Astronomy
Astronomy (from Greek: ἀστρονομία) is a natural science that studies celestial objects and phenomena. It applies mathematics, physics, and chemistry, in an effort to explain the origin of those objects and phenomena and their evolution. Objects of interest include planets, moons, stars, galaxies, and comets; the phenomena include supernova explosions, gamma ray bursts, and cosmic microwave background radiation. More generally, all phenomena that originate outside Earth's atmosphere are within the purview of astronomy. A related but distinct subject, physical cosmology, is concerned with the study of the Universe as a whole.
Astrophysics
Astrophysics is the branch of astronomy that employs the principles of physics and chemistry "to ascertain the nature of the astronomical objects, rather than their positions or motions in space". Among the objects studied are the Sun, other stars, galaxies, extrasolar planets, the interstellar medium and the cosmic microwave background. Their emissions are examined across all parts of the electromagnetic spectrum, and the properties examined include luminosity, density, temperature, and chemical composition. Because astrophysics is a very broad subject, astrophysicists typically apply many disciplines of physics, including mechanics, electromagnetism, statistical mechanics, thermodynamics, quantum mechanics, relativity, nuclear and particle physics, and atomic and molecular physics.
Astronomy
A fair number of people who go on to major in astronomy have decided on it certainly by the time they leave junior high, if not during junior high. I think it’s somewhat unusual that way. I think most children pick their field quite a bit later, but astronomy seems to catch early, and if it does, it sticks.
Nancy Roman, from interview by Rebecca Wright, Oral History Transcript (15 Sep 2000), on NASA website
Astronomy
And God made two great lights, great for their use
To man, the greater to have rule by day,
The less by night, altern.
John Milton, Paradise Lost (1667; 1674), Book VII, line 346.
Astrophysics
The greatest astronomers of the first half of the 20th century were the astrophysicists. For example, Arthur Eddington, Cecilia Payne, Hans Bethe, and Subrahmanyan Chandrasekhar elucidated the physical nature of stars using the new quantum theories of atomic, nuclear and particle physics. In recent decades, about half of the prizes of the American Astronomical Society are awarded for work in astrophysics and half in astronomy.
Gutti Jogesh Babu; Eric D. Feigelson (1 August 1996). Astrostatistics. CRC Press. p. 4. ISBN 978-0-412-98391-7.
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