Download - Stellar Evolution up to the Main Sequence
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Stellar Evolutionup to the Main Sequence
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Stellar EvolutionRecall that at the start we made a
point that all we can "see" of the stars is:
• Brightness• Color (Spectra)• Position• Distance (if we are lucky or clever)
Let's see if there are any correlations
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Stellar EvolutionUsing distance (when we know it) we can
convert the Brightness (apparent magnitude) into the absolute magnitude, or even the Luminosity
To make things easy we can write the luminosity relative to that of the Sun, L/L
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Stellar EvolutionThe Color, or spectra, we can convert to
– A Spectral Class – A Temperature– A B-V value
• V is the visible magnitude• B is the magnitude as seen on photographic
plates– Photographic plates are more sensitive to blue light –
blue stars will appear brighter• B-V gives a numerical "Color" index• For comparison
– the yellowish Sun (G2) has a B-V index of 0.656 and a surface temperature of about 6000K
– the bluish Rigel (B8) has B-V index of -0.03 and a surface temperature of about 11000K
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Stellar EvolutionWe can plot the Luminosity ratio versus the color:
O B A F G K M
1
.1
10
.01
1001000
The Sun would go here
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Stellar EvolutionThis plot was independently
discovered by Hertzsprung and Russell
It is now called the Hertzsprung-Russell, or H-R, Diagram
Ejnar Hertzsprung (1873-1967) Henry Norris Russell (1877-1957)
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The HR Diagram
O B A F G K M
1
.1
10
.01
1001000
The Sun
The 50 Nearest Stars
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The HR Diagram
O B A F G K M
1
.1
10
.01
1001000
The 50 Brightest Stars
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The HR Diagram
O B A F G K M
1
.1
10
.01
1001000
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The HR Diagram
O B A F G K M
1
.1
10
.01
1001000
There appears to be three main areas where the stars are grouped
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The HR Diagram
O B A F G K M
1
.1
10
.01
1001000
This curve is where 90% of the stars appear
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The HR Diagram
O B A F G K M
1
.1
10
.01
1001000
These are pretty dim, but also very hot…white hot
This implies that they are very small
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The HR Diagram
O B A F G K M
1
.1
10
.01
1001000
These are cool, but very bright - the size must be huge
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HR Diagram
O B A F G K M
1
.1
10
.01
1001000 Red Giants
White Dwarfs
Blue Giants
Main Sequence
Red Dwarfs
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Starbirth
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Protostars form in cold, dark nebulae
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The ‘central Star’ in Orion’s sword is a stellar nursery
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Evidence of Star Formation• forming stars are usually embedded in clouds
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Distant dark nebulae are hard to observe, because they do not emit visible light
However, dark nebulae can be detected using microwave observation, because the molecules in nebulae emit at millimeter wavelengths
Giant molecular clouds are immense nebular so cold that their constituent atoms can form molecules.
Giant molecular clouds are found in the spiral arms of our Galaxy.
Giant Molecular Clouds
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Star-forming regions appear when a giant molecular cloud is compressed
This can be caused by the cloud’s passage through one of the spiral arms of our Galaxy, by a supernova explosion, or by other mechanisms
Giant Molecular Clouds and Star-forming Regions
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Molecular Clouds
DisorderlyComplex
Giant Molecular Cloud in OrionInfrared view
From IRAS satellite
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Molecular or Dark Clouds
"Cores" and Outflows
Stages of Star Formation
Jets and Disks
Extrasolar System
1 pc
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Protostar:PROTOSTAR
SHRINKS AND HEATS UP
PROTOSTAR BEING ASSEMBLED
NUCLEAR FUSION STARTS
“pre-star” without nuclear fusion
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Stages of Starbirth# t to Next
Stage (yr)
Core Temp.
(K)
Surface Temp
(K)
Diameter(Km)
Description
1 2,000,000 10 10 100,000,000,000,000
Interstellar Gas Cloud
2 30,000 100 10 1,000,000,000,000 Cloud Fragment
3 100,000 10,000 100 10,000,000,000 Cloud Fragment
4 1,000,000 1,000,000 3,000 100,000,000 ProtoStar
5 10,000,000 5,000,000 4,000 10,000,000 ProtoStar
6 30,000,000 10,000,000 4,500 2,000,000 Star
7 10,000,000,000 15,000,000 6,000 1,500,000 Main Sequence
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Lifetime on the Main Sequence
• Luminosity basically describes how fast the star is ‘burning’ its fuel.
• This is clearly related to how much fuel there is because the greater the mass the higher the pressures and temperatures:
L M3
• Lifetime is “how much fuel / how fast it’s used”
T = M/L 1/M2
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Lifetime on the Main Sequence
Here are some comparison values:Mass(Msun)
Lifetime(Tsun)
Lifetime(years)
100 0.0001 1 million
10 0.01 100 million
1 1 10 billion
0.1 100 1 trillion
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Main Sequence Stars
TemperatureLu
min
osity highest mass
lowest mass
.
SUN
SPICA:
107 yr lifetime
PROXIMA CENTAURI
lifetime greater than age of universe
1010 yr lifetime
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The Path to the Main Sequence
O B A F G K M
1
.1
10
.01
1001000
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The T Tauri phase
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The T Tauri phase• Gravity causes the gas/dust cloud to condense. • The situation then usually becomes quite complex
• Some of the infalling gas is heated so much by collisions that it is immediately expelled as an outgoing wind.
• Jets and disks form as the infalling and outflowing gas collide and interact with changing magnetic fields.
• Temperatures and masses are similar to the Sun, but they are brighter
• They have fast rotation rates (few days)• Variable X-ray and radio emission• Not yet a 'star', but will be in a few million years
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During the birth process, stars both gain
and lose massIn the final stages of pre–main-sequence contraction, when
thermonuclear reactions are about to begin in its core, a protostar may eject large amounts of gas into space
Low-mass stars that vigorously eject gas are called T Tauri stars (age ~ 1 million year)
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A Magnetic Model for Jets (Bipolar Outflow)
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Jets: A circumstellar accretion disk provides material that a young star ejects as jets
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Jets: Clumps of glowing gas are sometimes found along these jets and at their ends
Known as Herbig-Haro Objects
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Starbirth in NCG 281
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M16 in Infrared
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Bok Globules
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Bok GlobulesNote how spherical some have become
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The Life of a StarGAS CLOUD
PHASES: WHAT CHANGES THINGS:
PROTOSTAR
gravity pulls part of cloud together
MAIN SEQUENCE STAR
nuclear reactions begin in star’s core