The photoionization process absorbs energy (light), it doesn't release it....

So how can some astrophysicists say that photoionization releases the light responsible for the coronal emission of a star going nova?

Are some atoms being immediately recombine with their loose electrons? Thereby releasing the near-infrared light?

  • 5
    $\begingroup$ Examples please. $\endgroup$
    – ProfRob
    Dec 10, 2021 at 7:03
  • 1
    $\begingroup$ Specifically, examples of published papers or textbooks $\endgroup$ Dec 10, 2021 at 14:14
  • $\begingroup$ Generally speaking, without or with specific relation to astronomy, emissions from an ion does not even imply recombination. The attained ion can be in an excited state and relax by emission, too. $\endgroup$
    – Alchimista
    Dec 12, 2021 at 8:39

2 Answers 2


You essentially answered your own question; the short answer is yes, recombination is the source of the referenced photons.

A substance in local thermodynamic equilibrium will find itself in one of three situations: completely ionized, partially ionized, or completely unionized. Statistically the third of these options is least likely if we’re dealing with stars (normally there’s a percentage ionized even if it’s really small), and the first option is only sustainable in really, really hot and constant areas, like O and B stars. The rest of our situations will be partially ionized material.

For a partially ionized material to be stable (e.g., not become completely ionized or completely unionized) the short term recombination and ionization rates must be the same. This may seem counterintuitive, but if you think about it, it makes sense in the short term (and in the long term if conditions aren’t changing), the contrary would mean that the material would eventually become completely ionized or unionized.

With that being said, now we consider emission lines. When these ionized atoms recombine, they’re going to give off emission lines in every direction. So a little piece of corona (which stretches far beyond the photosphere of a star) off to the side will give off emission lines that will be the results of atoms being photo ionized and then recombined, giving us the near infrared light that you’re referencing as emission lines.

The recombination may be immediate but it certainly doesn’t have to be and on average probably isn’t; it’s a process that happens over time.

If the bit about emission lines isn’t clear I can elaborate a great deal more on how they are distinguished from absorption lines, why they’re present, etc., I just didn’t want to go off on a tangent on a follow up question that you might not have! Just let me know in the comments if that bit isn’t clear.


Photoionisation is an absorption process. An atom interacts with a photon and the photon energy is used to remove an electron from the atom.

The reverse process, known as recombination, is where an electron is captured by an ion and a photon is emitted.

Since the free electron can have a continuum of energies, then the light emitted has a continuous spectrum. However, it is often the case that the electron is first recaptured into a high energy level and then subsequently makes a transition to a lower level in a spontaneous emission process. This results in the emission of photons at discrete energies/frequencies, which is likely what your source is referring to.

Why does your source say photoionisation is the origin of the emission? Perhaps because recombination needs free electrons. Photoionisation is one way of producing those free electrons.


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