microwave

Microwave is electromagnetic radiation (EMR) with wavelength in the range of 1 mm to 1 meter (frequency 300 MHz to 300 GHz). It comprises the shortest radio waves, adjacent to infrared.

The terms microwave astronomy and microwave radio astronomy are occasionally used and the more common term, millimeter astronomy spans the border between microwave and infrared. The atmosphere blocks much microwave EMR from reaching Earth, and space observatories have been used. 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 at least 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