[Alex Havers]

Alex Havers asked the Naked Scientists:
   
If supermassive black holes are so 'super massive', how can they sometimes have a density lower than air?

What do you think?

[graham.d]

There is much misunderstanding about Black Holes and our concepts are really theoretical because they have only been inferred by theory and observations of phenomena which we can explain by the presence of such objects. To answer your question we first need to decide what definition of density we should apply. When viewing a BH from a long way off we would only "observe" an Event Horizon. I say "observe" because really this is also a theoretical construct based on the idea that below this surrounding sphere nothing can escape the gravity of the mass within, not even light. And by escape, is meant escape to infinity (or at least a good distance away). There is usually an accretion disk surrounding the BH where matter is energetically orbiting. We could define density as the mass inside the BH divided by the volume contained within the Event Horizon though we cannot know what the distribution of matter within the BH is like. In any case, for most of the observed BH's (or what we think may be BH's) the density would be very high by this measure. On the other hand, if the BH were large enough (and was really super massive) the density could be quite low. You need a BH of around 10,000,000,000 solar masses (which would have a Schwarzchild radius of about 30,000,000,000 Km) which would result in a lower than air average density). It is thought that a supermassive BH may be at the centre of a Quasar discovered last year and that it weighs in a 18,000,000,000 Solar masses.