Life (and Death) as a High Mass Star

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) (2p, 2n).. create even numbered elements! KNOW THESE!

If a 24 Mg nucleus captures a helium nucleus, what would the product be? A) 20 Ne B) 28 Si C) 24 Si D) 28 S E) 32 S

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 c 2 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 10- billion-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 (Why not Europeans?)

Historically observed supernovae occurred in 1006, 1054, 1572, and 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. (Large Magellanic Cloud) Neutrino detectors in Ohio and Japan recorded a burst of neutrinos from this event. They confirmed that the explosion released most of its energy as neutrinos. 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! Faleen Colt Jessica Conner V.

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.