A. Dupree, R. Gilliland, FOC, HST, NASA
You've probably seen Betelgeuse, even if you didn't know it. In the Orion constellation, it's the red dot on the archer's left shoulder. It's also one of the largest stars astronomer have ever found. In 1993, scientists compared its girth to the size of Jupiter's orbit around our the sun.
But 15 years later, the same measuring tools say that Betelgeuse (which sounds a lot like "beetle juice") is about 15 percent smaller.
"The cause of the star's rapid contraction is a mystery," writes National Geographic. "But the team noted that they had observed an unusual big red spot on the star three years ago."
That spot might have caused the sudden contraction, but astronomers don't know how.
There are stars that regularly pulsate, known as Mira variables, but Betelgeuse doesn't really fit the description. Mira stars can lose as much as 25 percent of their volume, and much of their light, every other year. But they always return to their normal size and shine.
Betelgeuse, on the other hand, shows no signs of bouncing back and, on average, shimmers just as brightly as it did in 1993.
"Something unusual is happening with this star. The question is: What's going to happen next?" says Charles Townes, who heads a team observing the red supergiant at the University of California, Berkeley.
Mr. Townes told National Geographic that the star could go supernova at any moment. If it does, "the blast should be clearly visible from Earth."
[Editor's note: The original version of this story mistakenly said that the new size of Betelgeuse was similar to the orbit of Venus. While the star is shrinking, it's still many times larger than that. The volume lost was similar to the orbit of Venus.]