<human>: Give me a shortened version of the text

Explanation:

[NGC 4889, 21 Billion Solar Masses](

[21 billion solar masses is 4.2 * 10 ^ 40 KG](

[Schwarzschild radius is 6 * 10 ^ 13 meters](

[Volume is ~10^42 cubic meters](

[**Density is 0.042 kg/m^3**](

[Air at sea level has a density of 1.225 kg/m^3](

[That leaves us with a ratio of 1 to 29.2](

Basically, the most massive black hole is much less dense than air, and only 3 times as dense as the Martian atmosphere. This actually makes perfect sense because the Schwarzschild radius  increases linearly with mass while the volume increases cubicly with radius. So, if you double the mass, the volume of the black hole increases by a factor of 8. For reference, our sun would have a Schwarzschild radius of about 3 kilometers. It's density would be 1.8434×10^19 Kg/m^3.
<bot>: If you double the mass, volume of a black hole at the Schwarzschild radius  (event horizon) increases by a factor of 8.