What
is a Star?
The
basic difference between a star and
a planet is that a star emits light
produced in its interior by nuclear
`burning', whereas a planet only shines
by reflected light.
There seem to be an enormous number
of stars that are visible to the naked-eye
at a really dark site but, in fact,
the eye can only see about two thousand
stars in the sky at one time. We can
see the unresolved light of many thousands
more when we look at the Milky Way,
and the light of the Andromeda galaxy,
which can be seen by the eye, comes
from thousands of millions of stars.
The Sun is our own special star yet,
as stars go, it is a very average
star. There are stars far brighter,
fainter, hotter and cooler than the
Sun. Basically, however, all the stars
we can see in the sky are objects
similar to the Sun.
The Sun (and any other star) is a
great ball of gas held together by
its own gravity. The force of gravity
is continually trying to compress
the Sun towards its centre. If there
were not some other force counteracting
gravity, the Sun would collapse. Outward
pressure is produced by the radiation
from nuclear energy generation in
the Sun's interior.
How
do stars originate?
Stars form from concentrations in
huge interstellar gas clouds. These
contract due to their own gravitational
pull. As the cloud gets smaller it
loses some of the energy stored in
it as gravitational potential energy.
This is turned into heat which, in
the early days of the embryonic star,
can easily escape and so the gas cloud
stays cool. As the cloud's density
rises, it becomes more and more difficult
for heat to escape and the temperature
at the centre of the cloud rises.
If the cloud is big enough, the temperature
rise is sufficient for nuclear fusion
reactions to begin. These generate
more heat and the `burning' of hydrogen
into helium begins, as in the Sun.
The object is then a main sequence
star.
The Early Evolution of a Star
In its early stages the embryonic
star is still surrounded by the remains
of the original gas cloud from which
it formed. By this stage the cloud
remnant takes the form of a disk around
the star. The radiation from the star
gradually dissipates this disk, possibly
leaving behind a system of smaller
objects, planets.
The
Main-Sequence
The star now settles down to a long
period of stability while the hydrogen
at its centre is converted into helium
with the release of an enormous amount
of energy. This stage is called the
main-sequence stage, a reference to
the classical Hertsprung-Russell diagram
(see Figure). Most stars lie in a
well defined band in the diagram and
the only parameter that determines
where in the band they lie is the
star's mass.
The more massive a star is the quicker
it `burns' up its hydrogen and hence
the brighter, bigger and hotter it
is. The rapid conversion of hydrogen
into helium also means that the hydrogen
gets used up at a greater rate in
the more massive stars than the smaller
ones. For a star like the Sun the
main-sequence stage lasts about 10,000,000,000
years whereas a star 10 times as massive
will be 10,000 times as bright but
will only last 100,000,000 years.
A star one tenth of the Sun's mass
will only be 1/10,000th of its brightness
but will last 1,000,000,000,000 years,
longer than the current age of the
Universe.
Post
Main-Sequence Evolution
Stars do not all evolve in the same
way. Once again it is the star's mass
that determines how they change.
Small mass stars
Our knowledge of the evolution of
these stars is purely theoretical
because their main sequence stage
lasts longer than the present age
of the Universe, so none of the stars
in this mass range has evolved this
far! We believe that the evolution
will proceed as for the medium mass
stars except that the temperature
in the interior will never rise high
enough for helium `burning' to start.
The hydrogen will continue to `burn'
in a shell but will eventually be
all used up. The star will then just
get cooler and cooler ending up after
about 1,000,000,000,000 years as a
`black dwarf'.
Medium mass stars
Stars similar in mass to the Sun `burn'
hydrogen into helium in their centres
during the main-sequence phase but
eventually there is no hydrogen left
in the centre to provide the necessary
pressure to balance the inward pull
of gravity. The core of the star contracts
until it is hot enough for helium
to be converted into carbon. Hydrogen
continues to fuse into helium in a
shell around the core, but the outer
layers of the star have to expand.
This makes the star appear brighter
and cooler and it becomes a red giant.
During the red giant phase a star
often loses a lot of its outer layers
which are blown away by the radiation
coming from below. The star becomes
a planetary nebula (like M57, shown
above). Eventually the energy generation
will fizzle out and the star will
collapse to what is called a ` white
dwarf'.
The Hertzsprung-Russell diagram of
the nearest stars and the brightest
stars. The horizontal axis shows spectral
type and temperature from the hottest
stars on the left to the coolest on
the right. The vertical axis shows
the luminosity of the stars with those
1,000,000 times brighter than the
Sun at the top and those only 1/10,000th
of its brightness at the bottom. The
curved line marks the Main Sequence
- stars, including the Sun, which
are fusing hydrogen into helium. The
group at the top right, including
Betelgeuse and Aldebaran, are Red
Giants. The group at the bottom left,
including Sirius B, are White Dwarfs.
High mass stars
Hubble Space Telescope picture of
Eta Carina - a massive, highly evolved
star which is ejecting its outer atmosphere
in a series of violent outbursts.
(Image Credit: Jon Morse (Univ. of
Colorado) and NASA.)
There are very few stars with masses
greater than five times the mass of
the Sun but their evolution ends in
a very spectacular fashion. As was
said above, these stars go through
their evolutionary stages very quickly
compared to the Sun. Like medium mass
stars, they `burn' all the hydrogen
at their centres and continue with
a hydrogen `burning' shell and central
helium `burning'. They become brighter
and cooler on the outside and are
called red supergiants. Carbon `burning'
can develop at the star's centre and
a complex set of element `burning'
shells can develop towards the end
of the star's life. During this stage
many different chemical elements will
be produced in the star and the central
temperature will approach 100,000,000°K.
For all the elements up to iron, nuclear
fusion into heavier elements produces
energy and so yields a small contribution
to the balance inside the star between
gravity and radiation. Fusion of iron
into heavier elements, however, uses
energy rather than releasing it. Once
the centre of the star consists of
iron, no more energy can be generated,
and there is no longer a source of
pressure to counteract the crushing
pull of gravity. The star's core then
starts to contract rapidly, collapsing
on a timescale of less than one second.
The protons and electrons in the core
are crushed together to form neutrons,
releasing a flood of neutrinos, which
carry away most of the energy from
the explosion.
The core collapse in the dying star
releases a vast amount of gravitational
potential energy, sufficient to blow
away all the outer parts of the star
in a violent explosion, and the star
becomes a supernova. The light of
this one star is then as bright as
that from all the other 100,000,000,000
stars in the galaxy. During this explosive
phase all the elements with atomic
weights greater than iron are formed
and, together with the rest of the
outer regions of the star are blown
out into interstellar space. The central
core of neutrons is left as a neutron
star which could be a pulsar.
What is remarkable about this is that
the first stars were composed almost
entirely of hydrogen and helium and
there was no oxygen, nitrogen, iron,
or any of the other elements that
are necessary for life. These were
all produced inside massive stars
and were all spread throughout space
by such supernovae events. We are
made up of material that has been
processed at least once, and probably
several times, inside stars.
