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the branch of astronomy that deals with the physical properties of celestial bodies and with the interaction between matter and radiation in the interior of celestial bodies and in interstellar space.
Contemporary Examples

Thanks to astrophysicist extraordinaire Katherine Mack for helpful discussions in putting this article together.
Laniakea: The Milky Way’s Place in the Heavens Matthew R. Francis September 6, 2014

David Sumner, an American, is an astrophysicist with a grant to do research.
The Power of ‘Straw Dogs’ Malcolm Jones September 18, 2011

An astrophysicist with a poetic sensibility, she hooked me in our first interview with her direct manner and determination.
Lynn Sherr: Sally Ride’s Heroic and Trailblazing Life as an Astronaut Lynn Sherr July 23, 2012

I admire Neil deGrasse Tyson, the astrophysicist and author of Space Chronicles and Inexplicable Universe.
Vilified Bitcoin Tycoon After Losing $500 Million: My Life Is at Risk Nathalie-Kyoko Stucky September 16, 2014

Historical Examples

If Mars were incandescent, we should appeal to the astrophysicist for information2 regarding its chemical composition.
Mars and its Mystery Edward Sylvester Morse

Chances were there was no astrophysicist nearer than Ganymede.
Rip Foster in Ride the Gray Planet Harold Leland Goodwin

The astrophysicist continued, at considerable length, to establish the position of Ceti II to his own complete satisfaction.
Eight Keys to Eden Mark Irvin Clifton

noun
(functioning as sing) the branch of physics concerned with the physical and chemical properties, origin, and evolution of the celestial bodies
n.

1869, usually hyphenated at first, from astro- + physicist. Astrophysics is recorded from 1877.
astrophysics
(ās’trō-fĭz’ĭks)
The branch of astronomy that deals with the physical and chemical processes that occur in stars, galaxies, and interstellar space. Astrophysics deals with the structure and evolution of stars, the properties of interstellar space and its interactions with systems of stars, and with the structure and dynamics of clusters of stars such as galaxies.

The branch of astronomy devoted to the study of the physical characteristics and composition of objects in the sky. Typical concerns of astrophysics are how much light the stars give off and the size, mass, and temperature of planets and stars.