COTM: Mensa

John Howard

The World Wide Web is a wonderful thing and is full of startling facts. Where else could I have learned that Gamma Mensae is part of the Romulan Star Empire, and is sponsored via Mt Stromlo by Marnie Hill, or that "the constellation Mensa gives an ambitious, aspiring and proud nature. There will be many difficulties in the life, they will eventually be overcome and the latter years will be peaceful"?

Mensa hasn't been constellation of the month since April 1995, when it shared the limelight with Chamaeleon, but did get a thorough going over when Jenni Kay reviewed it in December 2003.  I observed it on 28 Feb, 2005, using a Vixen VCL200L on a Grand Polaris deluxe mount with Skysensor 2000 goto system, after a long wait for a clear night.

Abbé Nicolas Louis de Lacaille spent 1751 and 1752 in Cape Town, and named this part of the sky "Mons Mensae" after Table Mountain, which "has witnessed my nightly vigils and daily toils."  The IAU subsequently shortened the name.  You can see from the map of Mensa that the Large Magellanic Cloud forms a cloud cap to the constellation, just as Table Mountain is usually cloud-capped.

Mensa is notable for being the faintest of the 88 constellations, and my observations from downtown Downer were not helped by the glare of Civic in the same direction - these days I can no longer see the LMC naked eye from my backyard.  More notably, it is one of only two constellations to appear in the very large scale F-series of maps in the Herald-Bobroff Astroatlas. Also, it's the second-most-southerly constellation.

Galaxies:

Most of the galaxies in Mensa are very faint.  Jenni has covered the less faint ones, including the "bright" IC2051.  There is one extremely bright, extremely large one - the Tablecloth galaxy, a.k.a. the LMC.

Bright Nebulae:

NGC 2018:  Bit of a misnomer, as it's as dim as a Toc H light.  It sprang into view, however, when I moved the telescope.  This is a variation of the averted vision trick, where a moving faint or low contrast object is easier to see than a static one.  I saw it as large and hazy, with three stars seemingly embedded in the murk, although they would be foreground stars.

Double stars:

Gamma Mensae has an 11th magnitude widely separated companion, but I was unable to see it - the companion, that is.  This shows the amount of sky glow emanating from Civic, as 11th magnitude should be easy with an 8-inch aperture telescope.

h3783 was easier, although the companion at magnitude 10.5 was not visible with a 40mm eyepiece.  Shorter focal length eyepieces showed it clearly.  I used this double, which is on the long side of a prominent trapezium of stars, to fine align my goto system.  In this part of the sky, it's hard to tell one faint blob from another without this boon to navigation.

Variable stars:

AH Men:  The computer types at Mt Stromlo indulged their senses of humour in their programming so that when the 74-inch arrived at zenith, the control console displayed the object acquired as Kappa Cheeno, Alpha Romeo, Eta Boot, Nu Puppi or Rho Ubote.  Hence, when I saw the name AH Men on the target list of objects one night (I was observing with the RAPT in 2002 on the 74-inch), I assumed they were at it again.  Not so!  AH Men is a real star, and a cataclysmic variable at that.  (This is my favourite astronomical term - I rate it even higher than globular cluster.)

The Centre for Backyard Astrophysics on the WWW has a finder chart for this star, but at a magnitude varying from 13.2 to 14, I was unable to detect it. If it was in eruption on the 28th, it was a small cataclysm.

Globular clusters:

NGC 1841:  This is the most southerly globular cluster visible from Earth, and may be part of the LMC.  Try as I might, I just couldn't see it.  Jenni describes it as "exceptionally faint", and Albert had to use the Oddie to bag it in 1995.

LMC Clusters:

NGC 2031:  "Our clusters were chosen for their young ages, large brightnesses, smooth light distributions and relatively low background densities" is part of a submission to use the Hubble telescope to observe NGC2031 et al.  The LMC contains some clusters of stars that seem to be young globulars, or massive open clusters, or something in between.  They are also known as "blue globulars" or "young populous clusters".

From B. Dirsch, T. Richtler, W.P. Gieren, M. Hilker: Ages & Metallicities of LMC stars page 13:

All I could see of NGC 2031 was a faint but easy cluster in the same field as NGC 2018 which seemed to be on the edge of resolution.

NGC 2103:  I see it as quite large, oval shaped and faint, still visible albeit fainter through an OIII filter.  In the HBA, the symbol for this object, even in the E-series charts, is obscured by crowding in the field.

NGC 2058 and NGC 2065:  Kim Sebo of Mt Stromlo has written a thesis on these clusters and their many cepheids.

I could see six or seven smudges in a line parallel to a line of four or five field stars.  From the Astroatlas, they seem to be NGC 2046, 2047, 2057, 2058, 2059, 2065, 2066 and 2072.  OK, that's eight, but they're all blue globulars.

NGC 2075:  Very faint, medium sized smudge, still visible (barely) with OIII filter.  I had no luck "blinking" with the filter as I was getting too much reflection off the front, so had to screw it into the eyepiece each time.

That's all I saw.  I could have spent a few fruitful hours in the LMC part of Mensa, but the moon was rising by 10:30 so I called it quits.  The moon revealed that the seeing that night was, indeed, poor.  The sky transparency was quite good - maybe Antoniadi II.

I did get the chance to actually use the newly updated CAS resource, the Southern Cross titles list, which is a comprehensive index of all SC articles since 1988.  If you don't have a copy, buy one at the next meeting!

Upon reading Albert's April 1995 review of Mensa, I noticed that the article following it predicted the acceleration of the expansion of the Universe, three years before its discovery.  The author even calculated the acceleration: 2 x 10^-15 g.  Unfortunately, his total lack of understanding of the General Theory of Relativity meant that his prediction was based on false premises.

I've thoroughly enjoyed doing this COTM, and recommend the process as a good learning experience.  So, for a good time, ring Albert and ask him to allocate you a constellation.