Question: How large is the accretion disc?

Bella Boulderstone answered on 18 Jun 2019:

Great question! Accretion discs of black holes can vary in size depending on how big the black hole is and how it’s eating (or accreting).

There are generally two ‘flavors’ of black hole: stellar mass (5-50ish times the mass of our Sun) and supermassive black holes (hundreds-billions of times the mass of our Sun). (Where are the ones in between? We don’t know, they’re missing! People are looking into it right now!)

Accretion discs are nature’s way of funneling in stuff to the black hole and not all black holes have stuff around them so not all black holes have accretion discs.

However, the ones that do: stellar mass black holes can end up in pairs with a star that feeds it. The black hole rips material off the star and wraps it around itself, forming an accretion disc which funnels the star-stuff into the black hole. The size of these accretion discs depends on the distance between the black hole and its companion star which can be 1,000,000 – 10,000,000 km (thanks to my friend John for getting me that number!).

Supermassive black holes: these have an accretion disc and then around that is the dusty donut thing that I study. The size of these discs also depends on the size of the black hole, but they can be between 5-20 light days (the amount of distance that light covers in one day, like a light year). This doesn’t seem like much, but 10 light days is about 300,000,000,000 km!

I hope I’ve answered your question!