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May 17, 2014

A star that’s moving into the final stages of life arcs high across the south tonight. Although the star is bigger and heavier than the Sun, its fate is the same: It will end its life as a hot cosmic cinder called a white dwarf.

The star is Rasalhague, the brightest star in Ophiuchus, the serpent bearer. It’s quite low in the east as darkness falls, and climbs high across the south during the night.

Rasalhague is actually a binary — two stars that are bound together by their gravity. But only one of these stars is visible to the unaided eye. It’s more than twice as massive as the Sun, and about 25 times brighter. It’s much hotter than the Sun, too.

From this profile, astronomers deduce that the star is moving into the penultimate phase of its life: It’s becoming a giant. The star has consumed the hydrogen fuel in its core, so the core is shrinking and getting hotter. That will allow it to begin fusing heavier helium atoms. At the same time, the star’s outer layers are puffing up, making the star much bigger.

The star will undergo a series of changes during its giant phase. When that phase ends, it’ll cast its outer layers into space, leaving only its dense core — a white dwarf. It will no longer produce energy, but it will continue to shine because it’s extremely hot.

The Sun will undergo this same series of changes — beginning in several billion years.

We’ll talk about a star in Ophiuchus that’s already ended its life tomorrow.


Script by Damond Benningfield, Copyright 2003, 2009, 2014

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