THE STARS
What happens next depends mainly upon the star’s
initial mass. If it is less than one-tenth that of the Sun, the
core will never become hot enough for nuclear reactions to
begin, and the star will simply glow feebly for a very long
period before losing its energy.
If the mass is between 0.1 and 1.4 times that of the
Sun, the story is very different. The star goes on shrinking,
and fluctuates irregularly; it also sends out a strong stellar
wind, and eventually blows away its original cocoon of
dust. This is the so-called T Tauri stage, which in the case
of the Sun may have lasted for around 30 million years.
When the core temperature soars to 10 million degrees C,
nuclear reactions are triggered; the hydrogen-into-
helium process begins (known, misleadingly, as ‘hydrogen
burning’), and the star joins the Main Sequence. Hydrogen
burning will last for around 10,000 million years, but at
last the supply of hydrogen ‘fuel’ must run low, and the
star is forced to change its structure. The core temperature
becomes so high that helium starts to ‘burn’, producing
carbon; around this active core there is a shell where
hydrogen is still producing energy. The star becomes
unstable, and the outer layers swell out, cooling as they do
so. The star becomes a red giant.
This is as far as the nuclear process can go, because
the temperature does not increase sufficiently to trigger
carbon-burning. The star’s outer layers are thrown off, and
for a cosmically brief period – no more than about
100,000 years – we have the phenomenon of what is
termed a planetary nebula. Finally, when the outer layers
have dissipated in space, we are left with a white dwarf
star; this is simply the original core, but now ‘degenerate’,so that the atoms are broken up and packed closely together
with almost no waste of space. The density is amazingly
high. If a spoonful of white dwarf material could be
brought to Earth, it would weigh as much as a steam-
roller. The best-known white dwarf is the dim companion
of Sirius, which is smaller than a planet such as Uranus or
Neptune – but is as massive as the Sun.
Bankrupt though it is, a white dwarf still has a high
surface temperature when it is first formed; up to 100,000
degrees C in some cases, and it continues to radiate.
Gradually it fades, and must end up as a cold, dead black
dwarf; but at the moment no white dwarf with a surface
temperature of below 3000 degrees C has been found, and
it may be that the universe is not yet old enough for any
black dwarfs to have been formed.
With stars of greater initial mass, everything happens
at an accelerated rate. The core temperatures become so
high that new reactions occur, producing heavier ele-
ments. Finally the core is made up principally of iron,
which cannot ‘burn’ in the same way. There is a sudden
collapse, followed by an explosion during which the star
blows most of its material away in what is called a super-
nova outburst, leaving only a very small, super-dense core
made up of neutrons – so dense that a thousand million
tonnes of it could be crammed into an eggcup. If the mass
is greater still, the star cannot even explode as a superno-
va; it will go on shrinking until it is pulling so powerfully
that not even light can escape from it. It has produced a
black hole.
We have learned a great deal about stellar evolution
during the past decades, but many uncertainties remain.▲Stellar evolution. Star
formation begins with a
collapsing cloud of nebular
material (1). In the middle of
the cloud, the temperature
begins to rise and stars begin
to form (2). As they begin to
shine, the gas associated
with them is blown away
and a star cluster is produced
(3). This cluster is gradually
disrupted and becomes a
loose stellar association.
The evolution of a star
depends on its mass. A star
of solar type joins the main
sequence (4) and remains on
it for a very long period.
When its hydrogen ‘fuel’
begins to run low, it expands
(5) and becomes a red giant
(6). Eventually the outer
layers are lost, and the result
is the formation of a
planetary nebula (7). The
‘shell’ of gas expands and is
finally dissipated, leaving
the core of the old star as a
white dwarf (8). The white
dwarf continues to shine
feebly for an immense period
before losing the last of its
heat and becoming a cold,
dead black dwarf. With a
more massive star the
sequence of events is much
more rapid. After its main
sequence period (9) the star
becomes a red supergiant
(10) and may explode as a
supernova (11). It may end
as a neutron star (12) or
pulsar (13), although if its
mass is even greater it may
produce a black hole (14).23 4 5 6 7 891011121314E152- 191 UNIVERSE UK 2003mb 7/4/03 5:48 pm Page 175