A massive red star in the constellation Orion has shrunk in the past 15 years and astronomers don't know why.
Called Betelgeuse, the star is considered a red supergiant. Such massive stars are nearing the ends of their lives and can swell to 100 times their original size before exploding as supernovae, or possibly just collapsing to form black holes without violent explosions (as one study suggested).
Betelgeuse, one of the top 10 brightest stars in our sky, is a popular target among backyard skywatchers and was the first star ever to have its size measured, and even today is one of only a handful of stars that appears through the Hubble Space Telescope as a disk rather than a point of light. It was the first star (besides our sun) to have its surface photographed (by Hubble).
In 1993, measurements put Betelgeuse's radius at about 5.5 astronomical units (AU), where one AU equals the average Earth-sun distance of 93 million miles, or about 150 million km. Since then it has shrunk in size by 15 percent. That means the star's radius has contracted by a distance equal to the orbit of Venus.
"To see this change is very striking," said Charles Townes, a UC Berkeley professor emeritus of physics. "We will be watching it carefully over the next few years to see if it will keep contracting or will go back up in size." (Townes won the 1964 Nobel Prize in physics for inventing the laser and the maser, a microwave laser.)
"But we do not know why the star is shrinking," said Edward Wishnow, a research physicist at UC Berkeley's Space Sciences Laboratory. "Considering all that we know about galaxies and the distant universe, there are still lots of things we don't know about stars, including what happens as red giants near the ends of their lives."