supernova!

Warning: Astronomy themed posting

One evening in August of 1975 my mate Neil Bone phoned our house unexpectedly. We were in the same class in high school, geeky boys keen on science. Neil was already becoming a star, so to speak, in the amateur astronomy world, getting mentioned regularly in the "Circulars" of what was then the Junior Astronomical Society. He wanted me to run outside and confirm that the constellation Cygnus had unexpectedly gained a new star: a "nova". I knew Cygnus well enough and an extra star was obvious to the naked eye.

Eventually we heard that somebody else had spotted it an hour or two earlier. Nova Cygni 1975 did not become "Bone's Nova" but this was a pretty exciting episode all the same.

Just as Bone's Nova fell a little bit short for Neil, so Nova Cygni 1975 fell a little bit short of the ultimate astronomical event. A remarkable, dramatically brightening star, it was nonetheless not a supernova.

A white dwarf is an extremely exotic object, the mass of the Sun condensed to an object the size of the Earth. A spoonful does indeed weigh a ton. There are a few that are fairly easy to see for yourself, with a small telescope. Come to one of my classes and - weather allowing - I'll show you one; possibly the most exotic object I can let you see in a small telescope, and one of the most innocuous looking.

A white dwarf next to a normal star, in a binary system is in a difficult place. Gas falls onto it continually from the normal star. Maybe more and more hydrogen gas piles up on the surface of the white dwarf until there is a thermonuclear explosion, a naturally occurring hydrogen bomb on a huge scale. Then the star brightens up dramatically as a nova, a "new star". The accumulated gas is blown off in a huge explosion and the star settles back down to its previous timidity. This is what Neil and I - and of course many others - saw in 1975.

Much more desperate stuff can happen, much more rarely. If enough gas falls onto the white dwarf it may collapse catastrophically and blow itself completely to bits in an enormous nuclear explosion, a Type Ia supernova. Then this single star shines briefly with the brightness of all the stars of a galaxy. It is believed that these events all have more or less the same intrinsic brightness, so they play a vital role in estimating the distances to distant galaxies and thus estimating the age and future fate of the Universe.

I've been thinking of all this recently because there has just been a Type Ia supernova in M101, one of the nearest galaxies. People with modest telescopes may be able to spot it as it brightens over the next week or so, a single, ferociously luminous star 20 million light years away, a poor wee white dwarf driven beyond the Chandrasekhar limit and thus to fiery destruction; a much more dramatic event than the nova Neil spotted in 1975, but involving the same sort of object.

A supernova in one of the nearest galaxies: Neil would have loved this.