Image caption Nasa's Chandra X-ray Observatory views the supernova remnant called SN 1979C

US astronomers are confident an object studied for 30 years in the M100 galaxy is a black hole.

If so, it would be the "youngest, nearby" such object to Earth.

It is still a long way off in human terms - more than 470 million, million, million km. But given the size of the cosmos, that is effectively just like our back yard, says the Nasa team.

The object, bright in X-rays, is in the same place on the sky that a giant stellar explosion was observed in 1979.

Although the blast was observed to occur just 30 years ago, its light actually took 50 million years to arrive at Earth.

The supernova remnant, called SN 1979C, has been investigated by a series of telescopes including the US space agency's Swift satellite, the European Space Agency's XMM-Newton observatory, and the German Rosat spacecraft. The most recent study has been conducted using Nasa's Chandra facility.

What's really exciting about it is that we know the exact birthday of the black hole Kimberly Weaver, Nasa astrophysicist

All show the the source of X-rays has remained steady since 1995. This suggests strongly the object is a black hole.

"While it's been steady, it's also been extremely bright; and we explain this high luminosity as evidence of accretion of supernova material back on to the black hole," said Daniel Patnaude of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass, who led the latest research.

"As it's accreted on to the black hole, it has heated up to extremely high temperatures and become very bright in X-rays. We can use the brightness of the accretion to find out that this black hole probably has a mass of around five times the mass of our Sun."

The scientists think SN 1979C, which was first identified by amateur astronomer and schoolteacher Gus Johnson in 1979, formed when a star about 20 times more massive than the Sun collapsed in on itself.

This end-of-life event, which occurs when the nuclear fusion processes at a giant star's core can no longer support its great size, is thought to be the most common way of making a black holes.

Being able to view an example at such an early stage in its evolution, and reasonably close to Earth, should therefore prove a boon to researchers as they try to understand such objects better.

Nasa astrophysicist Kimberly Weaver commented: "It's not just that possibly we've found the youngest, nearby black hole. What's really exciting about it is that we know the exact birthday of the black hole. We've found for the first time, possibly, the true birthday of a black hole."

Jonathan.Amos-INTERNET@bbc.co.uk