Infrared radiation is electromagnetic radiation (i.e. light) that has a longer wavelength (shorter frequency) than optical (visible) light.
While there is some ambiguity to where the boundaries are for the various "forms" of electromagnetic radiation, the infrared band is typically taken to begin just beyond the red wavelengths of the optical band, around 0.74 micrometers, and extends up to 300 micrometers.
In terms of frequency, these wavelengths correspond to about 400 Terahertz (THz) and 1 Terahertz respectively.
Although infrared is "invisible" to the naked eye, actually more energy from the Sun reaches the surface of the Earth in this form than it does as visible or ultraviolet light.
Searching for Infrared Radiation from Outer Space
Infrared astronomy is conducted using the same technologies and instruments that are used in optical observatories, such as the Hubble Space Telescope (HST). So, infrared astronomy is sometimes classified as part of optical astronomy.
In fact, some observatories, like the HST, contain infrared detectors to supplement their optical capabilities.
But there are issues in conducting infrared astronomy research. First of all, heat is a primary emitter of infrared radiation. So a orbiting observatory has to shield the detectors from any heat sources, and also cool the detectors using liquid helium. As the instrument itself begins to heat up during use (like your home computer gets warm while you are using it) the infrared signature can interfere with the data acquisition.
This makes doing infrared astronomy difficult as well as limiting. Since infrared observatories are mostly in space, their operational lifetimes are determined by the amount of liquid helium that is onboard. As that supply runs out, the detector will no longer be able to discern the extraterrestrial sources of infrared radiation from that of the observatory itself. At this point, if there is no mechanism for re-fueling the satellite, it is decommissioned and will eventually crash back down to Earth. (Or in the case of observatories that are not bound in orbit around Earth, they will simply continue to orbit along their designated path.)
Types of Infrared Radiation in Astronomy
Those conducting research using infrared light undoubtedly focus on a subset of the possible infrared wavelengths. And depending on the type of working being done, there are various terms used to describe these subsets.
In astronomy the terms Near Infrared, Mid Infrared and Far Infrared are used. The exact cut-off points are variable, and usually depend on the type of instrument used and where the results are published. But, in general the breakdown is as follows:
- Near Infrared (NIR): Wavelengths ranging from 0.74 micrometers to 5 micrometers.
- Mid Infrared (MIR): Wavelengths ranging from 5 micrometers to about 35 micrometers.
- Far Infrared (FIR): Wavelengths ranging from about 35 micrometers to 300 micrometers.
Importance to Astronomy
In general, it is important in the study of astronomical bodies to construct a complete electromagnetic profile of the object. Whether one is studying a main-sequence star, a neutron star or a black hole, there is much information to be gleaned by studying the object in all wavelengths.
Specifically, however infrared studies of certain types of objects can be especially fruitful.
Studies of molecular clouds in space can reveal a star, hidden within, due to the infrared light that is emitted as the gas is heated up. In most cases the star would otherwise be invisible in the optical band.
Also stars that are still forming, called protostars, will emit infrared radiation long before the shine in the optical.
Infrared radiation is also key to detecting asteroids and other solar system objects that otherwise would not be visible. These objects are heated slightly by the warmth of our Sun, even out in the far reaches of our solar system. This tiny amount of radiation that is reflected back towards us, illuminates what would otherwise not be seen.
And on the other end of the spectrum, figuratively speaking, are the cores of galaxies. These violent places are often cloaked in gas and dust that would otherwise obscure their presence. Using infrared instruments the interactions of stars, black holes and supernova remnants can be seen interacting.
