BNGC 7. An Ancient Supernova Remnant

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By Steve Mencinsky, 1993

There is no truth to the rumour that the Treasurer's new telescope will not focus on anything closer than 200,000 light-years. Why, only recently he was using it to look at a very interesting and difficult object that lies within our own galaxy, and from his backyard he has been known to observe Mars!

Not too far away (in angular terms) from Sirius is the giant nebula IC2177, in apparent size 3 x 0.5 degrees. Well, even that is in dispute for there is a body of opinion that suggests that IC2177 itself is only a small part of this giant nebula. In any case it is believed that this nebula is a supernova remnant (SNR) some 840,000 years old - a truly venerable SNR, even possibly more so than our President (Editor's note: Tony has the right of reply).

As this SNR is only about 2 degrees from the galactic plane, it is in an extremely rich star field. Indeed, Uranometria map 273, shows 4 star clusters, 5 emission nebulae, and one variable star, all apparently associated with IC2177.

What can one observe here? Considering the low surface brightness that photography suggests, surprisingly much. From Cremorne, under a very milky grey-purple sky, all four clusters (NGC's 2335 and 2343, Collinder 465 and 466) were clearly visible through the 18" telescope with no filters. The very small but bright emission nebula NGC2327 was also easily seen, and very blue but of course much more obvious through the UHC filter. IC2177 itself and the Van den Bergh nebula (vdB93) remained completely invisible from Cremorne, as could be expected. But the greatest surprise - completely unexpected - was that Cederblad 90, the emission nebula at the southern tip of IC2177, was clearly visible from Cremorne!

From Ilford, naturally, very much more was visible. The clusters appeared very much richer. Cederblad 90 appeared much larger and brighter. A small portion of IC2177 itself was glimpsed, with great difficulty. vdB93 eluded successful observation until the February new moon weekend at Ilford.

Some 2 degrees to the west of IC2l77 lies a small emission nebula, name unknown, that appears to be another portion of this same SNR This nebula has remained unseen despite lots of filters/aperture/dark-sky/time.

The diagram below shows the general area Since this is such a star-rich field, the diagram is of very limited use. Probably your best bet would be to use your favourite chart to get into the general area, then use a photo or Uranometria to pick out the details.

While in this area, well worth observing through a large telescope and UHC filter are the nebula NGC2359 and associated IC468. This nebula is commonly referred to at Ilford as the Witch's Head nebula, but at the Texas Star Party (gosh. that was nearly a year ago!) I was introduced to this as the Florida nebula. From Ilford, NGC2359 appears as a mosquito wriggler, with IC468 as its two large feathery antennae - but then, my observing site at Ilford is downwind of Mick McCullagh's.

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