In the chair with G2’s discoverer

Astronomer Stefan Gillessen of the Max Planck Institute for Extraterrestrial Physics in Germany discovered the object G2 in 2011. He believes it is most likely a compact gas cloud

When G2 was seen heading towards Sgr A* in 2014, how surprised were you to see it emerge relatively unscathed?

I wasn't really that surprised – the simulations we carried out were not suggesting there would be a collision: they showed that the gas could run around Sgr A*.

What prevented the object from being torn apart in its entirety?

There’s a balance between centrifugal forces and gravitational forces as an object approaches a black hole, but it’s extremely difficult to fall in. For a circular object there's a very nice balance, but for an eccentric orbit there is swinging back and forth. Since the orbit of G2 is highly eccentric, you simply expect that it comes close to a black hole only for tidal forces to step in. The fact the object was tidally torn apart means it was not bound to anything else.

Would that dispel the theory there’s a stellar mass inside?

It's hard to prove. The problem is that you can make something hot enough or small enough for it not to be seen in theory, but it is very hard to test. I took the position that there could be a lump of gas because there is gas everywhere in the galaxy.

Are there strong arguments against a hidden stellar mass?

It has been shown that G2 slowed down, and this points to the gas not being gravitationally bound. You would expect the stellar source to continue on the original orbit at the same time as the gas is decelerating. But we haven't seen anything moving out. There is also a theory that this is a merged object – former binary stars colliding and shielding themselves in a cocoon – but six G objects would be too many merged objects out of the 40 stars we see in the galactic centre. Mergers are really rare and they'd need to live for millions of years.