Transcript Red Giants and White Dwarfs

Announcements
• Homework 10 due Monday: Make your own
H-R diagram!
Red Giants and White Dwarfs
3 November 2006
Today:
• Life cycles of stars
• Aging stars: red giants
• “Planetary” nebulae
• Spent stars: white dwarfs
Star Formation
Fusion of Hydrogen into Helium
4 1H (protons)
4He
This reaction powers all main-sequence stars.
The more massive the star, the more pressure
at its center and therefore the faster the reaction
occurs.
Sizes of Main-Sequence Stars
Hottest stars are
actually somewhat
larger
Should be white,
not green!
Reds are greatly
exaggerated!
Main Sequence Lifetimes
(predicted)
Mass
(suns)
25
15
3
1.5
1.0
0.75
0.50
Surface temp
(K)
35,000
30,000
11,000
7,000
6,000
5,000
4,000
Luminosity
(suns)
80,000
10,000
60
5
1
0.5
0.03
Lifetime
(years)
3 million
15 million
500 million
3 billion
10 billion
15 billion
200 billion
What happens when the core of a
star runs out of hydrogen?
• With no energy source, the core of the star resumes its
collapse…
• As it collapses, gravitational energy is again converted
to thermal energy…
• This heat allows fusion to occur in a shell of material
surrounding the core…
• Due to the higher central temperature, the star’s
luminosity is greater than before…
• This increased energy production causes the outer part
of the star to expand and cool (counterintuitive!)…
• We now have a very large, cool, luminous star: a “red
giant”!
Red giants are big!
Mars
Fusion of helium into carbon, oxygen
4He
12C
4He
4He
4He
16O
• 3 He nuclei must merge quickly, since 8Be is unstable
• Requires very high temperatures (100 million K) due to
greater electrostatic repulsion
• Produces less energy per kg than hydrogen fusion
• Can continue in core of a star for about 20% of mainsequence lifetime
Final stages
in the life of a low-mass star
• Core runs out of helium, again collapses and heats up
• Helium burning continues (quickly) in a thin, hot shell
surrounding the core; hydrogen burning continues in a
larger shell
• Instabilities cause inner temperature to fluctuate, which
causes outer layers of star to swell, pulsate
• Pulsations eject outer layers into space, gradually
expanding into a “planetary nebula”
• Eventually, energy production stops and a very dense
“dead” star is left behind: a “white dwarf”
“Planetary” Nebulae
Slowly expanding shells of gas,
ejected by pulsating stars, still heated
by what’s left of the star’s core
More Planetary
Nebulae
White Dwarf Stars
• “Dead” cores of former stars,
no longer burning nuclear fuel,
radiating away leftover heat
• Made mostly of carbon and
oxygen nuclei, in a diamond
crystal structure (“like a
diamond in the sky”)
• Crushed to incredible density by their own
gravity: the mass of the sun but the size of
the earth! (Higher-mass white dwarfs are
smaller!)
• Sirius B and Procyon B are nearby examples
H-R Diagram
Patterns
Luminosity
Luminosity =
(constant) x
(surface area) x
(temperature)4
For a given size, hotter implies
brighter.
A bright, cool star must be
unusually large (“red giant”).
A faint, hot star must be
unusually small (“white dwarf”).