microwave

Microwave is electromagnetic radiation (EMR) with wavelength in the general range of 1 mm to 1 meter or frequency in the range of 300 MHz to 300 GHz (various scientific/technology disciplines may use slightly different ranges). Microwave comprises the shortest radio waves, adjacent to infrared within the electromagnetic spectrum. The term microwave astronomy (and microwave radio astronomy) is occasionally used but a more common term is millimeter astronomy, for wavelengths on the order of a millimeter, that spans the threshold between microwave and infrared. The term microwave itself is likely used less often in astronomy than millimeter wave, but is commonly used in the phrase cosmic microwave background.

The atmosphere blocks much microwave EMR from reaching Earth, and space observatories have been used. (Water molecules in the atmosphere are what absorbs incoming microwave, and it is largely such microwave absorption by water molecules that heats food within a microwave oven.) However, the long-wavelength end of microwave is within the radio atmospheric window, multiple narrow windows allow some shorter microwaves through, and high-altitude observatories allow microwave telescopes to sense more signal. Some microwave sources, i.e., the subjects of microwave astronomy:

• cosmic microwave background (CMB) (which has inspired numerous CMB surveys, including three space missions)
• dust
• molecular clouds
• active galactic nuclei (AGNs)
• pulsars

Some mechanisms by which microwave is produced:

• black-body radiation (microwave is key for observing cool sources peaking within that band, and also useful for observing hotter sources, such as when they are obscured in other bands.)
• masers
• 21-cm line
• synchrotron radiation
• ionized plasma bremsstrahlung
• redshifted infrared sources