Stellar Evolution
For example,
red dwarf stars spend several hundred billion years as a main sequence
star, whereas the
much more massive stars
at the top of the main sequence spend only a few million years as a main
sequence
star. Stars with a
masses similar to that of the Sun have a main sequence lifetime of about
10 billion years.
Stellar evolution is the result of the interplay between gravity and gas pressure.
Gravity is always trying
to contract the star while gas pressure is trying to expand
the star. If gravity is
greater than the gas pressure,
a star contracts and heats up. If gravity is weaker than gas
pressure, a star
expands and cools.
If gravity and gas pressure are equal at every point in a star, the star
is said to be in
hydrostatic equilibrium,
it neither expands or contracts.
The mass
of a star determines how strong gravity is. The greater the mass of a star
the stronger gravity
is and the greater the compression
of a star. The greater the compression of the star, the higher its
temperature. The temperature
of a star determines how strong the gas pressure is and the rate at which
any
thermonuclear reactions
take place. The faster the TNF reactions take place, the faster a star
evolves.
The exact
sequence of evolutionary stages also depends on the mass of a star. Stars
similar in mass to
that of the Sun follow the
following sequence of stages:
T Tauri star, M-S (main sequence
stage), RG (red giant), Horizontal Branch, He TNF, Planetary Nebula
Stage, and WD (white dwarf).
For a star that is 10 times more massive than the Sun, the sequence is:
T Tauri star, M-S, RS (red supergiant), various other TNF reactions, supernova,
and then either WD,
neutron star, or black hole,
depending on the remaining mass of the star after the supernovae even.
B. Stages of Stellar Evolution:
1. Nebular Stage:
Stars
form from gas and dust clouds in the interstellar medium of a galaxy called
nebulae. This happens when
random motions within the
cloud leads to the formation of knots in the cloud. The knots are regions
of higher
density and gravity begins
to dominates over gas pressure. Gas pressure is small because the
temperature in a
nebula is very low. The
cloud then slowly begins to contract and heat up. Any random motions within
the cloud
become organized and the
protostellar cloud begins to rotate.
Dark
globule/ Bok Globule: The stellar nebula gravitationally contracts, gets
denser and becomes
opaque. These objects
are seen in silhouette against a background of fluorescing gas
(mostly hydrogen).
See Fig. 61.2 in S&A.
2. Protostar :
The contracting
cloud is converting gravitational potential energy into heat
or thermal energy, some of
which is converted into
EM radiation.
Protostars are living on their gravitational potential energy
The cloud remains opaque
until the temperature in the central region become sufficiently high
that the radiation
from this core ionizes
the gases and vaporizes the dust particles that are located in an envelope
surrounding the
central region. Light
from the core then penetrates the cloud or envelope and is able to
reach an observer.
Hence, the central region
or core begins to shine as a star. However, it does so erratically,
but the core of our
cloud is now a protostar.
Stars
identified in the later stages of the protostar stage are called T
Tauri stars. Such stars are said to be in a
state of quasi hydrostatic
equilibrium, because they contract very slowly now. However, they
can not stop
contracting completely because
they lose heat by radiation faster than they can generate heat by
contraction.
3. Main Sequence:
A star enters this stage when it initiates TNF of hydrogen nuclei into
helium nuclei in its core. This happens
when the core of the contracting
protostar reaches about 5 million Kelvins. This is because at this
temperature,
the collisions between protons
are so intense that the particles are able to overcome the repulsive
force of the
positive charges and get
sufficiently close for the strong or nuclear force to become effective.
The first step in a
series of steps that ultimately
produces a helium nucleus, is when 2 protons merge and form a deuterium
nucleus.
During this reaction, one
of the protons converts to a neutron by emitting a positron. A deuterium
nuclide then
consists of one proton and
one neutron. See page 416 in the
textbook for furter detaiils of the
steps for hydrogen
fusion in what is called the pp-chain. The thermonulear
fusion of H into He
reactions release a tremendous
amount of energy that percolates its way up through the star, heating it,
and
increasing the gas pressure
to the point that it balances gravity. Stars that are beginning the
main sequence
stage define what is called
the Zero Age Main Sequence or ZAMS,
which
is a unique curve or locus in the H-R
diagram .
Main
sequence stars are characterized by:
a. TNF of H into He in the core of the star.
b. Hydrostatic equilibrium: At every point in the star, gravity is balanced
by gas pressure. This happens
because the star does not radiate away its heat at the surface faster than
it generates it by TNF.
Some main sequence lifetimes for different stellar masses (Ms means solar masses):
25.0
Ms: 4 million years
15.0 Ms : 10-15 million years
5.0 Ms : 65 million years
3.0 Ms: 800 million years
2.25 Ms : 480 million years
1.5 Ms: 4.5 billionn years
1.00 Ms : 7-10 billion years
0.5 Ms: 25 billion years
0.10 Ms : 100 billion years
Objects
with a mass less than 0.06 solar mass units (the Kumar Limit) never initiate
TNF, and never
become true stars.
These objects are called Brown Dwarfs.
Since brown dwarfs are intrinsically faint objects,
it is difficult to detect
them. However, several nearby ones have been detected recently,
so their existence
has been verified.
Brown dwarfs, like stars, are comprised mostly of hydrogen and helium,
whereas
terrestrial-like planets
are comprised mostly of metals and silicates (rocks).
Jupiter, Saturn, Uranus, and
Neptune are comprised mostly
of hydrogen and helium, like stars, but they are planets. To be a
brown dwarf,
an object must be at least
17 times more massive than Jupiter but be below the Kumar Limt.
It may
be that brown dwarfs could be found to orbit a true star. If
this is so, a new class of satellites
orbiting stars, like the
TNOs and comets, would have to be recognized.
4. Red Giant or Supergiant
stage:
This
depends on the mass of the star. Upon exhaustion of H in the core, the
star begins to contract and get
hotter. TNF
of H into He then begins in a shell around the core. Meanwhile, the He
core continues to contract
and get hotter. The energy
released from the H fusion shell and the much hotter core heats the
outer layers of
the star thereby causing
these layers to greatly expand. The star then becomes a red giant or supergiant.
5. Core He fusion stage.
At 100
million Kelvins, He nuclei in the core collide and fuse to form C, then
O, and then Ne. The ignition of
He-fusion happens almost
explosively (helium flash), and the star
undergoes a rapid change to enter a stage of
hydrostatic equilibrium
again as it undergoes helium fusion in the core. The star is then
said to be a horizontal
branch
star, because stars of different mass in this stage of evolution
define a locus in the H-R diagram that is
is horizontal.
6. Other stages of TNF:
When all
the He is exhausted in the core, the star begins to contract again until
He fusion occurs in a shell
around the core, while hydrogen
fusion occurs in a layer above that. The star then expands to become
even
larger and enters the asymptotic
giant branch stage.
However, a star may
stop contracting before it can reach the ignition temperature of these
other fusion
reactions. How does this
come about? The temperature in a stellar cores is so large
that the gas is highly
ionized. The free
electrons now act as another gas in the star, separate from that of the
nuclei, and exerts
a separate pressure called
This pressure results from
the mutual repulsive forces between electrons and other quantum theory
aspects of
the energy states that the
electrons occupy when crowded close together. So, as the core contracts
and
heats up, the electron pressure
increases. Depending on the mass of the star, there will be a temperature
at
which electron pressure
may eventually balance gravity and the star halts its contraction.
At 600 million K, C
fuses into Mg. At higher temperatures, Mg undergoes TNF to form Si and
then Si fuses
to become Fe.
This last reaction happens only for very massive stars.
The following figure shows the post main sequence evolutionary tracks for stars with several different masses.
7. Planetary Nebula Stage:
For the
Sun, this happens before the carbon in the core reaches its fusion ignition
temperature. However,
the Sun is much hotter and
smaller now. The hot core now emits very intense radiation that causes
the Sun
to loose gravitational
control of its outer envelope. The envelope now begins to undergo
pulsations that
grow larger and larger.
Eventually, the outer hydrogen envelope of the Sun drifts away into space.
The Sun
then enters what is called
the planetary nebula stage. This name arose from the fact that
these expanding
shells appeared like fuzzy
planets to early telescopic observers, but they are not planetary phenomena.
More
massive stars will reach
the ignition temperatures of other nuclear reactions before electron
pressure can
stop further contraction.
The diagram below summarizes
the evolution of a star like the Sun in a temperatur luminosity diagram,
from the
main sequence to the white
dwarf stage. Not shown are the pre main sequence stages such as the
T Tauri
stage and other protostar
stages.
8. The Supernova Event:
Very massive
stars that convert Si into Fe run into a problem. Fe will not undergo
TNF. Instead, as the
iron core slowly contracts
and heats up, the iron nuclei absorb the gamma radiation, which is now
very intense.
This causes the iron
nuclei to undergo photodisintegration into
He, some neutrons, and copious amounts of
a very weakly interacting
particles called neutrinos. In addition,
many electrons are also forced to combine with
protons at the enormous
pressure inside the core of the star. This reactions also releases
copious amounts of
neutrinos. The neutrinos
quickly escape into space, taking with them a large amount of thermal energy
from the
core. So the core
cools very quickly with a concomitant reduction of pressure.
This brings about a sudden
collapse of the core which
then rebounds, and bumps into the outer layers of the star which are also
collapsing
onto the core.
The result is a shock wave that travels up through the star .
The shock wave also compresses
the upper layers of the
star to the ignition point of any nuclear fuels that are present in these
layers.
The released energy from
these reactions adds to the luminosity of the star. This all happens very
quickly, like an
explosion. The star
then brightens by millions of times so that, observationally, there appears
to be a
super bright new star in
the galaxy to which it belongs. Hence the name supernova. Nova
means new in Latin.
Remember, this only happens
for very massive stars. The Sun will not become a supernova. It and
other low
mass stars will slowly
shed their outer layers as they evolve to their final stage. The shedding
of the outer layer
non explosively is
called the planetary nebula stage.
In a supernova explosion,
the outer layers of the star blow off at very high speed, leaving behind
the core
of the star, which
is sometimes referred to as a supernova remnant. However, the term
supernova remnant
is sometimes meant to refer
to the blown off layers. The latter continue to expand
and eventually enrich the
interstellar medium with
the heavy elements that the star produced. So stars are slowly causing
a change in
the chemical composition
of a galaxy. Successive generations of stars will then have a higher abundance
of
heavy elements than
earlier generations or older stars. In this way, the age of a star may
be determined. Some
of the heavy elements in
the supernova remnant form microscopic particles that are called "dust".
Only in this
way does it become possible
to make planets and people. So we are made of "stardust."
9. Final Stages:
When a
star is prevented from collapsing so that it can not ignite another TNF
reaction it enters into a final
stage where it slowly cools
off and dies as a star. The possible final stages, depending on what stops
further
contraction are:
a. white dwarf, b. neutron star, and c. black hole.
Which
final stage a star eventually reaches depends critically on its mass. Very
massive stars may undergo a
supernova explosion and
then become either white dwarfs, neutron stars, or black holes, depending
on how
much mass the star has after
the explosion. Stars like the Sun and those less massive than the
Sun evolve
to become white dwarfs.
White Dwarfs :
In a white
dwarf, the repulsive forces between free electrons acts like a pressure
in the star. This pressure
can be very significant
because highly evolved stars are so hot that the gases are almost
completely ionized.
The electron pressure is
strong enough to balance gravity if the star's mass is less than 1.4 times
the Sun's
mass. This is known
as the Chandrasekhar limit. White dwars
are about the size of the Earth or somehat
larger. They have
an averge density 100 million times the density of water.
White dwarfs eventually cool
to become black dwarfs, but this takes about 50 billion years.
The universe is
insufficiently old for this
to have occurred for any star yet.
Neutron Stars
Stars
slightly more massive than 1.4 solar masses, but less than 3 solar masses,
contract to the point where
all electrons are forced
into the nuclei to combine with the protons and form neutrons.
The repulsive
force between neutrons now acts as a pressure that balances gravity.
When this happens,
the star becomes a
neutron star.
Neutrons
stars are very small, have intense magnetic fields, and spin very rapidly.
Charged particles in the
atmosphere of a neutron
star move along the magnetic field lines are focused towards the magnetic
pole
the star. As they
spiral towards the magnetic poles, the partilces emit all kinds of EM radiation,
even
radio waves. The radiation
emitted along the magnetic axis of a neutron star is very intense. If the
magnetic
axis is tilted with respect
to the rotational axis, one could observe the radiation coming from the
magnetic
poles of the star to be
in pulses, much like a rapidly rotating lighthouse. Such objects
have been detected and
are called "pulsars". Hence,
pulsars are evidence that neutron stars really exist. The first pulsar
was detected
in 1967 by Jocelyn Bell,
a grduate student in radio astronomy. Later, pulars were discovered
to be emitting
synchrotron radiation over
the entire EM spectrum. Read Units 62.1 and 62.2 in the textbook.
The explanation of the pulsar
phenomenon as a rapidly rotating neutron star was made by Pacini and Gold.
Black Holes
Stars that enter
the final stage of evolution with more than 3 solar masses become black
holes. Here the
gravity of the star is so
strong that light can not escape from the star, so it is said to be black.
Black Holes
are stars that have collapsed
inside their critical or "Schwarzschild Radius."
Once this happens, there is no
known force that can stop
the star from contracting and this is an untenable problem. Ordinary
classical
physics can not explain
what happens in a black hole and we must turn to the general theory of
relativity.
In general relativity,
very massive objects distort space and time. Gravity is then interpreted
as the curvature
of space-time.
Go to Chapter 11, which discusses General Relativity and Black Holes in more detail.
End of file
Copyright 2003, 2009, by R. J. Pfeiffer