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Stellar EvolutionChapter 12
Stars form from the interstellar medium and reach stability fusing hydrogen in their cores. This chapter is about the long, stable middle age of stars on the main sequence and their old age as they swell to become giant stars. Here you will answer three essential questions:
• What happens as a star uses up its hydrogen?
• What happens when a star exhausts its hydrogen?
• What evidence do astronomers have that stars really do evolve?
Guidepost
Stars evolve over billions of years because of changes deep inside. That raises an interesting question about how scientists can understand such processes:
• How can astronomers study the inside of stars?
This chapter is about how stars live. The next two chapters are about how stars die and the strange objects they leave behind.
Guidepost (continued)
I. Main-Sequence StarsA. Stellar ModelsB. Why is there a Main Sequence?C. The Upper End of the Main SequenceD. The Lower End of the Main SequenceE. The Life of a Main-Sequence StarF. The Life Expectancies of Stars
II. Post-Main-Sequence EvolutionA. Expansion into a GiantB. Degenerate Matter (簡併物質 )C. Helium FusionD. Fusing Elements Heavier than Helium
Outline
III. Evidence of Evolution: Star ClustersA. Observing Star ClustersB. The Evolution of Star Clusters
IV. Evidence of Evolution: Variable StarsA. Cepheid and RR Lyrae Variable StarsB. Pulsating StarsC. Period Changes in Variable Stars
Outline (continued)
The structure and evolution of a star is determined by the laws of
Stellar Models
• Hydrostatic equilibrium
• Energy transport
• Conservation of mass
• Conservation of energy
A star’s mass (and chemical composition) completely determines its properties.
That’s why stars initially all line up along the main sequence.
Maximum Masses of Main-Sequence Stars
a) More massive clouds fragment into smaller pieces during star formation.
b) Very massive stars lose mass in strong stellar winds
Example: Eta Carinae: Binary system of a 60 Msun and 70 Msun star Dramatic mass loss; major eruption in 1843 created double lobes
Mmax ~ 100 solar masses
Minimum Mass of Main-Sequence Stars
Mmin = 0.08 Msun
At masses below 0.08 Msun, stellar
progenitors do not get hot enough to
ignite thermonuclear fusion.
Brown Dwarfs
Brown DwarfsHard to find because they are very faint
and cool; emit mostly in the infrared.
Many have been detected in star forming regions like the Orion Nebula.
Evolution on the Main Sequence (1)
Zero-Age Main Sequence (ZAMS)
Main-Sequence stars live by
fusing H to He.
Finite supply of H => finite life time
Evolution on the Main Sequence (2)
Luminosity L ~ M3.5
A star’s life time T ~ energy reservoir / luminosity
T ~ M/L ~ 1/M2.5
Energy reservoir ~ M
Massive stars have short
lives!
Evolution off the Main Sequence: Expansion into a Red Giant
When the hydrogen in the core is completely converted into He:
H burning continues in a shell around the core.
He Core + H-burning shell produce more energy than needed for pressure support
Expansion and cooling of the outer layers of the
star Red Giant
“Hydrogen burning” (i.e. fusion of H into He) ceases in the core.
Expansion onto the Giant Branch
Expansion and surface cooling during the phase of an inactive He core and a H- burning shell
Sun will expand beyond Earth’s orbit!
Degenerate MatterMatter in the He core has
no energy source left.
Not enough thermal pressure to resist and
balance gravity
Matter assumes a new state, called
degenerate matter:
Pressure in degenerate core is due to the fact that
electrons can not be packed arbitrarily close together and have small
energies.
Red Giant Evolution
4 H → He
He
H-burning shell keeps dumping He
onto the core.
He-core gets denser and hotter until the
next stage of nuclear burning can begin in
the core:
He fusion through the
“Triple-Alpha Process”
4He + 4He 8Be + 8Be + 4He 12C +
Red Giant Evolution (5 solar-mass star)
Main Sequence
Expansion to red giant
Red giant
Helium ignition in the core
Development of Carbon-Oxygen
Core
Helium in the core exhausted;
development of He-burning shell
Fusion Into Heavier Elements
Fusion into heavier elements than C, O requires very high
temperatures; occurs only in very massive
stars (more than 8 solar masses).
The Life “Clock” of a Massive Star (> 8 Msun)
Let’s compress a massive star’s life into one day…
12 12
3
45
67
8
9
1011
12 12
3
45
67
8
9
1011
Life on the Main Sequence
+ Expansion to Red Giant: 22 h, 24 min.
H burning
H He
H He
He C, O
He burning:
(Red Giant Phase) 1 h, 35 min, 53 s
The Life “Clock” of a Massive Star (2)
H HeHe C, O
C Ne, Na, Mg, O
Ne O, Mg
H He He C, O
C Ne, Na, Mg, O12 1
23
4567
8
910
11
C burning:
6.99 s
Ne burning:
6 ms 23:59:59.996
The Life “Clock” of a Massive Star (3)
H HeHe C, O
C Ne, Na, Mg, ONe O, Mg
O burning:
3.97 ms 23:59:59.99997
O Si, S, P
H HeHe C, O
C Ne, Na, Mg, ONe O, Mg
Si burning:
0.03 ms
The final 0.03 msec!!
O Si, S, PSi Fe, Co, Ni
Summary of Post Main-Sequence Evolution of Stars
M > 8 Msun
M < 4 Msun
Evolution of 4 - 8 Msun stars is still uncertain.
Fusion stops at formation of C,O core.
Mass loss in stellar winds may reduce them all to < 4 Msun stars.
Red dwarfs: He burning never ignites
M < 0.4 Msun
Supernova
Fusion proceeds; formation of Fe core.
Evidence for Stellar Evolution: Star Clusters
Stars in a star cluster all have approximately the same age!
More massive stars evolve more quickly than less massive ones.
If you put all the stars of a star cluster on a HR diagram, the most massive stars
(upper left) will be missing!
HR Diagram of a Star Cluster
Example: HR diagram of the star cluster M3
Estimating the Age of a Cluster
The lower on the MS the turn-off point,
the older the
cluster.
Evidence for Stellar Evolution: Variable Stars
Some stars show intrinsic brightness variations not caused by eclipsing in binary systems.
Most important example:
Cephei
Light curve of Cephei
Cepheid Variables: The Period-Luminosity Relation
The variability period of a Cepheid variable is
correlated with its luminosity.
=> Measuring a Cepheid’s period, we
can determine its absolute magnitude!
The more luminous it is, the more slowly it
pulsates.
Cepheid Distance MeasurementsComparing absolute and apparent magnitudes of Cepheids,
we can measure their distances (using the 1/d2 law)!
The Cepheid distance measurements were the first distance determinations that worked out to distances beyond our Milky Way!
Cepheids are up to ~ 40,000 times more luminous than our sun
=> can be identified in other galaxies
Pulsating Variables: The Instability Strip
For specific combinations of radius and temperature, stars can maintain periodic oscillations.
Those combinations correspond to locations in the Instability Strip
Cepheids pulsate with radius changes of ~ 5 – 10 %.
Period Changes in Variable Stars
Periods of some Variables are not constant over time
because of stellar evolution. Another piece of evidence for stellar evolution