An international team reports the closest exploding star of its kind seen in four decades, offering insight into how massive stars perish.

The star, loc ated in the "Pinwheel Galaxy", M101, some 21 million light years away (one light year is about 5.9 trillion miles), belongs to a widely-observed Type 1a class of supernovas, or exploding stars.

"The best time to see this exploding star will be just after evening twilight in the Northern hemisphere in a week or so's time," said University of Oxford astronomer Mark Sullivan, in a statement. "You'll need dark skies and a good pair of binoculars, although a small telescope would be even better."

Such Type 1a stellar explosions are used as standard astronomical yardsticks to gauge how quickly distant galaxies are moving away from each other, observations that led to the discovery that the universe is moving apart at an accelerating rate in 1998, now ascribed to "dark energy" acting as an anti-gravity force across vast distances in the universe.

"Seeing one explode so close by allows us to study these events in unprecedented detail," Sullivan said in the statement. Astrophysicists are currently uncertain about the exact mechanism that triggers such standard-seeming supernovas, thought to result from either the merger of twin stars, or the accretion of gas from one star onto another.