Have you ever wondered why when some objects heat up, they begin to glow. Take for example this electric stove heating element. Why does it glow in the visible band when it becomes hot?
The answer to why the heating element glows the way it does, lies within the concept of blackbody radiation. A blackbody is a theoretical object that is a perfect emitter and absorber of radiation. Such an object would re-radiate its energy in a spectrum which depends only on the temperature of the body (and not, for example, on its composition). No object is a perfect blackbody because it does not re-radiate all the energy it absorbs. It is, however, possible to approximate most objects as blackbodies. A star, for example, acts almost exactly like a blackbody.
The foundation of blackbody radiation lies in the idea that radiation is released from blackbodies in the form of "quanta" or little discrete packets of light called photons. This is a different way of describing light than previously discussed. In this setup, the higher the energy of the photon, the bluer the light, which corresponds to a shorter wavelength. The lower energy photons, on the other hand, correspond to redder light or longer wavelengths. These photons only exist at certain quantized energy levels. However, because the increment between energy levels is so small, the spectrum being emitted by the blackbody appears to be continuous.
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Max Planck First introduced this idea in 1900 along with Planck's law for blackbody radiation, which is a function of temperature only. Although the function looks complicated, it is not so bad. Most of the variables are, in fact, constants: "h" represents Planck's constant, a fundamental constant of nature relating to the quantization of photons; "c" stands for the speed of light (300,000,000 m/s); "k" is Boltzmann's constant; the Greek letter lambda stands for the wavelength of light; and "T" stands for the temperature of the blackbody. |
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Lets go back to the heating element. Remember that when blackbodies emit radiation, they emit it in the form of a spectrum. In other words, they emit different energy (color) photons all at the same time or different wavelengths of radiation depending on how you want to think of it. Because the different wavelengths are emitted all at the same time, we see them "mixed" together and therefore, we only see one color of light. If we were to separate the light according to wavelength, we would see an optical spectrum like the one shown to the left. |
So how does this relate to infrared astronomy? Well, remember that there is a whole range of wavelengths redder than the eye can see. And those wavelengths that are just longer than the reddest color we can perceive with our eyes (.75-14.0 microns) corresponds to thermal or infrared emission. These wavelengths are just as much a part of the heating element's spectrum as the optical portion is, we simply can't see it.
The German physicist Wilhelm Wien was the first person to study thermal radiation in depth during the end of the 19th century. He and his contemporaries were the ones that discovered that all bodies constantly emit radiation and that the spectrum of this radiation does not depend on the composition of the object. He also derived Wien's Law:
This is not as intimidating as the Planck function but it reveals a fundamental truth of blackbody radiation. That is, the hotter a blackbody becomes, the shorter its wavelength of peak emission becomes. The wavelength of peak emission is simply the wavelength at which a blackbody emits most of its radiation. Lets look at a plot to make this clearer. Click here to view the plot.
Take a good look at the plot and the axes. What you are looking at is a plot of energy versus wavelength. In other words, the amount of energy emitted via electromagnetic radiation by a blackbody, in our case a star, versus the wavelength of that radiation. Think of it as a spectrum, like the one above, except it shows all wavelengths, even the ones we can't see. Furthermore, it also shows a quantitative measure of the amount of energy emitted at each wavelength, kind of like a brightness for each color (wavelength) of light. This type of plot is known as a spectral energy distribution or SED. This will become important when we discuss diagnosing disk geometries.
Now lets extract what information we can from the plot. Notice that as the temperature of the star (blackbody) increases, the wavelength of peak emission shortens. This is a direct result of Wien's Law. This means that as the temperature of the star increases, more radiation with wavelengths closer to the blue part of the spectrum will be emitted. In addition, there will be more radiation at wavelengths shorter than the eye can see emitted from the star, such as ultraviolet light. The opposite is true for cooler stars. The cooler a star is, the more radiation it will emit toward the redder part of the spectrum. This means that hotter stars will appear bluer and cooler stars will appear redder when we look at them with our eyes. This means that by examining a star's color you can determine it temperature.
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Although one can describe radiation emission in terms of it being redder or bluer, the range of visual wavelengths is very small, which is illustrated here. Its the same type of plot as before except it shows nicely the amount of radiation that is emitted at wavelengths we can't see with our eyes. Notice the 300K blackbody; it emits no radiation in the visual band but emits heavily in the infrared. It is not hot enough to emit wavelengths that small, which is why a heating element on an electric stove will get warm before it glows. It has to heat up to a certain temperature before it glows at wavelengths we can see. But until then, we can sense the infrared radiation as heat. |
This concept is key to understanding how planetary systems form. Because we believe that planets form from disks of dust and gas that exist around young stars, and because this dust emits heavily in the infrared, it is very important to have a firm grasp on these concepts.
To learn more about how these disks form click here!!!
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