A massive black hole tore apart a sun-like star and gobbled up part of it, astronomers say.

Two orbiting observatories detected a powerful flare of X-rays after the black hole swallowed some of the material, the first strong evidence for a long-held theory that black holes consume stars.

"The last cry of help before the matter falls into the black hole is radiated in X-rays, which we have detected," said Gunther Hasinger, of the Max Planck Institute for Extraterrestrial Physics in Germany.

Illustration of a doomed star approaching a black hole. (Courtesy: NASA/CXC/M.Weiss)

The flare was thousands of times brighter than the billions of other stars in the galaxy, astronomers said at a NASA news conference in Washington on Wednesday.

As the star approached the centre of a galaxy some 700 million light-years from Earth, the black hole stretched the star, tearing it apart.

"This unlucky star just wandered into the wrong neighborhood," said Stefanie Komossa of the Max Planck Institute in Germany. Komossa led the international team.

From optical telescopes on the ground, galaxy RX J1242-11 seems quiet.

Using NASA's Chandra X-ray Observatory and the European Space Agency's XMM-Newton X-ray Observatory, scientists were able to observe the outburst.

They say the flare was caused when gas from the ripped up star was heated millions of degrees as it was drawn into the black hole.

"In this case, David can't outwit Goliath and Goliath wins because ultimately, gravity wins here," said NASA astronomer Kim Weaver.

The force is an extreme example of tidal disruption, the same kind of gravitational pull the moon exerts on large bodies of water on Earth.

The black hole was a messy eater, consuming between one per cent and 25 per cent of the star.

The Milky Way galaxy has a black hole but astronomers say none of the stars at the centre of our galaxy in in immediate danger of being swallowed.