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Announcements• Pick up graded homework (projects, tests still in
progress)• Turn in Homework 10 by 5:00• Vote tomorrow!• Transit of Mercury (crossing in front of Sun),
Wednesday afternoon, roughly noon-5:00. We’ll have telescopes set up at observatory for viewing (weather permitting).
Today:
• Ages of star clusters
• Observation: Novae and Supernovae
• Theory: White dwarf explosions and deaths of massive stars
H-R Diagram Patterns
Lum
ino
sity
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”).
Main Sequence Lifetimes(predicted)
Mass (suns)
Surface temp (K)
Luminosity (suns)
Lifetime (years)
25 35,000 80,000 3 million
15 30,000 10,000 15 million
3 11,000 60 500 million
1.5 7,000 5 3 billion
1.0 6,000 1 10 billion
0.75 5,000 0.5 15 billion
0.50 4,000 0.03 200 billion
An old star cluster (Messier 3)
Main sequence “cuts off” above a certain point; plenty of red giants and white dwarfs
Oldest known cluster ages are about 12 billion years
Supernova Remnants
(False-color, x-ray images)
Typically expanding at about 1% of the speed of light
Planetary NebulaeSlowly expanding shells of gas, ejected by pulsating stars, still heated by what’s left of the star’s core
Transfer of matter to a white dwarf…
If enough hydrogen builds up, an explosive nuclear reaction can occur . . . a “nova”! (Not really a new star)
But white dwarfs can’t grow too massive
Just as relativity theory predicts that no signal can travel faster than the speed of light, it also limits the stiffness of materials. A white dwarf star of more than 1.4 solar masses (the “Chandrasekhar limit”) exceeds the stiffness limit and therefore implodes, shrinking to a much smaller size.
S. Chandrasekhar
Core of a supergiant (final stage)
(The theoretical astrophysicists can back all this up with equations and computer models.)