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Wolf Rayet (WR) stars

First identified by French Astronomers Charles Wolf (1827 - 1918) and Georges Rayet (1839 - 1906) in 1867, a Wolf-Rayet star is the candidate object to produce a sufficiently energetic supernova called a hypernova.

A Wolf-Rayet (WR) star has a mass at least 20 times that of the Sun and a surface temperature of 45,000 degrees Fahrenheit, significantly hotter than the Sun's 10,000 degree Fahrenheit surface temperature. WR stars also expel large amounts of heated gas such as helium in occasional bursts. In this way, WR stars are very "windy" with stellar wind velocities much greater than that of the Sun. The expulsion of so much gas means a WR star is surrounded by an "atmosphere" of gas that is comparable in size to the star itself. In this way, when astronomers peer at a WR star through their telescopes, they are really just seeing the ionized gas surrounding the star. This gas is so hot it emits visible light, along with a variety of other kinds of photons (radiation) ranging from less energetic radio waves and microwaves to more energetic ultraviolet light and X rays.

WR stars emit so much material because of the occasional upwelling of heavier elements fused inside the core, including carbon and oxygen. These heavier elements interrupt the energy flowing outward from the star by absorbing it. Eventually, this causes a tremendous outward pressure that takes the form of a powerful stellar wind blowing off the star. A typical WR star can lose a mass equivalent to that of the Earth in a year.

Because WR stars burn so brightly, fiercely, and erratically, they have very short life spans. A typical WR life span is just a few million years, compared to the billions years or so for more calmer, more stable stars such as our Sun (which is about midway through its 10 billion year life span).

When a WR star dies, it often undergoes a supernova detonation. Some of these supernovae are sufficiently energetic to classify as a hypernova. And some of these hypernovae are accompanied by a gamma-ray burst as the core undergoes an uneven gravitational collapse into the most awesome object in the Universe -- a black hole, an object whose gravitational pull within a certain distance is so powerful nothing, not even light, can escape.

* Return to "GRBs: A Collapse and the a Spectacular Explosion".

Imagine the Universe is a service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA's Goddard Space Flight Center.

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