sequence to become Red Giants, then die in various ways
• High mass stars evolve fast,… • Low mass stars evolve slowly
HR main sequence turnoff
The H-R Diagram of a Star Cluster
•All Stars born at the same time, only differ in their mass•Stars age at different rates, depending on their mass. More mass = faster evolution•Stellar Evolution web simulator
HR of star clusters vs age
M55 HR diagram
Evolution of Low Mass Stars• Note! I distinguish between low and medium
mass stars – the book calls all of them “low mass”.
• Begin with H burning in core• When H runs out, core collapses under
gravity, releasing grav potential energy, raising star’s luminosity
• Core collapse stops when “electron degeneracy” sets in. Electrons are “elbow to elbow” (in a quantum mechanical sense)
Layers; main seq vs. giant
Medium Mass Star Evolution
• H burning until all H is He, then core collapse, releasing gravitational potential energy, raising luminosity and expanding the star ~ x100 times
• Core density and temperature rises until 100 million K. Then…..
• Well, you tell me – what are the options for further fusion? We have H and He floating around in the core…
Be per nucleon
Helium burning layer
Sun to red giant cartoon
HR tracks to red giant
Sun and red giant side by side
Sun’s L vs time
We’re all doomed
HR with instability and variables
The End of the Line for Medium Mass Stars like the Sun…
• Added luminosity is so strong, it lifts the red giant’s low density outer envelope completely off the star.
• As it expands, its opacity drops and we see to a deeper and deeper and hotter and hotter depth, so the star moves left on the HR diagram
• Until… we see the electron degenerate core; the new white dwarf created at the center
• This core can now cool, as it can’t collapse further and it is exposed to the cold of outer space.
• Thus, it follows the cooling curve of a white dwarf; down and to the right on the HR diagram
• So, what we see is a hot stellar corpse surrounded by an expanding and thinning cloud of flourescent gas = a Planetary Nebula
HR track to PN stage
White dwarf->pN shell w velocity
Green fuzzball
PN misc young
Cateye nebula
Dumbell
Dumbell hst upclose details
Egg burst nebula
Helix Nebula
Ic 4406 P
Little Ghost PN
NGC 2346 pn
Pn abell 39
NGC 2440 pn
NGC 6751 PN (blue eye)
Ring Nebula
PN flying badminton
PN misc
Spirograph PN
Eskimo lowres
Eskimo hi res
Evolution of High Mass Stars – Short and Violent
Lives• Have enough mass to heat & compress core
to fuse all the way up to iron• Iron – the most stable, most tightly bound of
all nuclei• All fusion or fission involving iron will subtract
heat from the star’s core, not add to it. “Danger! Danger Will Robinson!”
Layers of a pre SN II
Eta Carinae
Ant nebula
Wolf-rayet star
The Death of High Mass Stars…
• When iron core exceeds about 1.4 solar masses, the temperature becomes high enough to cause nuclear reactions for iron
• Nuclear burning causes further core collapse, which raises the density and accelerates the nuclear reactions.
• In 0.2 seconds (!) the core collapses, fusing iron into lighter and also heavier elements
• This is the ONLY place in the universe that elements heavier than iron are made!
• Neutrinos produced, so vast in number that they blow apart the star…
Be per nucleon
• Be per nucleon
Supernova! (SN II)• 99% of energy release, the gravitational
potential of the star, goes into neutrinos• 1% goes into the explosion• 0.01% goes into visible light. Still, the light
is bright enough to equal the entire galaxy of 100 thousand million stars (Gah!)
• SN II are the only place in nature where the elements heavier than iron are produced
Let’s look at some ancient supernova remnants…
Cass A
Cass A colored
Cass A upclose
Kepler’s snr
LMC SNR
Another LMC SNR
SNR H-alpha
Pencil nebula snr
Veil Nebula (entire)
Cygnus loop SNR
Veil closeup1
Crab HST
Grav redshift
Neutron star layers
Egg nebula pulsar
Crab center w jet
Cerenkov radiation diagram
Crab center w jet sequence
Crab HST center upclos
Let’s look at another Pulsar. This one is in the globular star
cluster 47 Tucanae…
47 Tuc – ground based
47 Tuc HST
Millisecond pulsar
How to Detect Neutrinos?
• Like, neutrinos from supernova explosions• …or neutrinos from the sun (the strongest source
because it’s so close)• - once in a great while a neutrino will hit an
electron and deposit its energy, accelerating the electron to almost the speed of light. This rapid acceleration causes the electron to give of photons of light = synchrotron radiation
Sudbury neutrino detector
The Cosmic Abundances of the Chemical Elements
• Due to the nuclear fusion in the cores of stars• And… to supernova explosions• Remember – Supernova explosions are the ONLY
place in the universe where heavy elements are created!
• All the elements beyond Iron in the periodic table (gold, silver, uranium, copper…) are created ONLY in the core collapse of a supernova explosion.
Abundances of all elements
Abundances of all elements graph
Cosmic Rays…• The blast of a supernova explosion sends out
elementary particles at near the speed of light. These get further accelerated by magnetic fields in the galaxy.
• When they impact earth, they smash into our atmosphere and create cosmic ray air showers…
• Cosmic rays are a significant source of genetic mutations. Cancer odds are higher the higher the elevation you live, in part because of more cosmic ray exposure!
Cosmic ray airshower
What happens if the stars are in a close binary
system?
• This happens a lot! Nearly half the stars in our Galaxy are members of binary star systems
• Roche lobe defines gravitational “backyard” for each star
Mass transfer binary (art)
Mass transfer accretion disk
X-ray binary art
Nova sequence
But with all this mass falling onto the white dwarf, there’s
another possibility…
• … something more ominous… more terrifying… more…. Scary!
• What could that BE?!
Carbon Bomb Supernova (SN type I)
• If the white dwarf is close to the 1.4 solar mass upper limit that electron degeneracy can support…
• The added mass could push it past the limit before it gets hot enough to flash off
• Then, star collapses under the weight and because it is electron degenerate, energy created will not expand the star and shut off the fusion.
• So, entire star (carbon, mostly) undergoes fusion at once. What a star normally takes billions of years to burn, this star burns all at once. BIG explosion!
SN Ia sequence
Supernova! (SN I)• These are even brighter than SN II’s from
massive stars.• Very useful – they’re all the ~same – 1.4 solar
mass white dwarfs undergoing nuclear fusion. This turns out to mean they are…
• GREAT “standard candles” – objects of known luminosity, on which we can then use simple math to determine their distance.
• So, any SN I and its host galaxy, we can find it’s distance, even out to the edge of the observable universe, since they are so bright.
• Huge amount of observational effort today is going into discovering and charting the light curve of SN I’s throughout the universe!
SN Ia light curves
3 Possible Ends of a Stellar Corpse!
• If mass < 1.4 Msun = White Dwarf
• If 1.4Msun < M < 3 Msun = Neutron star
• If M > 3 Msun = Black Hole!
Tole cartoon