http://www.skyandtelescope.com.au 49by Alan Plummer VARIABLE STARSOverlooked no longer
V Pyxidis could be undergoing evolutionary change.
T
he bright southern variable
V Pyxidis has been brought to
my attention as a potentially
important and under-observed target.
V Pyx was first discovered as variable
in 1921 by Mary Applegate, Fellow in
Astronomy at the Harvard Observatory.
However, with only occasional study
over subsequent years, not a lot was
learned. What we know today is largely
thanks to regular observation by
members of the Royal Astronomical
Society of New Zealand’s Variable Star
Section, beginning in 1971.
V Pyx is now known to have a
pulsation period of 79.9 days and a
visual amplitude of 7.7 to 10.4. As it
pulsates, its spectral type changes
from a G8 to M1 supergiant. This
firmly classifies it as an ‘SRD’ semi-
regular variable. So why is it an
important target? As I write this, the
star is at magnitude 10.6 and may be
still falling, and its light curve shows it
to have dropped to magnitude 11.2 inthe past. Further, Sebastian Otero of
the American Association of Variable
Star Observers (AAVSO) notes that it is
showing characteristics of another class
of variable, the RV Tauri type. That
is, like an SRD but with alternating
shallow and deep minima. If a variable
star is seen to change classes, it
suggests its evolutionary status may
be changing.
The chart on this page shows both
Alpha and Beta Pyx, and spans about 5
degrees. The all-sky chart in the centre
pages of this issue plots these two
4th-magnitude stars, but they are not
always named. Still, they’re not hard
to find. Observe V Pyx with binoculars
or small telescope once per week and
please report to the AAVSO through
their website. Good observing!■ ALAN PLUMMER observes from the
Blue Mountains west of Sydney, and
can be contacted at alan.plummer@
variablestarssouth.orgW V Pyx is
located at 08h
53m 25.64s, –34°
49' 08.9" (epoch
J2000). This
chart (courtesy
of the AAVSO)
is approximately
5 degrees wide
with north up,
and has visual
magnitudes shown
with decimal
points omitted to
avoid confusion
with faint stars —
so 99 denotes a
magnitude 9.9 star.How comets
are named
We tend to make our comets more
cozy by using their common names:
Comet McNaught, Comet Thatcher,
Halley’s Comet, the Great Comet
of 1811... But astronomers follow a
different set of rules when referring
to these icy travellers. In 1994, the
International Astronomical Union
(IAU) codified a system meant to
eliminate confusion caused by
duplicate names. Let’s look at Comet
Thatcher’s official designation, C/1861
G1 (Thatcher), to explain the rules.
The most common prefixes for comet
designations are C and P. A preceding
P means you’re looking at a periodic
comet (any comet with an orbital period
of 200 years or less). The number
attached ahead of the P reveals the
order of discovery. For instance, Comet
1P/1882 Q1 (Halley) is so-called because
it was the first comet determined to be
periodic. These numbers are assigned
after a comet’s second perihelion
passage, which (ideally) confirms the
object’s periodicity.
A preceding C is used for non-
periodic comets, or comets with
periods longer than 200 years. Comet
Thatcher rounds the Sun every 415
years, so its designation begins with
a C. However, if you see an X, you’ll
know that the comet’s orbit couldn’t
be calculated. If you see an I, you’ll
know you have an interstellar object
(the IAU added I to the system last
year after the discovery of 1I/2017 U1
(‘Oumuamua) in October).
Thatcher’s 1861 is straightforward
— that’s the year of discovery. The G
tells us that the comet was discovered
in the first half of April (January =
A+B; February = C+D; March = E+F;
April = G+H; and so on). The 1 lets
us know that this was the first comet
discovered in the G period. And, of
course, we wrap up the designation
with the surname of the discoverer or
the object’s coziest call sign.
S. N. Johnson-Roehr