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Observing the Pinwheel's supernova
MARK ARMSTRONG
ASTRONOMY NOW
Posted: 26 August 2011


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Astronomers worldwide have been galvanised into action by the discovery of a very young supernova in the Pinwheel galaxy (M101) in Ursa Major. This supernova has been named PTF11kly and has real potential to become bright enough to be visible in large binoculars. Amateur astronomers are strongly urged to make as many observations as possible while it is ‘on the rise’, as such data is so valuable to on going research into these titanic explosions.

Professional astronomers conducting the Palomar Transient Factory discovered the supernova on 24 August at a relatively faint magnitude +17. The spectrum confirmed the object was a very young, normal type-1a supernova. Since the discovery announcement images have been pouring in and the supernova has brightened considerably in just a day to magnitude +14.8. The burning questions are how bright could this supernova become and how early was it discovered? The earliest detection of the supernova appears to be on August 23 and on an image taken the previous night, nothing was visible at the supernova’s position.

Its position is 58”.6 west and 270”.7 south of M101’s core at R.A. 14h 03m 05s.80, Decl. +54° 16’ 25”.3. Typical type-1a supernovae take 18 days or so to reach maximum brightness, so we could be talking well into the second week of September before PTF11kly peaks. M101 lies at a distance of 21 million light years and is one of the closest galaxies to us. Looking in the historical records, supernova 1937C was a normal type-1a that peaked at magnitude +8.4. Its host galaxy was IC4182, which is lies at around the same distance as M101. So we have the very exciting prospect of watching this supernova brighten over a couple of weeks to become visible first in small ‘scopes and then large binoculars. Watching it fade away too will be fascinating. Whatever happens, this will be the closest supernova of its type for years and should eclipse the great M81 supernova in 1993 (type-IIb), at least in terms of peak brightness.

Amateur astronomers are lucky that this is a type-1a supernova as opposed to a core-collapse type-II. Type-1as are not the result of the explosion of a massive star in excess of 8 solar masses but the destruction of a white dwarf star in a close binary with a younger but still evolved red giant star. Type-1a supernovae have a typical absolute magnitude (brightness as seen from 10 parsecs or 32.6 light-years) of -19.3. Type-IIs vary considerably depending on their precise classification, but the brightest examples have an absolute magnitude of -17 (by comparison, the Sun’s is +4.8!). So we should gain at least a couple of magnitudes on the type-II scenario.

M101 is a fabulous, grand design spiral galaxy that appears face on to our perspective. It is about the size of the full moon in diameter and although its low surface brightness makes it tough to see the spiral arms in moderate apertures, it produces magnificent images. Lying in Ursa Major just over five degrees north-west of Alkaid (eta UMa), the end star of the Plough, M101 is circumpolar from the UK. In late August, M101 is about 50 degrees up around 9.30pm as darkness falls, well past culmination in the northwestern sky. There will be about a two and a half hour observing window before M101 become less than ideally placed in the north-west and only 30 degrees up. The Moon is waning towards new on 29 August and won’t really have an impact until first quarter on 4 September.

Advanced amateurs with moderate to large apertures telescopes can do accurate photometry with CCD’s and filters so as to record the brightness of the supernova each night. If you have a large ‘scope, then visually you can make brightness estimates now using the field stars around M101 and as the supernova brightens smaller apertures come into their own too. Even if you have a CCD but no filters, then your images will still be valuable as a record of the rise and decline of this object. Observations are pretty worthless unless they are submitted along with those of others to appropriate organisations. The British Astronomical Association, The Society for Popular Astronomy and (The Astronomer Group) will be very happy to receive your observations, images and sketches. We will too here at Astronomy Now! (email your images to gallery2011(at)astronomynow(dot)com.

To follow the evolution of the supernova, visit David Bishop’s Bright Supernova page at http://www.rochesterastronomy.org/snimages/. For a great image of M101 and the supernova by the great British observer Denis Buczynski visit http://britastro.org/blog/?p=819.

Read more about the initial observing campaign of this supernova in our news story.

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Hubble Reborn
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