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

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

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

(False-color, x-ray images)

Typically expanding at about 1% of the speed of light

Crab Nebula

Other Supernova Remnants

“Veil Nebula” (Cygnus)

“Gum Nebula” (Vela)

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

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.