Activity #32, pages 111-112(pages 109-110 were done last Friday)

12.3 Life as a High-Mass Star

• Our Goals for Learning• What are the life stages of a high mass star?

• How do high-mass stars make the elements necessary for life?

• How does a high-mass star die?

What are the life stages of a high mass star?

High-Mass Stars> 8 MSun

Low-Mass Stars< 2 MSun

Intermediate-Mass Stars

Brown Dwarfs

High-Mass Star’s Life

Early stages are similar to those of low-mass star:

• Main Sequence: H fuses to He in core

High-mass stars don’t use proton-proton chain to fuse H into He. Instead, they use the CNO cycle - another way to fuse H into He, using carbon, nitrogen, and oxygen as catalysts

CNO cycle is main mechanism for H fusion in high mass stars because core temperature is higher

High-mass stars become supergiants after core H runs out

Luminosity doesn’t change much but radius gets far larger

High-Mass Star’s Life

Early stages are similar to those of low-mass star:

• Main Sequence: H fuses to He in core

• Red Supergiant: H fuses to He in shell around inert He core

• Helium Core Burning: He fuses to C in core (but no He flash like in low-mass stars)

How do high mass stars make the elements necessary

for life?

Big Bang made H, most He – stars made (still making!) some He and everything else… all the other elements.

Helium fusion can make carbon late in stars’ lives

Helium fusion requires higher temperatures than hydrogen fusion because larger charge leads to greater repulsion

Fusion of two helium nuclei doesn’t work, so helium fusion must combine three He nuclei to make carbon

CNO cycle can change C into N and O

Helium-capture reactions add two protons at a time

Helium capture builds C into O, Ne, Mg, …

Evidence for helium capture:

Higher abundances of elements with even numbers of protons

Advanced reactions make heavier elements

Cycling Between Core Fusion and Shell Fusion • As fusion of element A to element B occurs in the star's

core, the core temperature slowly increases• Once element A is no longer present in the core, fusion of A

to B moves to a high-temperature shell around the core• Shell fusion adds more element B to the core, which shrinks

and heats up even more to maintain its pressure• The shell temperature increases as the core shrinks, so the

rate of fusion increases; the star gets more luminous, and the core gets even more element B added to it; see previous step

• Eventually, the core temperature may get high enough for fusion of element B to element C; see first step above

• Low-mass stars only go through this cycle twice, but high-mass stars go through it many times

Advanced nuclear burning occurs in multiple shells

Advanced nuclear fusion reactions require extremely high temperatures

Only high-mass stars can attain core temperatures high enough for such reactions before degeneracy pressure stops the contraction of the core

Iron is a dead end for fusion because nuclear reactions involving iron do not release energy

(Fe has lowest mass per nuclear particle)

How does a high mass star die?

Iron builds up in core until degeneracy pressure can no longer resist gravity

Degeneracy pressure: no two particles can have the same velocity and position

Degeneracy pressure means there’s a limit to how dense objects can get

Core’s degeneracy pressure goes away because electrons combine with protons, making neutrons and neutrinos

Neutrons collapse to the center, forming a neutron star(supported by neutron degeneracy pressure)

Iron builds up in core until degeneracy pressure can no longer resist gravity. Core then quickly collapses to a new state [a neutron star], and outer layers rebound in a supernova explosion

Energy and neutrons released in supernova explosion enables elements heavier than iron to form

Elements made during supernova explosion

Crab Nebula: Remnant of supernova observed in 1054 A.D.

Supernova 1987A is the nearest supernova observed in the last 400 years

before after

The next nearby supernova?

What have we learned?• What are the life stages of a

high-mass star?

• A high-mass star lives a much shorter life than a low-mass star, fusing hydrogen into helium via the CNO cycle. After exhausting its core hydrogen, a high-mass star begins hydrogen shell burning and then goes through a series of stages burning successively heavier elements. The furious rate of this fusion makes the star swell in size to become a supergiant.

What have we learned?• How do high-mass stars make

the elements necessary for life?

• In its final stages of life, a high-mass star’s core becomes hot enough to fuse carbon and other heavy elements. The variety of different fusion reactions produces a wide range of elements— including all the elements necessary for life—that are then released into space when the star dies.

What have we learned?• How does a high-mass star die?• A high-mass star dies in the

explosion of a supernova, scattering newly produced elements into space and leaving a neutron star or black hole behind.

• The supernova occurs after fusion begins to pile up iron in the high-mass star’s core. Because iron fusion cannot release energy, the core cannot hold off gravity for long. In the instant that gravity overcomes degeneracy pressure, the core collapses and the star explodes.

Activity #29, part II (pages 100-101)