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Life (and Death) as a High Mass Star
Class viewing at the observatory TOMORROW, Saturday, Feb. 25
We’ll start at 7:00 p.m. and go until about 8.
This is weather
dependent!
A “high-mass star” is one with more than about
A) the mass of the Sun
B) 2 times the mass of the Sun
C) 4 times the mass of the Sun
D) 8 times the mass of the Sun
E) 16 times the mass of the Sun
Hydrogen fusion in a high mass star:
The CNO cycle
The CNO cycle has the same net reaction as
the proton-proton chain, but it involves
carbon, nitrogen, and oxygen isotopes as
stepping stones.
It requires higher temperatures, so it only
occurs in stars that are over 2 times larger
than the Sun.
Most importantly, it runs MUCH faster than
the proton-proton chain, allowing these larger
stars to burn through their available fuel much
more quickly.
These very high fusion rates generate enormous energies, creating strong radiation pressures that
blow much stellar material into space – a very strong stellar wind.
When a high mass star runs out of hydrogen:
It’s the same idea as with a low mass star:
• Core begins to collapse, and so heats up
• Fusion of hydrogen can begin in a shell
• This fusion drives the outer layers
outward, making a red supergiant
• When core is hot enough, helium fusion
begins there
• As that stabilizes, the star shrinks
• Hotter surface, smaller radius,
about the same luminosity
• When helium fuel runs out, core again
contracts…
This goes on through many cycles
Each time the cycle starts over, a heavier
element starts fusion in the center of the
core.
Also, another shell of fusing material is
added to the mix
In this way, many heavier elements
are created inside large stars:
(6p, 6n)
KNOW THESE!
(2p, 2n)
.. create even numbered elements!
If a 24Mg nucleus captures a helium nucleus, what would the product be?
A)
20Ne
B)
28Si
C)
24Si
D)
28S
E)
32S
Other reactions create elements up to iron.
(There are a lot more reactions than are listed here.)
Why does the fusion process end with iron?
Mass per particle:
Combining 4 hydrogens
creates 1 helium that has less
than the mass of the 4
hydrogens.
The missing mass has turned
into released energy.
E = m c2
Fusion beyond iron requires
energy to be absorbed by the
nucleus, essentially cooling
the plasma, rather than
keeping it hot.
Observed relative abundances of elements in the Milky Way
This is where the elements
come from.
Observing the relative
abundances of elements in
the universe (through
spectroscopy) confirms
our model of the origin of
elements in stars.
What reaction produces oxygen from carbon in high-mass stars?
A) helium capture
B) CNO cycle
C) proton capture
D) triple-alpha process
E) proton-proton chain
So the iron core will never fuse and generate energy.
When the silicon is all fused (and it only takes about a day!), the inert iron core will be crushed
by the weight of everything above it.
The degeneracy pressure of electrons is not strong
enough to hold up against gravity.
The result is the combining of an electron and a
proton, creating a neutron and a neutrino.
Now the core, which was about the mass of our Sun
and about the size of Earth, collapses into a ball of
neutrons about the size of a major city!
What remains of the core is called a neutron star: It is essentially a gigantic nucleus!
The energy given off by the collapse is
more than 100 times the total energy
the Sun will give off in its entire 10billion-year lifetime!
It drives the outer part of the star
outward in an explosion known as a
supernova.
This explosion spreads those heavy
elements back out into space:
Material for the next generation of
stars!
The Crab Nebula
Remains of a supernova explosion observed by Chinese,
Japanese, and Arabic astronomers in 1054.
(Why not Europeans?)
Historically observed supernovae occurred in 1006, 1054, 1572, and 1604.
The supernova of 1006 was visible in the daytime and cast shadows at night!
Supernova 1987A
It’s been observed regularly ever
since the explosion.
We’ve been able to see the nebula
expand over time.
Neutrino detectors in Ohio and Japan recorded a burst of neutrinos from this event.
(Large Magellanic
Cloud)of its energy as neutrinos.
They confirmed that the explosion
released most
This success has led to the development and construction of more “neutrino telescopes”.
The site of
Supernova
1987A
today
What is the heaviest element produced in the core of a high-mass star?
A) carbon
B) silicon
C) iron
D) lead
E) uranium
Astro-Cash Cab!
Isaiah
Availya
Zach
Patrick
1) A supernova explosion releases most of its energy as _______________ .
2) What is the source of energy in a massive star supernova?
Nuclear fusion
Nuclear fission
Gravitational potential energy
Chemical potential energy
3) What is the net product of the CNO cycle?
(Ignoring the energy and neutrinos)
helium
carbon
nitrogen
oxygen
all of the above
4) True / False
The CNO cycle is the main process whereby helium is created
from hydrogen in our Sun.