Unformatted text preview:

Chapter 13: Evolution of high-mass stars•high-mass stars: stars with masses greater than about 8 solar masses, live shorter lives than low mass stars, higher luminosities•more mass means stronger gravity and therefore more force on the core, more force means higher pressure, faster reactions, and greater luminosity13.1 High-Mass Stars Follow Their Own Path•Carbon-nitrogen-oxygen (CNO) cycle: hydrogen nuclei interact with carbon, which forms a nitrogen nucleus (first step)•in high mass stars, carbon is a catalyst for the fusion of hydrogen to helium•as the high-mass star runs out of hydrogen in core, the weight of outer star compresses it•before it’s electron-degenerate, the pressure and temp are high enough for helium burn-ing, and the star responds to temp increase, but luminosity changes very little, •once hydrogen core is degenerate, helium burns in core and hydrogen in shell, like a low-mass horizontal branch star, size grows, surface temp falls, moves off to the right on HR, leaves main sequence•stars with more than 10 solar masses will be red supergiants during helium burning•high-mass star exhausts helium, core collapses, temp is high enough to burn carbon, which can produce even more massive elements like sodium, neon, and magnesium•now the star has a carbon burning core, surrounded by a helium shell, and a hydrogen shell•from carbon, neon begins, then oxygen, many concentric layers fill the star•The H-R: high mass stars leave the main sequence, then move horizontally back and forthacross the diagram, as they pass through the instability strip, they become pulsating variable stars (Cepheid variables at top, RR Lyrae at bottom of strip), pulsate, growing larger and smaller, star alternately traps and releases thermal energy, causing it to expand and contract, gravity and pressure compete•the pulsations don’t affect nuclear burning, just the light that escapes, the star is brightest and bluest when it expands, faintest and reddest when it falls in •Cepheid variables: most luminous pulsating variable stars, periods of 1-100 days, the longer the period, the more luminous (this is the period-luminosity relationship, it’s consistent),can be used to find the distances to galaxies beyond our own•RR Lyrae variables: unstable low-mass horizontal branch stars, less luminous than Cepheid, can we used as standard candles•the pressure of the intense radiation on gas at the surface of a massive star overcomes the star’s gravity and causes winds with speeds of 3000km/s and huge mass loss13.2 High-Mass Stars Go Out with a Bang•high mass stars burn up from hydrogen to iron like an onion, nuclear reactions form all stable isotopes less massive than iron, but chain of nuclear fusion stops there •just like gasoline, which is self-sustaining once it starts, as hydrogen burns to helium, the energy released maintains the temp needed to keep the reaction going•binding energy: the energy bound up in an atomic nucleus, a nuclear reaction that in-creases it releases energy, decreasing it absorbs energy, moving up from helium to carbon, binding energy increases, so fusing helium to carbon releases energy, once you pass iron and go to heavier elements, fusing absorbs energy•iron fusion is not self-sustaining because it absorbs energy in the reaction•star’s balance is like a leaky inflatable ball, the larger the leak, the more rapidly you have to pump air into the ball, a star burning hydrogen/helium is like a ball with a slow leak, energy escapes by radiation and convection which is inefficient because the outer layers insulate the star, but in carbon burning, energy is carried in easily-escaping neutrinos•thus the outer layers fall in, the star is denser and hotter, and fuses faster, as “neutrino cooling” goes on, each element’s fusion goes faster than the one before•even though silicon burning gives off 200 million times more energy than helium, the starisn’t much more luminous because of neutrino cooling•once iron core is formed there’s nothing to replenish energy escaping in neutrinos, iron core is not burning so it collapses, causing gravity/density/temperature to skyrocket, Earth-size core is degenerate, high pressure of outer layers caused temp and density to rise•at these high temperatures1) photons can break iron nuclei (photodisintegration), reversing nuclear fusion2) high density forces electrons to combine with protons to form neutrons•these processes absorb rest of star’s energy, as well as neutrino release•collapse accelerates in less than a second•material in core is denser than an atomic nucleus, the force becomes repulsive•half of core slows in its inward fall, other half slams into the innermost part of the star and bounces, sending shockwaves out; in a second 20% of core becomes neutrinos (some don’teven escape because of such high densities)•energy of these neutrinos drives up the pressure and temp, inflating a bubble of extremelyhot gas and intense radiation around the core, in a minute the shock wave has pushed its way out through the helium shell, in a few hours, it reaches the surface, blasting material outward ina Type II supernova- RECAP: Not even electron degeneracy pressure can stop the collapse of an iron ash core. As the core collapses, the temperature is so high that thermal gamma-ray photons photodisinte-grate iron and the core is so dense that electrons and protons form neutrons and release neutri-nos. Photodisintegration and electron absorption rob the core of pressure support. The collapse accelerates until nuclear forces suddenly become repulsive. The overcompressed core bounces, driving its outer layers outward through the star. The expanding shock is strengthened by thepressure of a hot bubble of trapped neutrinos from the core. The shock continues through the outer layers of the star, blasting forth in a Type II supernova and leaving behind the collapsed remains of the core, a neutron star. 13.3 The Spectacle and Legacy of Supernovae•Type II supernovas are huge expanding bubbles of million-kelvin gas that glow in X-raysand drive visible shock waves into the surrounding interstellar medium, bubbles remain after thousands of years, explosions compress nearby clouds, triggering the initial collapse that be-gins star formation•only the least massive elements were formed at the beginning of the universe, hydrogen, helium, lithium, beryllium, boron; all the rest through nuclear


View Full Document

FSU AST 1002 - Chapter 13

Documents in this Course
Chapter 1

Chapter 1

27 pages

Notes

Notes

3 pages

Chapter 1

Chapter 1

31 pages

Exam 3

Exam 3

2 pages

Chapter 1

Chapter 1

27 pages

Chapter 1

Chapter 1

15 pages

Exam 4

Exam 4

2 pages

Chapter 1

Chapter 1

27 pages

Sun

Sun

44 pages

Exam 1

Exam 1

5 pages

Exam 3

Exam 3

10 pages

ASTRONOMY

ASTRONOMY

24 pages

Load more
Download Chapter 13
Our administrator received your request to download this document. We will send you the file to your email shortly.
Loading Unlocking...
Login

Join to view Chapter 13 and access 3M+ class-specific study document.

or
We will never post anything without your permission.
Don't have an account?
Sign Up

Join to view Chapter 13 2 2 and access 3M+ class-specific study document.

or

By creating an account you agree to our Privacy Policy and Terms Of Use

Already a member?