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June 12, 2016

The star Arcturus is a little bit heavier than our own star, the Sun. Yet that small difference has a big effect on the star’s evolution: Arcturus entered a final stage of life billions of years earlier than the Sun will.

Arcturus is one of the brightest stars in the night sky, and shines yellow-orange. Both of those characteristics are a result of its stage in life, which was triggered by its mass.

Mass is the key to how quickly a star consumes the hydrogen in its core. The gravity of a heavier star squeezes its core more tightly, making it hotter. That revs up the star’s nuclear engine, causing it to “burn” through the hydrogen more quickly than a less-massive star.

When the hydrogen is gone, the core shrinks and gets even hotter. That causes the star’s outer layers to expand and cool, making its surface look redder. In other words, the star becomes a red giant.

And that’s what’s happened with Arcturus. So even though it’s only about eight percent more massive than the Sun, it’s 25 times wider and a couple of hundred times brighter. And its surface is cooler, so it looks redder than the Sun.

Arcturus is about seven billion years old. That’s older than the Sun, but not as old as the Sun will be when it becomes a giant: It won’t reach that stage of life until it’s more than 10 billion years old.

Look for Arcturus high in the southern sky at nightfall, far to the upper left of the Moon — a puffy star that’s nearing the end of its life.

Script by Damond Benningfield

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