Download Stellar Evolution

Stellar Evolution
Please press “1” to test
your transmitter.
What is the expected mainsequence life time of the sun?
1.
2.
3.
4.
5.
10 million years
100 million years
1 billion years
10 billion years
100 billion years
Evolution on the Main Sequence
Main
Sequence
evolution
Zero-Age
Main
Sequence
(ZAMS)
Main-Sequence
stars live by fusing
Hydrogen (H) to
Helium (He).
Finite supply of H
=> finite life time.
Evolution on the Main Sequence
Evolution off the Main Sequence:
Expansion into a Red Giant
H in the core completely
converted into He:
“H burning” (i.e.
fusion of H into He)
continues in a shell
around the core.
Expansion and
cooling of the outer
layers of the star →
Red Giant
Question:
B
A
C
X
E
D
Which way will a
star at point X
move in the HR
diagram when it
swells up to a red
giant, so that its
luminosity
increases slightly,
and its color
changes to red?
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!
Red Giant Evolution
4 H → He
He
He-core gets
denser and hotter
until the next stage
of nuclear burning
can begin in the
core:
He fusion:
3 4He → 12C
“Triple-Alpha
Process”
Fusion of Helium
into Carbon
Red Giant Evolution
(5 solar-mass star)
C, O
Inactive He
Nuclear fusion can, in principle,
continue (i.e., produce energy) until
which element is reached?
H → He
He → C, O
C → Ne, Mg, O
Ne → O, Mg
…
1.
2.
3.
4.
5.
Helium
Carbon
Oxygen
Iron
Plutonium
The Life “Clock” of a
Massive Star (> 8 Msun)
Let’s compress a massive star’s life into one day…
H → He
11 12 1
Life on the Main Sequence
+ Expansion to Red Giant:
22 h, 24 min.
2
10
9
3
4
8
7
H burning
6
5
H → He
He → C, O
11 12 1
He burning:
(Red Giant Phase)
1 h, 35 min, 53 s
2
10
9
3
8
4
7
6
5
H → He
He → C, O
11 12 1
C → Ne, Na, Mg, O
2
10
9
C burning:
6.99 s
H → He
He → C, O
3
4
8
7
6
5
C → Ne, Na, Mg, O
Ne → O, Mg
Ne burning:
6 ms
23:59:59.996
H → He
He → C, O
C → Ne, Na, Mg, O
Ne → O, Mg
O → Si, S, P
O burning:
3.97 ms
23:59:59.99997
C → Ne, Na, Mg, O
H → He
He → C, 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
Fusion proceeds to
formation of Fe core.
M > 8 Msun
Fusion
stops at
formation
of C,O
core.
M < 4 Msun
M < 0.4 Msun
Evolution of
4 - 8 Msun
stars is still
uncertain.
Red dwarfs:
He burning
never
ignites
Evidence for Stellar Evolution:
Star Clusters
Stars in a star cluster all have
approximately the same age!
Take all stars of a 5-billion-year-old cluster,
and put them onto a Hertzsprung-Russell
diagram. Do you expect to see a complete
Main Sequence?
1.
2.
3.
Yes
No, stars near the
upper end will be
missing.
No, stars near the
lower end will be
missing.
High-mass stars evolve off the main
sequence (to become red giants)
earlier than low-mass stars.
=> For a given age, low-mass stars
are still on the MS, while high-mass
stars are already red giants!
Example:
HR diagram of the star cluster M 55
High-mass stars
evolved onto the
giant branch
Turn-off point
Low-mass stars
still on the main
sequence
What can we infer from the location
of the turn-off point?
1.
2.
3.
4.
5.
The average mass of
stars in the cluster.
The distance of the
cluster.
The age of the cluster.
The size of the cluster.
All of the above.
The lower
on the
MS the
turn-off
point, the
older the
cluster.
Evidence for Stellar Evolution:
Variable Stars
Some stars show periodic
brightness variations.
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!
What can we infer from a Cepheid’s
absolute magnitude? (Of course, we
can also measure its apparent
magnitude.)
1.
2.
3.
4.
5.
Its distance.
Its age.
Its mass.
Its temperature.
Its radius.
Comparing absolute and apparent
magnitudes, we can measure a star’s
distance (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.