Novae
O
ccasionally a bright star will flare up where no star
has been seen before. Naturally enough, this is known
as a ‘nova’, from the Latin for ‘new’, but the name is
misleading; a nova is not really new at all. What has
happened is that a formerly dim star has suffered an out-
burst and brightened up to many thousands of times its
normal state. Its glory does not last for long; in a few days,
weeks, or a few months at most it will fade back into its
previous obscurity.
It now seems certain that a nova is the result of an out-
burst in the white dwarf component of a binary system.
The other member of the pair is a normal star, which has
not yet evolved to the white dwarf condition, and is of
relatively low density. The white dwarf has a very power-
ful pull, and draws material away from its companion;
as time goes by, a ring or ‘accretion disk’ builds up around
the white dwarf. As more and more material arrives in the
accretion disk, the temperature rises. In the lower partof the disk mild nuclear reactions are going on, but are
‘blanketed’, so to speak, by the non-reacting material
above. This cannot last indefinitely; eventually the tem-
perature builds up to such an extent that there is a violent
nuclear explosion, and material is hurled outwards at
speeds of up to 1500 kilometres (over 900 miles) per
second. At the end of the outburst, the system reverts to its
original state. Though the outburst releases a tremendous
amount of energy – perhaps equivalent to a thousand
million million nuclear bombs – the white dwarf loses
only a tiny fraction of its mass.
Some novae may become very brilliant. GK Persei of
1901 reached magnitude 0.0; at its peak it must have been
200,000 times as luminous as the Sun, and it remained a
naked-eye object for four months. As it faded, it was seen
to be surrounded by nebulosity which gave every impres-
sion of expanding at a speed equal to that of light, though
in fact the material had been there all the time and was
merely being illuminated by the brilliance of the nova. It
was only later that the actual nebulosity associated with
the outburst became visible; the present magnitude of the
star is about 13 – the same value as it had been before
the explosion. In 1918, Nova Aquilae flared up abruptly,
and outshone every star apart from Sirius; it was discov-
ered on 8 June, and did not fade below the sixth magnitude
until the following March. Spectroscopic research showed
that it threw off shells of gas, and nebulosity became
visible; this gradually expanded and became fainter, finally
disappearing. At present, the old nova is of the 12th mag-
nitude, and seems to be smaller but denser than the Sun.
DQ Herculis 1934 was discovered by an amateur,
J. P. M. Prentice. It ranks as a ‘slow nova’, and was aATLAS OF THE UNIVERSE
BRIGHT NOVAE, 1600–1999
The following list includes all classical novae which have reached
magnitude 4.5 or brighter:Year Star Discoverer Max.
magnitude
1670 CK Vulpeculae Anthelm 3
1848 V841 Ophiuchi Hind 4
1876 Q Cygni Schmidt 3
1891 T Aurigae Anderson 4.2
1901 GK Persei Anderson 0.0
1912 DN Geminorum Enebo 3.3
1918 V603 Aquilae Bower 1.1
1920 V476 Cygni Denning 2.0
1925 RR Pictoris Watson 1.1
1934 DQ Herculis Prentice 1.2
1936 V630 Sagittarii Okabayasi 4.5
1939 BT Monocerotis Whipple and 4.3
Wachmann
1942 CP Puppis Dawson 0.4
1963 V533 Herculis Dalgren and Peltier 3.2
1967 HR Delphini Alcock 3.7
1970 FH Serpentis Honda 4.4
1975 V1500 Cygni Honda 1.8
1992 V1974 Cygni Collins 4.3
1993 V705 Cassiopeiae Kanatsu 5.4
1999 V382 Velorum Williams and Gilmore 2.5
1999 V1994 Aquilae Pereira 3.6Nova (HR) Delphini 1967,
discovered by the English
amateur George Alcock. It
reached naked-eye visibility.
This photograph was taken on
10 August 1967 by
Commander H. R. Hatfield.
The quadrilateral of Delphinusis shown; HR is the brightish
star near the top of the picture.
The maximum was unusually
prolonged, as the light-curve
shows; fading was gradual,
and by 2003 the magnitude
had returned to its pre-
outburst value of about 13.152-191 Atl of Univ Phil'05 7/6/05 1:23 pm Page 180