# Do black holes have a characteristic size?

First off, I know that a black hole can theoretically exist at any size, and I know about the Schwarzschild radius, etc. Snowflakes could also exist at just about any size, and they of course vary considerably in different environments. But there's a characteristic area, say ~1mm$^2$, associated with snowflakes; nobody ever saw a 1 m$^2$ snowflake fall from the sky.

So: is there a meaningful average or characteristic size for black holes that exist in nature? Or do we even have enough data to answer this question?

For example astronomers associate stars with the solar mass, although stars can of course be a couple orders of magnitude more/less massive. Galaxies also have a characteristic size, varying a few orders up or down from the size of our own.

My best guess (at least for a lower limit) is the size associated with the CMB temperature, as black holes smaller than this in nature should evaporate quickly, as they would Hawking-radiate more energy than gained through photon absorption. But I don't know if this is a meaningful consideration.

Your lower bound has certainly been considered. Even Wikipedia does the calculation in a number of places, noting that a black hole will be in equilibrium with the $2.7\ \mathrm{K}$ CMB at about the mass of the Moon.

The thing about black holes, though, is that there are multiple channels for them to be produced in nature. The remnants of collapsed stars should have masses around $1\ M_\odot$. This number probably varies far less than the masses of stars themselves vary, but it is set by very complicated physics involving degenerate matter and stellar evolution as a whole.

On the other hand, supermassive black holes in the centers of galaxies have masses well over $10^9\ M_\odot$. So right there you're looking at at least $9$ orders of magnitude of observed variation. Add to that another $7$ or so on the lower end if you believe lunar-mass black holes exist, and you can see why there isn't a standard $1\ M_\mathrm{BH}$ unit.

It should be noted that most all black hole properties of interest scale perfectly in proportion to some power of the mass, usually just the first power. Thus one often normalizes quantities to the mass of the particular black hole in question.

• Thanks for your insight! To clarify, your answer lies more towards "there is no meaningful size", rather than "we do not (yet) know of a meaningful size." – chase Feb 6 '14 at 8:57
• @chase Exactly. The only way you could get meaningful sizes is by limiting yourself to a particular class of black hole. – user10851 Feb 6 '14 at 15:35
• @chase Note this is also sort of implicit in your snowflake example. There's nothing stopping us from making a 1m^2 ball of snow. The characteristic size of snowflakes comes from the details of the formation process. – AGML Oct 4 '16 at 21:48

For essentially thermodynamic reasons, in dense stellar environments (globular clusters and galactic cores) heavy objects like black holes are supposed to sink into the centre of the gravitational potential, where they tend to merge with one another, forming black holes with larger masses. Over millennia as galaxies and clusters collide, these large central objects repeatedly merge, forming eventually supermassive black holes of $10^7$ - $10^{10}$ solar masses.