Thanks to Robert for posting the images of Wd 1 and Wd 2. While Simmo and others have reported Westerlund 2, Westerlund 1 is one of the less-reported objects in the amateur astronomy repertoire. It is easy to locate but difficult at best to see. Once found and triangulated with nearby guide stars for future visits, the more we look the easier it gets. I use Jose Torres’ TriAtlas B chart no. 174; look for WESTR1, bottom centre, in Image 01 below.
Wd 1 requires more patience even in visual mag 7 skies than any other object I’ve tackled. Navigate straight south (poleward) from the comet-like flare of the Table of Scorpius to the loose cluster NGC 6240. Shift westward (to the right) 1.5 degrees in R.A., and voila. Or point your finder to the northern tip of a small Ara star cloud which terminates near Hogg 22 at the cloud’s northern tip. Look for the nearly equilateral triangle shown in the WikiSky image #02 below.
At 75x the triangle will occupy the central half of an 82° AFOV eyepiece. A pretty little chain of 11th to 12th mag stars forms the S (trailing) side. Wd 1 lies in a region overabundant in <13 mag stars to the W compared with nearby fields. The dark lane at the far left of the WikiSki image is quite pronounced in dark skies but much more difficult or undetectable if there’s light pollution.
To the SE (upper left) of the triangle is a pleasing catenary chain of 12.5 to 13.8 stars. The stars between the catenary and the triangle are all sub mag 14. This field is a handy place to estimate seeing depth at the time. My 20 cm reaches about mag 15.3 in this area. Then cross the triangle to the region of the square in the Wiki image. Wd 1’s feeble evanescence looms out of the Mag 13-14 star field for only brief glimpses in my 20 cm scope at 120x, then subsides into the background glow. The evanescence may not come back for a couple of minutes at a time, and then for only a few seconds. The eye will eventually assemble enough remembered photons to manifest as a 4’ by 3’ glow with no discernible edge, no core, and no granularity from stars just under the visual threshold. Wd 1 doesn’t show at all in my 180mm scope at 90x. Despite Wd 1’s frail luminosity, I managed to log it five times over the most recent Mar 24 – Apr 2 2014 dark moon cycle.
The hesitant, faint patch we see visually is actually radiating the luminosity equivalent of over 100 million times solar. Less than half of it is in the visual band. Wd 1 radiates so much energy in the UV and IR that relatively little of its light comes to us in the visual band. Only two of its stars, W7 and W33, reach into the visual mid 15th magnitude range. Three others are mag 16-17. After that, it’s the basement down to mag 25. Westerlund 1 is probably the most visually dimmed cluster we amateur folk are likely ever to encounter. It was discovered and identified as ‘a heavily reddened star cluster in Ara’ by Bengt Westerlund in 1961.
In Westerlund’s initial report
Wd 1 barely rated a squib. The first report notice started with the sentence ‘A highly reddened cluster in Ara.’ Later that year he followed up with a 6-page paper describing his observations of 80 stars. In retrospect that paper was prescient in its predictions, but at the time it raised little attention. Wd 1 went back to sleep for thirty years. Westerlund himself went on to a much more luminous career than his clusters did. (
Check ADS using his name in the search box—his first paper was published in 1950.)
The discovery paper and first analysis for Wd 1 are no longer available on ADS but appeared in Publications of the Astronomical Society of the Pacific, Vol. 73, No. 430, p.51.
Westerlund’s 1987 follow-up study of 258 Wd 1 stars initiated the big surge in Westerlund 1 lore we find in an ADS or arXiv search today. Its light passes through both the Sagittarius Arm and Scutum-Crux arms of the Milky Way before it arrives on our tender shores. Wd 1 also has its own natal gas and dust to deal with. Hence Wd 1’s light is reddened by 12.9 magnitudes before we see it. For every Wd 1 photon we see, 143,149 photons have been absorbed. That turns a mag 2.7 star in our night sky into a mag 15.6 star like Wd 1’s most luminous offspring W7. The only reason we can detect anything of it at all is because Wd 1’s 10,000+ stars are jam-packed into a sphere that could slip easily between us and Alpha Centauri. It’s a good thing for us that only our imaginations can go there.
Image 03 below is the Wd 1 visual field to the 19th magnitude.
Wd 1’s statistics are enough to make you give up statistics. Wd 1 is the most massive young cluster known in our galaxy. To we eyepiece huggers, Wd 1 is very challenging to observe because its light is reddened by the gas and dust of three complete Galactic arm crossings. Its light starts out some 24,000 light years away, passes twice through the Sagittarius and Scutum-Carina arms, and eventually arrives at our modest apertures dimmed by almost 13 magnitudes. If we were in the middle of it we’d have no trouble reading newspapers at night under 60 OB supergiants, 24 Wolf-Rayet stars, 6 yellow hypergiants, 4 red supergiants, a luminous blue variable, and an unusual Be supergiant which probably the result of a stellar merger.[5] In addition, X-ray observations have revealed the presence of the anomalous X-ray pulsar CXO J164710.2-455216, a slow rotating neutron star that must have formed from a high-mass progenitor star. Westerlund 1 is believed to have formed in a single burst of star formation, implying the constituent stars have similar ages and compositions. Astronomers classify Wd 1, Wd 2, and nearby NGC 3603 in Carina as ‘super star clusters’. This term is usually applied to clusters so massive and coherent they may evolve into globular clusters. The elongated, lumpy ball we call Wd 1 may end up in 100 million years a young globular with a multibillion-year future ahead of it.
Good hunting!
=Dana in SA
