Why Do Stars Show Both Emission and Absorption Spectra?

[Nishantkumar19]

When you heat things up, they emit specific wavelengths of light, right? Like when you heat up sodium, it emits yellow.

But don't things emit shorter wavelengths of light at higher temperatures? Like how hotter stars are blue and colder stars are yellow. Since stars are mostly hydrogen, shouldn't they be a mixture of just the colors we see in Hydrogen's emission spectrum?

When we see an absorption spectrum of sunlight, there are black gaps for the different elements. But those same elements are heated up in the sun, so they should be emitting light too, right?

Is it that the outer layers of stars are cooler? So the inner layers emit the light, and the outer layers block specific wavelengths of light by absorbing and then scattering them? But surely even the outer layers would be hot enough to be emitting their own light, right?

Please help!

 

[DrClaude]

When you heat things up, they emit specific wavelengths of light, right? Like when you heat up sodium, it emits yellow.

Be careful here: that is the case of you heat up a low-pressure sodium vapor. In other words, you have to be in conditions where atoms can still be seen as independent of one another. Simply going to high pressures, where collisions between atoms is more important, will give light that is closer to white. The Wikipedia article on sodium vapor lamps has nice illustrations.

But don't things emit shorter wavelengths of light at higher temperatures? Like how hotter stars are blue and colder stars are yellow. Since stars are mostly hydrogen, shouldn't they be a mixture of just the colors we see in Hydrogen's emission spectrum?

Shorter wavelengths are due to higher-energy electronic excitations. Again, because of the conditions in which the atoms are, each line gets broaden due to collisions, the Doppler effect, etc. Also, in the case of a star, you have a plasma, where many atoms are ionized: it is not only hydrogen (and helium) atoms anymore. Stars are almost perfect blackbodies.

When we see an absorption spectrum of sunlight, there are black gaps for the different elements. But those same elements are heated up in the sun, so they should be emitting light too, right?

Is it that the outer layers of stars are cooler? So the inner layers emit the light, and the outer layers block specific wavelengths of light by absorbing and then scattering them? But surely even the outer layers would be hot enough to be emitting their own light, right?

Emission is mostly from the plasma, which gives wide-spectrum blackbody radiation. That light passes through the star's atmosphere, which is much less dense and atoms can be seen as independent, that's why you get a discrete absorption spectrum. There will be also emission due to the atmosphere, but it is isotropic, so overall there is light "missing." This is the same thing you get in absorption spectroscopy in the lab.

 

[Nishantkumar19]

Alright that clears it up a lot. Thanks!

 

[Ken G]

Is it that the outer layers of stars are cooler?

It depends on the line. Some absorption lines are due to scattering, as already explained above, but others (like the famous Sodium "D" lines in the Sun) are due to absorption of the light from the hotter photosphere and replacing it with emission from cooler regions overlying the photosphere, just as you are thinking. Indeed, some lines even receive contribution from the chromosphere, which is hotter than the photosphere, and when the chromosphere gets thick enough (as can happen in solar flares), these lines are actually seen in emission.

So the inner layers emit the light, and the outer layers block specific wavelengths of light by absorbing and then scattering them? But surely even the outer layers would be hot enough to be emitting their own light, right?

Yes, but it's less than what they absorb, if the temperature is lower, and scattering always reduces the light because it bounces some of the light back down were it can be reabsorbed.