Post on 22-Feb-2016
description
Chapter 17: Evolution of High-Mass Stars
Massive stars have more hydrogen to start with but they burn it at a prodigious
rateThe overall reaction is still 224 eHeH
There are 3 gamma ray photons instead of two
and it consumes hydrogen much faster
but requires higher temperatures.
High-mass stars have convection
in their cores
Because of convection in their core, high mass stars never develop a degenerate helium core so they never have a helium flash.
As high mass stars evolve off the main sequence they move
back and forth on the H-R diagram
For high-mass stars the core temperature will get hot enough to fuse carbon
Carbon will start fusing when the core temperature reaches 800,000,000 Kelvin. Unlike hydrogen fusion which only produces helium or helium fusion which only produces carbon, carbon fusion can produce lots of different elements.
As the temperature gets higher,
heavier elements start
to fuseThe products of one fusion will form the elements for the next level of fusion making the core start to look like an onion with multiple shells of fusion
As they evolve, some stars move into the “instability
strip”
In the instability strip stars can begin to pulsate. The first pulsating star to be discovered was Delta Cephei so we call this type of star a Cepheid variable
For Cepheid variables, the brightness varies in a regular
way
Another type of pulsating variable is RR Lyrae
The pulsations are due to a layer of
helium that
becomes ionized
The period of their variation is related to their
luminosity
Once an iron core starts to form, the end comes quickly
Each stage of fusion requires higher temperatures,
releases less energy and lasts less time
Higher fusion also releases
most of its energy as neutrinos
Since neutrinos don’t interact with normal matter much, they quickly escape the star and don’t help inbalancing the inward crush of gravity.
Because of the nuclear force, fusing iron requires energy
rather than releasing it
At this point temperature in the core is nearly 5 billion K
The peak of the blackbody curve that is being produced is in the high energy gamma ray range
Due to degeneracy, extreme temperature and pressure, the core starts to collapse
g + 56Fe→134He + 4n
Once the iron becomes degenerate, the core starts to collapse as more mass is added. As it collapses, it gets hotter. As it gets hotter, the gamma rays get more energetic until they have enough energy break up the iron. The process is called photodisintegration
After photodisintegration, reverse beta decay relieves the electron degeneracy pressure
Things get so crowded the electrons are squeezed into the nucleus where they combine with protons to make neutrons in a process called Reverse Beta Decay
Reverse Beta Decay causes neutrinos to pour
out of the coreThe conversion of electrons and protons into neutrons causes the core to rapidly shrink in size which produces a blast of heat which forms a shock wave. In addition, huge amounts of energy is being carried off by the neutrinos
The shock wave
pushes the rest of the star
outward
The result is a Type II Supernova
Supernovae come is several types
Type Ib through Type II are from the core collapse of a massive star. Type Ia is the thermonuclear detonation of a white dwarf star.
The last naked-eye supernova was SN 1984A
The first supernova to be observed in 1987, it was visible to the naked eye for several weeks if you were in the southern hemisphere. It occurred in the Large Magellanic Cloud, a satellite galaxy 180,000 ly away.
Type II Supernovae are important for the elements they
produce
The Solar System formed from the debris of several
supernovae
After the supernova, a neutron star is left behind
Because of the peculiarities of their emissions, they are called Pulsars
Pulsars are spinning Neutron StarsThe Lighthouse
Model
The Crab pulses in all wavelengths
Pulsars emit by synchrotron radiation
Charged particles spiraling in a magnetic field produce synchrotron radiation. The stronger the magnetic field, the shorter the wavelength emitted
If a pulsar is radiating, where is the energy coming
from?
Pulsars in a binary system will emit x-rays and gamma
rays
How do we know all this stuff about stellar evolution?
Since the stars in a cluster are all born at about the same time, we can study
clusters to learn how stars evolve
The H-R Diagram of a cluster shows which stars are starting
to evolve off the main sequence
The actual H-R
diagram of clusters match the
theory very well
By comparing clusters of different ages we
can understand how stars
evolve