The Super-Massive Black Hole Stays on a Diet


At the very center of our galaxy lives a gigantic black hole with a mass more than four million times that of the Sun. The existence of such an immense monster does not constitute an anomaly in our Universe but rather a common occurrence since most (if not all) galaxies host huge black holes in their center. These super-massive black holes exhibit two different kinds of behavior: they are either actively swallowing large amounts of matter or they are quiet with almost no material falling into them.

The super-massive black hole of our galaxy is of this second kind and it is well known to astronomers because it is surrounded by several stars that orbit around it. It is thanks to the orbital motion of these stars that it has become possible to measure the huge mass of this monster. Indeed since the black hole is not devouring any material, it is basically impossible to directly spot it. Gas falling into a black hole would become extremely luminous and emit radiation in a process known as accretion. 

However, In 2011 a surprising discovery was made by a team of astronomers lead by Stefan Gillessen: a big cloud of gas was found to be headed towards the black hole at high speed, with a closest approach predicted to be happening in early 2014. The cloud was seen to be moving in an orbit that would have brought it so close to the black hole to be tidally disrupted by the huge gravitational force exerted upon it. This event sprouted a lot of excitement among astronomers because part of the disrupted cloud could have fallen into the black hole and/or could “light-up” in the process, revealing precious information about the super-massive monster and the environment around it.

The closest approach of the cloud is finally happening in these days and the first surprising results have been reported in a brief communication released today and lead by astronomer Andrea Ghez: with great surprise the cloud of gas has remained intact after its closest encounter with the super-massive black hole. This implies that the cloud is really not what it seems and the most likely hypothesis now is that a star is embedded in the center of the cloud. The gas obscures the light emitted by the star thus making it invisible, but the gravity of the star has kept the gas together even after passing very close to the black hole.

Now astronomers are following the behavior of the cloud (plus the embedded star) at Keck observatory, hosting one of the biggest telescopes on the planet. More exciting and probably unexpected news will certainly come in the next future as the interaction between these two objects keeps developing: stay tuned for more news!

Black Hole Information Paradox: Firewalls or Chaos ?

singularityA few weeks ago Stephen Hawking released a paper discussing the black hole information paradox that triggered an intense debate in the scientific community and had a large impact on the public. I have discussed his paper on this blog and I refer to that post for further reference. In short: Stephen Hawking suggested a solution to the black hole information paradox that invokes the existence of apparent horizons and the lack of firewalls. Continue reading

Do Mid-sized Black Holes Exist?

Black-Hole-WallpaperWe know that black holes exist in Nature with at least two different sizes: super-massive black holes, that live in the centre of galaxies and stellar-mass black holes. The origin and evolution of these two types of black holes are very different. The millions of stellar mass black holes that populate our galaxy are mostly formed during the gravitational collapse of a massive star. The formation of super-massive black holes is much less well understood. Continue reading

The Clouds Around the Monster

storm-clouds-chile_66705_990x742Neutron stars are few kilometres sized objects that come to life when a massive star dies in a devastating supernova explosion. A few months ago a very peculiar neutron star has been discovered in an even more peculiar location of our Galaxy. A magnetar is neutron star surrounded by a magnetic field so intense that it is billion of times stronger than the strongest magnetic field ever produced in a physics lab on Earth. Place yourself in such a strong magnetic field and you won’t survive a whole second.One such magnetar has been discovered in the Galactic Center, very close to the supermassive black hole that exists there. Continue reading

First Black Hole Mass and Spin Measured with Timing

586384-3x2-940x627Imagine to have a black hole spinnig around a normal companion star like our Sun. At some point of the evolution the black hole can start removing and devouring matter from the outer layers of its companion star, in a process known as accretion. During this phase the matter becomes so hot that it shines X-rays that are detectable from X-ray space telescopes orbiting around the Earth.

Continue reading