Lloyd said:
Doesn't supercritical mean above a critical temperature and pressure? What are the critical temperature and pressure of hydrogen? Is the critical temperature below 100K, which Verschuur thinks is the temperature of the filaments?

"Supercritical" literally means above the "critical point", which is where the temperature and pressure conspire to create a new property set. The matter is denser than a liquid, though it is compressible like a gas. And the electron shells merge, creating a shared electron cloud, like in a solid. According to the following diagram, the critical point of hydrogen is at around 10^1.2 K and 10^1.1 bar. But blackbody radiation only starts happening when you get into the thousands of K.