A Milky Way Galaxy Galactic Mystery

Astronomers have learned that the centre of our Milky Way galaxy harbours a long-sought black hole. But the finding has raised even more questions than before.

In the most suspenseful detective stories, the mystery deepens even as the plot reveals more clues. So has it been in real life for astrophysicists investigating the centre of our Milky Way galaxy. They hoped that NASA's Chandra X-ray Observatory would reveal a long-suspected black hole there - and indeed it did. But Chandra's revelations have raised new questions that baffle scientists perhaps even more than before.

A black hole is an object both so massive and so compact that not even light itself can escape its staggering gravity. For decades, theorists have argued that giant stars (ones at least 10 times as massive as our Sun) routinely end their lives as supernovas - catastrophic explosions that spray matter light-years through interstellar space, leaving behind only a dense remnant of the original star. If the remnant exceeds about 3 solar masses, it will become a black hole.

In 1974, Astronomer Royal Sir Martin Rees proposed that supermassive black holes - ones with a million or even a billion solar masses - might exist within the centres of some galaxies. The galaxies he had in mind have impressively active nuclei (centres) that shine as brightly as 30 billion or more Suns. They glow, unsteadily flickering, at all wavelengths from radio to gamma rays, and they spew powerful jets of charged particles into space. Rees reasoned that black holes gobbling matter were the sources of such turmoil.

"There's no other way we can think of that active galactic nuclei (AGNs) could put out so much energy," says Donald Kniffen, Chandra program scientist at NASA's Office of Space Science at NASA headquarters. "The only accepted theory is black holes." Furthermore, there's a dawning awareness that active galaxies aren't the only ones that might harbour such "monsters in the middle." Ordinary galaxies like the Milky Way have them, too.