You may have seen this video showing the relative sizes of different stars, and how massive some of them are:
What’s often not appreciated is that as well as being mind-bogglingly large, the largest supergiant stars are also incredibly tenuous.
Just how tenuous? Let’s take Betelgeuse as an example. There’s some uncertainty about the exact figures, but it has a mass about eleven times that of the sun, and a radius of about 900 solar radii. In other words:
On earth, a density of just 2×10-5 kg/m3 would be considered a hard vacuum. It is just one sixty thousandth of the density of the earth’s atmosphere, 1.2 kg/m3. The density of the largest known star in the video, VY Canis Majoris, is roughly similar.
Yet despite this, this gives Betelgeuse an escape velocity of about 70 km/second at its outer extremity. Betelgeuse’s enormous size is due to extreme nuclear fusion reactions in its core, where helium is converted into carbon and oxygen, and then subsequently into heavier elements, culminating in the conversion of silicon to iron. Once its supply of silicon is exhausted, nuclear fusion is no longer possible, the star collapses in on itself, and a supernova explosion results. For Betelgeuse, it is estimated that this will happen sometime in the next million years or so.