Evolution of High Mass Stars AST 112

High Mass Stars So… what exactly do high mass stars do? The same thing as low mass stars: they get on the Main Sequence and convert H to He. Then they blow up!

Life From Stars Need low mass stars for life – They live long enough to allow life to flourish Need high mass stars for life – They produce the elements heavier than carbon

High Mass Stars: Main Sequence Low mass stars fuse H into He through the proton – proton chain – Slow! High mass stars fuse H into He through the CNO cycle – Fast!

The CNO Cycle Recall that nuclear reactions happen when nuclei have enough kinetic energy to overcome electric repulsion High mass stars heat the cores to a higher temperature – H nuclei can now react with carbon, oxygen and nitrogen

The CNO Cycle Carbon, nitrogen and oxygen act as catalysts – C, N, and O don’t get consumed; they just “help out” This is why high mass stars shine bright and die young.

The CNO Cycle Text, Page 574: Did the first high-mass stars in the history of the universe produce energy through the CNO cycle?

Hydrogen Exhaustion 25 M Sun star uses up its hydrogen in a few million years Quickly develops a hydrogen burning shell, outer layers expand Helium gradually begins to burn (no helium flash)

Supergiant Core collapses, outer layers swell At this point, it’s a supergiant star.

Burning Helium Star burns He for few hundred thousand years Runs out of He – Inert carbon core begins collapse Similar to low-mass star thus far

Burning Carbon High-mass stars: HOT! – Easily reach 1,200,000,000 o F for carbon fusion Fuses carbon for a few hundred years, runs out

He-Capture Reactions Helium nucleus fuses with heavier nuclei – Carbon to Oxygen – Oxygen to Neon – Neon to Magnesium Helium Capture Reactions

Heavy Nucleus Reactions In the core: – Carbon + Oxygen -> Silicon – Oxygen + Oxygen -> Sulfur – Silicon + Silicon -> Iron Heavy-Nucleus Reactions

What can you think of that came from the inside of a dying high-mass star?

Advanced Nuclear Burning The core fuses elements, runs out, shrinks, heats, and fuses new elements This results in layers of heavy elements

High Mass Stars: Advanced Nuclear Burning These sequential shells result in a zig-zag path about the HR diagram Most massive stars: outer layers don’t have time to respond!

High Mass Stars: Advanced Nuclear Burning Iron starts to accumulate in the central core – Elements lighter than iron release energy when fused – Elements heavier than iron release energy when split

High Mass Stars: Advanced Nuclear Burning Not energetically advantageous for iron to fuse / split …so it doesn’t.

High Mass Stars: Advanced Nuclear Burning Iron is not undergoing nuclear reactions Doesn’t collapse – Electron degeneracy pressure (cramming too much stuff together) Iron keeps on piling up…

Death of a High Mass Star A good way to remove electron degeneracy pressure: Get rid of the electrons!

Death of a High Mass Star … and piling up and piling up… Conditions such that electrons combine with protons – Forms neutrons, releases neutrinos – Degeneracy pressure vanishes instantly

Death of a High Mass Star In a split second, an iron core the size of Earth collapses into a sphere of neutrons 5-10 miles across and releases a torrent of neutrinos. This releases 100x the energy released by our Sun in its entire lifetime!

Supernova

Outer layers of the star get blown away – Mostly due to neutrinos – 6000 miles / second (3% speed of light!) The leftover core is either: – A neutron star if it’s small enough – A black hole if it’s large enough

Supernova A supernova is so bright it can briefly outshine an entire galaxy! Bright for about a week, fades over months

Neutron-Capture Reactions Where do elements heavier than iron come from? Rare reactions that capture a neutron – Neutron changes to proton – Repeats Requires high energy – Only happens close to and during supernova

Nuclear Reactions: Observational Evidence Look at composition of stars, gas, dust in Milky Way Look at C, O, or Ne – Even number of protons – Come from He capture (+2 protons) – These can fuse together Elements heavier than iron are rare

Notorious Supernova Remnants Messier 1, The Crab Nebula (in Taurus) Growing several thousand miles per second! Neutron star lives inside

Notorious Supernova Remnants Re-tracing the Crab Nebula’s expansion puts the supernova at 1100 A.D. In the first year of the period Chih-ho, the fifth moon, the day chi-ch’ou, a guest star appeared approximately several [degrees] southeast of Thien-kuan. After more than a year it gradually became invisible. July 4, 1054 Taurus

Notorious Supernova Remnants Supernova 1987A occurred in the Large Magellanic Cloud 150k LY away – Did the star explode in 1987?

Milky Way Supernovas Four in the last 1000 years: – 1006 (So bright it cast shadows at night!) – 1054 (Just did that one) – 1572 (Tycho Brahe saw it) – 1604 (Kepler saw it)

Betelgeuse The size of the star extends out past the orbit of Mars Its shape is pulsating 600 LY away… it’s safe.