Supermassive Black Hole

I mentioned supermassive black holes briefly in my last post as one of the potential causes of the power outputs of ultraluminous galaxies. However, I don't actually know a lot about them, so I went to my favorite place for quick information, Wikipedia.

http://en.wikipedia.org/wiki/Supermassive_black_hole

According to this esteemed database (don't worry, the article appears to be properly referenced), while stellar black holes have masses on the order of those of large stars (makes sense), supermassive black holes may have millions of solar masses. More interesting, though, is the claim about the density of supermassive black holes:

The average density of a supermassive black hole (defined as the mass of the black hole divided by the volume within its Schwarzschild radius) can be much less than the density of water (the densities are similar for 10⁸ solar mass black holes^[5]). This is because the Schwarzschild radius is directly proportional to mass, while density is inversely proportional to the volume. Since the volume of a spherical object (such as the event horizon of a non-rotating black hole) is directly proportional to the cube of the radius, average density decreases for larger black holes, being inversely proportional to the square of the mass.

The Schwarzchild radius is the distance from the center of an object such that, if an object's total mass is contained within the sphere of the Schwarzchild radius, the necessary escape velocity would be the speed of light. Which, unless you are some sort of rule-breaking neutrino, is against the law.

A black hole occurs when the radius of the object is less than that of its Schwarzchild radius. Fortunately for us, most things (like, say, the Milky Way) are not so compact.

But, the larger the Schwarzchild radius, the less compact (ie, dense) and object needs to be to still fulfill the criteria to be a black hole. And since the Schwarzchild radius is proportional to mass, the more massive an object, the less dense it needs to be. Wow.

Further links in case you're interested.