# Why is the spectrum of a star pretty much continuous? [duplicate]

I was reading about the development of the quantum theory when I got to the explanation for spectral lines. It's a topic that I've revisited many times but I came up with a question. I know that in some interactions with matter the amount of energy absorbed or transmitted in the form of photons is quantized but then what processes lead to the wide spectrum of frequencies that we observe in an object's electromagnetic spectrum? For example in a star what is going on in the nucleus that we get radiation in most frequencies?

• What you are talking about is atomic emission, which is a purely quantum mechanical process. But there is also a "classical" way through which charged particles (e.g., electrons) emit light, this happens when the particle is accelerated. As you probably know, the matter in the sun is a plasma which is comprised of charged particles (ions and electrons, not bound to each other) and when these accelerate they can emit a photon, whose frequency depends on the acceleration, not on the spectra of atomic states.
– user137661
Aug 8 '19 at 18:56
• @SV Sun's spectrum is close to a Planck spectrum, which doesn't reflect whether or not the particles are ionized, but simply reflect its temperature. In fact, although the interior of the Sun is ionized, its surface — which is what we see — is mostly neutral.
– pela
Aug 8 '19 at 19:33
• Is there reason to think that when free electrons are accelerated and emit light, that this light is quantized into discrete photons? Aug 8 '19 at 20:54
• @SV Don't thank me, it seems I'm wrong :)
– pela
Aug 9 '19 at 8:26
• @Pela Mostly neutral is correct. But if it were entirely neutral the photosphere would be much hotter, because we would see to greater depths. Hydride ions start to form below about 10,000K. The hydride opacity also has a very steep temperature dependence, which is why the photosphere is thin. Aug 9 '19 at 11:03

The main source of opacity that defines the visible photospheric continuum of the Sun is that due to H$$^{-}$$ ions. This has been understood for 80 years (Wildt 1939). These ions are in an equilibrium with neutral hydrogen atoms, where the additional electron can be ejected due to the absorption of photons (bound-free absorption) with a continuum of energies and vice-versa can be formed via the emission of a continuum (free-bound emission) of photon energies.