Harvard-Smithsonian Center for Astrophysics|
Release No.: For Release: 9:20 a.m. EST, January 13, 2000
ATLANTA, GA--Astronomers have long known that the atmospheres of pulsating stars either expand or contract over time, but, thanks to new spectra and images of Betelgeuse taken with the Hubble Space Telescope (HST), they have now discovered small regions on that star's surface where gas is sometimes being expelled at one side -- while simultaneously splashing down at the other.
Alex Lobel and Andrea Dupree of the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge and Ronald Gilliland of the Space Telescope Institute in Baltimore, announced their results here today at the 195th meeting of the American Astronomical Society.
Their observations provide the first direct evidence for such complex flows in the gas surrounding cool oscillating stars and will help to solve the persistent question: "What physical mechanism drives these amazing dynamics?".
From early 1998 to spring 1999, the Space Telescope Imaging Spectrograph (STIS) was used to scan Betelgeuse's disk four times and to record spectra from small slices cut across its surface.
"We found the upper atmosphere or chromosphere warmer than the region below itand we observed that it also contracted and expanded during this period", says Lobel, an astrophysicist at the CfA. "But, most surprisingly, a scan in fall 1998 suggests streams of gas that are heading in opposite directions --with velocities of about 10,000 miles per hour."
The astronomers can measure these differences in direction because moving material scatters light out of bright spectral emission lines that emerge from the chromosphere, leaving two distinct peaks. By the Doppler effect (like the change of pitch while a jet flies over or a train passes by), when the left peak is higher than the right peak, gas is falling to the star, and the reverse when it is blown off into space.
Throughout the same period, the Faint Object Camera (FOC) was used to make new images of Betelgeuse's chromosphere in ultraviolet (UV) light. While the intensity of UV emitted by the chromosphere varied with the star's pulsation, brighter and rather subtle intensity patterns appeared at different locations on the stellar disk. Although an FOC observation by Dupree and colleagues in 1995 revealed a bright spot-like area on Betelgeuse's surface, the new images -- with their improved resolution -- now also revealed thata bright 'arc-like' structure spanned a large portion of the disk in September 1998.
"As astonishing as these images may be, they also show us that the precise locations of the brighter regions remain unresolved," says Lobel. "Sharper and more frequent images obtained during one pulsation cycle are needed to pinpoint their physical origin".
The team is considering several explanations for the brighter structures seenin the images. For example, huge convection cells produced deep in the photosphere during the oscillations could propel denser and hotter gas into the chromosphere. Or, enormous shock waves generated by the pulsation couldfragment into smaller "shock trains" that climb into the chromophere and then migrate randomly across the surface, leaving extra UV light in their hotter colliding wakes. Or, perhaps, unexpected strong magnetic fields could form long hollow tubes that pierce the chromosphere, thus allowing hotter gas to pour in from below.
"For all these scenarios, the new images show that the upper atmosphere changes in a rather unordered manner just like the simultaneous up- and down-flows seen in the STIS spectra," says Dupree.
One out of a million stars in our galaxy is a supergiant and fewer yet are cool supergiants. Not only is Betelgeuse a cool supergiant,but it is also the seventh brightest star visible in the northern hemisphere, appearing in the shoulder of the constellation Orionat a distance of 425 light-years from Earth. Because its surface temperature can drop to below 3,000 K degrees, it shines reddish. Its atmosphere is like a big puffy cloud, ten million times less dense than our Sun; and, under such conditions, slight perturbations have dramatic effects on movements of its atmospheric gases.
Indeed, this star is so big that, if it replaced the Sun at the centerof our Solar System, its pulsating atmosphere would extend almost to the orbit of Jupiter. Additional measurements by Lobel, Dupree, and Gilliland revealed that the star is wrapped in an even bigger and less dense envelope of warmer gas. Its chromosphere extends up to 5,000 times the radius of our Sun, or out to Neptune's orbit, where the temperature can increase to about 5,000 K degrees.
This work is supported by NASA through the Space Telescope Science Institute and the Smithsonian Astrophysical Observatory.
EDITORS: Images and caption of this result are available at:
Harvard-Smithsonian Center for Astrophysics