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. Astrophysics was born as the application of physics to the phenomena observed by astronomy, this was only possible 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. Nearly all astronomers now have a strong background in physics, and the results of observations are always put in an astrophysical context, so the distinction between astronomy and astrophysics almost doesn't exist anymore.


In the early part of its history, Astronomy involved only the observation and predictions of the motions of the objects in the sky that could be seen with the naked eye. Astronomers were also usually priests, and for a long time it was believed that celestial phenomena had an influence on events on earth. Greeks made some important contributions to astronomy, but the progress almost stopped during the middle ages, except for the work of some Arabic astronomers. The renaissance came to astronomy with the work of Copernicus, who proposed a heliocentric system. His work was defended, expanded and corrected by the likes of Galileo and Kepler. Newton created celestial dynamics with his law of gravitation, that finally explained the motions of the planets. Stars were found much later to be far away objects, and with advent of spectroscopy it was proved that they were similar to our own sun, but with a range of temperatures, masses and sizes. The existence of our Galaxy, the Milky Way, as a separate group of stars was only proven in the 20th century, along with the existence of "external" galaxies, and soon after, the expansion of the universe seen in the recession of most galaxies from us. Cosmology, a discipline that has a large intersection with astronomy, made huge advances during the 20th century, with the model of the hot big bang heavily supported by the evidence provided by astronomy and physics.

For a more detailed history of astronomy, see the history of astronomy.

Division by way of obtaining information

Given its huge scope, astronomy is divided into 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). The most common tool is the telescope.
  • Infrared Astronomy deals with using infrared radiation (wavelengths longer than the red light). Again, the most common tool is the telescope, but at longer wavelengths. Space telescopes are also used to eliminate noise ( electromagnetic interference) from the atmosphere.
  • Radio Astronomy uses completely different instruments 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). See also Radio Telescopes.

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 or in space.

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).

Division by subject

Astronomers study many objects including planets, stars, novae, star clusters, galaxies, nebulae, etc. but not every astronomer observes every kind of object. A different division can be made using the regions of space and problems addressed; among them

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

Also, there are other disciplines that may be considered part of astronomy, or are interdisciplinary sciences with astronomy one of the disciplines:

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

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


International Astronomical Union American Astronomical Society National Optical Astronomy Observatories European Southern Observatory

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

Basic Concepts
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See also space science.

What are our priorities for writing in this area? To help develop a list of the most basic topics in Astronomy and Astrophysics, please see Astronomy and Astrophysics basic topics.

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