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. Well 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)
Surface temp (K) Luminosity (suns)
Lifetime (years) 25 15 3 1.5
1.0 0.75 0.50 35,000 30,000 11,000
7,000 6,000 5,000 4,000 80,000 10,000
60 5 1 0.5 0.03 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)
Tychos 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 whats left of the stars 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 cant 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
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