What are the sub-fields of Astronomy?

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Astronomy is the study of everything beyond the Earth's atmosphere. It applies physics, biology and geology to explain the origin and evolution of space, stars and celestial bodies. Individuals thinking of going into this field of study may wish to focus specifically on one sub-field.

A telescope used to see into space.

The four sub-fields of Astronomy are: Astrophysics; Astrometry; Astrogeology; and Astrobiology. The following gives a brief description of each sub-field and its focus.

Astrophysics, as a scientific discipline, was born in mid-nineteenth century Europe. Observational astrophysics focuses on recording data by using telescopes and other astronomical equipment to observe celestial objects. Theoretical astrophysics focuses on creating theoretical models and figuring out the observational implications and consequences of those models.

Similar to geophysics, which is the study of Earth's physics, astrophysics is the branch of astronomy that applies the laws of physics to explain the birth, life, and death of objects in the universe (such as planets, stars, galaxies and nebulae). Interacting with objects in space is done by studying the amount of radiation they emit. These emissions given off by planets, stars etc., are examined by looking at certain properties, such as temperature, density, luminosity, and chemical composition.

Much of astrophysics is focused on developing theories that will help us understand how radiation is produced. Astrophysicists apply many disciplines of physics to do this, including nuclear and particle physics, atomic and molecular physics, electromagnetism, relativity, thermodynamics, classical mechanics, statistical mechanics, and quantum mechanics.

Astrophysics is very tightly knit with both astronomy and cosmology. The differences between the three are: astrophysics creates physical theories of small to medium-size structures in the universe; astronomy calculates motions, positions, and luminosities; and cosmology creates physical theories of the largest structures in the universe and studies the expansion and evolution of the universe as a whole.

Astrometry is the branch of astronomy that focuses on the precise measurement of where stars and other celestial bodies are positioned and move in space. It is the oldest scientific method used to map and detect the positions and movements of extrasolar planets (an extrasolar planet is any planetary body that is outside the solar system and that usually orbits a star other than the Sun). Astrometric measurements can provide invaluable information on the movements and origin of the Solar System and the Milky Way, a frame of reference for the movement of stars and individual objects in space, and can also help to determine the distribution of dark matter in the galaxy.

Another aspect of astrometry is error correction, as there are a few factors that can introduce errors into the measurement of a star's position. These factors include: errors made by the observer, imperfections in the measuring instruments, and atmospheric conditions. Instrument improvements and making compensations to the data can reduce these errors. The results can then be studied and analyzed by using statistical processes to calculate data estimates and error ranges.

Astrogeology can be viewed as the parent science of the Earth sciences. It is very much like the Earth sciences, but for other bodies in our solar system. Astrogeology (also known as planetary geology or exogeology), focuses on the geology (rocks, terrain, and material) of the planets and their moons, asteroids, comets, and meteorites. Astrogeology looks at understanding what the internal structure is of the terrestrial planets (terrestrial planets are planets that are mainly composed of rocks or metals, such as Mercury, Venus, Earth, and Mars) and looks at volcanoes, lava flows, impact craters, rift valleys, and wind activity on these planets. The structure of the giant planets and their moons as well as the make-up of the minor bodies of the Solar System are also studied.

Research in this field is ongoing, and every discovery helps scientists to better understand the Earth's evolution in comparison with that of its neighbours in the solar system. Every planet in our solar system has unique geological features which scientists have uncovered over the years through telescope observations or through data returned by space probes.

Each planet in the solar system has its own specialized study:
Heliology - the study of the Sun
Hermeology - the study of Mercury
Cytherology - the study of Venus
Selenology - the study of the Moon
Areology - the study of Mars
Zenology - the study of Jupiter
Kronology - the study of Saturn
Uranology - the study of Uranus
Poseidology - the study of Neptune
Hedeology - the study of Pluto

Astrobiology (formerly known as exobiology), is the branch of astronomy that focuses on the search for life outside Earth. It is the study of the origin, evolution, distribution and future of life in the universe, and considers the question of whether extraterrestrial life exists, and if it does, how humans can detect it. NASA’s current astrobiology program addresses three fundamental questions: How does life begin and evolve? Is there life beyond Earth and, if so, how can we detect it? What is the future of life on Earth and in the universe?

While astrobiology is very much an emerging and developing field, the question of whether extraterrestrial life exists elsewhere in the universe is a justified theory and therefore a valid scientific inquiry. Planetary scientist David Grinspoon calls astrobiology "a field of natural philosophy, grounding speculation on the unknown, in known scientific theory". The field of astrobiology has made an enormous effort to underline the importance of education, both to train the next generation of scientists, and to also keep the public aware of any breakthroughs.

This field was once considered outside the mainstream of scientific inquiry, but has now become a formalized field of study. Twenty years ago, no universities had dedicated degree programs in astrobiology and very few even offered a course in this field. Today, every major university in the country has at least one course in astrobiology and many have degree programs.

Missions are just starting to take astrobiology to new levels of understanding. Astrobiology will endure long into the future given the endless fascination with questions about the origins and prevalence of life.

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