ESO/Gravity Consortium/L. Calçada

This ball of gas is racing around the black hole at our galaxy’s heart

Earlier this year, astronomers were looking for signs that S2, the star with the closest known orbit to the supermassive black hole thought to be at the center of the Milky Way, might—as predicted by Albert Einstein—deviate from the orbital path prescribed by Newtonian gravity.

But while they were watching, they spied something else: three bright infrared flares unrelated to the star (visualization above). Those flares, the researchers reveal today, are the signs of superheated gas racing almost as close to the black hole as possible without getting sucked in—at 30% the speed of light.

Observing the action so close to the galactic center, known as Sagittarius A*, is extremely challenging because it is distant, small, and shrouded in gas and dust. The team used the world’s largest optical instrument, the European Southern Observatory’s Very Large Telescope in Chile, and combined the light of its four 8.2-meter mirrors to get the resolution of a 130-meter virtual telescope using a new instrument called GRAVITY.

Following up on the serendipitous discovery, the astronomers saw the three flares move in small 45-minute orbits, and the polarization of their light rotated full circle in the same period. The scientists calculated that this must be material circulating around the black hole, just outside the closest orbit in which objects can move without being sucked in. The finding, the team says, is another firm piece of evidence that Sagittarius A* is the galaxy’s fathomless dark heart.