Death of stars

• Final evolution of the Sun• Determining the age of a star cluster• Evolution of high mass stars• Where were the elements in your body made?

• Reading 21.3, 22.1-22.10

White dwarf• Star burns up rest of hydrogen• Nothing remains but degenerate core of Oxygen

and Carbon• “White dwarf” cools but does not contract

because core is degenerate• No energy from fusion, no energy from

gravitational contraction• White dwarf slowly fades away…

Evolution on HR diagram

Time line for Sun’s evolution

When hydrogen burning in the core stops, a star like the Sun

begins to evolve

1. toward the upper right in the H-R diagram.2. toward the zero age main sequence stage.3. up the main sequence to become an O star.4. toward the lower left in the H-R diagram.

Higher mass protostars contract fasterHotter

Higher mass stars spend less time on the main sequence

Determining the age of a star cluster

• Imagine we have a cluster of stars that were all formed at the same time, but have a variety of different masses

• Using what we know about stellar evolution is there a way to determine the age of the star cluster?

Turn-off point of cluster reveals age

The HR diagram for a cluster of stars shows stars with spectral types A through K on the main sequence and stars of type O and B on the (super) giant branch. What is the approximate age of the cluster?

1. 1 Myr2. 10 Myr3. 100 Myr4. 1 Gyr

Higher mass stars do not have helium flash

Nuclear burning continues past

Helium

1. Hydrogen burning: 10 Myr2. Helium burning: 1 Myr3. Carbon burning: 1000 years4. Neon burning: ~10 years5. Oxygen burning: ~1 year6. Silicon burning: ~1 day Finally builds up an inert Iron core

Why does nuclear fusion stop at Iron?

Fusion versus Fission

Core collapse• Iron core is degenerate• Core grows until it is too heavy to support itself• Core collapses, density increases, normal iron

nuclei are converted into neutrons with the emission of neutrinos

• Core collapse stops, neutron star is formed• Rest of the star collapses in on the core, but

bounces off the new neutron star (also pushed outwards by the neutrinos)

If I drop a ball, will it bounce higher than it began?

Do 8B10.50 - Supernova Core Bounce

Supernova explosion

Crab nebula

Cas A

In 1987 a nearby supernova gave us a close-up look at the death of a

massive star

Neutrinos from SN1987A

Where do the elements in your body come from?

• Solar mass star produce elements up to Carbon and Oxygen – these are ejected into planetary nebula and then recycled into new stars and planets

• Supernova produce all of the heavier elements– Elements up to Iron can be produced by fusion– Elements heavier than Iron are produced by the

neutrons and neutrinos interacting with nuclei in the supernova explosion

Review Questions• How does the evolution of a high mass star

differ from that of a low mass star?• How can the age of a cluster of stars, all formed

at the same time, be determined?• Why does fusion stop at Iron?• How are heavy elements produced?