Black-body radiation and visible spectrum of stars

Question

I've done some reading on black bodies and black body radiation, after reading that stars are considered black bodies. As such I think the black bodies absorb all light, but also put off a radiation that reflects the color the star is, based on mass, etc.

Our sun is a yellow dwarf star, and this link, in reference to our sun, says "light is emitted at almost all energies in the visible spectrum".

Are we missing some energies in our visible spectrum from our sun, if so what are we missing and do we know why we are missing them? If we were in a different solar system with a different type of star (e.g. red giant, blue dwarf, etc.) would they be missing any of the visible spectrum?

Answer 1

The tricky bit is that there are other effects happening in stars that lead to the spectrum being "incomplete". Quoting the last paragraph of the link mentioned by OP,

When the light leaves the surface of the Sun, it is very nearly a continuous spectrum. However, as it passes through the Sun's atmosphere, gasses present in that atmosphere absorb specific wavelengths of light, leaving the pattern seen in the spectrum above. By studying those lines and matching them to known lines from elements, astronomers are able to determine what gasses are present in the Sun's atmosphere.

Light will leave the surface of the Sun at pretty much a black body spectrum, but the Sun contains multitudes of hydrogen and helium, for example. These elements (just like any other) have quite specific spectral lines: the differences in energies between their energy levels are quite specific and they can absorb only photons with those precise energies. Due to this effect, the frequencies corresponding to these precise energies will end up deprecated on the spectrum we can detect on Earth. Essentially, when we look at sunlight, we see there are a few frequencies missing.

These effects were first noticed, if I recall correctly, by Fraunhofer. By cataloguing the spectral lines of different elements, one becomes able to identify the composition of bodies by looking at their spectra.

If we had a different star, the spectrum could be different, since it depends on the specific chemical composition of the star's atmosphere.

Answer 2

Light is emitted at all energies in the visible spectrum of the Sun, nothing is missing. It is just that the solar spectrum is darker at some wavelengths; these are associated with discrete electronic transitions in atoms, ions and molecules in the photosphere.

To understand why, consider the Sun as a big ball of hot gas that gets hotter the deeper you go into it. The radiation we get from the Sun arises from material at a depth that is shallow enough that the photons stand a good chance of escaping. This is how the photosphere is defined.

That depth is determined by the opacity of the overlying material at any particular wavelength. If the opacity is larger at some wavelengths then the photospheric emission at those wavelengths comes from higher in the solar atmosphere and hence from material at cooler temperatures. The Sun is mainly composed of hydrogen and helium, but there are important traces of heavier elements and these provide increased opacity at discrete wavelengths (with some broadening due to Doppler motions and collisions between atoms) corresponding to electronic transitions between their individual energy states (that are characteristic of each element).

Since the brightness of the emitting material scales as $T^4$, then where the opacity is high and photons are arriving from higher, cooler material, the Sun appears relatively dark at those wavelengths - known as absorption lines. Note that it is relatively dark, not missing - you are still looking at temperatures of thousands of Kelvin even at the centres of strong absorption lines.

A natural question is then to ask, why can't we see to the centre of the Sun at other wavelengths? The answer to that is that there are various mechanisms that provide (lower) opacity over a broad range of wavelengths too. These include the photoelectric effect in metal atoms like sodium and potassium and the continuum absorption provided by the ionisation of H$^{-}$ ions. These generally limit us to being able to see to depths in the Sun that reach about 6000 K.