Download Introduction to Astronomy

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).
A giant star spot
Supernovae and Neutron Stars
6 November 2006
Today:
• Ages of star clusters
• Observation: Novae and Supernovae
• Theory: White dwarf explosions and
deaths of massive stars
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”).
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
A young star cluster (Pleiades)
Main sequence only, no
red giants or white dwarfs
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
Nova (“New Star”)
Nova (“New Star”)
“Tycho’s Supernova” (1572)
Supernova 1987a
A supernova in another galaxy
Supernova Remnants
Typically expanding at about 1% of the speed of light
(False-color, x-ray images)
Crab Nebula
Other Supernova Remnants
“Veil Nebula” (Cygnus)
“Gum Nebula” (Vela)
Planetary Nebulae
Slowly 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
Type I and II Supernovae (theory)
Final days of a massive star
Core of a supergiant (final stage)
(The theoretical astrophysicists can back all
this up with equations and computer models.)
Nuclear binding energies
The greater the binding energy per nucleon, the “more stable” the
nucleus is. Fusion reactions release energy only when the products
have more binding energy than the reactants.