Transcript Stellar Evolution III

Stellar Evolution III
Post-MS Evolution of Low Mass Stars
(M < 8MSUN)
He
Ashes
5 MSUN Star
C, O
Ashes
He Ashes
•Star will spend about 10% of its lifetime as a red giant
(e.g. The Sun: MS lifetime – roughly 10 billion years
RG lifetime – roughly 1 billion years)
If lifetime is one day,
RG phase starts at 9:36p,
8
Planetary Nebulae
• During the last stages of the RG phase, outer atmosphere of star
Is ejected
Planetary Nebulae
White Dwarfs
• All stars smaller than 8 solar masses end up as white dwarfs
• Core of star left behind
• Made of Helium (outer parts), Carbon, Oxygen (inside)
• Supported by pressure of degenerate electrons: the properties
of the star are determined by the properties of matter at subatomic
Scales
• Size: similar to Earth
• Mass: around that of our Sun
• Density: about 3,000,000 g/cc
• Blue-white color: temperature
About 25,000 K at birth
• Does not produce any
energy: simply cools off
• Not really a star: a compact object
Post-MS Evolution of High Mass Stars
(M > 8MSUN)
Let’s compress a massive star’s life into one day…
11 12 1
H → He
Life on the Main Sequence
+ Expansion to Red Giant: 22 h,
24 min.
25 MSUN: 7,000,000 yr
2
10
9
3
8
H burning
4
7
6
5
H → He
He → C, O
11 12 1
2
10
He burning:
(Red Giant Phase)
25 MSUN: 500,000 yr
1 h, 35 min, 53 s
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
3
8
4
7
6
5
25 MSUN: 600 yr
C → Ne, Na, Mg, O
H → He
He → C, O
Ne → O, Mg
Ne burning:
6 ms
25 MSUN: 1 yr
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
25 MSUN: 0.5 yr
H → He
He → C, O
C → Ne, Na, Mg, O
Ne → O, Mg
O → Si, S, P
Si → Fe, Co, Ni
Si burning:
0.03 ms
25 MSUN: 1 yr
The final 0.03
msec!!
FUSION CANNOT CREATE NUCLEI HEAVIER
THAN IRON (Fe)
Chandresekhar Mass
• If you add mass to an object supported by pressure from
degenerate electrons, gravity squeezes more tightly and its radius
decreases
• Theoretically, when the mass reaches 1.4 MSUN, the radius reaches
zero – this is the Chandrasekhar mass
• Stars less than 8 MSUN produce cores less than 1.4MSUN
• What about the final iron cores of massive stars?