快猫短视频

Red giants, white dwarfs, black holes 鈥� there seems to be a bewildering
variety of stars in the sky. Yet astronomers have now explained all known
types of star in terms of a single theory. It reveals not only how a star
shines, but also how stars are born, live and die

WHEN we look up at the stars in the sky, we get the impression that they are
changeless. Certainly, the sky we see today is not very different from the
view that our ancestors had 5000 years ago, when they first connected up the
stars into constellations: Ursa Major, the great bear; Taurus, the bull;
Cancer, the crab, and so on.

But the stars do change. Rather like human beings, they are born; they live;
and they die. A star鈥檚 lifetime is very long compared with ours 鈥� many
millions of years 鈥� so we only rarely see individual stars changing before our
eyes.

Astronomers can, however, work out the story of a star鈥檚 life by picking out
stars at different stages of their lives. An analogy can give an idea of the
method. Imagine a Martian who lands on Earth in a crowded shopping centre. It
stays for only a few minutes, so that nobody ages in front of its eyes. But it
sees human beings at many stages of development: babies, old people, children,
the middle aged, adolescents, pregnant women. From these data, the Martian
could work out how humans are born, live and eventually come to the end of
their lives.

Similarly, astronomers can pick out stars that are being born, stars in the
prime of life and stars that are dying. How do they know what stage a star has
reached in its life? The answer comes from a detailed theory of star life,
which is based on the laws of physics. This theory of 鈥渟tellar evolution鈥� is
one of the great achievements of science in the 20th century.

From the infinity of space

A star is born

STARS are born from the tenuous gas which fills the whole of space. This gas
is composed mainly of hydrogen atoms, with a sprinkling of helium. In some
places, the gas clumps together in rather more dense interstellar gas clouds.

According to gravitational theory, the gas cloud鈥檚 own gravity makes it
attract itself. This should pull it in on itself, compressing the cloud to
ever higher densities. The centre of the cloud should be the most compressed
region. Here, astronomers expect some of the gas to condense into individual
鈥渂lobs鈥�, each held together by its own gravity. When a gas is compressed, it
becomes hotter. So, the temperature at the centre of each blob rises to 10
million degrees Celsius 鈥� hot enough to start nuclear reactions. These
reactions turn hydrogen to helium and create vast amounts of energy. As a
result, the blob begins to shine: a star is born.

Unfortunately, ordinary telescopes cannot actually show us stars being born in
the interstellar gas clouds. The problem is particles of dust. Mixed in with
the gas in space are small particles of dust 鈥� similar in size to the
particles in cigarette smoke. In the denser clouds of gas, where the dust is
more concentrated, dust particles absorb light passing through the cloud. As a
result, we can see the clouds as dark silhouettes against a background of
distant stars. The most famous dark cloud is The Coalsack, which is visible to
the naked eye from the southern hemisphere. The dust also prevents us from
seeing what is happening inside the dark cloud 鈥� the region where stars are
being born.

In recent years, astronomers have solved this problem. They have built
telescopes that pick up infrared radiation instead of light. The dust
particles in space do not absorb infrared radiation, so the infrared
telescopes can pick up radiation coming from within the dense clouds, and can
鈥渟ee鈥� the stars being born there. The most successful infrared telescope was
on board a satellite which was launched into orbit in 1983. The Infrared
Astronomical Satellite (IRAS) found thousands of young stars hidden deep
within the interstellar clouds.

Astronomers have found that the blobs of gas collapse in rather an odd way.
The central parts of a blob fall inwards rather quickly, while the outer parts
follow at a more leisurely rate. The blobs are also rotating, quite slowly,
but as the outer parts fall inwards, they begin to spin more rapidly 鈥� just as
ice-skaters spin more quickly when they draw in their arms. As a result, the
infalling gas forms a disc around the newly born star at the centre, where the
gas is compressed enough for nuclear reactions to start. Within this disc, the
gas and the dust that is mixed in it eventually form into a set of planets
orbiting the new star.

Once the star is shining, it produces a powerful 鈥渨ind鈥� of hot gas that forces
its way outwards in opposite directions, above and below the disc. This wind
drives away most of the original gas cloud that hides the star from view. Now
we can see the young stars with an ordinary telescope. They light up the final
tatters of gas from the original cloud, making it glow as a bright nebula.

Nebulae, each surrounding a 鈥渘ursery鈥� of young stars, form some of the most
beautiful sights in the sky. Most famous is the Orion Nebula, which you can
spot from Europe with the naked eye during the winter months, as a misty patch
below the stars of the belt in the constellation of the mighty hunter.

In the prime of life:

The main sequence

WHEN a star is born, it is a ball of hot gases, composed mainly of hydrogen.
It shines because nuclear reactions at its centre are turning hydrogen into
helium. To this extent, all new-born stars are the same. The main thing that
marks out one star from another is its mass 鈥� the amount of matter it
contains. The mass of a star is fixed at its birth and it determines both a
star鈥檚 lifetime and its ultimate fate.

Our Sun is a very typical star, currently in the prime of its life, and so it
makes a convenient yardstick for measuring other stars. Instead of saying a
star weighs 20 000 million million million million tonnes, for example, we can
say it is as massive as 10 suns. On this scale, newborn stars cover a wide
range, from as light as 0.07 suns to as heavy as 100 suns.

The nuclear reactions run fastest in the heaviest stars, because their centres
are hottest and most compressed. So the heavier stars are the brighter stars,
with hotter surfaces. We can arrange these stars in a definite sequence,
called the main sequence of star types. At one end are the lightweight stars,
which are much dimmer than the Sun and with a surface temperature of only 3000掳C. The Sun
is in the middle, with a temperature of 6000掳C. At the top end of the
range are heavyweight stars shining as brightly as 100 000 suns, and with a
surface temperature of 30 000掳C or more.

A star spends most of its life turning hydrogen into helium, so the period of
the main sequence period is really the prime of its life. The length of its
life depends very critically on how heavy the star is. A heavyweight star uses
up its nuclear fuel so rapidly that it soon exhausts its supplies of hydrogen.
A lightweight star, even though it has a smaller supply of fuel to start with,
uses it much more gradually, and so lasts for a much longer time.

A star鈥檚 lifetime is too long for us to appreciate easily, so again we can use
the Sun as a comparison. According to theory, the Sun will spend 10 000
million years altogether as a main sequence star. The heaviest stars survive
for only one-thousandth of this time. The very lightweight stars can last for
a hundred times longer than the Sun.

When a star like the Sun dies, it doesn鈥檛 just fizzle out. Instead, it
experiences a kind of 鈥渕iddle age spread鈥�, and expands to become a red giant,
about a hundred times its previous size.

The reason for this behaviour lies in the star鈥檚 very centre, its core.
Reactions here have turned the original hydrogen into helium. Like the ashes
in a fire, this central region produces no energy. Nuclear reactions are still
going on in a thin 鈥渟hell鈥� around the helium core, and calculations show that
these reactions produce more energy than before. As this extra energy pushes
up through the star, it makes the outer parts of the star swell up. As its
outer layers cool down, the star shines a red colour: hence the name 鈥渞ed
giant鈥�. If we could cut a section through a red giant, we would find that it
has a very small and dense core, and a huge outer region of very thin gas 鈥�
much more rarified than the Earth鈥檚 atmosphere.

Compared with the main sequence stars, red giant stars are not very common;
however, because they are very large they appear bright and stand out
conspicuously in our skies. The most famous is Betelgeuse, in the constellation Orion; another is Antares, in Scorpio: the
Greek name means 鈥渢he rival of Mars鈥� because of its brilliant red colour.

A red giant finds it difficult to hold onto its huge outer regions. The star
becomes unstable and eventually the outer gas drifts off into space. Before
completely disappearing, the gas forms a bubble around the dying star 鈥� the
effect is like a glowing smoke ring in space. Astronomers call these bubbles
鈥減lanetary nebulae鈥�, because they look rather like a planet when you observe
them with a small telescope.

After the star鈥檚 outer regions have disappeared, we can see the tiny, very hot
core. It is only one-hundredth the diameter of the Sun 鈥� no larger than planet
Earth 鈥� and is so hot that it shines white hot. Astronomers call this a 鈥渨hite
dwarf鈥�. Because white dwarfs are very small, they appear as a rather dim
object in the sky and thus are difficult to find.

Astronomers have been highly successful in tracking down white dwarfs when
they are a companion to another star. The first to be discovered was the
companion to Sirius, the brightest star in the sky. Because Sirius is known as
the Dog Star, its small companion is often called 鈥渢he Pup鈥�.

A white dwarf is no longer producing any energy. It shines merely because it
began life so hot. As time passes, it gradually cools down, fading through
yellow, orange and red, until 鈥� like a dying ember in a fire 鈥� it fades from
sight altogether.

Supernova!

An explosion in space

A HEAVYWEIGHT star has a much more dramatic end 鈥� as astronomers in the
southern hemisphere saw in 1987. A star previously visible only through a
powerful telescope suddenly exploded, and shone so brightly that it was easily
visible to the naked eye. The star had died as a supernova.

The build-up to a supernova starts after a heavy star has lived out its main
sequence of life. Now that the star has used up its central supplies of
hydrogen, it expands to become a red giant, with a central compact core of
helium. But this is not the end of the story. In the middle of such a massive
star, the pressure and temperature keep on rising until helium atoms begin to
fuse into a heavier element, carbon. This reaction produces extra energy to
keep the star shining. Eventually, the increasing temperature and pressure
force the carbon to change to even heavier elements, such as neon, silicon, and iron.

At this point, the star鈥檚 core is like an onion, with concentric layers (from
the inside out) of iron, silicon, neon, carbon, helium and hydrogen. But the
process cannot carry on indefinitely. If you try to fuse together iron nuclei,
the reaction does not produce energy 鈥� in fact, it takes in energy. So the
star鈥檚 centre is now unstable. In just a few seconds, it collapses entirely. A
wave of energy from the collapsing core blows the star apart, in the explosion
of a supernova.

Neutron stars

and black holes

BUT what happens to the collapsing core of a supernova? In the 1930s, two
astronomers working in the US, Fritz Zwicky and Walter Baade, suggested that
it shrank into a small ball, smaller than a white dwarf, made entirely of the
subatomic particles called neutrons.

For decades, this was just a theoretical idea 鈥� until one autumn day in 1967.
Two radio astronomers at Cambridge, Tony Hewish and Jocelyn Bell, picked up
regular signals coming from the sky. They dismissed the idea that it might be
鈥渓ittle green men鈥� trying to contact the Earth, and realised that instead they
had foundsome kind of natural lighthouse in space.

The lantern of a lighthouse sends out beams of light that seem to flash as the
lantern rotates. The signals picked up at Cambridge must have come from a
cosmic lighthouse that was emitting beams of radio waves, and spinning about
once a second. From our present knowledge, only one kind of star was small
enough to spin so rapidly 鈥� a neutron star.

Radio astronomers have now located hundreds of spinning neutron stars (also
known as pulsars, because of their regular 鈥減ulses鈥� of radio waves). One of
them lies at the centre of the Crab Nebula, the twisted gas cloud thrown out
by a supernova that exploded 900 years ago.

A neutron star is only about 25 kilometres across, and the material inside it
is so tightly packed that a pinhead of matter from a neutron star
would have a mass of a million tonnes. Its gravity is so strong that an
astronaut who tried to land on its surface would be crushed and spread out to
form a layer only one atom thick!

White dwarfs and neutron stars may seem very bizarre, but theory predicts an
even odder type of 鈥渟tar corpse鈥�: a black hole. If the collapsing core of a
supernova is too massive (heavier than three suns), it cannot end up as a
neutron star. Its own gravity is so powerful that the core continues to
shrink, until it becomes a mathematical point, with no size at all and an
infinite density. Surrounding this point is a region a few kilometres across
where gravity is so strong that nothing can escape 鈥� not even light. This
region is a black hole. It is 鈥渂lack鈥� because it does not let light escape;
and even if you tried to illuminate it, the hole would swallow up the beam
from your torch. It is a 鈥渉ole鈥�, because anything you throw into it can never
emerge again, however powerful the rocket engines you might strap to it.

As with neutron stars, astronomers first predicted black holes in the 1930s.
Only in the past few years have they found some evidence for them. In the
constellation Cygnus (the swan), there is a powerful source of X-rays, named
Cyg X-1. Astronomers have found a star at this point in the sky. The star
itself is quite ordinary, and cannot be producing the X-rays. But it is not on
its own. It is swinging around a companion star that is invisible in ordinary
telescopes. By observing the visible star carefully, astronomers found that
its invisible companion was exerting the gravitational pull of an object as
heavy as 10 suns. This is much too heavy to be a neutron star, and so the only
possibility is that it is a black hole.

Rebirth!

Cocktail of new elements

SUPERNOVAE do not represent just death and destruction. The blast from a
supernova sweeps up the gases in space, compressing them into dense clouds.
Here gravity can get to work, making the gas clouds shrink, and condense into
a new star. So a star is like a phoenix: the death of a star as a supernova
can trigger off the birth of a new generation of stars.

When a star dies 鈥� as a planetary nebula or a supernova 鈥� it seeds space with
the new elements that it has created during its lifetime or in its death
throes 鈥� elements such as carbon, iron, gold and even uranium and other
radioactive elements. So the newly born stars will contain slightly less
hydrogen, and rather more of these exotic elements.

Astronomers now believe that when the Universe began, in the big bang, the
gases consisted almost entirely of hydrogen and helium. Dying stars have
formed all the other elements, including the silicon, oxygen and iron that
form the Earth, and the carbon and other elements in our bodies. So we owe our
very existence to the life and death of countless past generations of stars.

Classifying the stars

THE STARS in our skies have a bewildering range of properties: there are
giant stars and dwarf stars; bright stars and dim stars; hot stars and cool
stars. Astronomers make sense of the stars by plotting them on a graph. A
Danish astronomer, Ejnar Hertzsprung, and an American, Henry Norris Russell,
found the most useful kind of graph back in 1914, and astronomers still call
it a Hertzsprung Russell, or H-R, diagram.

On an H-R diagram, the vertical axis represents the luminosity or brilliance
of a star. The horizontal axis represents the star鈥檚 temperature: for
historical reasons, this is plotted with high temperatures to the left, and
low temperatures to the right.

When Hertzsprung and Russell plotted the positions of stars on their diagram,
they found that most of them occupy a narrow strip running from top left to
lower right, the 鈥渕ain sequence鈥�. We now know that these are stars that derive
their energy by turning hydrogen into helium.

Life and death of the Sun

OUR KNOWLEDGE of other stars gives us a means of predicting the fate of the
star that is most important to us: the Sun. A comparison with other stars with
the same mass tells us that the Sun is a main sequence star, shining because
it is turning hydrogen into helium at its centre.

The Sun was born about 5000 million years ago, and we predict that it has
enough hydrogen to carry on much as it is for another 5000 million years. Then
it will begin to swell.

As the Sun turns into a red giant, it will swallow up Mercury, and then Venus.
The bloated Sun will boil away the Earth鈥檚 oceans, destroying any life that
has not fled to another planetary system. Then the Sun will engulf the Earth
itself.

Eventually, the Sun鈥檚 outer layers will puff away as a beautiful planetary
nebula, leaving a white dwarf at the centre of what is left of the Solar
System, circled forlornly by the charred remnants of its remaining planets.

Further reading

100 Billion Suns, by Rudolf Kippenhahn (Weidenfeld & Nicolson), is an
excellent introduction to the birth, life and death of stars. Superstars, by
David Clark (Dent), provides a popular level introduction to supernova,
written before the bright supernova of 1987. 鈥淔irst light on starbirth鈥� and
鈥淪upernova: the cosmic bonfire鈥� are articles in 快猫短视频 that present the
latest results on the birth and death of stars (27 August, 1987, p 46; 5
November, 1987, p 52).