chapter 21: stellar explosions stellar explosionsvazct/stars/ch 21.pdfwhite dwarfs, binaries and...

Chapter 21: Stellar Explosions

© 2017 Pearson Education, Inc.

Stellar Explosions

Chapter 21: Stellar Explosions

• The Final Fate of Stars

• Low Mass Stars

– Forming White Dwarfs

– The Fate of a White Dwarf

• Novae

• High Mass Stars

– Fusion of Heavy Elements

– Collapse of the Iron Core

– Photodisintegration

• Supernovae

• Making elements heavier than Iron

• The Cycle of Stellar Evolution© 2017 Pearson Education, Inc.

Final Fate of Stars

• A low mass star (up to ¼ solar masses) cannot achieve

the temperatures required to fuse He. After the main

sequence

– 4 11H → 4

2He + 2 e+ + 𝜈𝑒It forms a Helium white dwarf.


• A star of mass greater than ¼ solar masses but less

than 8 solar masses will achieve the temperatures

required to fuse He to C and O

– 3 42He → 12

6C

– 42He + 12

6C → 168O

and forms a Carbon-Oxygen white dwarf


Final Fate of Stars

• Even larger stars, of mass greater than 8 solar

masses but less than 12 solar masses can go on to

fuse O with He to form Ne

– 42He + 16

8O → 2010Ne

ending up as Neon-Oxygen white dwarfs.

• Stars of mass greater that 12 solar masses will have

a very different history. Electron degeneracy

pressure cannot sustain the core

• This leads to a catastrophic explosion that leaves

behind a neutron star.

• The explosion is called a supernova.


Final Fate of Stars


White Dwarfs, Binaries and Novae

• As the white dwarf cools, its size

does not change much; it simply

gets dimmer and dimmer, and

should finally cease to glow (black

dwarf). White dwarfs are sustained

by electron degeneracy.

• However, we sometimes see white

dwarfs suddenly brighten, i.e., turn

nova.

• Nova Persei, a WD that suddenly

brightened by a factor of 40,000!

• In general, a nova is a star that

flares up very suddenly and then

returns slowly to its former

luminosity.



• We know today that a nova is a white dwarf that is

undergoing an explosion on its surface

• The explosion results in a rapid and temporary increase

of its luminosity

• What causes a white dwarf to undergo surface

explosions?



• A white dwarf that is part of a semidetached binary

system can undergo repeated novas.



• Material falls onto the white dwarf from its main-

sequence companion.

• This forms a swirling disk of matter around the white

dwarf. The disk gets hotter and hotter (by friction or

viscosity) even as it falls in and becomes more and

more luminous.

• The gas becomes denser as it builds up on the

surface because of the great mass and small radius

of the white dwarf.

• When enough material has accreted, fusion can reignite very suddenly, burning off the new material.



• The sudden start of fusion can generate shock waves that blow the surface layers into space.

• Material keeps being transferred to the white dwarf, and the process repeats.

• A nova represents one way in which a dead star in a binary system can extend its life.


Supernovae

• Supernovae are far more catastrophic and

luminous explosions than novae.

• Supernovae are not “super” versions of novae.

These explosions have a very different origin.

• They can achieve luminosities that are over millions

of times that of the sun.

• They are one-time events. Once they occur, there is

little or nothing left of the progenitor star.

• Supernovae are classified by duration (light-curves)

and by their absorption spectra. The most important

consideration is the presence or absence of

Hydrogen.


Supernovae

• There are two types of supernovae, both equally

common, which exhibit very different light-curves:

– Type I, is a carbon-detonation supernova

– Type II, is a core-collapse supernova that occurs at

the death of a high-mass star.


Type I: Carbon Detonation Supernova

Carbon-detonation supernova:

• Each time a white dwarf in a binary goes nova, it ejects

some of the matter it has collected from its companion.

• Still, not all of it is eliminated and the white dwarf grows

in size.

• In time this white dwarf will accumulate too much mass

from its binary companion.

• If the white dwarf’s mass exceeds 1.4 solar masses

(Chandrashekar limit), electron degeneracy can no

longer keep the core from collapsing.

• Carbon fusion begins throughout the star almost

simultaneously, resulting in a carbon explosion.



• This graphic illustrates this mechanism.

• Nothing is left of the white dwarf after the Carbon

Detonation. Type I supernovae are Hydrogen poor.

• In this way, the heavier elements are transferred

back to the Interstellar medium



• On the right is an image of

the supernova remnant G

299, which resulted from a

Type I supernova explosion.

• (Type I supernovae are

used to measure the

expansion of the universe)

• The supernova remnant SN

1572. It resulted from a Type

I supernova, visible to the

naked eye and observed in

1572, in particular by Tycho

Brahe. © 2017 Pearson Education, Inc.

Larger Mass Stars

• Larger mass stars are able to fuse heavier elements.

The sequence goes like this

– 42He + 20

10Ne → 2412Mg

– 42He + 24

12Mg → 2814Si

• This is the He capture sequence. It requires higher

temperatures at every step and each step occurs faster.

The last two stages occur appreciably only in stars of

mass > 12 solar masses. Advanced nuclear fusion of the

heavier elements then starts to occur:

– 126C + 16

8O → 2814Si

– 42He + 28

14Si → 3216S

– 2814Si + 28

14Si → 5628Fe


Larger Mass Stars

• Core temperatures of stars of mass greater than 12

solar masses allow the fusion of elements up to Fe.

• Other pathways also occur, e.g., an intermediate mass

element like Nitrogen is formed during the following

reaction:

– 11H + 12

6C → 137N → 13

6C + n + 𝑒+ + 𝜈𝑒

– 136C + 1

1H → 147N

• All these reactions release energy.

• Iron is the heaviest element that is energetically

favorable to fuse.

• Fusion to even heavier elements obviously does occur,

but now it removes energy from the star.


Larger Mass Stars

• But He capture continues, releasing energy, as well:

– 42He + 28

14Si → 3216S

– 42He + 32

16S → 3618Ar

– 42He + 36

18Ar → 4020Ca

– 42He + 40

20Ca → 4422Ti

– 42He + 44

22Ti → 4824Cr

– 42He + 48

24Cr → 5226Fe

• and… at the cost of energy:

– 42He + 52

26Fe → 5628Ni

– 42He + 56

28Ni → 6030Zn

– Etc.


Large Mass Stars


Nuclear Binding Energy


Fate of the Iron Core

• Iron lies at the lowest point of the curve, so with the

appearance of substantial amounts of iron, fusion

ceases.

• The iron core has no way to generate the energy

required to sustain itself and it begins to collapse.

• With the collapse, the core temperature rises to

about 10 billion degrees K.

• Gravitational energy is released as heat in the form

of photons.

• According to Wien’s law these photons have a very

small wavelength


Fate of the Iron Core


Photodisintegration

• Wien’s Law:

𝜆 =2.898 × 10−3

𝑇= 2.898 × 10−13 m

• They are smaller than the nucleus and extremely

energetic. They act as high energy bullets and begin

a process of photodisintegration of the heavy

elements in the core.

• In less than one second the process undoes all the

effects of fusion, splitting the Iron nuclei into smaller

and smaller pieces.

• This process cools the core, reduces the pressure

and accelerates the core’s collapse.


Photodisintegration

• Soon the core ends up where it began, made of

protons, electrons and neutrons, but at much

higher densities.

• The gravitational force (weight of the core) is far too

much for electron degeneracy to counterbalance it.

• The electrons then begin to combine with the

protons to form neutrons. This process is called

neutronization

𝑝 + 𝑒− → 𝑛 + 𝜈𝑒• Since there are equal numbers of protons and

electrons the central region of the core is now made

of neutrons.


Type II: Core Collapse Supernova

• When the neutronization process begins, the core

density is about 1012 kg/m3

• A very large number of neutrinos are produced in this

process. These neutrinos carry out about 1046 Joules

of energy (in about 10 seconds) because they interact

weakly with matter.

• As they get closer together, the neutrons begin to

exert an outward degeneracy pressure on the core.

• The core continues to collapse, compressing the

matter to a density of 1017 kg/m3, at which point the

neutron degeneracy pressure causes the collapse to

halt. The outer layers rebound off the inner core.


Type II: Core Collapse Supernova

• A massive shock wave sweeps through the star,

ejecting all the outer layers (including the heavy

elements just formed) into space. This is a Core-

Collapse Supernova.

• A neutron core survives this type of a supernova

explosion. Type II supernovae are Hydrogen rich.


Supernova Remnants

• Supernovae leave

remnants—the

expanding clouds of

material from the

explosion.

• The core is bounded by an

expanding shock wave,

which is expanding into

space and sweeping up

material from the ISM

Electrons ejected from the

core are in the middle,

emitting radio synchrotron

radiation.© 2017 Pearson Education, Inc.

The Crab nebula

resulted from a Type II

Supernova (2 kpc).

Supernova Remnants

There are in general three kinds

of remnants:

• Crab-like remnants (Crab)

• Shell-like remnants like

Cassiopeia on the right (3.4

kpc, Type II). A shock wave

plows through space.

Sometimes the shells contain a

central neutron core like Vela,

on the right (294 pc, Type II).

• Composites: appear shell-like

in radio and crab-like in X-rays,

or crab-like in both frequencies

but also have shells.


Formation of Elements

• He capture (also known as the alpha process) can

only lead to the formation of elements with atomic

mass divisible by four.

• This is the most common process, so these

elements will be more abundant than others.

• But it is not the only way elements can form.


Formation of Elements

• Elements can also form by direct capture of free protons

and neutrons by heavy nuclei to form even heavier

nuclei.

• By the time 2814Si appears the temperature in the core is

about 1 billion K. At this point, the photons are so

energetic that photodisintegration begins and there is a

competition between forming heavier nuclei and

breaking them up.

• The result is that some heavy nuclei (like 2814Si) are

destroyed creating new 42He and promoting He capture

to form even heavier elements.

• The presence of a variety of elements, He, protons and

neutrons then produces other, less common, elements.


Formation of Elements Heavier than Iron

• He capture cannot proceed after Iron is formed.

Making elements heavier than iron occurs via

neutron capture.

• Neutrons have no charge, so they are easier for

heavy elements to capture.

• Neutron capture is a very slow process and is often

referred to as the s-process.

• Being slow, the heavier nuclei have time to decay

into lighter nuclei, so it ends up being a competition

between capture and decay.

• This is why there are only trace quantities of the

heaviest elements.


Formation of the Heaviest Elements

• Neutron capture, or the s-process, can function to

produce only relatively stable nuclei.

• Any element with an atomic mass greater than 209

(Bismuth) cannot be produced by neutron capture

because the decay rate is greater than the capture rate.

• However, in the last 15 minutes of a supernova

explosion there are so many neutrons that the capture

rate exceeds the decay rate as the neutrons are jammed

into the nuclei.

• This is known as the rapid process (r-process)

because it proceeds very rapidly.

• So the heaviest elements are formed after the star has

died!


The Cycle of Stellar Evolution

• Star formation is

cyclical: Stars form,

evolve, and die.

• In dying, they send

heavy elements into the

interstellar medium.

• These elements then

become parts of new,

next generation stars.

• And so it goes.


Summary

• Once hydrogen is gone in the core, a star burns

hydrogen in the surrounding shell. The core

contracts and heats; the outer atmosphere expands

and cools.

• Helium begins to fuse in the core as a helium flash.

The star expands into a red giant as the core

continues to collapse. The envelope blows off,

leaving a white dwarf to gradually cool.

• A nova results from material accreting onto a white

dwarf from a companion star.

• The same white dwarf may undergo many novae.


Summary, cont.

• A Type I supernova is a carbon explosion, occurring

when too much mass falls onto a white dwarf.

• Very massive stars become hot enough to fuse

carbon, then heavier elements, all the way to iron. At

the end, the core collapses and rebounds as a Type

II supernova.

• All heavy elements are formed in stellar cores or in

supernovae.

• The heaviest elements are formed after the star is

dead.

• Stellar evolution can be understood by observing

star clusters.


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