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white dwarf star, Milky Way Galaxy, gravitational waves, observable universe, Supernovas

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Supernova, violent explosion that occurs when gravitation causes a star to collapse onto itself. Supernovas, also called supernovae, can be brighter than all the stars in an entire galaxy combined and can shine for weeks or months. The extreme conditions in supernova explosions forge atomic particles into chemical elements—all the atoms in the universe that are heavier than iron were formed in supernovas. The explosions also spread gas and dust into space where the chemically enriched interstellar material can form new stars and planets—almost every substance on Earth is made in part or in whole from the ashes of supernovas.

Supernovas can leave behind some of the strangest objects in the universe—neutron stars and black holes—and may give off bursts of gamma rays and cause ripples in space-time called gravitational waves. A supernova also unleashes intense electromagnetic radiation that could destroy all life on Earth if one exploded too near to our solar system. The extreme brightness of some supernovas allows astronomers to measure the distances and motions of galaxies far back into time, indicating how fast the universe is expanding.

Supernovas are very rare events and the vast majority of stars, including our Sun, do not have enough mass to reach a supernova stage. Because the universe is filled with billions of galaxies, astronomers are able to observe a few hundred distant supernovas a year. Throughout the observable universe, in fact, a supernova goes off about every second. However, most of these events are not detected because astronomers cannot scan the entire universe at once. Over the past 1,000 years only about a half-dozen supernovas have exploded in our own Milky Way Galaxy.

Supernovas are distinct from novas (from the Latin term nova stella meaning “new star”). Novas occur in double-star systems that have a normal star and a white dwarf star orbiting close together around a common center of gravity. Gas drawn off the normal star explodes on the surface of the white dwarf star in a nuclear reaction that brightens then fades. The process can happen repeatedly. A supernova is a one-time event that affects the core of the star, not only its surface, and is billions of times brighter than a nova.


Pasachoff, Jay. M.m A.B., A.M., Ph.D.

Field Memorial Professor of Astromony and Director of the Hopkins Observatory, Williams College. Author of "Astronomy: From the Earth to the Universe, 6th ed."; "Field Guide to the Stars and Planets, 4th ed."; "Fire in the Sky"; and "Nearest Star: The Exciting Science of Our Sun".

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