The Electromagnetic Spectrum
In order to understand infrared astronomy, a good place to start is understanding the electromagnetic spectrum. The electromagnetic spectrum is basically a way for astronomers and other scientists to group all types of radiation together. Every type of radiation from gamma rays to radio waves is part of the electromagnetic spectrum. Our eyes, in fact, are nothing more than detectors of a particular type of electromagnetic radiation: visible light. When our eyes detect visible light, we interpret different wavelengths as different colors. The longer the wavelength, the redder the light is and the shorter the wavelength, the bluer the light is. Infrared radiation is simply "visible light" at wavelengths longer than the eye can see.
As you have probably already guessed, astronomers separate the electromagnetic spectrum into sections according to wavelength (or frequency depending on how you want to think about it). It is, in fact, the only difference between different types of radiation. Gamma rays and X-rays have relatively short wavelengths, while infrared and radio waves have relatively long wavelengths. Visible light exists somewhere in the middle and it spans a very small portion of the spectrum. Infrared astronomers are merely those astronomers interested in the infrared portion of the spectrum and what it can tell us about the universe. Infrared radiation spans wavelengths of .75 microns to 14.0 microns where 1 micron=0.000001 meters. (Micron is short for micrometer).
Because visible light is one of the few types of radiation that can penetrate our atmosphere, our eyes naturally evolved to detect it. The Earth's atmosphere actually blocks out most of the electromagnetic spectrum. It only allows certain wavelengths to pass through. This presents a major difficulty for infrared astronomers because wavelengths longer than 1 micron have a hard time getting through. There are specific wavelengths in the infrared though called "bands" that can reach the surface; but for the most part, infrared light is absorbed by water vapor and carbon dioxide in the atmosphere. J, H, and K are bands often observed in the infrared because they correspond to wavelengths that are observable on the surface. They correspond to wavelengths of 1.25, 1.65, and 2.20 microns respectively.
Because most of the infrared radiation that hits the Earth is absorbed in the atmosphere, the surface of the Earth is not a very good place to observe objects in the infrared. So, astronomers have to put infrared telescopes into space, above the atmosphere, in order to observe objects fully in the infrared. The Spizter Space Telescope will be the newest of these satellite telescopes and will observe at wavelengths from 2 to 180 microns.
Properties of Infrared Radiation
Any object that has a temperature over absolute zero (-459.67 degrees Fahrenheit or -273.15 degrees Celsius or 0 degrees Kelvin), emits infrared radiation. And although infrared light is not visible to our eyes, we can still sense it as heat. When you look at a camp fire your eyes are detecting the visible light that is being emitted by the flames. However, when you warm yourself by that same fire, your body is sensing the infrared radiation emitted by it.
Even objects that we think of as very cold emit significant amounts of infrared radiation. To the left is an image of a melting ice cube taken in the infrared. It is color coded so that different colors correspond to different temperatures. The purples and blues correspond to the coldest temperatures where the ice is still solid, while the warmer temperatures, corresponding to the yellows and reds, appear further from the cube. Notice that even though an ice cube is very cold it still emits a detectable amount of infrared radiation.
Why Use Infrared Radiation to Study the Universe?
Because infrared radiation corresponds to wavelengths longer than the human eye can see, it allows us to look at the universe in a different way. Because of this, we are often able to detect objects that have been invisible to us in the past. A quick example will make this clear. Take, for instance, this picture of a dark fenced area.
The only thing that we are able to detect with our eyes is an opened chain linked fence. There is, however, more we cannot see. Taking an image in the infrared reveals that there is actually an ostrich within the fenced area. There is simply not enough visible light for our eyes to see the ostrich. However, because the ostrich is warm, it emits a large amount of its own infrared radiation. The ostrich is easily detected by an infrared camera, but had we only used a camera sensitive to optical wavelengths, we would have never known the ostrich was there. Applying this concept to astronomy, we can imagine that taking images in the infrared will allow us to learn more about the universe just as taking this simple picture allowed us to learn more about the fenced area.
Take a look at this image of the constellation Orion. The image on the left was taken in the optical, while the image on the right was taken in the infrared. One can easily see that the image taken in the infrared reveals much more about this portion of the sky than the optical image does. The infrared image reveals the presence of dust which glows brightly in the infrared. This is caused by the dust grains absorbing ultraviolet light from the surrounding stars. This, in turn, causes the dust to heat up and re-emit infrared radiation. The dust is not hot enough to glow in the visible range so we don't see it in the optical image. But the infrared image shows us that a large amount of dust is present here. The color coded picture illustrates how much infrared radiation is being emitted from this dust. So this picture, in fact, is showing us the amount of heat that is being emitted .
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