THE STARS
is blown away, leaving the neutron-star so dense that at
least 2,500 million tonnes of its material could be packed
inside a matchbox. The peak luminosity may be around
5000 million times that of the Sun.
A neutron star is an amazing object. Its diameter may
be no more than a few kilometres, but its mass will be
equal to that of the Sun. The gravitational pull is very
strong (objects would weigh a hundred thousand million
times more on the surface of a neutron star than they would
on the surface of the Earth), and so is the magnetic field.
The rate of rotation is very fast, and beams of radio radia-
tion come out from the magnetic poles, which are not coin-
cident with the poles of rotation. If a radio beam sweeps
across the Earth, we receive a pulse of radio emission; the
effect may be likened to the beam of a rotating lighthouse
illuminating an onlooker on the seashore. It is this which
has led to some neutron stars being known as pulsars.
Many supernovae have been seen in outer galaxies, but
in our own Galaxy only four have been seen during the last
thousand years; all these became brilliant enough to be
seen with the naked eye in broad daylight. The brightest of
all was seen in 1006 in the constellation of Lupus, the Wolf
(Map 20); it is not well documented, but appears to have
been as bright as the quarter-moon. We know more about
the supernova of 1054, in Taurus (Map 17), because it has
left the gas-patch known as the Crab Nebula, which con-
tains a pulsar spinning round 30 times a second; this is oneof the few pulsars to have been optically identified with a
very faint, flashing object. The Crab is 6000 light-years
away, so that the outburst actually occurred before there
were any astronomers capable of observing it scientifically.
The supernova of 1572, in Cassiopeia (Map 3), is
known as ‘Tycho’s Star’, because it was carefully studied
by the great Danish astronomer. The distance is 6000
light-years; there is no pulsar, but radio emissions can be
picked up from the wisps of gas which have been left. This
is also true of the 1604 star, observed by Johannes Kepler.
The radio source Cassiopeia A seems to be the remnant of
a supernova which flared up in the late 17th century, but
was not definitely observed because it was obscured by
interstellar material near the plane of the Galaxy.
There have been two particularly notable supernovae
since then. In 1885 a new star was seen in the Great
Spiral in Andromeda (Map 12), which is over two million
light-years away; it reached the fringe of naked-eye visi-
bility, and is remembered as S Andromedae. Unfortunately
nobody appreciated its true nature, because at that time it
was not even generally believed that the so-called ‘starry
nebulae’ were external systems.
Then, in 1987, came a flare-up in the Large Magellanic
Cloud, which is the nearest of the major galaxies and is a
mere 169,000 light-years away. The maximum magnitude
was 2.3, so that the supernova – 1987A – was a con-
spicuous naked-eye object for some weeks. Surprisingly,
the progenitor star – Sanduleak 69º202 – was not a red
supergiant, but a blue one, and the peak luminosity was
only 250 million times that of the Sun, which by super-
nova standards is low. It seems that the progenitor, about
20 million years old and 20 times as massive as the Sun,
was previously a red supergiant; it shed its outer layers and
became blue not long before the outburst happened. The
ejected material spread out at 10,000 kilometres (over
6000 miles) per second, subsequently lighting up clouds of
material lying between the supernova and ourselves. As
yet no pulsar has been detected, but if one exists – as is
very likely – it should become evident when the main
debris has cleared. European astronomers lament the fact
that the supernova was so far south in the sky, but at least
it has been available to the Hubble Space Telescope,
which has taken remarkable pictures of it.
We cannot tell when the next supernova will appear in
our Galaxy; it may be tomorrow, or it may not be for many
centuries. Astronomers hope that it will be soon, but at
least we have learned a great deal from Supernova 1987A
in the Large Magellanic Cloud.Supernova in Messier 81.
M81 is a spiral galaxy
in Ursa Major, 8.5 million
light-years away (not so
very far beyond the Local
Group); it is a very easy
telescopic object. In 1993
a supernova flared up in it.
This picture was taken by
R. W. Arbour when the
supernova (arrowed) was
at maximum brightness. Supernova 1987A in the
Large Cloud of Magellan.
This photograph was taken
from South Africa when
the supernova (lower right)
was near its maximum
brightness.▲ Ring round a supernova.
These images from the
Hubble Space Telescope
show a light-year wide ring
of glowing gas round the
Supernova 1987A, in the
Large Cloud of Magellan.
The HST spectrograph
viewed the entire ring
system, and produced a
detailed image of the ring
in each of its constituent
colours. Each colour
represents light from a
specific element: oxygen
(single green ring), nitrogen
and hydrogen (triple orange
rings), and sulphur (double
red rings). The ring formed
30,000 years before the
star exploded, and so
is a fossil record of the
final stages of the star’s
existence. The light from
the supernova heated the
gas in the ring so that it
now glows at temperatures
from 5000 to 25,000°C.E152-191 UNIVERSE UK 2003mb 7/4/03 5:49 pm Page 183