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.



HII 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 HII region.


An HII 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 Strömgren sphere. The most abundant element here is, as one might easily guess, Hydrogen. The Hydrogen here is not only photoionised but also recombined. So, what happens? The hydrogen is excited to n=3 with the UV, and then, the elctron comes back, but to n=2. this releases energy in the optical range, and we see this as the H-alpha emission.

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