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# Astronomy and Astrophysics

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Astronomy, which etymologically means laws of the stars, is the science whose subject is the observation and explanation of events outside the earth. Once it was understood that the elements that made up the "celestial objects" were the same that made up the earth, and that the same Laws of physics applied, Astrophysics was born, as an application of physics to the phenomena observed by astronomy. Most (if not all) astronomers now have a strong background in physics, and observations are always put in an astrophysical context, so the distinction between astronomy and astrophysics almost doesn't exist anymore.

Astronomy is one of the few sciences where amateurs still play an active role.

The study of the planets of the solar system is (in recent times) sometimes considered a different discipline, called Planetary Sciences or Planetology.

For a history of astronomy click here.

Given its huge scope, astronomy is divided in different branches. The divisions are not unique, however, and the intersections, as well as astronomers who work in several areas, are the rule more than the exception.

In astronomy, the main way of obtaining information is through the detection and analysis of electromagnetic radiation. A traditional division of astronomy is given by the region of the [electromagnetic spectrum]? observed:

- [Optical Astronomy]? refers to the techniques used to detect and analize light in and slightly around the wavelengths than can be detected with the eyes (about 400 - 800 nm).
- [Infrared Astronomy]? deals with using infrared light (wavelenghts longer than the red).
- [Radio Astronomy]? uses completely different techniques: to detect radiation of wavelengths of mm to cm, the receivers are similar to those used in radio broadcast transmission (which uses those wavelengths of radiation).

Optical and Radio astronomy can be done using ground based observatories, because the atmosphere? is transparent at those wavelengths. Infrared light is heavily absorbed by [water vapor]?, so infrared observatories have to be located in high, dry places.

The atmosphere? is opaque at the wavelengths used by [X-ray Astronomy]?, [gamma-ray Astronomy]?, [UV Astronomy]? and [Far Infrared Astronomy]?, and so observations can be carried out only from balloons or [space observatories]?.

All the previous disciplines are based on the detection of photons, but we also receive information from outside the earth carried by [cosmic rays]?, neutrinos, and, in the near future, [gravitational waves]? (see LIGO?).

A different division can be made using the regions of space and problems addressed; some of these are:

- [Galactic Astronomy]?
- [Extragalactic Astronomy]?
- Galaxy Formation and Evolution
- [Star Formation]?
- [Stellar Astronomy]?
- Cosmology
- Astrometry

If your favorite area of research is not mentioned, feel free to add it.

International Astronomical Union

See also Astronomers and Astrophysicists, Amateur Astronomy,/History.

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Last edited August 15, 2001 1:05 pm (diff)
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