Betelgeuse, normally one of the brightest stars in the sky, has started to behave strangely.

Astronomers think the unusual dimming could be a sign that it is about to explode into a spectacular supernova – though nobody quite knows for sure what will happen to the famous star.

The dimming has been noted in recent weeks and have been a gradual process. Each time astronomers observed the star through December it seemed to be less bright, to the point that it has become dimmer than ever before.

Now astronomers think that the behaviour could indicate that something might be about to happen to the star.

Betelgeuse is already approaching its death: it is relatively young, but red giants like Betelgeuse burn bright and then die away in a spectacular supernova.

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If that happens, it will be visible in detail from Earth. Betelgeuse is only 700 light years away, and so its explosion into a supernova would be clearly visible from Earth, giving astronomers a chance to watch one of the universe's most intense and spectacular events from relatively nearby.

But while the dimming is clear – it has dropped by a factor of two, enough to be seen with the naked the eye – there is nothing to say for sure that it could be a sign that it is about to go supernova. And even if it does, there is no indication that will happen any time soon.

There are also a variety of other explanations for the dimming. The star is already seen as semi-variable, pulsating as a variety of vast processes happen around it, and the latest changes could be a more dramatic version of the same natural changes.

Betelgeuse is famous in part because it is so bright. It is the biggest of any star we can see apart from our own sun, and is traditionally one of the 10 brightest stars in the sky.

But its recent unusual behaviour have led it to drop out of that chart. It has become so dim that it has lost its place even in the top 20, astronomers say.