One of the brightest stars in the sky is mysteriously shrinking, astronomers have discovered. Betelgeuse marks the armpit of Orion the hunter and is easy to spot twinkling in the night sky.

Some astronomers believe the star is a ticking time-bomb on the verge of destroying itself in a massive explosion.

It would become the most spectacular supernova ever seen, shining more brightly than the Moon for months on end and beating a cosmic blast in Cassiopeia in the 16th century.

Betelgeuse is a red giant so big that it is five times the size of the Earth’s orbit around the Sun and would stretch out as far as Jupiter.

It is one of the few stars visible as a disk rather than a point of light with the Hubble space telescope. But its diameter has shrunk by more than 15 per cent since 1993.

The change was spotted by astronomers monitoring Betelgeuse with a triple telescope called the Infrared Spatial Interferometer on Mt Wilson in California. However, they say the star’s brightness over the same period has shown no significant dimming.

Nobel prize winner Professor Charles Townes, 94, of the University of California Berkeley, said: “To see this change is very striking. We will be watching it carefully over the next few years to see if it will keep contracting or will go back up in size.”

Colleague Edward Wishnow said: “We do not know why the star is shrinking. Considering all that we know about galaxies and the distant universe, there are still lots of things we don’t know about stars, including what happens as red giants near the ends of their lives.”

The brightest supernova of recent times happened in a small companion galaxy of the Milky Way in 1987.

Picture: Professor Townes with one of the telescope’s three main components. (Photo: Cristina Ryan).

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