Supernova Remnants

Dave Green, an astronomer at the Mullard Radio Astronomy Observatory, in Cambridge, lists 235 galactic supernova remnants on his web site. Unfortunately, although this may be the true at radio wavelengths, the amateur visual observer or imager is rather hard done by and has to make do with only a handful of objects.

M1, the first object in Messier’s catalogue was discovered by John Bevis in 1731 but also discovered independently by Messier in 1758. It is an easy object in almost any instrument and can be glimpsed in 10×50 binoculars under a good sky. Many people think that filters make little difference to its appearance, but there may be some subtle changes. I’d be interested to know what you think. Imagers should be able to show the fibrous nature hinted at visually. Another relatively easy object, although because of its complexity there are some more difficult parts to it, is the Veil Nebula in Cygnus.

More challenging targets, visually at least, are IC443 and Simeis 147 (also known as Sh2-240). IC443 in Gemini is an extremely difficult object in my 14-inch telescope, although it seems to image relatively easy. It is located at RA 6h 17m 52s and Dec. +22° 46′ (2000.0) with a size f 27′ x 7′. Simeis 147 in Taurus is an even more difficult object visually, although a web search will bring up some observations and sketches. Again, it is an object that images well, showing fantastic detailed filamentary structure, and many images from amateurs have appeared in Sky & Telescope over the years. Mind you, these have usually been long exposures under pristine skies, so imaging it from the UK may be quite another story. It was discovered in 1952 at the Crimean Astrophysical Observatory in Simeis in the Ukraine and so is named after a place, not a person. An image of Simes 147 appears in Burnham’s Celestial Handbook Vol. 3 page 1887. Located at RA 5h 39m and Dec. +28° it occupies over 3 degrees of sky, so will suit those people who image with CCDs and telephoto lenses. I understand from Owen Brazell that Sh 2-91 in Cygnus has recently been classified as an SNR. It is brighter than might be expected and was observed by him (20-inch Obsession + OIII filter) at the Equinox Star Party in Thetford. It is located at RA 19h 35m 31s and Dec. +29° 44′ 40″.

There are two other objects that frequently appear in lists of SNRs. They are NGC 6888, the Crescent Nebula in Cygnus and IC 2118, the Witch Head Nebula in Eridanus. NGC 6888 is an emission nebula surrounding Wolf-Rayet star HD 192163. Images from IRAS have shown a nebula shell beyond the Crescent and it has been suggested that this is the result of a supernova explosion many years ago. However, other astronomers have disputed this, and suggest instead that this is just an earlier ejection during the star’s red giant stage. Whatever the explanation, the Crescent Nebula is a fascinating object and I’d be delighted to receive images of it – even if it isn’t a supernova remnant!

The Witch Head Nebula, named because of its appearance in long exposure photographs, is another intriguing object, but it does not appear to be a SNR. Most references call it a bright nebula and leave it at that – although bright in this context is very relative. The SIMBAD web site lists it as a reflection nebula, which is interesting as some observers say it responds to nebula filters suggesting there is an emission region to it.

In the January 2005 Sky & Telescope Sue French, in her monthly Deep-Sky Wonders column, mentions the Witch Head. She calls it a SNR and says that it was visible (just) in her 4.1-inch (105mm) refractor from the Florida Keys Winter Star Party. I haven’t seen this object (in fact I haven’t looked for it) but I’d be interested to know how easy it is from our mediocre skies. It is a large object, covering approximately 3o x 1o and centred on RA 5h 4m 46s and Dec. -7° 13′ (2000.0).

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