Astronomy, which etymologically means laws of the stars, is the science whose subject is the observation and explanation of events outside the earth. Astrophysics is the part of astronomy (and physics) that deals with the application of physics to the phenomena observed
by astronomy. Nearly all astronomers now have a strong background in physics, and the results of observations are usually put in an astrophysical context, so astronomy
and astrophysics are used with a very close meaning.
Astronomy is one of the few sciences where amateurs still play an active role, especially in the discovering and monitoring of transient phenomena.
Astronomy is not to be confused with astrology, a pseudoscience which attempts to predict a person's destiny by tracking the paths of astronomical objects. Although the two fields share a common origin, they are quite different; astronomy embraces the scientific method, while astrology does not.
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.
A first distinction is between theoretical and observational astronomy and astrophysics. Observers use a variety of means to obtain data about different phenomena, data that is then used by theorists to create and constrain theories and models, to explain observations and to predict new ones. Fields of study are also categorized in another two main ways: by subject, usually according to the region of space (e.g. Galactic astronomy) or problems addressed (such as star formation or cosmology); and according to the means of obtaining the data (e.g. optical astronomy or radioastronomy)
Many of the important subfields of astronomy have their own Wikipedia articles, such as the following:
See list of astronomical topics for a more exhaustive list of astronomy-related pages.
electromagnetic radiation, photons, but we also receive information from outside the earth carried by cosmic rays, neutrinos, and, in the near future, gravitational waves (see LIGO and LISA[?]).
A traditional division of astronomy is given by the region of the electromagnetic spectrum observed:
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, except for a few wavelength "windows", Far infrared astronomy , and so observations can be carried out only from balloons or space observatories.
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. The ancient Greeks made many important contributions to astronomy, among them the definition of the magnitude system. They also defined the Zodiac, a band of twelve bright constellations.
The study of astronomy 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 model of the Solar System. His work was defended, expanded upon, and corrected by the likes of Galileo Galilei and Johannes Kepler. The latter of these was the first to provide a system which described correctly the details of the motion of the planets with the Sun at the center. However, Kepler did not understand the reasons behind the laws he wrote down. It was left to Newton's invention of celestial dynamics and his law of gravitation, the final explanation of the motions of the planets. Astrophysics was a later development, which only became 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.
Stars were found to be far away objects, and with the advent of spectroscopy it was proved that they were similar to our own sun, but with a wide 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, such as the cosmic microwave background radiation, Hubble's Law and cosmological abundances of elements.
For a more detailed history of astronomy, see the history of astronomy.