http://www.skyandtelescope.com.au 49by Alan Plummer VARIABLE STARSO
ne of the pleasures of variable
star observing is getting to know
a small patch of sky really well,
andespeciallywhensaidpatchisone
of the great showpieces of the sky. The
chartabovehasa2°fieldofviewwith
four variable stars identified, alongf
with a sequence of non-variables that
can be used to estimate their brightness.
Thefirstthingtounderstandaboutthis
field is that it’s right on the galactic
equator,wherestar-formingregions,
nebulae, young open clusters and high-
mass stars abound. In this direction we
look down the length of the Carina-
SagittariusArmoftheMilkyWay.
Our targets areUCar,IX Car,AG Car
andEta Car, all of them either super- or
hypergiants. Thanks to the Gaia mission
we can reliably place U and IX at around
7,000 light years away, on the near side of
the Carina-Sagittarius Arm. AG and Eta
arethoughttobeatasimilardistance.Ofsuddenly faded as it raced past the Sun?
Apparently not, as two days later it was
again spotted in the twilight, almost as
bright as Jupiter.
Over the following weeks, Seki-Lines
became a prominent sight in the night
sky, displaying a beautiful dust tail,
down the centre of which lay a kind of
dark ‘shadow’ which effectively divided
the tail into two sections. By calculating
the paths of dust particles that left
the comet’s nucleus at different times,
astronomers found that this ‘shadow’ lay
where those particles emitted at the time
of perihelion should have been located.
For some reason, these particles had
gone missing! This neatly corresponds
with the apparent ‘reverse outburst’
or temporary fading of the comet at
perihelion. Somehow, the comet seems
to have partially switched off — or at the
very least, toned down — activity during
its closest approach to the Sun.
This switching off could be explained
if Seki-Lines released a dense cloud of
particles from its nucleus just prior
to perihelion. According to this line
of thinking, such a cloud would have
temporarily blocked the Sun’s light and
heat from the nucleus surface, causing a
brief shut-down of activity. Then, as the
cloud dispersed, the nucleus once again
became exposed to the Sun’s radiation
and the comet’s activity returned to
normal. Another suggestion might be
that the fierce heat experienced at just
0.03 a.u. from the solar centre may have
destroyed any particles as soon as they
were released, effectively stripping the
comet of all but the bare nucleus.
Like Arend-Roland, Seki-Lines also
displayed an anti-tail, but this was a very
different feature from that of the earlier
comet — far less prominent, shorter and
not as spike-like. More importantly, it
could not have been a perspective effect
as it appeared at a time when the Earth
was not crossing the comet’s orbital
plane. This appears to have been a true
sunward drift of heavy particles emitted
by the comet.
■ DAVID SEARGENT is the discoverer of
comet 1978 XV.
the huge number of known variables in
this field, I’ve chosen these few because
they are bright and worth observing.
U is a Cepheid variable that ranges
from 6th to 7th magnitude and can be
observed every clear night. AG and Eta
are both unpredictable luminous blue
variables. The ‘book’ on LBVs is being
re-written as we speak, so they are well
worth observing every clear night. IX is
a red supergiant ranging over 7th to 8th
magnitude, and needs observing only
once per fortnight at most.
50mm binoculars or telescope is all
that needed for these stars. For guidance
in making and lodging variable star
estimates, you can email me or visit the
website of the American Association of
Variable Star Observers (aavso.org).■ ALAN PLUMMER has racked up more
than 60,000 variable observations. He can
be contacted at [email protected]A Carina quadrella
Four easy stars in the same southern field.