Astronomers say they have discovered the youngest known supernova in the galaxy, giving them a rare opportunity to study a star as it dies.

The supernova, or stellar explosion, occurred sometime around 1868, or 140 years ago, scientists at the University of Cambridge and North Carolina State University said Wednesday.

The explosion of the star would have made a bright flash but could not be seen because the star was lying close to the centre of the Milky Way galaxy and was embedded in a dense field of gas and dust. This made the object about a trillion times fainter to optical light sensors than an unobscured supernova.

It led scientists who first spotted the object in 1985 to estimate its age at 400 to 1,000 years old.

The dying star's true age was discovered using NASA's Chandra X-ray Observatory and the National Radio Astronomy Observatory's Very Large Array, both of which revealed that its rate of expansion was too fast to be anything but a young star.

The discovery helps settle a mystery for astronomers concerning the lack of supernovas spied in the Milky Way.

Observations of other galaxies suggest stars in the galaxy should be going supernova at a rate of three every century, yet before this discovery, the last known supernova occurred around 1680, an estimate based on the expansion of its remnant, Cassiopeia A.

"If the supernova rate estimates are correct, there should be the remnants of about 10 supernova explosions that are younger than Cassiopeia A," said David Green of the University of Cambridge in the United Kingdom in a statement. "It's great to finally track one of them down."

Robert Kirshner, a Harvard University astronomer who is not affiliated with the study, said during a teleconference with reporters that discovering such a young supernova will give scientists a better understanding of a process crucial to forming the more complex elements in the universe.

"You're actually getting to see the rock that made the splash, not the wave that's going out into the pond," Kirshner said.

With files from the Associated Press