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The Pencil Nebula By David Malin (David Malin Images)

NGC 2736, the Pencil Nebula was first noted by John Herschel on March 1, 1835, during his four-year stay in South Africa. He observed NGC 2736 (= h3145) only once. He described it "...as an extraordinary long narrow ray of excessively feeble light". Though there are much more conspicuous and extensive regions of the Vela SNR visible on modern wide angle photographs they were not seen by Herschel, probably because of their more diffuse nature and the rich starfield in which they lie. However, he did find several more objects in this part of the sky that eventually appeared as extended objects in the New General Catalogue (NGC) of 1888, so he was looking hard.

The Pencil Nebula is almost exactly on the Galactic equator. This places it several degrees from where one might expect to find an outlying wisp of the Vela supernova remnant if you assume that the arc of nebulosity seen in this image of the Vela supernova remnant is the north-west quadrant of a roughly spherical shell outlining the expansion of the supernova explosion.

 

Vela Supernova Renmant and the Pencil Nebula


(Images by David Malin (DMI) and the AAO)

 

It is also much further east of where one might expect to find nebulosity if you assume that the Vela pulsar is close to the center of the nebulous shell. However, it is known that the Vela pulsar has a highproper motion. Even so, from the purely optical images alone, the association of the Pencil nebula with the Vela supernova remnant begins to look a little doubtful.

As is so often in astronomy, there is more here than meets the eye, and at X-ray wavelengths, the full extent of the Vela SNR becomes evident in pictures from the ROSAT satellite (at left). The Pencil nebula coincides with a 'blister' on eastern edge the X-ray image. This blister is one of several symmetrical features on the X-ray photograph that probably originate in the supernova explosion itself. However, its corresponding western counterpart, far away from the Galactic equator, is very diffuse and extended. The eastern part, which is close to the optical position of the Pencil nebula, is compact and bright at X-ray wavelengths. This is probably because it is expanding into much denser interstellar material close to the Galactic plane.

All this suggests that much of the Vela SNR is hidden by dust at optical wavelengths, and that we are fortunate to have a clear patch through which we see it. The nebula is probably 'reddened' by intervening interstellar matter, which will have the effect of diminishing the blue light from the excited gas rather more than the red light.

For more information, visit the
Vela Supernova Remnant File by Bill Blair (JHU)

 

SuperCOSMOS Ha Survey of the Vela Supernova Region
The H-alpha survey was undertaken by the Anglo-Australian Observatory on behalf of the UK and Australian communities and had been scanned and put on line under the auspices of the Wide Field Astronomy unit. The Vela mosaic has been put together from this data by Mike Read and Quentin Parker. This image has been labled with several features and shows the location of the Pencil Nebula image by David Malin. Use of this image is courtesy of the UK Schmidt Telescope (copyright in which is owned by the Particle Physics and Astronomy Research Council of the UK and the Anglo-Australian Telescope Board) and the H-alpha Sky Survey as created by the SuperCOSMOS measuring machine and is reproduced here with permission from the Royal Observatory Edinburgh.

Rancho Del Sol Observatory Image of the Pencil Nebula

Image courtesy of K. Crawford (Camino, CA)