Space Telescope Science Institute
January 13, 2000
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 it and 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 that a 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 seen in 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 could fragment 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 Orion at 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 center of 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.
The photo-montage made in ultraviolet light shows the pulsating atmosphere of Betelgeuse. The star is scanned with the Space Telescope Imaging Spectrograph. Seven spectra are recorded through a small field of view shown by the rectangle at different positions on the disk, marked by the crosses. The thick yellow curves show the shape of a double-peaked emission line which emerges from Betelgeuse's chromosphere. The depression between the peaks is formed far out in this warmer envelope. The spectra in the top images of January and March '98 reveal that this outer region collapses since the left-hand peak of the emission line is everywhere stronger than the right. But the STIS scan in the lower left image of September '98 unveils how atmospheric gas begins to move up in the fifth scan position, where the right-hand peak exceeds the left. Here gas streams in opposite directions at the same time through the star's outer atmosphere. The spectral scan in the lower right image of March '99 shows that the expanding trend proceeds and extends further across the chromosphere. The arrows in the images point North in the plane of the sky and should be aligned when comparing the location of surface details. Note that the mean intensity levels of the four images have been equalized to bring out these details. The top images are actually observed brighter than the lower ones. Note also that some spectral scans are taken a week apart from the images.
National Radio Astronomy Observatory
Socorro, New Mexico 87801
April 8, 1998
Jeremy Lim of the Academia Sinica Institute of Astronomy & Astrophysics in Taiwan; Chris Carilli, Anthony Beasley, and Ralph Marson of the National Radio Astronomy Observatory (NRAO) in Socorro, NM; and Stephen White of the University of Maryland studied the red-supergiant star Betelgeuse, about 430 light-years away in the constellation Orion. They reported their findings in the April 9 issue of the scientific journal Nature.
"These radio-telescope images confirm that Betelgeuse -- already more than 600 times larger than our Sun -- has a dense atmosphere that extends to many times larger still than the star itself," said Lim. "The highest-resolution image shows the star's atmosphere to have a remarkably complex structure." "To our surprise," added White, "the images also show that most of the gas in the atmosphere is only about as hot as that on the surface. Previously, all of it was thought to be very much hotter."
The astronomers used the VLA to make images of Betelgeuse at a variety of radio frequencies. The series of radio observations measured the temperature of the star's atmosphere at different heights. Previous observations with the Hubble Space Telescope (HST) at ultraviolet wavelengths showed that the star's atmosphere contains very hot gas at about twice the surface temperature. The VLA images showed that there also is lower-temperature gas throughout the atmosphere. This gas is near the surface temperature at low heights and decreases in temperature progressively outwards. Although its existence was not previously suspected, this lower-temperature gas turns out to be the most abundant constituent of Betelgeuse's atmosphere.
"This alters our basic understanding of red-supergiant star atmospheres," explains Lim. "Instead of the star's atmosphere expanding uniformly because of gas heated to very high temperatures near its surface, it now appears that several giant convection cells propel gas from the star's surface into its atmosphere. This creates the complex structure we observe for Betelgeuse's atmosphere."
Betelgeuse can be likened to an enormous "boiling" ball of gas heated by the release of energy from nuclear fusion in its core. The circulating boiling pattern -- convection -- appears as large regions of hot upwelling gas on the star's surface. "The idea that red-supergiant stars have enormous convection cells is not new," noted Marson. "This was suggested by Martin Schwarzschild more than 20 years ago, and was seen in optical images of Betelgeuse's surface in 1990."
The new picture of Betelgeuse's atmosphere also helps resolve the mystery of how massive amounts of dust and gas are expelled from red supergiant stars, an important source of enrichment for the interstellar medium. If their atmospheres were entirely very hot at lower levels, dust grains would not be able to condense there. Dust grains could possibly condense at higher levels, but there they would not get enough "push" from the star's radiation to explain their outward movement. In the new picture, the relatively cool environment at lower levels allows dust grains to condense effectively; here they can be strongly propelled by the more-intense starlight, carrying gas with them. Indeed, dust has previously been inferred to form sporadically near Betelgeuse's surface, but its presence there was difficult to reconcile with the old picture.
"This method for propelling the mass outflows of red giant and supergiant stars was proposed by Sun Kwok in the same year that Martin Schwarzschild postulated the existence of large convection cells on these stars," Lim said. "But it has taken us more than 20 years to realize their symbiotic relationship in structuring Betelgeuse's atmosphere, and perhaps also the atmospheres of other such stars."
"Just the fact that we can image the surface of a star other than the Sun blows me away," said Carilli. "We used the highest frequencies available on the VLA, the largest spacing of VLA antennas, and new techniques for calibrating data to produce an image with extremely high resolution."
The bright red star in the shoulder of Orion, the Hunter, Betelgeuse is a red supergiant about 10 times more massive than the Sun. In 1836, Sir John Herschel noticed that Betelgeuse, known since ancient times, actually changes its brightness. Astronomers now know that these regular brightness variations are caused by Betelgeuse's rhythmic change in size over an approximately six-year cycle. If at the location of the Sun, Betelgeuse -- one of the largest stars in our galaxy -- would extend to the asteroid belt, swallowing Mercury, Venus, Earth, and Mars. The VLA measured the temperature of Betelgeuse's atmosphere to a height beyond the orbit of Uranus, more than six times larger still than Betelgeuse itself.
The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.
An image supporting this release is available.