A Milky Way Galaxy Galactic Mystery

In 1974, even as Rees was speculating about black holes in active galaxies, American radio astronomers Bruce Balick and Robert Brown were observing the relatively quiet centre of our own galaxy. There they discovered a compact and variable radio source that looked much like a faint quasar - a type of far-away AGN that astronomers normally find near the edge of the observable Universe. But this object was "only" 26,000 light-years away, in our own cosmic backyard! Because it appeared to be inside a large, extended radio source already known as Sagittarius A, they named it Sagittarius A* (pronounced "A-star").

Over the next two decades, astrophysicists painstakingly observed Sagittarius A* at radio, optical, and near-infrared wavelengths. The breakneck speed (up to 1400 km/second) of gas and stars swirling around in the centre of the Milky Way began to convince them that something small yet massive - some 2.6 million solar masses - was indeed lurking in our galaxy's centre Was it a supermassive black hole, or just millions of closely packed more-or-less ordinary stars?

Only X-ray observations could provide definitive evidence - both because X-rays are a characteristic final silent scream of matter as it is engulfed forever by a black hole, and only X-rays can penetrate the thick gas and dust obscuring our direct view of the galactic centre Thus, a race was on to be the first to detect X-rays from Sagittarius A*.