THE STARS
Gas shell around Nova
Cygni 1992. A Hubble Space
Telescope (HST) image of a
rapidly ballooning bubble of
gas blasted off a star. The
shell surrounds Nova Cygni
1992, which erupted on
19 February 1992, and the
image was taken on 31 May
1993, 467 days after the
event. The shell is so young
that it still contains a record
of the initial conditions of
the explosion.Nova GK Persei 1901.
This nova rose briefly to
magnitude 0.2, and
subsequently declined to its
pre-outburst magnitude 13.
This image, taken with the
WIYN telescope in 1994,
shows the expanding
nebulosity round the old
nova.Light curve for a nova.
HR Delphini observed bythe author from discovery
in 1967 to 1974.4.0
5.0
6.0
7.0
8.0
9.0
10.0
11.0
July 1967
TimeMagnitudeJuly 1968 July 1969 July 1970 July 1971 July 1972 July 1973 July 1974naked-eye object for several months; it is now an eclipsing
binary system with a period of only 4 hours 39 minutes.
HR Delphini of 1967 was even slower, and took years to
revert to its original magnitude of about 12. On the other
hand V1500 Cygni of 1975 rocketed up to prominence
in only a few hours, and had faded below naked-eye
visibility in a few nights; it is now excessively faint.
A few stars have been known to show more than one
outburst; these are known as recurrent novae. Thus the
‘Blaze Star’ T Coronae Borealis, in the Northern Crown
(Map 4) is usually of about the tenth magnitude, but flared
up to the second in 1866, and again brightened in 1946.
The interval between the explosions was 80 years, and
astronomers will be keeping a careful watch on it around
2026 to see if it will provide a repeat performance.
There is a definite link between novae and cataclysmic
variables of the SS Cygni or U Geminorum type which are
often termed dwarf novae. The exceptionally luminous
and unstable P Cygni, in the Swan (Map 8) flared up to
the third magnitude in 1600, and was once classed as a
nova, but now seems to be a special type of variable star;
for many years now its magnitude has hovered around 5,
though it may well brighten again at any moment.
Novae show interesting changes in their spectra, and it
is very important to start observing them as soon as possi-
ble after the start of the outburst. This is where amateurs
come into their own, because they know the night sky far
better than most professionals, and have a fine record of
nova discovery. For example, an English schoolmaster,
George Alcock, discovered five novae (as well
as five comets); he used powerful binoculars, and could
identify some 30,000 stars on sight, so that he could recog-
nize a newcomer at once.
Telescopic novae are not uncommon. Most of them
appear in or near the Milky Way, and enthusiastic nova-
hunters concentrate upon these regions of the sky. One
never knows when a brilliant new star may burst forth
without the slightest warning.152-191 Atl of Univ Phil'05 7/6/05 1:24 pm Page 181