New discoveries Milky Way’s black hole surrounded by ring of not-so-hot gas Read time: 3 minutes There’s a big black hole in the core of the Milky Way. A huge region in space where gravity is so strong that nothing can escape, not even light. This monster black hole is four million times more massive than our Sun. Luckily, it’s also very far away: 26,000 light-years. It can’t do us any harm. In the past, astronomers have discovered that the black hole in the Milky Way center is surrounded by huge clouds of gas. Some of this gas falls towards the black hole. Before it disappears over the edge, it heaps up in a flat, rotating disk. But the ins and outs of this process (known as ‘accretion’) are not well understood. New ALMA observations of the Milky Way’s supermassive black hole may help to solve the mystery. Astronomers using ALMA made a surprising discovery: they found a ring of relatively cool gas orbiting the black hole. Astronomers already knew that there’s a lot of extremely hot gas surrounding the black hole. This gas has a temperature of some ten million degrees. It is so hot that it emits energetic X-rays. It can only be observed by X-ray telescopes in space. Meanwhile, millimeter-wave telescopes like ALMA (but much smaller) had also discovered cooler hydrogen gas, within a few light-years from the black hole. This gas has a temperature of ‘only’ some ten thousand degrees. Thanks to its sharp eyesight, ALMA has now found a rotating ring of this not-so-hot gas, pretty close to the edge of the black hole. The ‘cool’ gas is orbiting at a distance of just one-hundredth of a light-year. That’s about a thousand times the distance between the Sun and the Earth. It’s not a huge amount of hydrogen, by the way. You would need ten times as much to create a giant planet like Jupiter. But the ‘cool’ rotating ring is a surprising find. After all, the extremely hot, X-ray emitting gas doesn’t show any rotation. Hopefully, the discovery will help astronomers to better understand how gas – both hot and not-so-hot - accumulates in the so-called ‘accretion disk’ that surrounds the black hole. What? Our Sun is one of a few hundred billion stars in the Milky Way galaxy. In the very center of the Milky Way galaxy is a bright source of radio waves. It is known as Sagittarius A*, after the constellation in which it is located. Over the past decades, it has become clear that Sagittarius A* is a so-called supermassive black hole, weighing in at four million times the mass of the Sun. With huge infrared telescopes, astronomers have seen giant stars whirling around this invisible monster black hole. Now, ALMA has also found a rotating ring of relatively cool hydrogen gas orbiting the black hole. Who? The ALMA observations of the Milky Way’s central black hole were carried out by Elena Murchikova of the Institute for Advanced Study in Princeton (USA). For this research, Elena teamed up with Sterl Phinney of the California Institute of Technology (where she also works), Anna Pancoast of the Harvard-Smithsonian Center for Astrophysics, and Roger Blandford of Stanford University. The four astronomers published their results in the scientific journal Nature on June 5, 2019. ALMA URL