It鈥檚 a meal the size of a solar system. The supermassive black hole at the centre of our galaxy has started chowing down on a passing cloud of gas three times as massive as Earth 鈥� an event that has been hotly anticipated.
In 2011, astronomers spotted that a gas cloud called G2, was on a collision course with the black hole, which has the mass of four million suns and is known as Sagittarius A* (Sgr A* for short). Today, the European Southern Observatory has released the first image of G2 slamming into the material surrounding the black hole. The researchers at ESO report that the front part of the cloud has passed its point of closest approach, and that the tail is still falling towards Sgr A*.
The collision is producing an interstellar light show 鈥� and at only 25,000 light-years away, we鈥檝e got front-row seats. Here are some of the questions astrophysicists hope the display will help us to answer.
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1. Why do quasars shine so brightly?
Quasars 鈥� explosively brilliant objects at the edge of the observable universe 鈥� are thought to be powered by supermassive black holes much like Sgr A*, yet our galaxy is far fainter than a quasar. That鈥檚 partly because Sgr A* doesn鈥檛 have as much gas and dust to munch on as its supermassive brethren did billions of years ago. But that鈥檚 not the whole story: even if you take the different sizes of their diets into account, Sgr A* wouldn鈥檛 be as bright as a quasar, pound for pound. A close-up view of G2鈥檚 close encounter with Sgr A* may give us a better understanding of quasars鈥� brilliance.
2. Does the Milky Way teem with baby black holes?
As G2 speeds through the centre of the galaxy, it may brush past black holes that are 鈥渙nly鈥� a few times the mass of the sun, and that were formed by collapsing stars. Some of its gas may be captured by these black holes, releasing bursts of X-rays as it swirls down their gravitational drain.
Computer simulations suggest that the centre of the Milky Way should be rife with these modest black holes 鈥� so .
3. Are there chinks in Einstein鈥檚 theory?
Einstein鈥檚 theory of gravity, general relativity, has passed every test physicists have thrown at it for nearly 100 years. But there鈥檚 good reason to suspect that it is incomplete: it doesn鈥檛 play nicely with quantum mechanics, the other pillar of modern physics.
G2鈥檚 close encounter with Sgr A* may let physicists study the behaviour of gravitational fields far stronger than any seen in our own stellar neighborhood 鈥� and therefore give us a glimpse of a chink in relativity鈥檚 armour. Earlier this year researchers showed that if Sgr A* does not obey general relativity, there could be surprisingly stable regions close to Sgr A*, where gas particles could hover. When G2 hits it, these ring-like structures may show up in the radio, infrared and X-rays signals given off during the collision.
4. Who blew the Fermi bubbles?
In 2010, the Fermi gamma-ray telescope spotted huge bubbles of hot gas spewing forth from the centre of the Milky Way. Astronomers are still puzzled by them, but one theory is that they are powered by high-energy 鈥渏ets鈥� shot out from the intensely hot disc of material surrounding Sgr A*. Such jets have been seen on a much larger scale in quasars, but how they form is still not entirely clear. And even if jets are not the culprit, Sgr A* is still the prime suspect for the formation of the bubbles. G2鈥檚 close approach may illuminate efforts to understand these strange superheated structures.
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