Introduction
Where
do stars come from ? What happens to them during their lifetimes, and
how do they die ? These are questions which have concerned astronomers
for many years.
Human lifetimes are
over in the blink of an eye when compared to the ages of the stars,
which can live for many billions of years. The problem of determining
the life history of stars by looking through our telescopes is like
that of an imaginary alien, who comes to Earth for a day and takes
photographs of people, to find out how humans develop. The alien will
have photographs of small people and large people, as well as
photographs showing people of different skin colour and so on. Our
imaginary visitor must piece together these pictures to tell the story
of human evolution. Do the large people turn into small people, or is
it the other way around? Do the people with dark skin eventually change
colour? How long does it take for them to develop? The problem is very
complicated. In the same way, astronomers must solve the problem of
fitting together their observations into a sequence which correctly
tells the story of a star's life. Many theories have been developed to
explain the stars which we see through our telescopes; these theories
have been tested, modified and often completely rewritten. Today we
have a better idea of a star's life story - but many questions remain
unanswered. This page sets out to tell the story of stars, from birth
to death, as we now believe it occurs.
The birth of a star
Stars
are created out of nebulae - vast clouds of gas in space. The gas
(mainly hydrogen) may continue to exist in a cloud for millions of
years, but if it is disturbed (by colliding with other clouds, or from
the blast from a nearby supernova explosion for example) the rotating
cloud may begin to collapse in on itself. As the centre of the cloud
becomes more dense, the collapse accelerates due to the increasing
gravitational attraction of the gas; the central region collapses
faster than the outer areas, and so the outlying gas is left rotating
about the centre. obscuring our view of what occurs there. If the cloud
is very large, it may fragment into smaller pockets of gas, each pocket
also collapsing. The collapsing clouds mark the beginning of the
formation of a star.
As the collapse
continues, the gas in each cloud begins to warm up; slowly at first,
but steadily until the centre begins to glow; dim at first but becoming
brighter. Eventually the central temperature is so high that nuclear
reactions begin to occur, and the object evolves from what astronomers
call a protostar
into a true star. The star is still orbited by the remaining gas and
dust from the original interstellar cloud. This material can then begin
to form planets; but some is driven away from the region by a strong
"wind" of particles from the star. We call such stars T Tauri
stars, named after the first star of this type to be observed by
astronomers. By blowing away the surplus gas and dust, the young star
is no longer shrouded from our view, and now astronomers can begin to
study the star directly.
Because stars can form out
of very large interstellar clouds which may fragment into many smaller
clouds, we often see stars forming in groups called clusters. One such cluster is the Pleiades.
An
interesting feature of this image is the haze of nebulosity surrounding
each of the young, hot blue stars. This haze is a remnant of the gas
cloud from which the stars formed, now made visible by reflecting
starlight.
How long does the process of formation take ? The length of time depends on the star's mass
- i.e. how much material it contains. For small low mass stars this
phase can take billions of years, but for much more massive stars it
can be over in as little as several hundred thousand years.
More material means shorter life
Now
that the star has blown away its dusty shroud and started to produce
energy by the process of nuclear fusion, it settles into the most
stable part of its life, converting hydrogen gas into helium.
Astronomers call this period the main sequence. However, the
way the star evolves depends on how massive the star is (i.e. how much
material it contains). Since the life stories are so different, we will
look at three examples which together cover the possible evolution
sequences. These examples cover the development of
(1) Stars similar to our own Sun.
(2) Stars several times more massive than the Sun.
(3) Very massive stars.
The evolution of a Sun-like star
When
a Sun-like star settles on the main sequence, it is turning hydrogen
into helium; about three quarters of its material is hydrogen, the
remainder being helium and very small quantities of other, heavier
elements. The star will remain in this phase for around 10 billion
years (our own Sun is roughly half way through this period). As more
and more of the heavier helium is produced in the core of the star, the
central regions begin to contract, and the temperature in the centre of
the star increases. The effect of this is to cause the star to increase
in brightness or luminosity. Eventually the centre of the star
is made entirely of helium, and the nuclear reactions which give the
star energy by converting four atoms of hydrogen into one of helium
occur in a shell surrounding the helium core. As more helium is made,
the central helium region gets larger and the shell moves closer and
closer to the surface of the star. The energy given off by this shell
pushes the very outer layers of the star outward, and these layers
cool. The star expands and becomes a Red Giant. At this stage the star can be hundreds of times larger than it was when it first entered the main sequence.
During this process, the helium core has beome more and more dense as
material is added to it, and so the temperature has increased.
Eventually the temperature becomes so great that the star can begin new
reactions, turning helium into carbon. The shell of hydrogen which
pushed the outer atmosphere of the star away gradually becomes weaker
and the star shrinks slightly - it is no longer a red giant. The star
continues to obtain its energy by turning helium into carbon, and in
the same way that hydrogen burning produced a helium core and a
hydrogen-burning shell, this helium burning produces a carbon core and
a helium burning shell; once again the star becomes a red giant.
However, this time the situation is different, because the temperature
in the very dense central carbon region never becomes great enough to
begin a new phase of nuclear reactions. Around one million years after
the star becomes a red giant for the second time, nuclear reactions
finally end in the core. Only the shells of hydrogen and helium burning
closer to the surface now produce energy for the star.
Over
a period of thousands of years, the star's central region shrinks and
heats up, blowing the outer regions off. Astronomers can see the
effects of this process; many objects are seen which are made up of a
very small, dense central star surrounded by a shell of gas which
appears to be expanding. These objects are called planetary nebulae, because the round shape of the gas cloud can look like a planet as seen through a telescope.
The
image shown here (courtesy of the Mount Palomar Observatory) shows the
"Ring Nebula" found in the constellation of Lyra. The central star
responsible for the surrounding ring of gas is clearly visible. This is
one of the most spectacular planetary nebulae to be seen in the night
sky. When the outer layers of the star's atmosphere are blown away to
form the nebula, the object seen at the centre of the gas cloud is the
core of the original star. It is still very hot - perhaps as high as
100,000K. But the material gradually cools and contracts, to become a
tiny dim object called a white dwarf, which will, over a period of billions of years, cool to become a black dwarf.
This, then, is the fate of our Sun and the stars like it - ending its
life as a dense body not much larger than the Earth. But before the sun
reaches this stage, the Earth and all the inner planets will have been
consumed in the fiercely hot atmosphere of the red giant which our star
will first become.
The evolution of stars with several times the Sun's mass
In
general, the more massive the star, the more rapid its sequence of
evolutionary stages become. However, that is not the only difference
between "light" and "heavy" stars; more massive stars go through stages
which their less weighty counterparts do not.
The
more massive star will enter the main sequence with a higher
temperature (typically over 10,000K for a star of over 4 times the mass
of our Sun) and greater luminosity than the Sun like star. The central
core regions of these stars are also far hotter than in the previous
class, and this causes the star to "age" more rapidly since, although
the larger star has more fuel, it uses this fuel much quicker. For
example, a star with five times as much mass as the Sun will evolve
into a red giant one hundred times quicker ! The star completes its
hydrogen and helium burning stages in the same way as the Sun-sized
star described above. However, because the star is hotter, it can
burn carbon when the helium supply becomes scarce. At this point,
helium is burned around a carbon core, sending the star into a red
giant stage. Carbon is then burned, then neon, and each successive
element is burned to exhaustion. At each stage when the star switches
from burning one element to another, it again goes through the red
giant phase, before returning to the main sequence for another period.
However, when the star gets so far into this process that it develops a
core of iron, no further reactions are possible because iron cannot be
converted into the next element without huge amounts of energy being
consumed.
The death of stars more massive than the sun
So
far we have looked at the entire evolutionary path of stars like the
Sun, and we've looked at the way stars with several times the Sun's
mass expend their nuclear fuel. What next? In fact, the last section
applies not only to stars a few times more massive than the Sun, but to
the most massive stars in the galaxy. The next major difference in
behaviour comes when we consider how stars more massive than the Sun
end their lives.
The mass of a star at the end of
its nuclear burning phase determines what happens to it next. As we've
already seen, stars like the Sun will end their days as a white dwarf.
In fact, stars which are slightly more massive than the sun also reach
this point; the only difference is that since these stars managed to
produce elements other than helium and carbon, the resulting white
dwarf will be made of a different mixture of materials - but a white
dwarf it is, nonetheless.
If,
on the other hand, a star is so massive that even at the end of its
main sequence lifetime, it still has more than 1.4 times our Sun's
current mass, then the situation changes dramatically. In such cases,
the star suffers a supernova explosion. The cores of these
stars contain iron, which cannot be used to produce more energy. As
more and more material is added to the core, the density gets higher,
until it equals that found in white dwarfs.
Eventually
the supply of nuclear fuel runs out, and in a few seconds the star's
atmosphere falls rapidly towards the centre under the influence of the
powerful gravitational field. The central region is compressed to form
an incredibly dense object called a neutron star, and the
infalling atmosphere "bounces" off this object, and the star explodes;
for a brief moment in time, the star can outshine all the other stars
in the galaxy put together. The result of such a supernova explosion is
the creation of the neutron star, and a rapidly expanding cloud of gas
- once the stars atmosphere - called a supernova remnant. This
image (taken with the Anglo Australian Telescope) shows the supernova
which appeared in the Large Magellanic cloud (visible from the southern
hemisphere) in 1987. Comparison of the left hand image with the one on
the right showing the same star before the explosion, illustrates just
how powerful a supernova event is.
This
image shows an artist's impression of the death of a star in a
supernova explosion, seen from within the star's planetary system. The
planets in the foreground will be destroyed by the immense amount of
energy from the event - but there can be no life on the planets to
witness the event, since the surfaces of these worlds were baked as the
star passed through its red giant phase. (Courtesy of SEDS)
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Star Trek style sick bay featured on BBC’s The One Show
