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Transcript
BENDIGO U3A 2012
Keith Thompson
THE LIFE CYCLES OF STARS (5)
RECAPITULATION
In the first session we saw how stars condense out of huge gas and dust clouds in the
interstellar medium of the Milky Way, our galaxy, and how they are born as clusters
of stars.
In the second session we saw how astronomers learned to find the distance to (nearby)
stars using parallax and how this enabled them to know the intrinsic brightness
(absolute magnitude) deduced from apparent brightness corrected for distance.
Combining this absolute magnitude with temperature deduced from colour
measurements or spectral type they created the Hertzsprung Russell diagram. This
showed that there is a systematic trend from hot bright massive blue stars to cool dim
low mass red stars forming a continuous line called the main sequence.
In the third session we looked at the source of energy of a star, nuclear fusion, and in
the fourth deduced how long various mass stars should live on the main sequence
illustrating this with HR diagrams of various open clusters of different ages.
In this fifth session we shall look at the various ways stars die and what is the result.
EXTREMELY LOW MASS STARS-Brown dwarfs
Extremely low mass stars less than 0.08 of a solar mass cannot raise the core
temperature enough to start nuclear hydrogen burning. These never make it to the
main sequence but shine dimly deriving energy from slow collapse.
Such are
called brown dwarfs.
FAINT STARS
Faint stars do not get much press, they are a bit boring, but there are more of them
than we might think just because we do not see them.
Here are two HR diagrams One for visually bright, the other for nearby stars within 5
parsecs.
1
The bright stars get noticed even when they are far away. The dim ones are not seen
even though nearby. It is a lesson that when you take statistics be careful how you
pick your sample.
LOW MASS STARS-Red Dwarfs  White Dwarfs
These are low mass stars which ignite Hydrogen burning, but when they are due to
leave the main sequence and become giants, having used up about 10% of their fuel
are not massive enough to raise the Helium core to 100 million K.
They never
ignite Helium burning and never become red giants. They fade quietly shrinking
under gravity generating energy just from gravity collapse. They get dimmer and
dimmer but at the same time get smaller and smaller so they have little surface area to
radiate away so become hot and blue. They end in the lower left corner of the HR
diagram as White Dwarfs. A white dwarf is a star supported by what is called
electron degeneracy pressure. This is a quantum mechanics phenomenon in which
the electrons resist being pushed too close together to occupy the same quantum state.
(It is not merely electrostatic repulsion; there is a similar phenomenon with neutrons)
It results in the star ending up incredibly hot surface temperature, about 100,000 K,
and about the size of the Earth. Because so hot it looks bright blue but because it is so
small it is very dim. It has an enormous density, about 1 ton/cc.
SOLAR TYPE STARS ~ 1 M o
These stars move to the red giant phase up into the red giant region then burning
helium to Carbon. Helium burning happens suddenly and explosively and much of
the envelope surrounding the core is ejected as a gas cloud. The star then moves left
across the HR diagram getting bluer as the hot core is exposed. The hot core emits
strong UV light which ionizes the gas and causes it to fluoresce. Red is from H II
Blue-green from O III and N III
They are called Planetary Nebulae.
They have nothing to do with planets but when they were first observed as dim grey
smudges they looked like planets.
NGC 7293
M27 The Dumbbell Nebula
The gas is moving away from the star at 10 to 30 km/sec and the whole cloud will last
about 50,000 years.
2
NGC 1514
NGC 6751
NGC 2438 in cluster M46
M57 Ring Nebula
A sketch by William Herschel of the Dumbbell Nebula NGC 6853
Every planetary nebula has a central star which is in the process of becoming a white
dwarf supported by electron degeneracy pressure. Here is an HR diagram especially
for these central stars. You see that they are hotter than O and B type stars but
dimmer. That is because they are so small. A white dwarf usually ends up about the
size of the Earth.
3
STARS WITH ZERO AGE MAIN SEQUENCE (ZAMS) LESS THAN 8 M O
As these stars evolve up the asymptotic giant branch they pulsate and lose mass. The
mechanism is not understood. It may be the large outer envelope is so far from the
massive core that surface gravity is weak, it may be just a powerful stellar wind, or it
may be the pulsations emit puffs of gas.
Being giants they pulsate very slowly and are known as Long Period Variables (LPV)
(period ~ a year)
MIRA THE WONDER STAR Omicron Ceti
 Ceti is an LPV (Long Period Variable) with a pulsation period of about 11 months.
When bright (for about 2 months) it is about 3rd or 4th magnitude. When dim (most of
the time) it is invisible. There would be some years when it was not seen at all,
hence its name.
This photograph in UV was taken in 2007 and shows a long trail of gas and dust
emitted from the star. The star is moving at 130 km/sec through the inter-stellar
medium and in the last 30,000 years has laid this trail 13 LY long.
LPV stars are known to lose mass at a typical rate of 10 4 M O /year. In 30,000 years
this could amount to 3 M O
Because it is cool (3000 K) the mass loss will include dust grains of carbon and
silicate. .
4
These all return material to the interstellar medium, the clouds of gas and dust we
started with to make stars. However this material has heavy elements in it Carbon
and Oxygen.
HEAVY WEIGHT STARS with zero age mass more than 8 M O
The main sequence life burning hydrogen lasts only 100 million years instead of 10
billion for the sun. These are O and B type stars. Even before leaving the main
sequence these stars emit material from their surface due to sheer radiation pressure.
The strong light radiation carries gas with it.
Our sun emits a solar wind of protons
and electrons which can cause aurorae and in certain cases disrupt our electricity
supply lines.
In its later post main sequence life the star will go through the giant stages of burning
helium, then carbon, oxygen neon and other elements.
During these giant phases the star continues mass loss by stellar wind. This mass
loss may be quite significant. If after consuming all fuels and not having enough
mass to initiate further fuel burning, if after all this the mass is less then 1.4 solar
masses the star will end as a white dwarf. That is the collapse is stopped by electron
degeneracy pressure. If the final mass is between 1.4 and 2 or 3 solar masses it
becomes a neutron star. In a neutron star the core is massive enough to build the
sufficient gravity pressure to initiate the nuclear reaction called reverse  decay.
In normal  decay a proton may decay to become a neutron and unload its positive
charge by emitting a positive electron (positron). We saw this in the proton-proton
chain of nuclear fusion. Two protons collide and together form a deuteron with one
proton undergoing beta decay and emitting a positive electron (positron). In reverse
 decay the pressure forces the electrons and protons to combine to become a neutron
plus a neutrino which escapes.
p  + e   n + e
This removes most of the protons and electrons from the star. Some may still exist on
the surface but in the body of the star the pressure will win. A typical neutron star is
about the size of Melbourne say 10 km across. Its collapse is stopped by neutron
degeneracy pressure. It still has stellar type mass so its density is enormous, about
10 15 gm/cc. = 10 15 tonnes/cubic metre .
Here are a couple of hot mass-loss stars.
Wolf Rayet star HD 56925 NGC 2359
HD 148957 NGC 6164-5
5
EXTRA HEAVY WEIGHT STARS
Stars with initial mass so great that after mass loss and late in life it is more than 8 or
10 solar masses undergo a violent death. The star has burned all possible fuels up to
creating iron in the core. Because these elements are near the peak of the mass defect
curve the energy gain per reaction is very low. Consequently, in order to maintain
the core temperature and support the rest of the star, the reactions proceed at a
tremendous rate. In an extreme case of a 20 M O star the stages are as follows.
THE STAGES OF A 20 M O STAR
Main Sequence Life
10 Million Years
Helium burning
1 Million Years
Carbon Burning
300 Years
Oxygen Burning
200 days
Silicon Burning
2 days
It is unlikely that a star will reach this stage and still be so massive. It will probably
lose mass first.
The core temperature will reach 8 billion degrees and the photons (  rays of high
intensity) are powerful enough to break up heavy nuclei. Up until now the star has
been trying to support itself by producing heavier and heavier nuclei. The core is
partially supported by electron degeneracy pressure as in a white dwarf.
The

powerful rays begin to undo this process. The iron nuclei are split into lighter
elements. Since this is going the other way on the mass defect curve, it absorbs
energy, produces free protons which capture the electrons, not to become hydrogen
again but to become neutrons by reverse  decay. This removes the electrons with
their electron degeneracy pressure which were supporting the core. The neutrinos
escape completely from the core taking a large amount of energy with them. The
radiant energy flux of neutrinos is about 10 million times more than the normal star
energy flux.
The inner core collapses suddenly and within about 1 second the core has collapsed
from about the size of the Earth to about 50 km radius. It is only stopped by the
6
neutron degeneracy pressure and briefly by the neutrons in the inner core actually
touching each other so the strong interaction becomes repulsive. The inner core stops
and begins to rebound in a shock wave meeting the outer core still falling in. The
rest of the star, the outer envelope has been left behind as the core collapsed and now
is without support. The outer envelope of the star begins to free fall inwards and
meets the core rebounding from its crush.
There is a sudden brightening of the star so it briefly rivals the rest of the galaxy in
brightness. It the tremendous flux of energy the elements in the outer envelope are
built up into heavy elements and blasted out into space.
The crab nebula was observed to
explode in AD1054 by the Chinese
and the American Indians. In the
intervening years the material
ejected has expanded and is still
expanding outwards. The central
star is a neutron star about 10-15
km in size and spinning 30 times a
second, (a pulsar). It is magnetic
and acts like a dynamo which
powers the surrounding nebula.
M1 The Crab Nebula a supernova remnant
The Veil Nebula in Cygnus
The Vela Supernova remnant.
HIGH MASS LOSS CREATES NEW STARS
There are collections of hot O and B type stars known as OB Associations (Not Old
Boys Associations) High mass loss stars such as these create winds of fast particles
which propagate through the interstellar medium. If these collide with a molecular
cloud they can compress the cloud and stimulate it to collapse initiating newborn
stars.
7
COMETARY GLOBULE
Cometary globules have nothing to do with comets apart from a similarity in shape.
They are gas and dust clouds being illuminated and eaten away by hot O and B type
stars with strong stellar wind and light pressure. The galaxy is a background object.
The hot O and B type stars are mass loss stars with high stellar winds. This wind
blows against the molecular cloud compressing it and causing it to collapse and
generate more O and B type stars. This is a continuous process as long as the cloud
lasts.
Another possible cause of cloud collapse and new star generation is the shock waves
produced by supernovae explosions. Let us look at some spiral galaxies.
8
THE GALAXY NGC1300.
This is somewhat like what our own Milky Way galaxy would look like were we able
to get outside it. It shows the young hot bright O and B stars as bluish white and the
H II regions as red.
Why do we see stars only in the spiral arms? Because we see only the young hot O
and B stars in the arms where they are born and where they die. There are stars in the
gaps between the arms but they are too faint by comparison. There is a compression
shock wave which circulates around the galaxy. As the shock wave passes through it
initiates collapse in the molecular clouds stimulating star creation. We see the newly
created O and B stars as the shock wave initiates the compression of the molecular
clouds. The Galaxy is rotating. Our Milky way galaxy rotates once in about 200
million years. Our sun is 5 thousand million years old and has made about 25 trips
round the galaxy. This makes our sun 25 galactic years old.
Hot O and B type
stars live only a few million years. They do not live long enough to orbit the galaxy
even once, they never see their first galactic birthday, they live and die roughly where
they were born while the compression wave passes on to initiate further star formation
and elsewhere.
9
RELENTLESS GRAVITY
All this has come about because of the relentless pressure of gravity. This is strange
because of all the forces of nature, (there are four, gravity, electrostatics, weak nuclear
force in Beta decay, and the strong nuclear force). Of these only gravity and
electrostatics are long range forces.
The other forces only work over a short range.
Electrostatics can be neutralized by mixing positive charges with negative so atoms
are essentially neutral. Gravity cannot be neutralized. There are no positive and
negative aspects of gravity. This means that given enough mass, gravity can win over
the other forces despite being the weakest force of all.
WHAT HAPPENS TO THE NEWLY CREATED HEAVY ELEMENTS?
Every time a supernova explodes or a mass loss star loses mass. The interstellar
medium (ISM) is enriched a little with heavy elements. Elements like carbon,
oxygen, magnesium neon iron calcium sodium silicon and many more. Subsequent
stars which condense out of this enriched material will themselves be enriched with
heavy elements.
Those who remember their chemistry will remember the period table of the elements.
Astronomers have a much simplified version of this; it has only three members,
Hydrogen, Helium, and Metals. All elements heavier than Helium are called metals.
Carbon is a metal, Oxygen is a metal and so on. When we looked at the composition
of the sun in Lecture 1 it was by weight 74% hydrogen, 25% helium and 1%
everything else. The “everything else” is the metallicity of the sun.
This means
that the stuff which condensed and formed the sun had already been through stars in a
previous existence. Our sun is made of secondhand material. In fact all the stars
we have looked at so far are composed of stuff which has already been cooked in the
hot furnace of stars. If we go into the subject of cosmology we find the current belief
10
is the universe was born about 13.7 billion years ago in a violent explosion out of
nothing. The only elements made were hydrogen and helium (with some traces of
Lithium). No other elements were created in the big bang. All others have been
created in stars. We are basically made of hydrocarbons, vast numbers of various
organic molecules basically carbon and hydrogen with some other important
elements. Every molecule of oxygen you are breathing, every molecule of nitrogen
that stops us having oxygen poisoning, every atom of iron in our blood cells, carbon
in our various cells, calcium in our bones, and so on, was made in a star furnace
millions of years ago. Without the stars in the distant past we would not exist.
People sometimes wonder why the universe has to be so big. I don’t know about the
whole universe but it had to be able after the big bang to create galaxies which could
rotate and have giant clouds of hydrogen and helium which condensed into the first
stars which in turn produced heavy elements which were processed again and maybe
again being each time enriched. Eventually in the gas clouds around stars the heavy
elements coalesced into solid bodies called planets, and on at least one of these life
evolved.
CAN WE SEE SOME EARLIER STARS WITH NO HEAVY ELEMENTS?
Almost but not quite. We have looked at a few open clusters.
These are characterized by five things
1.
They are all around the plane of the Milky Way.
2.
They have at most a few hundred stars
3.
Their spectra show they have a high metallicity (1% to 3%)
4.
They have a loose open structure.
5.
They are relatively young.
11
There is another type of cluster I have not yet shown you.
Globular clusters are characterized by
1.
They cluster as a spherical halo around the centre of the galaxy
not in the plane of the Milky Way
2.
They have about a million stars
3.
Their spectra show very little metallicity.
4.
They have tight compact spherical structure
5.
They are old.
The American Observer Walter Baade discovered there were two distinct
populations of stars. What he called Pop 1 were the young stars in the open clusters,
in the spiral arms of the galaxies, all the stars we have looked at in this course are Pop
1 stars. The sun is a pop 1 star.
Pop 2 stars have much lower metallicity. They had less of the heavier elements.
Pop 2 stars occur in globular clusters and in the core of a spiral galaxy.
Even Pop 2 stars contain some metallicity.
Are there Pop 3 stars which are truly
primordial and contain nothing but hydrogen and helium ?
So far none has been
found but if we look back in time to the beginning maybe we might see them.
12
A STELLAR LIFETIME PERSPECTIVE
It helps if we can place these various stages of the stars life into a perspective. For a
solar mass star we shall run the "tape" in "fast forward" at the rate 5 years/second.
This is about 6 million times faster than normal.
Gestation period: from the hydrogen/dust cloud ; 50 million years  4 months
Main Sequence life: 10 Billion years  60 years
Red Giant phase: about a Billion years  5 years
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