Observational astronomy

Main article: Observational astronomy

In astronomy, information is mainly received from the detection and analysis of light and other forms of electromagnetic radiation.^[13] Observational astronomy may be divided according to the observed region of the electromagnetic spectrum. Some parts of the spectrum can be observed from the Earth's surface, while other parts are only observable from either high altitudes or space. Specific information on these subfields is given below.

[edit] Radio astronomy

Radio astronomy studies radiation with wavelengths greater than approximately one millimeter.^[14] Radio astronomy is different from most other forms of observational astronomy in that the observed radio waves can be treated as waves rather than as discrete photons. Hence, it is relatively easier to measure both the amplitude and phase of radio waves, whereas this is not as easily done at shorter wavelengths.^[14]

Though some radio waves are produced by astronomical objects in the form of thermal emission, most of the radio emission that is observed from Earth is seen in the form of synchrotron radiation, which is produced when electrons oscillate around magnetic fields.^[14] Additionally, a number of spectral lines produced by interstellar gas, particularly the hydrogen spectral line at 21 cm, are observable at radio wavelengths.^[7][14]

A wide variety of objects are observable at radio wavelengths, including supernovae, interstellar gas, pulsars, and active galactic nuclei.

Infrared astronomy

Infrared astronomy deals with the detection and analysis of infrared radiation (wavelengths longer than red light). Except at wavelengths close to visible light, infrared radiation is heavily absorbed by the atmosphere, and the atmosphere produces significant infrared emission. Consequently, infrared observatories have to be located in high, dry places or in space. Infrared astronomy is particularly useful for observation of galactic regions cloaked by dust, and for studies of molecular gases.

[edit] Optical astronomy

The Subaru Telescope (left) and Keck Observatory (center) on Mauna Kea, both examples of an observatory that operates at near-infrared and visible wavelengths. The NASA Infrared Telescope Facility (right) is an example of a telescope that operates only at near-infrared wavelengths.

Historically, optical astronomy, also called visible light astronomy, is the oldest form of astronomy.^[15] Optical images were originally drawn by hand. In the late nineteenth century and most of the twentieth century, images were made using photographic equipment. Modern images are made using digital detectors, particularly detectors using charge-coupled devices (CCDs). Although visible light itself extends from approximately 4000 Å to 7000 Å (400 nm to 700 nm),^[15] the same equipment used at these wavelengths is also used to observe some near-ultraviolet and near-infrared radiation.