The youngest known galactic supernova remnant

That would be Cas A, the strongest radio source in the sky (besides the sun). Based on the expansion rate of the filaments light from this supernova must have reached earth ~340 years ago ( roughly 1667). Although it was not recognized as a nova at the time, it may have been catalogued as a 6th magnitude "star" by Astronomer Royal John Flamsteed in 1680, as there is no known counterpart today for his "star" Cassiopeia 3.

Available images have traditionally been at radio wavelengths (http://images.nrao.edu/395) or x-ray (http://chandra.harvard.edu/photo/1999/0237/), though HST captured it in 2006 (http://spacetelescope.org/news/heic0609/). So, it was never known as a visual target and the first observation was probably made in the last couple of years. So, this object was high on my observing list earlier this month when I observed with Ray Cash, Mark Wagner, Richard Navarette, Ken Archuleta and Mina Reyes at Lassen on the peak of the Perseids. But I really didn't know what to expect.

Here's the good news -- using a 21mm Ethos (108x) and an OIII filter, I was surprised to easily pick up the brightest filament in my 18-inch Starmaster. With this combination a small, elongated, fairly low surface brightness glow was immediately noticed, ~1.2'x0.4', extended ~E-W. The SNR was not seen unfiltered at 108x or at 175x, so it responded strongly to the OIII filter. The visual filament I picked up forms the northern edge of the 4' diameter supernova remnant. The glow is situated at the vertex of an isosceles triangle with a mag 11.4 star 2.5' SW and a mag 11.7 star 2.5' NNW. The 56' field includes a number of mag 10 stars and is fairly rich, but there are no brighter stars. Give it a try!

--Steve

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