COTM:
-Triangulum Australe-TrA
John Howard
Many moons ago, when Albert
was allocating COTMs for this year, he kindly gave me a very
easy one: Triangulum Australe. The name is Latin for, guess
what, Southern Triangle, and the three brightest stars of the
constellation do, in fact, form a perfect triangle. It's abbreviation
is TrA, which is hard to spell but quite easy to sing.
It's also easy to find: start at the Southern Cross, move your
eyes to the Pointers, then move them a bit further in the same
direction to a large triangular formation. At this time of the
year, it's high in the southern sky.
I swept the whole constellation late last February using a
12-inch
Newtonian, and found it rather interesting. Being in the plane
of the Milky Way, it has plenty of stars, many of which form
great chains and loops.
The chains of stars made me wonder whether those formations
are artefacts of the imagination, or actual physical associations,
like clusters. They seem far too numerous to be the result of
line-of-sight co-incidence. You will recall that it was the
great William Herschel who statistically analysed the separations
of double stars, and decided that they were too numerous to
be due to chance alignment. Luckily, during the last few years,
the tools for answering the riddle of the chains have become
available to all - Hipparchos, Tycho
and the internet - so I leave the solution as an exercise for
the reader.
I saw NGC6025 - a very fine open cluster -
for the first time, and I was impressed. Shaped like an S with
a bar through it (sideways, like a pound sign, not vertically
like a dollar sign), it's worth putting on your list of objects
to return to. Being fairly bright, it's also good for moonlit
nights.
By a stroke of good fortune, Jenni Kay reported on several
objects from this constellation last month, so see her description
of NGC6025 on page 5 of the July journal.
My next session on TrA was a month later at Wiruna with a 10-inch
Dobsonian-mounted Newtonian telescope. Cloud, rain and dew slowed
proceedings for a couple of nights, but the night of 30th March
provided some good, dark-sky views, with the five brightest
stars easily visible to the naked eye.
It was on that night that I "discovered" NGC6101
just outside the borders of TrA in Apus - a grey blob like the
Crab Nebula, but fainter. Not having an atlas immediately to
hand, I wandered the observing field looking for a DSC (digital
setting circles) or GOTO equipped telescope. I found one, with
an observer kind enough to identify my discovery for me. It's
a globular cluster 45,000 light years away, and is completely
invisible from my backyard, even with a 12-inch telescope.
I'm resisting the temptation to digress here into the etiquette
and niceties of using GOTO equipped astronomers as an on-tap
resource for identifications. They don't seem to mind (at this
stage of the spread of the technology). No doubt, rules will
develop in time, as has happened with, for example, mobile phone
etiquette.
Knowing full well that double stars are a traditional part
of any report on constellations (how do these rules develop?),
I thought I'd better bite the bullet and find some, even though
I've had little luck with doubles in the past. Either I can't
find 'em, or can't split 'em. There are four doubles listed
in TrA in Hartung, with separations of 1 arcsec, 1".8,
1" and 13". After a couple of observing sessions,
my record was intact: couldn't find 'em or couldn't split 'em.
After the extended weeks of cloudy weather, and thinking that
this report was due last month, I panicked and bought a Vixen
Skysensor 2000 system so I could find the elusive doubles. (This
is a slight exaggeration). Success!
Having had a look at Alpha Centauri (separation
14 arcsec) and Alpha Crucis (separation 4 arcsec)
as gauges, and asking my telescope mount to go to the appropriate
co-ordinates, I was able to just split the star Rmk
20 (and an infinitive), whose components are 1.8 or
2.4 arcseconds apart, depending on the book you read. The big
advantage of the goto system here is that you know the star
in the middle of the eyepiece is the one you're after - you
don't have to check every one in the field at high power, wondering,
"Is it elongated? Is that a floater or a nebulous halo?"
The components being nearly equal in magnitude also helped for
this one.
I332 proved more difficult. One component
is two magnitudes fainter than the other, and the stars are
only 0.8 arcsec apart. I couldn't split them with the 8-inch.
Slr11 was another success - barely. I could
just make out the mag. 8.8 companion 1.2 arcsec from the mag.
6.5 primary. Two other stars close by to the NE and SW made
this quite an interesting view. Perhaps it's a quadruple system?
Iota Trianguli Australis was dead simple -
the faint, mag. 9.4 companion is currently 13 arcsec distant
from the primary, about the same separation as the components
of Alpha Centauri. It would have been even easier for John Herschel
in 1836, when the stars were 25 arcsec apart. The primary star
is a spectroscopic binary, but I seem to have mislaid my spectroscope.
I did see a third, somewhat fainter star about another 15 arcsec
out from the secondary.
I was unable to detect any colour differnces in the doubles.
More aperture and better eyesight would no doubt help in that
regard.
The two planetary nebulae listed in Hartung are rather faint
- NGC5844 at mag. 12.5 is so faint it doesn't
even get into the Herald-Bobroff Atlas. I couldn't see them,
but Jenni describes them as "obvious".
Even when I knew the planetary nebulae NGC5844
and NGC5979 were in the field of view (not
the same field!), I couldn't see them from my backyard in Downer
using an 8-inch Cassegrain.
My intention was to lavishly illustrate this article with CCD
images of the planetary nebulae and galaxies in this constellation,
but owing to the wind and rain, and as my new pier has not yet
been commissioned, and six month's notice is not enough, perhaps
I can just refer you to the pictures in Jenni's article.
What's in Hartung is all I'm ever likely to see, and even some
of those targets need a dark sky and good observing technique.
I am most impressed by NGC6025, however, and
recommend that everyone have a look at it.
An interesting object not mentioned in any reference appeared
briefly in TrA a month or two ago. As Mike Nelmes walked out
my front door, he looked up and there was an Iridium flare flaring
away within its borders. |
