Astronomy can be generally defined as the study of space beyond the Earth's atmosphere, focusing on the location, size, distribution, motion, composition, and evolution of the sun, planets, stars and other celestial phenomena. Almost all astronomers have an undergraduate degree in astronomy or astrophysics, and many have graduate degrees. Astronomers usually specialize in one or more of the major sub-fields of astronomy; typical job titles include planetary astronomers, stellar astronomers, solar astronomers, galactic and extragalactic astronomers and cosmologists who study the origins and evolution of the universe.

Planetary Astronomers

Planetary astronomers study both the planets in our own solar system and extra-solar planets. The research of astronomers involves using telescopes and other instruments to measure the motion, dimensions and composition of planets and other planetary bodies such as moons and asteroids. Planetary astronomers also study the distribution and types of planets found in different solar systems.

Radio Astronomers

Radio astronomers specialize in the use of radio telescopes. Radio telescopes are basically huge radio wave receivers that detect naturally occurring radio emissions from stars, galaxies, quasars, and other celestial objects. Radio astronomers use radio telescopes to learn about extremely distant celestial objects, and have developed sophisticated software to help them separate the radio signals from these objects from background noise up to a million times greater.

Solar and Stellar Astronomers

Solar and stellar astronomers are related, but their jobs are quite different. Solar astronomers study our sun, and their research focuses on the composition of the sun, sunspots, solar flares, eclipses and other solar phenomena. Stellar astronomers study the suns of other solar systems. One of the main jobs of stellar astronomers is to identify the type and composition of the suns in distant solar systems. Some solar systems even have more than one sun.

Galactic and Extra-Galactic Astronomers

Galactic and extra-galactic astronomers study extremely distant objects. This requires the use of sophisticated tools such as radio telescopes and space telescopes as atmospheric interference limits the value of traditional ground-based telescopy. Their research typically involves identifying patterns in the distribution, composition and conditions of stars and gases that reveals information about the history of our galaxy or neighboring galaxies.

Cosmologists

Cosmologists are theoretical astronomers and their work often overlaps that of physicists. They are often defined as astronomers who study the evolution of the universe and the nature of space and time. Cosmology grew out of the historical discipline of celestial mechanics, and came into its own in the 1920s with Einstein's theory of general relativity and George Lemaitre's postulation of the big bang theory. Cosmologists tend to build their theories from the data of other astronomers rather than making the observations themselves.