Life story of a star Micro-world Macro-world Lecture
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 Suns 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 fusionlarger
charge leads to greater repulsion Fusion of two helium nuclei doesnt 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 theres a sudden flash when a large part of the Helium gets burned all at once. We dont see this flash because its 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 stars 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 Suns 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 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
Keplers Supernova Oct 8, 1604 Chosun Silok Keplers Supernova today Light curve from Keplers 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
3m Sun 300,000 x Earth
3 km Mearth = 6x10 kg 24 Normal density R=6400km
If the Earth was the density of a white dwarf If the Earth was the density of a neutron star
R10km R2.5m If the Earth was Compressed into A Black Hole
Rhoriz1cm 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
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