THE SUN
particles called neutrinos, which are difficult to detect
because they have no electrical charge. The Sun appeared
to emit far fewer neutrinos than predicted but the recent
discovery of their small but non-zero mass has resolved
the discrepancy.
If a neutrino scores a direct hit upon an atom of chlorine,
the chlorine may be changed into a form of radioactive
argon. Deep in Homestake Gold Mine in South Dakota,
Ray Davis and his colleagues filled a large tank with over
450,000 litres of cleaning fluid, which is rich in
chlorine; every few weeks they flushed out the tank to see
how much argon had been produced by neutrino hits. In
fact the numbers were strikingly less than they should
have been, and similar experiments elsewhere confirmed
this. (It was essential to install the tank deep below the
ground; otherwise the results would be affected by cosmic
ray particles which, unlike neutrinos, cannot penetrate far
below the Earth’s surface.) Japan’s Super-Kamiokande
detector, 1000 metres down in the Mozumi mine, uses
50,000 tons of pure water; it too detects fewer neutrinos
than had been expected.
Like all other stars, the Sun began its career by con-
densing out of interstellar material, and at first it was not
hot enough to shine. As it shrank, under the influence of▲ The Sun in the Galaxy. The
Sun lies well away from the
centre of the Galaxy; the
distance from the centre is
less than 30,000 light-years,
and the Sun lies near the
edge of one of the spiral
arms. This picture shows the
Milky Way in infra-red, as
imaged by the COBE satellite. Projecting the Sun. The
only safe way to view the
Sun is to project it through
a telescope on to a screen.
John Mason demonstrates!Homestake Mine, in South
Dakota, site of the world’s most
unusual ‘telescope’ – a large
tank of cleaning fluid
(tetrachloroethylene), rich in
chlorine to trap solar neutrinos.
The observed flux is only aboutone-third as great as predicted.
The same has been found by
investigators in Russia, using
100 tonnes of liquid scintillator
and 144 photodetectors in a
mine in the Donetsk Basin, and
at Kamiokande in Japan.The Super-Kamiokande
neutrino detector, Japan,
consists of an inner and an
outer volume which contain
32,000 tons and 18,000 tons of
pure water respectively. The
outer volume is shielded
against cosmic rays. The inner
has 11,200 photomultiplier
tubes which detect the pale
blue light known as Cerenkov
radiation emitted by particles
moving as fast as light in
water. The Super-Kamiokande
came into operation in 1995.gravity, it heated up, and when the core temperature had
risen to 10 million degrees nuclear reactions were trig-
gered off; hydrogen was converted into helium, and the
Sun began a long period of steady emission of energy. As
we have seen, it was not initially as luminous as it is now,
and the increase in power may have had disastrous results
for any life which may have appeared on Venus. But at the
moment the Sun changes very little; the fluctuations due to
its 11-year cycle are insignificant.
However, this will not last for ever. The real crisis will
come when the supply of available hydrogen begins to
become exhausted. The core will shrink and heat up as dif-
ferent types of reactions begin; the outer layers will expand
and cool. The Sun will become a red giant star, and will be
at least 100 times as luminous as it is at present, so that the
Earth and the other inner planets are certain to be
destroyed. Subsequently the Sun will throw off its outer
layers, and the core will collapse, so that the Sun becomes
a very small, incredibly dense star of the type known as a
white dwarf. Eventually all its light and heat will leave it,
and it will become a cold, dead globe – a black dwarf.
This may sound depressing, but the crisis lies so far
ahead that we need not concern ourselves with it. In our
own time, at least, there is no danger from the Sun.E152-191 UNIVERSE UK 2003mb 7/4/03 5:41 pm Page 155