The biggest known star in the cosmos is in its death throes and will eventually explode, astronomers said on Wednesday.

Using a telescope at the European Southern Observatory in Chile, the astronomers said they had spotted telltale signs in a star called W26.

Located about 16,000 light years away in the constellation of Ara, or The Altar, the star has a diameter 3,000 times that of the Sun.

W26, first observed in 1998, is a "red supergiant", a term for a star that is as big as it is short-lived.

Stars of this kind typically have lifetimes of less than a few million years before they exhaust their nuclear fuel and explode as supernovae.

W26 is becoming unstable and shedding its outer layers, a key step in the death process, according to the paper, published in the British journal Monthly Notices of Britain's Royal Astronomical Society (RAS).

W26 is "the largest known star in the Universe", the RAS said in a press release.

The observations suggest "W26 is coming towards the end of its life and will eventually explode as a supernova".

W26 is surrounded by a cloud, or nebula, of glowing hydrogen gas whose atoms have been stripped of their electrons.

A similar cloud was found around the remnant of a star that became a supernova in 1987.

"The presence of the nebula, high stellar luminosity and spectral variability suggest that W26 is a highly evolved RSG [red supergiant] experiencing extreme levels of mass loss," says the paper.

W26 is located in a star cluster called Westerlund 1, home to hundreds of thousands of stars. It is the most massive stellar group in our galaxy, the Milky Way.

Observing Westerlund 1 can be a problem because light from it is affected by clouds of gas and dust.

But the ionised gas around W26 is a boost for visibility, which should make it easier to monitor what happens next.

When a red supergiant sheds its outer layers, it does so after enriching the material from nuclear reactions deep within the star.

The stuff that is spewed out includes many elements, such as magnesium and silicon, that are necessary for forming rocky planets like Earth.

"How this material is ejected and how this affects the evolution of the star is however still a mystery," the RAS said.