Betelgeuse is Shrinking: Why? Good Question

"Popular Giant Star Shrinks Mysteriously "
Space.com (June 9, 2009)

"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)...."

The article includes links to related articles on the Space.com website, and gives a pretty good overview of what we know about Betelgeuse: and what we don't. Betelgeuse's shrinkage isn't slight: it's lost about 15% of its diameter in the last 15 years.

One thing that's interesting about Betelgeuse's shrinking - apart from astronomers not knowing what's causing it - is that the star's brightness has been pretty much constant over the last 15 years.

Supergiant stars that change over time is nothing new. Mira, or Omicron Ceti, fluctuates on about a 332-day cycle. But, unlike Betelgeuse, Mira's brightness changes: from magnitude 3.4 to 9.2. (Observing Variable Stars Telescope.com)

More:

• "Lemming Tracks: All Betelgeuse, All the Time"
  (June 10, 2009)