WASHINGTON (AFP) -- A massive bright reddish star in the Orion constellation has mysteriously shrunk by over 15 percent in the last 15 years and astronomers have not yet determined why, according to a study released Tuesday.

Betelgeuse, considered a supergiant star, is so large that it would reach to Jupiter's orbit in our solar system. But at a radius of about five astronomical units, the star has shrunk in size since 1993 by a distance equivalent to Venus's orbit.
“To see this change is very striking,” University of California, Berkley professor Charles Townes, who won the 1964 Nobel Prize in physics for inventing the laser, said in a statement.
“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.”
According to Townes, the star's size diminished “smoothly, but faster as the years progressed.”
The findings were presented Tuesday at a meeting of the American Astronomical Society in Pasadena, California, and were based on readings collected at UC Berkeley's Infrared Spatial Interferometer (ISI) atop Mount Wilson in Southern California.
Edward Wishnow, a UC Berkeley research physicist who worked with Townes on the study, said the researchers did not know why the star was shrinking.
“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,” Wishnow said.
Red supergiant stars are suspected to explode into type-II supernovas, cosmic explosions due to a massive star's internal collapse.
Townes observed a bright spot on the star's surface in recent years, although the star appears spherically symmetrical for now.
Betelgeuse, which is among the eight brightest stars in our sky, was the first to ever be measured and is still only one of a handful of stars that appear as a disk rather than a point of light through the Hubble Space Telescope. Despite Betelgeuse's diminished size, Wishnow noted that its visible brightness had showed no signs of significant dimming over the past 15 years.