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 MSun 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 oF 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 -> IronHeavy-Nucleus Reactions

What can you think of that camefrom 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

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 becameinvisible.

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.