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Astronomy can be divided into in to eight main fields. They are Observational Astronomy, Theoretical Astronomy, Planetary Astronomy, Solar Astronomy, Stellar Astronomy, Galactic Astronomy, Extragalactic Astronomy, Cosmology. Each of these fields have their own area of study, methodology, tools and assumption. While there is some over lap they are each field is interesting in its own way.
Radio Astronomy deals with the observation of radio wave from space. It involves wave lengths grater than one millimeter. Infrared Astronomy deals with the observation of infrared light which has a wave length longer than red. It is basically observing the heat emissions in the universe. Optical Astronomy deals with observations in light visible to the human eye. It is the oldest from of Observational Astronomy and is the type common among amateur astronomers.
Ultraviolet Astronomy deals with the observation of ultraviolet light which has a wave length just shorter than blue light. Because these wavelengths are absorbed by the Earth's atmosphere observations done from from space. X-ray Astronomy deals with the observation of X-rays, which are produced in space by objects with temperatures above 10 million degrees. Gamma-ray Astronomy deals with the observation of Gamma-rays, source objects include pulsars, neutron stars, and black holes.
Neutrino Astronomy involves detecting neutrinos from space with heavily shielded underground detectors consisting of large amounts of water. Gravitational Wave Astronomy attempts to detect gravitational waves using huge interferometers. Astrometry involves measuring the positions of celestial objects such as the Sun, Moon, planets and stars. It is one the oldest fields of science.
This is the theoretical wing of Astronomy. It works with observational astronomy which is used to gather information on which to develop theories as well as testing them. The main difficulties with Theoretical Astronomy are the philosophical starting assumptions used and the problem of using theory to interpret observations rather than using the observations to test the theory.
Another issue in Theoretical Astronomy is the tendency to patch popular
existing theories when they fail to predict actual observations rather than
questioning the original theory. This is most often done in dealing with
theories of origins. A major example is the invention of the notion of dark
energy to save the Big Bang from Type Ia Supernovae observations.
Planetary Astronomy is the study of planetary bodies. It includes planets, moons, dwarf planets, comets, and asteroids. This not only includes bodies orbiting the Sun, but also extrasolar planets as well. In fact the discovery of extrasolar planets has greatly extended Planetary Astronomy.
Solar astronomy is the study of the Sun. Not only is the Sun important for sustaining life on Earth but the Sun is also the closest star to Earth making it the easiest star to study. It uses reflecting telescopes that produce images of sun for study.
Stellar astronomy is simply the study of stars. The down side is that their distance makes direct study difficult. This leads to a lot of theoretical interpretations of observations. The discovery of extrasolar planets has been a result of Stellar astronomy.
Galactic Astronomy is the study of our galaxy the Milky Way. This would include the Milky Way's contents and structure. This process is made more difficult by the fact that we can can only see it from the inside. Despite this fact we know that the Milky Way is a barred spiral galaxy and has a 4.6 million solar mass supermassive black hole at the center.
Extragalactic Astronomy is the study of galaxies outside the Milky Way. This
includes the study of their contents and structure.Studying the general
structure of relatively nearby galaxies is fairly easy since they can by seen in
telescopes in great detail.
Cosmology is the study of Universe. This is includes its structure, composition, history and origin. The problem is that there are limits to how much of the universe we can directly observe. As a result Cosmology is highly influenced by philosophical assumptions. By applying different philosophical assumptions to observations of the universe one can get different cosmologies. For example based on observation the Earth seems to be near the center of an expanding universe. There are two main ways this observation can be interpreted depending upon philosophical assumptions one uses to interpret the observation.
If one starts with atheistic philosophical assumptions since it is highly unlikely that we would be near the center of the expansion it leads to the assumption of an unbounded expanding universe without a center, such that every point in the universe looks like the center of the expansion. The result is the Big Bang cosmology, which had to be modified by with the artificial addition of so called dark energy to explain Type Ia Supernovae observations. The result is that to save the result of atheistic philosophical assumptions a totally ad hoc addition is needed to make the Big Bang cosmology fit reality.
If however one starts with Biblical assumptions one can not only accept the observational evidence that we are near the center of the expansion at face value but one can conclude that the universe is bounded by a layer of water. (Psalm 148:1-4) This bounded Bible based cosmology explains the Type Ia Supernovae observations without ad hoc additions.
This comparison shows the philosophical problem in cosmology. Because of the philosophical assumptions behind the Big Bang, its total failure to the predict the results of the Type Ia Supernovae observations was ignored. Instead of recognizing this major failure the totally ad hoc addition of imaginary dark energy was made to make the Big Bang fit reality.
Like all major fields of Science, Astronomy has sub fields. It is important to understand the assumptions commonly used by those sub fields. Observational Astronomy is one of the few areas of science that benefits form a significant amateur participation where the amateurs often contribute meaningful observations. The sub fields of Astronomy each make their own contribution to Astronomy as whole which is where the big picture comes together.