Dark lines in a continuum spectrum is common. But are there bright lines too?
By the stimulated emission process, the light can also be amplified, right?
In this case, we should get bright lines instead of dark lines.
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$\begingroup$ Can you provide a bit more context? Half of astronomy is based on the existence of emission lines, both from stimulated emission, but also from other types of emission processes, like recombination and collisions. Is this what you mean? $\endgroup$– pelaCommented Feb 11, 2016 at 14:36
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$\begingroup$ yes : stars.astro.illinois.edu/sow/spectra.html#nebulae $\endgroup$– James KCommented Feb 12, 2016 at 14:12
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$\begingroup$ The dark lines on the emission spectrum of the sun are called Fraunhofer lines. en.wikipedia.org/wiki/Fraunhofer_lines $\endgroup$– MaxWCommented Apr 15, 2016 at 0:00
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2 Answers
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The key thing about lines is that they have a higher opacity than the continuum, so they are generally formed in layers that are above where the continuum formed. Thus there are two general ways to get emission lines (i.e., lines that are brighter than the continuum), one is to make the higher regions hotter than what is below, and the other is to make them physically larger. The first means that at line wavelengths, the object looks brighter, and the second means that at line wavelengths, the object looks larger. The first happens in what is called a "chromosphere" of the star (a shell that is hotter because of some additional heating going on above the photosphere of the star), and the second happens if the star has a wind or anything else that physically extends it in ways that are transparent in the continuum but can be seen in lines.
The third way to get emission lines is if the continuum is not optically thick (i.e., the object is transparent in the continuum), as happens with various low-density nebulae, but your question seems to stipulate that there is a bright continuum there, so there is an opaque object emitting something akin to blackbody radiation.
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$\begingroup$ And perhaps it should be added that you can get emission lines a fourth way, by stimulated emission as mentioned in the question, but this is quite rare as it requires a "population inversion" like you need in lasers. When this does happen, which is not often, it can happen in the microwave regime, so you will hear of astrophysical "masers," but few emission lines are formed that way. $\endgroup$– Ken GCommented Jan 2, 2017 at 16:54
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Ionised gas will emit radiation at particular frequencies, Nebulae that generate their own light this way are called "Emission nebulae" and they have spectra that are characterised by a collection of bright lines. They contrast with reflection nebulae that are lit by bright stars.
Stars also have part of their spectrum that exceeds what would be expected from a blackbody due to emission from gas in their photosphere's
