One of the twins of Gemini is getting bigger. The star is already about nine times the diameter of the Sun. But changes in its core will cause it to puff up to dozens of times the Sun’s diameter. And that’ll cause it to slow down.

Pollux is Gemini’s brighter twin. It’s quite close above the Moon at first light tomorrow. Castor, the other twin, is farther above Pollux.

Pollux has passed the end of its “normal” lifetime. It’s “fused” the original hydrogen fuel in its core to make helium. Now, it’s fusing the helium to make heavier elements. As part of that process, its core got smaller and hotter, so it radiates more energy into the surrounding layers of gas. That’s pushed on the star’s outer layers, causing Pollux to puff up like a giant balloon.

As a star gets bigger, it spins more slowly. Today, a spot on the surface of Pollux rotates at 6300 miles per hour or faster — a little faster than the Sun rotates. But Pollux will get much bigger in the future. As it does, it’ll spin more slowly.

That’s thanks to a law of physics known as the conservation of momentum. The most common example of how it works is a figure skater. When a skater pulls in its arms, it spins faster. But if the skater then extends the arms, it spins more slowly. So over millions of years, Pollux will figurately extend its own arms — causing the giant star to slow down.

Tomorrow: a spacecraft sets a collision course with an asteroid.
 

Script by Damond Benningfield