H-two, Help-too. Kick-two, Kry-too.

Hydrogen (he/him) sees a UV photon. He is excited. So excited that his electron jumps up two levels. Being far from home, the electron cries. So, the Hydrogen lets the electron come one level closer, still keeping a bit of the excitement. H II regions are cool. I mean, temperature-wise. It could be seven thousand to fifteen thousand kelvin. And this depends on what's in there and what causes it. So, what causes the temperature? Photoionisation. And who causes the photoionisation? A star! A massive central star. So, let me introduce to you, an H II region. An H II region is a region of photoionised gas surrounding a massive star. A sweet help by the star to its surrounding region. So, what really happens? There's a big star. There's gas around it. The star radiates UV rays. And what does UV do? They ionise the ambient medium. i.e., the UV photons are absorbed by the gas, and hence a spherical region is formed. This is a Str ömgren sphere. The most abundant...