Reference Terms
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Astronomy is the scientific study of celestial objects (such as stars, planets, comets, and galaxies) and phenomena that originate outside the Earth's atmosphere (such as the cosmic background radiation).

It is concerned with the evolution, physics, chemistry, meteorology, and motion of celestial objects, as well as the formation and development of the universe.

Astronomy is one of the oldest sciences.

Astronomers of early civilizations performed methodical observations of the night sky, and astronomical artifacts have been found from much earlier periods.

However, the invention of the telescope was required before astronomy was able to develop into a modern science.

Historically, astronomy has included disciplines as diverse as astrometry, celestial navigation, observational astronomy, the making of calendars, and even, at one time, astrology, but professional astronomy is nowadays often considered to be identical with astrophysics.

Since the 20th century, the field of professional astronomy split into observational and theoretical branches.

Observational astronomy is focused on acquiring and analyzing data, mainly using basic principles of physics.

Theoretical astronomy is oriented towards the development of computer or analytical models to describe astronomical objects and phenomena.

The two fields complement each other, with theoretical astronomy seeking to explain the observational results, and observations being used to confirm theoretical results.

Amateur astronomers have contributed to many important astronomical discoveries, and astronomy is one of the few sciences where amateurs can still play an active role, especially in the discovery and observation of transient phenomena.

The most frequently studied star is the Sun, a typical main-sequence dwarf star of stellar class G2 V, and about 4.6 Gyr in age.

The Sun is not considered a variable star, but it does undergo periodic changes in activity known as the sunspot cycle.

The study of stars and stellar evolution is fundamental to our understanding of the universe.

The astrophysics of stars has been determined through observation and theoretical understanding; and from computer simulations of the interior.

Star formation occurs in dense regions of dust and gas, known as giant molecular clouds.

When destabilized, cloud fragments can collapse under the influence of gravity, to form a protostar.

A sufficiently dense, and hot, core region will trigger nuclear fusion, thus creating a main-sequence star.

Almost all elements heavier than hydrogen and helium were created inside the cores of stars.

Note:   The above text is excerpted from the Wikipedia article "Astronomy", which has been released under the GNU Free Documentation License.
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