Can I see the M101 supernova?

The supernova in M101 is undeniably exciting, the nearest and brightest since SN1987a. What a COMPLETELY MIND-BOGGLING thing this would be to see with your own eyes! The media have, quite rightly, been encouraging people to go and look for it themselves. But I do think they're overstating how easy it will be to see. It's great that lots of people who don't normally look at the sky will be doing so. How likely is it that they will find and recognise the supernova?

Here's the bottom line: it can be seen in binoculars, but bigger, more powerful ones than most (non-astronomer) people will have handy. The supernova will be one of many wee, faint stars in the field of view; working out which one might need a wee bit of care and effort. It might not be as easy as it sounded on TV, but it's still more than worth the effort. The rest of the post fleshes out this view.

Astronomers use a funny system called magnitude to talk about the brightness of objects in the sky. Faint objects have large values of magnitude, bright objects have smaller values. The bright stars have magnitudes between 0 and 1. The brightest star, Sirius, has a negative magnitude, -1.4. The faintest stars you can see with the naked eye, in a very dark place a long way from street lights, will have magnitudes somewhere between 6 and 7, depending on exactly how dark it is. In my suburban back garden one can't usually see stars fainter than about magnitude 5. Sometimes it's worse than this. In the city centre light pollution will hide most stars, even those with magnitudes of 3 or 4.

The supernova has a magnitude of about 10, which means it is much too faint to be seen with the naked eye. You need some sort of optical aid, binoculars or a telescope. 10 is a lot bigger than 6, so tiny wee binoculars, like you might stick in your pocket to take to a big gig or the theatre, won't be big enough.

Two numbers describe your binoculars: magnification; and diameter of the lenses (in mm). So, 7 by 50 binoculars (for example) have lenses of 50 mm diameter and they magnify everything 7 times (i.e. make things 7 times bigger). If they magnify more than 10 times, or have lenses bigger than 50 mm, they're physically difficult to use without a tripod to steady them.

Here is a very detailed study of how faint you can see with binoculars of various sizes and magnifications. You probably won't want to trawl through it, so let me skip straight to the bottom line. 10 by 50 binoculars are the absolute minimum that might show the supernova. You will need to be somewhere that you can see stars at least as faint as mag. 5.5 with the naked eye - certainly not in a big town, or the poor wee supernova will just be lost in the general sky glow. If your binoculars need cleaned, or weren't high quality to begin with, they won't do the trick. You have a much better chance with e.g. 20 by 80 or 25 by 100 binoculars, big beasts that are too heavy to just hold without a tripod, and sufficiently specialised to be found in very few houses. Here I'm in complete agreement with the Berkeley Lab video about the supernova.

With my own 7 by 50 binoculars and in my suburban back garden, I know I will struggle to see things fainter than about magnitude 9. To see the supernova I will have to use a telescope, or borrow bigger and more powerful binoculars from somebody.

How would I know where to point the binoculars? That's explained quite nicely in this video, from the Lawrence Berkeley Lab in the USA. Actually what's explained is how to find M101, the galaxy in which the supernova has taken place. If your sky is dark enough and your telescope or binoculars big and powerful enough for M101 to be visible, a wee faint star amidst the glow of the galaxy will be unmistakable. If you have a telescope with "GoTo" technology - and you know how to use it - you can just tell the telescope to find M101.

But, as the Sky and Telescope article mentions, the galaxy M101 is quite large, as such objects go. Its surface brightness is low, even although its total magnitude is quite large, and it may be hard to see against the glow of the sky, especially if there is light pollution. In my back garden it is invisible in binoculars, and even with a very high quality 70 mm telescope (much more detail in this excellent book). So the galaxy may not be visible as a marker. Then the question becomes: "which of the several wee faint stars in my binocular field of view is the one I'm interested in?" In my opinion, this needs some preparation. You will not just point your binoculars in the right place and go, "oh wow, there's the supernova". One possibility is to study the Berkeley Lab Youtube video. Freeze it and practice recognising the stars immediately around M101 and the supernova. Stellarium is wonderful, free software that shows you what's in the sky. It will help you find M101 but I don't think it includes stars as faint as 10. Cartes du Ciel will do this for you, if you download all the possible star catalogues with it, but it is maybe not quite as easy to use as Stellarium. You could use the link to the AAVSO website in the Sky and Telescope article but the resulting chart is also a wee bit technical in nature.

I think TV and some newspapers have made seeing the supernova sound easier than it really is, at least for people who haven't previously looked much at the sky. If you're willing to point a (big enough) pair of binoculars to the right part of the sky and be happy that one of those little points of light is probably the supernova, that's not so tricky. Really finding the supernova and being confident that you have seen it is also perfectly possible, but needs more attention and probably a bit of preparation before you go outside. Serious amateur astronomers will know all this already, so perhaps the best solution is to get in touch with your local Astronomy society. There you'll find people who can do these things confidently.

It's great that people are getting fired up to look at the sky and that the phenomenon of the supernova has caught so many peoples' imagination. It may be harder to see than the media have suggested, but what more amazing incentive could there be for a little bit of care and attention?

I had useful discussions on Twitter with Robert Massey and Pete Lawrence.

(added the summary paragraph near the top, 9 Sept)