http://www.skyandtelescope.com.au 49by Alan Plummer VARIABLE STARSThe dog and
the dwarf
S
irius, the ‘dog star’ in Canis
Major, is one of the most
underrated telescopic showpieces
of the sky, shining the purest brightest
white. When observing this issue’s
target variable star, however, it is a
luminous obstacle to be overcome.
The variable star in question, HL
Canis Majoris, is only 8 arcminutes
south of Sirius and some 800 light-
years further away. Because of this
proximity to its famous neighbour, the
HL CMa remained undiscovered until
1980 and even then was noticed first
at X-ray wavelengths by the Einstein
X-ray Observatory, after which it was
located visually on pre-existing survey
photographs. As well as being an X-ray
source, HL CMa turns out to be one
of the brightest and most active dwarf
novae in the sky, outbursting up to 11th
magnitude every 15 to 20 days.
Dwarf novae are strongly interacting
binary stars comprising a white dwarfprimary drawing mass from a lower-
mass secondary through an accretion
disk. The optical radiation originates
inthedisk;boththequiescent14th
magnitude (in HL CMa’s case) and
in outburst. As such systems go, HL
CMahasaratherhighaccretionrate,
working out to 10 billion tonnes of gas
per second impacting the white dwarf.
The physics of accretion disks are not
well understood, and this system is a
member of a very small class of objects
whose light curve shows a remarkable
phenomenon... outburst standstills. The
first four months of 2018 saw HL CMa
stuck at 12th magnitude.
You’ll need high magnification for
theresultingsmallfieldofview,and
recent published papers do in fact use
visual observations submitted to the
AAVSO (aavso.org)database.‘Fainter
than’estimatesareusefulalso,soifyou
can only see to 13th magnitude, that’s
still good data.■ALAN PLUMMER
observes from the
Blue Mountains west
of Sydney, and can
be contacted atalan.
plummer@variable
starssouth.orgW HL Canis Majoris
is located at 6h 45m
17.21s, –16° 51' 34.6"
(epoch J2000). This
chart (courtesy of the
AAVSO) shows the
star just 8 arcminutes
south of Sirius. Visual
magnitudes shown with
decimal points omitted
to avoid confusion
with faint stars — so
104 denotes a 10.4
magnitude star.Spot HL Canis Majoris
in outburstit was in the pages of this book that
he describes “a comet or stream in the
West, the head end under the Horizon”
and compared its appearance with that
of December 1680. The latter was a
sungrazer, and displayed the typical
appearance of a sungrazer, although
it was not related to the Kreutz group.
Knox notes that the comet of 1666
lasted “about the space of one month”.
This report, apparently standing on
its own, has long been controversial.
For a long time the ‘accepted’ version
was that Knox had mistaken the year
and that the object he observed was
actually that of 1668. If that were true,
however, he must have been mistaken
about the month as well as the year,
as the later comet appeared in March
whereas Knox clearly states that the
one he saw was in February.
There are two further reasons for
doubting that Knox was mistaken. In
his account, he mentions that the sight
of the comet “did much daunt both
King and People, having but a year
or two before felt the sad event of a
Blazing Star in this Rebellion which I
have now related”. The “Blazing Star”
to which he is here referring was the
comet of 1664. That he recalls this as
“one or two years” and not “three or
four years” previously surely favours the
earlier date.
Moreover, it seems that Knox’s
record may not, after all, have been the
only one. A catalogue of Korean comet
and nova observations compiled by R.
Sekiguchi in 1917 records a “comet ...
seen in the winter of the seventh year of
Hyonjong,” which translates to the first
months of 1666. Although we cannot be
certain, in view of the scant information
provided by this record, it is reasonable
to think that this was the same
object seen by Knox. We may, I think,
cautiously add the comet of February
1666 to the list of suspected members of
the Kreutz group of sungrazers.
■ DAVID SEARGENT is the discoverer
of comet 1978 XV. His book on comets,
Snowballs in the Furnace, is available
from Amazon.com