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Stellar Evolution
We have lots of information about stars, but we still
need to consider two more areas before we begin
to put this all together and see if we can see some
kind of “stellar life cycle” (also called stellar
evolution). Those last two areas are
interstellar material: atoms, dust, and nebula;
and variable stars.
Stellar Evolution
The Crab Nebula, M1, as imaged by Hubble Space Telescope and the Mount Palomar telescope.
How do we know what is in
interstellar space?
Gas and dust in space can:
scatter light
absorb light, heat up, and then re-emit light
Scattered Light
In scattering light, blue light scatters more
than red light. This gas and dust will then
tend to “redden” starlight that passes
through it. This effect is seen on the earth –
the sky is blue because the blue light is
scattered more than the red light; but the
sunrise and sunsets appear red because most
of the blue has been scattered out of the
direct sunlight.
Absorb Light
Atoms will selectively absorb light of
particular frequencies – called an absorption
spectrum. They will later re-emit that light,
but in different directions – the emission
spectrum.
Dust particles will absorb light of most any
frequency and tend to heat up. They will
emit “blackbody” radiation based on their
temperature.
Nebula
This combination of absorption
and emission of light by gas
and dust results in different
types of nebula (areas of
Image of the youngest known planetary
nebula, the Stingray nebula (Hen-1357).
relatively high gas and dust):
dark nebula and glowing nebula.
See web sites:
http://nssdc.gsfc.nasa.gov/photo_gallery/photogallery-astro-nebula.html
http://www.robgendlerastropics.com/Nebulas.html
Interstellar Space
Liquid water has about 3 x 1022 water molecules
per cubic centimeter (in English about 30 billion
trillion). Most solids and liquids have similar
numbers.
At the earth’s surface our atmosphere has about 2.4
x 1019 molecules per cubic centimeter (about a
thousand times less dense than liquid water).
In most of interstellar space, there is about 1
hydrogen atom per cubic centimeter.
There are regions of interstellar space, though, that
have much higher densities. In nebula, that
number can reach a million atoms per cubic
centimeter.
Variable Stars
While most stars appear to be quite stable, at
least on a human time frame, some stars do
show variations in brightness. A few show
huge changes that appear to be catastrophic
events. Others have brightness changes in a
very periodic manner, some on the order of
seconds, others on the order of days.
Cepheid Variables
One type of star, a Cepheid Variable, has a
brightness that varies by up to about half a
magnitude with a period that ranges from 1 to 100
days.
By looking at star clusters where all of its stars
appear to be about the same distance away, we
find that the period of the star is related to the
luminosity of the star! The lower the period, the
lower the average luminosity. Cepheid variables with
a period of 1 day have an absolute magnitude (luminosity)
of about –2. On the other end, Cepheid variables with a
period of 100 days have an absolute magnitude of about
–8.
Cepheid Variables
These are all very luminous stars (giants), and can be
seen from very far away.
What makes this important are the following
relations. We can always measure brightness. It is
also easy to measure the period of a Cepheid
Variable. With the period-luminosity relationship,
we can then get the luminosity. Finally, knowing
the brightness and luminosity, we can calculate the
distance!
This distance determination will be an important tool
in Part 5 of the course where we look at the
overall size and structure of the universe.
Stellar Evolution
Let’s now try to put all of this info together
into a theory that will explain our
observations and lead us to make further
observations to support, refine, or refute our
theory.
1. Beginning:
Gravitational formation
Most of space is fairly empty of matter and
cold. However, there are areas of relatively
high gas and dust (nebula). Over time,
gravity will tend to pull the gas and dust
together. As it does, it will tend to convert
the gravitational energy into heat energy
(the speed of falling is converted into heat).
1. Beginning:
Gravitational formation
The “cloud” of gas and dust will tend to get smaller
and hotter. A smaller size tends to reduce the
luminosity, but hotter tends to increase luminosity.
The position of the newly forming star on the H-R
diagram will move to the left as it heats up but
wander up and down somewhat as its size shrinks.
This process takes about 50 million years for a star
like the sun, but may take a much shorter time for
a more massive star since there will be more
gravity. A ten solar mass star will only spend
about 200,000 years in this initial stage.
H-R Diagram
Gravitational formation
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2. Nuclear Fusion of Hydrogen
Stability on the Main Sequence
When the temperature and pressure at the core
of the newly forming star reaches a certain
point, the hydrogen atoms will collide with
one another so hard that nuclear fusion will
occur (basically four hydrogen atoms
combine to form one helium atom plus
LOTS of energy). This “hydrogen bomb”
process tends to blow the star apart, but
gravity continues to try to collapse the star.
2. Nuclear Fusion of Hydrogen
Stability on the Main Sequence
The result of these competing tendencies is a stable
star, both in size and in temperature (and hence in
position on the H-R diagram on the Main
Sequence).
More massive stars have more fuel, but they also
have more gravity that causes the core to burn the
fuel at a faster rate than less massive stars. The
result is that more massive stars are hotter and
more luminous and are higher on the Main
Sequence than less massive stars, and they remain
stable on the Main Sequence for less time.
2. Nuclear Fusion of Hydrogen
Stability on the Main Sequence
A star like the sun will last about 10 billion
years on the Main Sequence.
A star with 15 times the mass of the sun will
only last about 10 million years on the
Main Sequence. In the same way, stars with
less mass then the sun will stay on the main
sequence much longer than 10 billion years.
3. Red Giant
Nuclear Fusion of Helium
When the hydrogen starts to run out in the core, the
explosive energy production of nuclear fusion no
longer can balance the gravitational tendency to
collapse, and so the core of the star will again start
to collapse while hydrogen is still burning on the
outside of the core. This gravity collapse of the
core will again heat up the core, and this extra heat
will cause the star’s surface to expand. As the
surface expands, it will tend to cool. The result is
a red giant state – higher luminosity but a little
cooler surface.
3. Red Giant
Nuclear Fusion of Helium
For a star like the sun, this expansion of the
surface will be large enough to reach the orbit
of Venus or even the Earth.
When the core gets hot enough, it will start to have
the helium atoms (ashes of the hydrogen fusion)
combine in nuclear fusion to form carbon and
release energy.
This process takes roughly about 10% of the time of
the Main Sequence hydrogen burning.
H-R Diagram
Red Giant
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4. Unstable stars
After the helium fuel in the core runs out, there
are different scenarios for different masses of
stars.
For a star with about the mass of the sun or
less, the core will again collapse and the
gravitational energy of the collapse will eject
some of the outer layers of the star (called
planetary nebula ejection) and the core (now
at about 0.6 of the original mass of the star)
will heat up (move to the left and tend to
move up) and shrink (tend to move down).
H-R Diagram
Unstable
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Cepheid
Variables
eject planetary nebula
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Sun = G2 at +4.8 Magnitude
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4. Unstable stars
For more massive stars, the situation is more
complicated. With the higher gravity, the
core can get hot enough to start burning the
carbon to get even heavier elements. This
proceeds until the core turns into iron.
Since the nucleus of iron is tightest bound
of all atoms, iron cannot undergo nuclear
fusion to release energy like the less
massive atoms can.
4. Unstable stars
When the core cannot continue with fusion,
there is nothing to balance gravity, and the
core will totally collapse. The implosion of
the core will release so much energy that it
will blow the outer parts of the star
completely away in a supernova explosion.
Final Stage – Death of the Star
There are three possibilities for the collapsed
core depending on the mass of the
remaining core:
1) If the final mass after the planetary nebula
release is less than 1.4 solar masses, the
remaining mass of the star will collapse
down to a size about that of the earth. It
will be a white dwarf star, and then as it
cools it will become a brown dwarf and
then eventually cool even further.
H-R Diagram
Final Stage: Death
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Cepheid
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eject planetary nebula
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White dwarf
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Final Stage – Death of the Star
2) If the final mass (after the supernova
explosion) is more than 1.4 solar masses
but less than about 3 solar masses, the
core will stop collapsing when the atoms
are so compacted that the electrons are
shoved into the protons and the whole
mass becomes neutrons that stick together
by gravity. This is called a neutron star.
It’s diameter is only about 20 kilometers
(compared to about 12,000 kilometers for a
white dwarf!).
Pulsar
2-continued) If the original star had an appreciable
magnetic field and a rotation, the resulting neutron
star may still have that magnetic field and it will
have a much higher rotational speed due to the
collapse. The magnetic field may cause light to be
emitted in a beam, and with the rotation this beam
may rotate at a high angular speed. We have seen
pulses of light with periods of a few seconds from
these spinning neutron stars and so we call them
pulsars.
Final Stage – Death of the Star
3) If the final mass of the core after the
supernova explosion is more than about 3
solar masses, then gravity is so strong it will
collapse the matter even beyond the neutron
star size. We know of nothing that would
stop the collapse. This is called a black
hole.
Black Holes
Note that the mass of a black hole is still there
and its gravity will affect things around it.
But gravity is so strong near it that even light
can be trapped so that it does not escape
from the black hole.
Further away, though, other stars will feel the
gravity just like they feel the gravity of
other massive objects.
Mass back to nebula and space
In the ejection of the planetary nebula and in
supernova explosions, some and sometimes
most of the mass of the star is ejected back
into space. There is a difference, though.
The initial mass of the collapsing nebula
consisted of mostly hydrogen. The final
mass of the expanding nebula is enriched in
the heavier elements. The energy in a
supernova is so high that elements heavier
than iron are made.