An exploding star in a galaxy far from Earth should be visible from Britain soon after twilight this evening. Amateur astronomers will be able to observe the flicker of light from its violent death throes through a good pair of binoculars or a telescope, clear skies permitting.

The supernova – the nearest of its kind to be spotted in 40 years – lies in the Pinwheel galaxy, 21m light years from Earth.

The distance means observers on Earth will see the death of the star as it played out 21m years ago, the time it has taken the light from the exploding star to reach our planet.

Under clear skies, the supernova can be found by looking first for the Plough (also called Ursa Major or the Big Dipper) in the sky immediately after sunset. The "handle" of the Plough has three stars.

"As you look at the sky, draw an imaginary line through the second and third stars in the handle and follow that line up and left. The supernova is four degrees along, or around the distance taken up by five full moons in the sky," said Dr Mark Sullivan, an astrophysics research fellow at Oxford University.

The supernova will appear in the sky as a bright star on the edge of one of the Pinwheel galaxy's spiral arms. "Whilst it looks more or less like just another bright star, unlike its companions this supernova will soon fade away, and after a few days it will only be visible with larger telescopes," Sullivan added.

The Oxford team spotted the supernova on 24 August. Working with a group called the Palomar Transient Factory (PTF) collaboration, the scientists used the Hubble space telescope to observe the supernova.

The team noticed the supernova about five days after the star exploded and will continue observations until mid-October.

The discovery is particularly important because it is a type 1a supernova, the kind used to measure the expansion of the universe.

"For many people it could be a once in a lifetime chance to see a supernova of this kind blossom and then fade before their eyes; we may not see another one like it for another 40, or perhaps over 100 years," Sullivan said.