Lives and Deaths of Stars
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Transcript of Lives and Deaths of Stars
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28,000 light years200 billion stars
Milky Way Galaxy
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100 billion galaxies in the observable Universe
10 day exposure photo!
Over 1500 galaxies in a spot 1/30 the diameter of the Moon
Farthest and oldest objects are 12-13 billion light years away!
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Proxima Centauri (Alpha Centauri C)Closest star (4.2 light-years from the Sun)
Voyager 1: 12 light-hours from the Sun (90 AU)
Launched in 1977
The most distant human-made object in space
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How can we learn about the life of stars??
• Our life span is ~ 80 years
• Human civilization exists ~ 5000 years
• Our Sun exists at least 4.6 billion years!
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Star Clusters – “School Classes” Star Clusters – “School Classes” for Starsfor Stars
They consist of stars of the same age !
Open clusters100’s of stars
Globular clusters100,000 of stars
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Giant molecular clouds – stellar nurseries
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Great Orion Nebula
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The Horsehead Nebula
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Star Forming Region RCW 38
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Coldest spots in the universe:T ~ 10 K
Composition:• Mainly molecular hydrogen• 1% dust
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Protostars: warm clumps of gas surrounded by infalling matter
Disks: planet formation?!
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• The matter stops falling on the star• A star becomes hot enough to sustain the pressure of gravity
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Contraction stops when the gravity is balanced by thermal pressure
Stars are held together by gravity. Gravity tries to compress everything to the center. What holds an ordinary star up and prevents total collapse is thermal and radiation pressure. The thermal and radiation pressure tries to expand the star layers outward to infinity.
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Surface temperature 6000 KTemperature at the center 14,000,000 K!
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A puzzle: the Sun and other stars radiate away huge amounts of energy. They should lose all their heat in less than a million years!
There must be an internal energy source: nuclear fusion reactions
However, the Sun lives 4.6 billion years
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“Planetary” model of atom
Proton mass: 1.7x10-27 kgElectron mass: 9x10-30 kg
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Nuclear reactionsNuclear reactions• Fission: decay of heavy nuclei into lighter fragments
•Fusion: synthesis of light nuclei into a heavier nucleus
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A star will live until all hydrogen is exhausted in its core
star mass star mass (solar masses)(solar masses)
Lifetime Lifetime (years)(years)
6060 3 million3 million
3030 11 million11 million
1010 32 million32 million
33 370 million370 million
1.51.5 3 billion3 billion
11 10 billion10 billion
0.10.1 1000's 1000's billionsbillions
Our Sun will live 5 billion years more
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What happens when all hydrogen is converted into helium in the core??
Mass defines the fate of the star
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Fate of the collapsed coreFate of the collapsed core
White dwarf if the remnant is below the White dwarf if the remnant is below the Chandrasekhar limit 1.4 solar massChandrasekhar limit 1.4 solar mass
Neutron star if the core mass is less than Neutron star if the core mass is less than ~ 3 solar masses~ 3 solar masses
Black hole otherwiseBlack hole otherwise
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Death of StarsDeath of Stars
“All hope abandon, ye who enter here” Dante
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Outer layers expand due to radiation pressure from a hot core
The star becomes a Red Giant
• Surface temperature drops by a factor of ~ 2• The radius increases by a factor of ~ 100• Luminosity increases ~ R2 T4 ~ 100-1000 times
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In only about 200 million years it will be way too hot for humans on earth. And in 500 million years from now, the sun will have become so bright and big, our atmosphere will evaporate, the oceans will boil off, and surface dirt will melt into glass.
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What is left??
A stellar remnant: white dwarf, composed mainly of carbon and oxygen
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It is extremely dense
All atoms are smashed and the star is supported by pressure of free electrons
White dwarf
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White DwarfsWhite DwarfsDegenerate stellar remnant (C,O core)
Extremely dense:1 teaspoon of WD material: mass ≈ 16 tons!!!
White Dwarfs:
Mass ~ Msun
Temp. ~ 25,000 K
Luminosity ~ 0.01 Lsun
Chunk of WD material the size of a beach ball would outweigh an ocean liner!
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As it cools, carbon crystallizes into diamond lattice.Imagine single diamond of mass 1030 kg!Don’t rush, you would weigh 15,000 tons there!
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White dwarfs in a globular cluster
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(SLIDESHOW MODE ONLY)
Death of a massive star
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The iron core of a giant star cannot sustain the pressure of gravity. It collapses inward in less than a second.
The shock wave blows away outer layers of a star, creating a SUPERNOVA EXPLOSION!
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For several weeks the supernova outshines the whole galaxy
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Eta Carinae: will explode soon
Distance 7500 ly
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Supernova RemnantsSupernova Remnants
The Cygnus Loop
The Veil Nebula
The Crab Nebula:
Remnant of a supernova
observed in a.d. 1054
Cassiopeia A
Optical
X-rays
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Crab nebula: the remnants of supernova 1054
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Formation of Neutron StarsFormation of Neutron StarsCompact objects more massive than the
Chandrasekhar Limit (1.4 Msun) collapse further.
Pressure becomes so high that electrons and protons combine to form stable neutrons throughout the object:
p + e- n + e
Neutron Star
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Properties of Neutron StarsProperties of Neutron Stars
Typical size: R ~ 10 km
Mass: M ~ 1.4 – 3 Msun
Density: ~ 1014 g/cm3
Piece of neutron star matter of the size of a sugar cube has a mass of ~ 100 million tons!!!
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Isolated neutron stars are extremely hard to observe
Neutron stars have been theoretically predicted in 30s.Landau, Oppenheimer, Zwicky, Baade
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However, there are two facts that However, there are two facts that can help:can help:
Neutron stars should rotate extremely fast Neutron stars should rotate extremely fast due to conservation of the angular due to conservation of the angular momentum in the collapsemomentum in the collapse
They should have huge magnetic field due They should have huge magnetic field due to conservation of the magnetic flux in the to conservation of the magnetic flux in the collapsecollapse
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Jocelyn Bell
Discovery of pulsars: Bell and Hewish, 1967
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When the core is too massive, nothing can prevent collapse into a black hole
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2
2
c
GMRs
Schwarzschild radius: event horizon for a nonrotating body
To make a black hole from a body of mass M, one needs to squeeze it below its Schwarzschild’s radius
Rs
Gravitational collapse: the body squeezes below its event horizon
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Black holes are NOT big cosmic bathtub drains!
Far from a black hole R >> Rs weak field
Approaching a black hole R Rs (strong field): gravity pull very strong
If our Sun collapses into a black hole, we won’t see any difference in the gravitational pull (but it will be VERY cold)
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How to observe a black hole if it How to observe a black hole if it does not emit any radiation? does not emit any radiation?
Good news: most stars are in binary systemsGood news: most stars are in binary systems
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The Universe in gamma-ray eyes
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Gamma-ray bursts
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Gamma-ray bursts
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Our Earth and our bodies are made of atoms that were synthesized in previous generations of stars