Chapter 17: Evolution of High-Mass Stars

Massive stars have more hydrogen to start with but they burn it at a prodigious

rate

The 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 Stars

The 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