Kirchhoff's Laws and Star Spectra

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  • 0:01 How Spectra Are Formed
  • 1:14 A Continuous Spectrum
  • 1:48 An Emission Line Spectrum
  • 2:31 An Absorption Line Spectrum
  • 3:29 Putting It All Together
  • 5:54 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov
This lesson will define for you three laws associated with spectra and how they relate to the composition of stars, as well as how they relate to atoms and wavelengths.

How Spectra Are Formed

Another lesson explains to you how it is that a unique spectrum of a star is formed thanks to its particular makeup. In case you haven't watched that video, no worries; I'll give you a quick summary of it all.

A photon will be absorbed or emitted when an atom's electron transitions from one energy level to another. The precise wavelength of the photon that is absorbed and emitted depends on the actual energy difference between the two levels the electron jumps to and from. A spectral line is formed when this happens and represents an electron's jump between two specific energy levels.

Combinations of lines found in a star's spectrum are a clue to the elements found in a star, something akin to a barcode line being linked to a specific product, or fingerprint ridges being linked to a unique person. That's because each unique atom will produce different spectral lines.

Thanks to the understanding of this all, scientists were able to boil down the complexities of how spectra are formed into three laws: Kirchhoff's Laws. These laws point to different kinds of spectra, which we'll be defining.

A Continuous Spectrum

Kirchhoff's First Law states that a hot solid, liquid, or dense gas produces a continuous spectrum. A continuous spectrum is a complete arrangement of colors, like that of the rainbow, devoid of spectral lines. An example that can show this would be something like an incandescent light bulb. If you were to take such a light bulb, turn it on, and pass its light through a prism, you'd get this pretty-looking, complete rainbow of colors - that's a continuous spectrum, simple as that.

An Emission Line Spectrum

Kirchhoff's Second Law states that a thin, hot gas, produces an emission line spectrum. An emission line spectrum is a spectrum with bright spectral lines juxtaposed against a dark background. An example of this would be a sign using a gas, like neon, for a neon sign.

Basically, an excited gas, in this case neon, will emit photons from its excited atoms. These photons come out as bright lines of a specific wavelength unique to the atom that's producing it, on an emission line spectrum. Because of these bright lines, emission line spectra are also known as bright-line spectra.

An Absorption Line Spectrum

Kirchhoff's Third Law, the most important one for our lesson on star spectra, tells us that a thin cool gas, in front of a source of continuous spectrum, will form an absorption line spectrum. An absorption line spectrum refers to dark spectral lines interspersed on a continuous spectrum. For a particular gas, the dark lines of its absorption spectrum will appear in the same wavelengths as that same gas's bright lines of its emission spectrum.

In this case, it's as if our light bulb, from the first law, is now surrounded by a cloud of gas. Atoms of that gas will absorb photons of wavelengths particular to each kind of gas. Because these photons are absorbed, their wavelengths are missing from the spectrum, creating dark lines. This is why absorption line spectra are also called dark-line spectra.

Putting It All Together

What Kirchhoff's laws mean, when put together, is that if white light passes through a gas, that gas's atoms will absorb specific wavelengths from the white light. If you were to use a prism or a spectrograph and observe the light straight on, you would get a continuous spectrum produced by that light, but one that is missing certain wavelengths because the gas that surrounds that light is absorbing specific wavelengths due to the atoms in the gas.

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