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.
The birth of a star
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.
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
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
The evolution of a Sun-like star
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.
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.
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.
The evolution of stars with several times the Sun's mass
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 death of stars more massive than the sun
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.
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)