The life history of a star depends primarily on its mass A little bit on its metallicity ( Z )
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• The life history of a star depends primarily on its mass • A little bit on its metallicity (Z) • Sometimes influenced by nearby stars • Low mass stars (M < 8M Sun ) live a long life and die slowly • High mass stars (M > 8M ) live fast Stars form from cool clouds of gas called molecular clouds • Gravity overcomes pressure, and several stars begin to form • Usually get multiple stars in the same region, about the same age • Called clusters • Initially, the stars are all moving together at the same speed • Lots of stars with low mass, few with large mass • Lowest mass: about 0.08 M sun • Highest mass: about 100 M sun
description
Stellar Evolution. Stars form from cool clouds of gas called molecular clouds Gravity overcomes pressure, and several stars begin to form Usually get multiple stars in the same region, about the same age Called clusters Initially, the stars are all moving together at the same speed - PowerPoint PPT Presentation
Transcript of The life history of a star depends primarily on its mass A little bit on its metallicity ( Z )
• The life history of a star depends primarily on its mass• A little bit on its metallicity (Z)• Sometimes influenced by nearby stars
• Low mass stars (M < 8MSun) live a long life and die slowly• High mass stars (M > 8MSun) live fast and die violently• The more massive a star is, the faster it does everything
Stars form from cool clouds of gas called molecular clouds• Gravity overcomes pressure, and several stars begin to form• Usually get multiple stars in the same region, about the same age• Called clusters
• Initially, the stars are all moving together at the same speed• Lots of stars with low mass, few with large mass• Lowest mass: about 0.08 Msun
• Highest mass: about 100 Msun
Molecular Clouds:
Stellar EvolutionMolecular Clouds
ProtostarMain Sequence Star
Red Giant starHorizontal Branch
Asymptotic Giant BranchPlanetary Nebula
White Dwarf
ProtostarMain Sequence StarSupergiant StagesType II Supernova
Neutron Star or Black Hole
Low
Mass Stars
M < 8M
SunH
igh
Mas
s Sta
rsM
> 8M
Sun
Stars are powered by nuclear fusion• The combining of simple nuclei to make more complex ones• Stages are defined by what is going on at the center
Main Sequence Stars: IntroductionA Main Sequence Star is a star that is burning hydrogen to helium at its center• This is nuclear burning, not combustion• No oxygen• We don’t care about the details
• This process is extremely efficient• It can go for a long time
• During this stage, the structureof the star hardly changes• Small increase in
luminosity• Spectral class
stays almost thesame
• Small motionupwards in theH-R diagram
4 1H 4He
Main Sequence Stars: Mass DependanceEverything about the star depends on mass• Higher mass stars have:• Larger radius
• Somewhat higher temperature
• Much higher luminosity
R M
0.4T M
2 4L R T 3.5M
Type MassO5 60B0 18B5 5.9A0 2.9A5 2.0F0 1.6F5 1.3G0 1.05G5 .92K0 .85K5 .74M0 .51M5 .21M8 .06
60 MSun
1 MSun
0.08 MSun
The main sequence is a band because• Stars have variable metallicity• Stars are different ages
Main Sequence Stars: Lifetime
A star stays as a main sequence star until it runs out of hydrogen• The amount of fuel is proportional to its mass:• The rate it consumes fuel depends on its mass:• How long it lasts depends on mass:
F M3.5L M
FTL
3.5
MM
2.5M
Big Stars Die Fast
The Sun lasts about 10 Gyr on main sequence
2.5
10 GyrMSMTM
Giant StarsMolecular Clouds
ProtostarMain Sequence Star
Red Giant starHorizontal Branch
Asymptotic Giant BranchPlanetary Nebula
White Dwarf
ProtostarMain Sequence StarSupergiant StagesType II Supernova
Neutron Star or Black Hole
Low
Mass Stars
M < 8M
SunH
igh
Mas
s Sta
rsM
> 8M
Sun
The stars run out of hydrogen to burn to helium• Low mass stars burn helium to produce carbon and oxygen (Z = 6,
8)• High mass stars also produce elements through iron (Z = 26)• These produce much less energy than hydrogen• The fuel is used faster and runs out faster
• All giant stages together last about 10% of the previous stages
Giant Stages: Movement on HR diagram• Low mass stars get cooler and more luminous• Up and right on the HR diagram
• High mass stars get cooler• Right on the HR diagram
• The high mass stars move off from the main sequence first• You can estimate the age of a cluster by which stars
have left the main sequence• The turn off point• More about this later
Cepheid Variable Stars• Not all stars are constant luminosity• There is a region of the HR diagram where
stars pulsate, called the instability strip• Not Main Sequence stars
• The temperature, size, and luminosity all vary periodically• Many Cepheids are extremely bright -
much more luminous than typical main sequence stars• We can see them far away, even in nearby
galaxies• One of the biggest motivations for the
Hubble telescope was to study Cepheids in galaxies a few Mpc away
Cepheid Variable Stars
Cepheid Variable Stars• Bigger stars pulsate more slowly• Bigger stars are more luminous• There is a simple relationship
between the period and the luminosity• If you know the period, you know
the luminosity• If you measure the brightness,
you can then get the distance
2.81log 1.43M P
1510 pc
m M
d
P is period in days
• Complication – modified by metallicity• Must be compensated for
Planetary NebulaMolecular Clouds
ProtostarMain Sequence Star
Red Giant starHorizontal Branch
Asymptotic Giant BranchPlanetary Nebula
White Dwarf
ProtostarMain Sequence StarSupergiant StagesType II Supernova
Neutron Star or Black Hole
Low
Mass Stars
M < 8M
SunH
igh
Mas
s Sta
rsM
> 8M
Sun
Low Mass stars end their lives as planetary nebulas• Outer layer is expelled from the star• This mixes carbon/oxygen/helium back into interstellar space
• Inner super-hot layer gradually revealed• This star is now radiating in the ultraviolet – visible luminosity is low• But the ultraviolet light excites the atoms in the gas that has been expelled
HydrogenHelium
Carbon/Oxygen
Asymptotic Giant
Ultraviolet
Planetary NebulaWhite Dwarf
Ring Dumbbell
Cat’s Eye
Helix
M2-9
Hourglass
NGC 6751Eskimo
EggNGC 2440
Spirograph Abell 39
Planetary Nebula (2)• The planetary nebula glows brightly in visible light• An emission-line spectrum
• How bright any given planetary nebula is is difficult to predict• Statistically, there are more dim ones than bright ones• However, there is an approximate maximum luminosity:
* 4.47 0.05M • Almost independent of metallicity• Can be used to measure distance to objects containing many planetary nebulae• Such as a galaxy
White DwarfMolecular Clouds
ProtostarMain Sequence Star
Red Giant starHorizontal Branch
Asymptotic Giant BranchPlanetary Nebula
White Dwarf
ProtostarMain Sequence StarSupergiant StagesType II Supernova
Neutron Star or Black Hole
Low
Mass Stars
M < 8M
SunH
igh
Mas
s Sta
rsM
> 8M
Sun
A white dwarf is a burned out star consisting of carbon and oxygen• Dead – dim and getting dimmer• Mass of Sun, size of Earth• Gravity is opposed by degeneracy pressure• Quantum mechanical effect due to Pauli Exclusion principle
• There is a maximum mass – called the Chandrasekhar mass: about 1.42 MSun
Type Ia SupernovaA white dwarf with a giant companion can gain mass from its companion• As it gains mass, gravity increases – it shrinks• When it reaches Chandrasekhar mass it collapses, catastrophically• Temperature increases drastically• Fusion begins again in the core• The entire star explodes, all at once• Star is super bright – as bright as an entire galaxy• We can see them most of the way across the universe
• All type Ia supernovae have nearly identical precursors• They should blow up almost exactly the same way• They should have almost uniform luminosity• For reasons that aren’t well understood, they are not• Some are more efficient than others• Amount of time they take to get bright seems to predict luminosity
Supernovae in GeneralMolecular Clouds
ProtostarMain Sequence Star
Red Giant starHorizontal Branch
Asymptotic Giant BranchPlanetary Nebula
White Dwarf
ProtostarMain Sequence StarSupergiant StagesType II Supernova
Neutron Star or Black Hole
Low
Mass Stars
M < 8M
SunH
igh
Mas
s Sta
rsM
> 8M
Sun
• High mass stars end their life as Type II supernovae• Or occasionally, type Ib or Ic
• These stars are very complex, containing numerous elements• All supernovae mix “metals” back into inter-stellar medium• They already contain many elements• More are made during the explosions
Type II Supernova Hydrogen
HeliumCarbon/Oxygen
Red Supergiant
NeonSilicon
Iron
Supernovae
SN1987A
SN1994 D
Supernova Remnants
The Crab Nebula
Tycho’s Supernova Remnant
Puppis AVeil Nebula
Vela Nebula
Kepler SNR
CygnusLoop
W49B
N49
Black HolesMolecular Clouds
ProtostarMain Sequence Star
Red Giant starHorizontal Branch
Asymptotic Giant BranchPlanetary Nebula
White Dwarf
ProtostarMain Sequence StarSupergiant StagesType II Supernova
Neutron Star or Black Hole
Low
Mass Stars
M < 8M
SunH
igh
Mas
s Sta
rsM
> 8M
Sun
A black hole is an object where gravity overcomes all other forces• Mostly come from very heavy stars (> 25 MSun)• Gravity becomes so strong nothing can escape• Relativity says nothing can go faster than light:• If you get too close, the escape velocity equals speed of light• Called the Schwarzschild radius
2esc
2GMvR
2 2
S
GMcR
2
2S
GMRc
