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Exam #3: May 3 (Chp 12, 13)
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HR Diagram of the Brightest Stars
1.E+06
1.E+05
1.E+04
1.E+03
1.E+02
1.E+01
1.E+00
1.E-01
1.E-02
1.E-03
1.E-04
1.E-05
35000
2
30000
25000
20000
15000
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HR Diagram of the Closest Stars
1.E+06
1.E+05
1.E+04
1.E+03
1.E+02
1.E+01
1.E+00
1.E-01
1.E-02
1.E-03
1.E-04
1.E-05
35000
3
30000
25000
20000
15000
10000
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1.E+06
1.E+05
1.E+04
1.E+03
1.E+02
1.E+01
1.E+00
1.E-01
1.E-02
1.E-03
1.E-04
1.E-05
35000
4
30000
25000
20000
15000
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Conclusions:
• Low mass stars are more common in our
galaxy (not a lot of O and B-type Main
Sequence Stars in the HR Diagram of the
closest stars).
• Stars spend most of their time on the Main
Sequence (not a lot of supergiants and giants
in the HR Diagram of the closest stars).
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Chapter 13
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Introduction
• Where do stars come from?
Giant Molecular Clouds
Bok Globules
Interstellar Medium (ISM)
Protostars
Pre-Main Sequence Stars
• How do they age (evolve)
• What is their fate?
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Bi-polar jets
Herbig-Haro objects (HH objects)
Brown Dwarfs
Contraction timescales depend on mass
Hydrostatic Equilibrium
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Sun becomes a Red Giant
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Planetary Nebulae:
Typical size: 0.25 ly
Typical velocity of
expanding material:
20 km/s
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At the end of its life, a star like the Sun will shed its
outer layers.
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Collapse of Massive Stars:
As
the star’s core shrinks, protons and electrons merge to form
neutrons and the core is transformed into a sphere of neutrons.
p+ + e-  n + 
The
loss of electrons in the creation of the neutrons causes the
core pressure to drop suddenly – nothing remains to support the
star, so its inner layers collapse
In a matter of seconds the Earth-sized iron core is
transformed into a 10-km, extremely dense ball of neutrons
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Collapse of Massive Stars:
The
outer layers of the star, now not supported as well, collapse
and heat to billions of degrees as they slam into the neutron
core.
The
gas pressure surges and thrusts the outer layers back into
space in a gigantic explosion – a supernova.
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Supernova Explosions:
In
a few minutes, more energy is released than during the star’s
entire life.
brightens to several billion times the luminosity of the Sun –
a rate larger than all the stars in the Milky Way combined.
It
Speeds
may exceed 10,000 km/sec
Free
neutrons from the explosion synthesize heavier
elements (e.g., gold, platinum, uranium)
Materials
mix with interstellar matter to be recycled into a new
generation of stars
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NGC 4725 (late 1940’s)
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Large Magellanic Cloud: SN1987A
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Supernova Remnants
(Vela, Crab)
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Crab Nebula:
Noted by Chinese Astronomers in 1054 AD
Constellation of Taurus
6000 LY away
6 LY across
Expanding at 3 million MPH
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NGC 3603: 2 million years old
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Stars like the Sun probably do not form iron cores
during their evolution because
a) all of the iron is ejected when they become
planetary nebulae
b) their cores never get hot enough for them to
make iron by nucleosynthesis
c) the iron they make by nucleosynthesis is all
fused into carbon
d) their strong magnetic fields keep their iron
in the atmosphere
e) none of the above
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