http://www.skyandtelescope.com.au 49by Alan Plummer VARIABLE STARSLong-period luminaries
Three stellar targets in the heart of an open cluster.
W SY Sco is located at 17h 53m 48.82s, –34°
24' 02.9"; BN Sco at 17h 54m 10.57s, –34° 20'
27.3"; and V407 Sco at 17h 52m 25.52s, –35°
03' 17.4" (epoch J2000). This chart (courtesy of
the AAVSO) shows stars to about magnitude
12.5. Visual magnitudes shown with decimal
points omitted to avoid confusion with faint
stars — so 60 denotes a magnitude 6.0 star.M
essier 7 in Scorpius is also
known as Ptolemy’s Cluster,
named for the 2nd century
European astronomer. This cluster is
found in a rich field — in addition to the
globular cluster NGC 6453 far in the
background, there are also a couple of
Barnard’s dark nebulae and many double
stars. And there are hundreds of variable
stars within the 1.3 degree field of M7,
although most of them are not suitable
for visual observers. But let’s look at
three very different long-period variables
that most certainly are suitable.
BN Sco is a rare example of a Mira-
type variable with a double maxima. It
is very red, and has a period of 620 days
with a magnitude range from 9.8 to less
than 14.5. Nearby is SY Sco, a Mira-
type with a period of 230 days, ranging
from magnitude 8.5 to 14.5. And to the
south of the cluster is V407 Sco, a starfor whose understanding we owe thanks
to Australian observer Peter Williams.
Because of his work we can now say that
V407 Sco has a period of 401.5 days
with a magnitude range of 10.2 to less
than 14.5.
All three of these stars are in need
of visual observations, but nothing
more strenuous than once per week or
two. To begin, don’t look for the targets
themselves, but rather find their fields.
The stars themselves may or may not be
visible. The magnitudes on the chart on
this page only give a rough idea of what
you’re looking at; it’s recommended that
you generate your own charts through
the AAVSO website (aavso.org).■ ALAN PLUMMER observes from the
Blue Mountains west of Sydney, and
can be contacted at alan.plummer@
variablestarssouth.orgSPOT ON
JUPITER IS STILL worth your
attention, even though it is now a
couple of months past opposition.
Here are the times, in Universal
Time, when the Great Red Spot
should cross the planet’s central
meridian. The dates, also in UT,
are in bold.
July 1, 0:10, 10:06, 20:01; 2,
5:57, 15:53; 3, 1:48, 11:44, 21:40;
4, 7:36, 17:31; 5, 3:27, 13:23,
23:18; 6, 9:14, 19:10; 7, 5:06,
15:01; 8, 0:57, 10:53, 20:48; 9,
6:44, 16:40; 10, 2:36, 12:31, 22:27;
11, 8:23, 18:19; 12, 4:14, 14:10;
13, 0:06, 10:01, 19:57; 14, 5:53,
15:49; 15, 1:44, 11:40, 21:36; 16,
7:32, 17:27; 17, 3:23, 13:19, 23:15;
18, 9:10, 19:06; 19, 5:02, 14:58;
20, 0:53, 10:49, 20:45; 21, 6:41,
16:36; 22, 2:32, 12:28, 22:24; 23,
8:19, 18:15; 24, 4:11, 14:07; 25,
0:02, 9:58, 19:54; 26, 5:50, 15:45;
27, 1:41, 11:37, 21:33; 28, 7:28,
17:24; 29, 3:20, 13:16, 23:11; 30,
9:07, 19:03; 31, 4:59, 14:55.
These times assume that the
spot will be centred at System II
longitude 290°. If the Red Spot has
moved elsewhere, it will transit 1^2 / 3
minutes earlier for each degree less
than 290° and 1^2 / 3 minutes later
for each degree more than 290°.