Giant star Betelgeuse mysteriously shrinking: study

AFP on Yahoo ^ | 6/9/09 | AFP

[NormsRevenge]

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 whon 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.

[SunkenCiv]

Ooops, that earlier APOD link was from 1998! This is from info from 1995:

http://seds.org/hst/Btlgeuse.html

The Hubble image reveals a huge ultraviolet atmosphere with a mysterious hot spot on the stellar behemoth’s surface. The enormous bright spot, more than ten times the diameter of Earth, is at least 2,000 Kelvin degrees hotter than the surface of the star.

The image suggests that a totally new physical phenomenon may be affecting the atmospheres of some stars. Follow-up observations will be needed to help astronomers understand whether the spot is linked to oscillations previously detected in the giant star, or whether it moves systematically across the star’s surface under the grip of powerful magnetic fields.

[SunkenCiv]

http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/970616b.html

[Tim Kallman for Ask an Astrophysicist]

Betelgeuse (also known as alpha ori) is a very large star, an M supergiant... For stars on the main sequence, which includes our Sun, there is simple proportionality between size and mass, and also a simple scaling for luminosity. For evolved stars the situation is less simple. Betelgeuse is more than 1000 times larger than the Sun, and 50000 times as luminous, but only about 20 times as massive. Most of the light from Betelgeuse comes out in the infrared, however, which is very different from the Sun. One consequence of the advanced evolutionary state of Betelgeuse is that it probably was much more massive when it was on the main sequence, and has already lost a significant fraction of its mass (probably more than half) in a stellar wind... A very crude estimate is that such stars spend 1% of their lives as supergiants, which would suggest 10,000,000 stars similar to Betelgeuse in our galaxy... Another one is Mira, in the constellation Cetus. Mira is probably larger than Betelgeuse, so large that it is thought that the outer layers of the star are barely held together by gravity. Mira is known to pulsate and eject its outer layers, probably in large part because of its weak gravity. Possibly the most massive known star is eta carina, which may have been 150 times as massive as the Sun when it first formed, and may be 50 - 60 times as massive as the Sun currently. In the 1830s eta carina underwent a tremendous outburst during which it became a brilliant naked eye object and ejected an amount of gas with mass approximately equal to the mass of the Sun. It is likely that the minimum main sequence mass for a star which will eventually make a black hole is 8 - 10 times the mass of our Sun. This is quite a bit less than Betelgeuse had when it was on the main sequence, and there are many such stars in our galaxy.

[SunkenCiv]

http://www.solstation.com/x-objects/betelgeuse.htm

Past observations have indicated that, partially because of roiling convection cells beneath its surface, the star’s surface has been observed to “wobble in and out” along with two pulsations, one restarting annually while the other cycles over six years. Based on long-term monitoring at 11.15 micrometers using the Infrared Spatial Interferometer at Mount Wilson Observatory, however, the star’s diameter appears to have progressively shrunk from 11.2 to 9.6 AUs; as the star’s radius is now just under about five times Earth’s orbit distance, having shrunk by a distance equal to the orbit of Venus. It is still unclear whether the star is experiencing a long-term oscillation in its size, undergoing initial contractions towards a collapse or a blow off of material related to its impending death as a red supergiant via a supernova, or simply rotating to present a different side of its bumpy surface (and so appear to change in size). Although the star appears spherically symmetrical at present, Townes and former graduate student Ken Tatebe had observed a bright spot on the surface of Betelgeuse in recent years.