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Astronomical theorists have been saying for decades that it should happen, but not until yesterday were scientists able to report direct proof that when a giant star explodes, super-dense black holes or neutron stars form.

Combining data from a variety of radio telescopes, Canadian and American astronomers have found the bright burst of radio signals previously associated with black holes and neutron stars coming from the middle of Supernova 1986 J. The supernova is the remains of a star 30 million light years away, which was an estimated 20 to 25 times the size of our sun.

Although light coming from SN 1986 J was first observed in 1986, the dust and gas of the exploding star at first blocked astronomers' ability to see characteristic radio waves.

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Recently, computer-based technology let them pierce the fog the stellar explosion created.

"The trick is that by combining telescopes from all over the world, we can synthesize a telescope the size of the Earth and thereby get a resolution about 100 times better than the Hubble Space Telescope," said Michael Beitenholz, a research astronomer at York University in Toronto.

Observations taken in 2002 and 2003 using this synthetic telescope found evidence of either a black hole or neutron star at the centre of where SN 1986 J once was.

The problem is to tell which it is, because the two have similar origins.

At the end of its life, after it has burned up all its available atomic fuel, a star will both explode and contract.

While its outer layers are ejected into space, theory says the core of the star should collapse down to an extremely dense object known as a neutron star. Neutron stars have diameters of a few tens of kilometres across (one-70,000th that of our sun), but the average mass of neutron stars is roughly that of our sun.

If a neutron star has a mass much larger than our sun, the gravitational forces it generates cause it to collapse and shrink into a black hole.

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A black hole is an intense gravitational sink in space with attractive powers so great that not even nearby light can escape its pull.

Seeing the bright radio signals that are the fingerprints of a black hole or neutron star was something of a shock for astronomers, even though they had been looking for them.

"It was expected to not be visible for another couple of decades," Dr. Beitenholz said.

At this point, they cannot tell which of the two patterns the star they are observing followed. Characteristically, a neutron star gives out a narrow, defining pulsed beam of radio energy known as a pulsar.

If the pulsar is facing away from Earth, its distinctive signature won't be seen.

Nonetheless, more refined observations, particularly with X-rays, should in the relative short term resolve the question of whether the star's death gave birth to the youngest black hole or the youngest neutron star ever observed by Earth astronomers.

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"If [the supernova]stays bright, definitely within a decade we should be able to say whether it is a black hole or a neutron star," Dr. Beitenholz said.

For the time being, the chief significance of the finding, which is published in today's issue of the magazine Science, is that the supposed theory of how black holes and neutron stars form is supposed no more.

"It is a textbook theory now witnessed for the first time," said Norbert Bartel, a York University astronomer professor and co-author of the paper.

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