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June 6, 2015

The Sun is a middle-aged star. It’s four-and-a-half billion years old, and its “normal” lifetime is expected to last about five billion years more.

Another middle-aged star is one of the brightest lights of summer nights. Vega is low in the east-northeast as darkness falls right now, and will stand even higher at nightfall by the time summer arrives in a couple of weeks.

Both Vega and the Sun are roughly half-way through their time on the “main sequence” — a time when they’re steadily “fusing” the original hydrogen in their cores to make helium. Yet Vega is only one-tenth as old as the Sun.

The reason for the difference is mass: Vega is more than twice as heavy as the Sun. The extra heft gives Vega a stronger gravitational pull. That squeezes its core more tightly, which makes it millions of degrees hotter than the Sun’s core. The higher temperature revs up the nuclear reactions, so Vega burns through its hydrogen much faster than the Sun does.

When Vega uses up the hydrogen, it’ll move off the main sequence and into a new phase of life — the giant phase. Its core will shrink and get hotter, triggering a new round of nuclear reactions. Its outer layers will puff outward, so the star will swell to many times its current size.

And when that phase is done, Vega’s outer layers will blow out into space, leaving only its hot but now-dead core. Yet the Sun will shine on — outlasting its bigger, brighter neighbor by billions of years.


Script by Damond Benningfield, Copyright 2015


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