Transcript Why is there a main sequence?

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Chapter 12
Stellar Evolution
Guidepost
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 four
important questions:
• Why is there a main sequence?
• Why is there a relationship between the masses
and luminosities of main-sequence stars?
• How does a star change as it exhausts its hydrogen
fusion fuel?
• What is the evidence that stars really do evolve?
Guidepost (continued)
This chapter is about how stars live. The next two
chapters are about how stars die, and the strange
objects they leave behind.
Outline
I. Main-Sequence Stars
A. Stellar Models
B. Why is there a Main Sequence?
C. The Upper End of the Main Sequence
D. The Lower End of the Main Sequence
E. The Life of a Main-Sequence Star
F. The Life Expectancies of Stars
II. Post-Main-Sequence Evolution
A. Expansion into a Giant
B. Degenerate Matter
C. Helium Fusion
D. Fusing Elements Heavier than Helium
Outline (continued)
III. Star Clusters: Evidence of Evolution
A. Observing Star Clusters
B. The Evolution of Star Clusters
IV. Variable Stars: Evidence of Evolution
A. Cepheid and RR Lyrae Variable Stars
B. Pulsating Stars
C. Period Changes in Variable Stars
Stellar Models
Maximum Masses of Main-Sequence Stars
Mmax ~ 100 solar masses
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
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 Dwarfs
Hard 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)
Main-Sequence
stars live by
fusing H to He
Zero-Age Main
Sequence (ZAMS)
Finite supply of H
=> finite life time
Evolution on the Main Sequence (2)
A star’s life time T ~ energy reservoir / luminosity
Energy
reservoir ~ M
Luminosity
L ~ M3.5
T ~ M/L ~ 1/M2.5
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:
“Hydrogen burning”
(i.e. fusion of H into He)
ceases in the core
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
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 Matter
Matter 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
H-burning shell
keeps dumping He
onto the core
4 H → He
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”
He
4He
+ 4He  8Be + g
8Be
+ 4He  12C + g
Helium Flash
The onset of Helium fusion occurs very
rapidly, in an event called the
Helium Flash.
Red Giant Evolution
(5 solar-mass star)
Helium in the core
exhausted;
development of
He-burning shell
Development of
Carbon-Oxygen
Core
Helium ignition in
the core
Red giant
Expansion to
red giant
Main Sequence
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)
H  He
Let’s compress a massive star’s life into one
day…
11 12 1
Life on the Main Sequence
+ Expansion to Red Giant:
22 h, 24 min.
2
10
9
3
4
8
H burning
7
6
5
H  He
He  C, O
11 12 1
2
10
He burning:
(Red Giant Phase)
1 h, 35 min, 53 s
9
3
4
8
7
6
5
The Life “Clock” of a Massive Star (2)
H  He
He  C, O
C  Ne, Na, Mg, O
C burning:
6.99 s
H  He
He  C, O
11 12 1
10
9
2
3
8
4
7
6
5
C  Ne, Na, Mg, O
Ne  O, Mg
Ne burning:
6 ms
23:59:59.996
The Life “Clock” of a Massive Star (3)
H  He
He  C, O
C  Ne, Na, Mg, O
Ne  O, Mg
O  Si, S, P
O burning:
3.97 ms
H  He
He  C, O
23:59:59.99997
C  Ne, Na, Mg, O
Ne  O, Mg
O  Si, S, P
Si  Fe, Co, Ni
Si burning:
0.03 ms
The final
0.03 msec!!
Summary of Post Main-Sequence
Evolution of Stars
Supernova
Fusion
proceeds;
formation
of Fe core.
M>8
Msun
Evolution of
4 - 8 Msun
stars is still
uncertain
Mass loss in
stellar winds
may reduce
them all to <
4 Msun stars
Fusion
stops at
formation of
C,O core.
M < 4 Msun
M < 0.4 Msun
Red dwarfs:
He burning
never
ignites
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:
d Cephei
Light curve of d Cephei
Cepheid Variables:
The Period-Luminosity Relation
The variability period of
a Cepheid variable is
correlated with its
luminosity.
The more luminous it
is, the more slowly it
pulsates.
=> Measuring a
Cepheid’s period, we
can determine its
absolute magnitude!
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