Eyeing the Centre of the Milky Way

Q: To start, can you tell us a bit about the environment at the centre of the Milky Way?

A: In the galactic centre, we know of a radio source: Sagittarius A*. It was discovered in 1974 but it turned out to be quite difficult to reliably determine the mass of this very compact radio source.

For example, the centre is hidden behind dense dust clouds, making it impossible to see in visible light. However, with the advent of infrared detectors, we can now look through these clouds, and so we now know that there are actually thousands of stars. We can observe these stars and see them moving individually.

Q: What about the galactic centre was your team observing and why?

A: We want to learn more about the black hole. We cannot observe a black hole directly — it is black, not even light can escape from it. However, we can study the surroundings of the black hole. Imagine you would like to observe a lion at a waterhole. Usually, it is hard to spot when it is lying below a bush. However, all the other animals that you actually can see start behaving differently. The lion influences its environment — and so does a black hole. The stars close to a massive black hole feel the strong gravity, and do not move on straight trajectories, but on Keplerian ellipses. By now, we know of 45 stars orbiting the black hole at the centre of our Milky Way. It is just like the planets are orbiting the Sun in our Solar System. The difference is that the stellar orbits around the Milky Way centre are randomly oriented (as opposed to an almost flat plane like our Solar System) and the orbital periods are somewhat longer, in the range of tens or hundreds of years.

A panorama of the Milky Way arching above the platform of ESO´s Very Large Telescope (VLT) on Cerro Paranal, Chile.

Credit: John Colosimo (colosimophotography.com)/ESO

Q: What’s so extraordinary about the star S2 and why is it useful for studying SgrA*?

GRAVITY shortly after installation at the Very Large Telescope (VLT). One of the instrument’s main science objectives is observing the environment around Sagittarius A*.

Credit: ESO/GRAVITY consortium

A: The star S2 is special in that its orbit is very close and that it is actually bright enough for making detailed measurements. S2 completes a revolution in only about 16 years — this means that we can actually study a full orbit (or more) in one astronomer’s lifetime. This is exactly what we did. Starting in 1992 we began observing its orbit, including the closest approach in 2002. In 2008, we had the first full revolution completed and have continued observations, covering now a second pericenter approach in May 2018.

Q: Where did you go for observations and why did you need to go there?

A: The galactic centre is located in the skies of the southern hemisphere, so the best thing to do is to go south. As the stars are quite faint we need a large telescope and for the best view, it helps to be high up and in a dry environment. And, most crucial, we need the images to be as sharp as possible since the stars in the galactic centre are very densely packed. That’s why we use the VLT operated by ESO in the Chilean Atacama Desert. It offers all the requirements we need.

Q: What needs to be researched after these observations?

A: After the discovery of the black hole, the next logical step was to investigate this black hole in more detail. Due to the extremely strong gravitational field, we expected to see the effects of general relativity — but only if we can look close enough. This is why we needed to push the technology. Our team has developed SINFONI and GRAVITY. With SINFONI we can measure the radial velocity of stars very accurately and GRAVITY gives us extremely sharp images and accurate positions.

Q: Why should we study the galactic centre?

A: Ever since the discovery of the radio source in the galactic centre, there have been discussions on its nature, and, in particular, if it could be the counterpart of a supermassive black hole, which is also the source type speculated to be at the centre of quasars. Unlike those galaxies, however, the Milky Way centre is right on our doorstep — and this makes it possible to study it in exquisite detail. And it is very a unique laboratory - where does one otherwise have access to a massive black hole to study the extreme physics close to an event horizon?