Betelgeuse

STScI, NASA





Larger image.





Betelgeuse is the bright,
orange-colored star in
Constellation Orion, at
top left (another image
with enhanced colors).

Like many of the stars of Orion, it was probably formed a few million years ago from the molecular clouds observed in Orion but has evolved rapidly due to its unusually high mass. The star is a member of the "Orion OB1 Association," where massive young objects with over 10 times the Sol's mass can be found in abundance (more on OB associations and stellar nurseries). No stellar companions observed in past centuries have been confirmed, including speckle detections of two close objects at eight and 67 AUs (Karovska et al, 1986; and 1985).

In 1836, Sir John Frederick William Herschel (1792-1871), portrait), the son of Sir William Friedrich Wilhelm Herschel (1738-1822, portrait), noticed that Betelgeuse had changed in brightness. Astronomers now know that these regular brightness variations are caused by changes in the actual size of the star's atmosphere. According to Robert Burnham, Jr. (1931-93), Betelgeuse is the only marked variable among the first magnitude stars as well as the 11th brightest star in the sky. It is a well known semi-regular, pulsating variable, whose main period appears to be around 5.7 years with shorter periods of 150 to 300 days superimposed. The star has a peak magnitude of 0.2 in 1933 and again in 1942. At minimum brightness, as in 1927 and 1941, the magnitude may drop below 1.2, a difference in light intensity of about two times.

The Star

Betelgeuse is a red supergiant star of spectral and luminosity type M1-2 Ib. The star may have between 12 and 17 times Sol's mass (ESA), over 650 times Sol's diameter, and around 40,000 to 100,000 times its bolometric luminosity. The European Space Agency's astrometry satellite HIPPARCOS has measured the star's distance from Earth to be around 430 light years, giving it an absolute visual magnitude of -5.1 (based on a HIPPARCOS Vmag= 0.45) and a visual luminosity of 9,400 times Sol's. Its 3,100-degree-Kelvin surface, however, radiates mostly infrared wavelengths, so that only 13 percent of its radiant energy is emitted in the form of visible light.

NASA Observatorium




As a red supergiant,
Betelgeuse emits a lot
of infrared radiation
as well as red light.




See a discussion of
"core helium burning"
as part of stellar
evolution and death.

Various measures, particularly those made at higher energy wavelengths where the star's outer gases are particularly opaque, suggest an enormous size (more discussion). In red light, Betelgeuse has an angular diameter of 0.054 arcseconds at 720 nm (Wilson et al, 1992), so that it would extend to 3.6 AUs into the Main Asteroid Belt within the Solar System (for a diameter of 7.2 AUs in red light). In more energetic ultraviolet light, its radius has been measured at over eight AUs out (for a diameter of 16 AUs), or more than twice that at optical wavelengths. Finally, the large radio photospheres of red giant and supergiant stars give rise to detectable thermal radio emission, so that in radio wavelengths, the star's has been found to extend beyond Saturn's orbit (more discussion). Further ultraviolet and far ultraviolet images of the star revealed a chromospheric diameter as large as 260 to 270 milli-arcseconds, or 34 to 35 AUs -- between the orbits of Neptune and Pluto (Timothy et al, 2002; and Uitenbroek et al, 1998).

Andrea Dupree, Ronald Gilliland,
STScI, NASA, and ESA




Larger ultraviolet image





The increased opacity of Betelgeuse's
atmosphere in ultraviolet wavelengths
suggests that the disk of Betelgeuse
extends well beyond Jupiter's orbit and
that a huge convection cell has created
an enormous (yellowish) hotspot -- more
from CfA and STScI.

Betelgeuse is surrounded by shells of dust and gas that it has already blown off through a very strong stellar wind. There is a partial ring of dust lying out at around three times its radius, but more was found at around 650 AUs (James Kaler, 2001; and Rinehart et al, 1998). Even more dust is found around 12,000 AUs out, or 3,000 times the stellar radius. At about 36,000 AUs (or 1,000 times the stellar radius) or so, there is also abundant gas (James Kaler, 2001). In addition, there is a great, assymmetric shell of dust extending to at least 3.3 light-years or one parsec from the star (Baud et al, 1984). Although Betelgeuse is very large in diameter, astronomers believe that it contains no more than 20 Solar-masses so that the star has an average density less than air (University of Leicester Astronomy Society).

Massive stars use their fuel quickly and do not live very long. Although Betelgeuse may only be around six million years old, core hydrogen for thermonuclear fusion has already run out at its core, so that gravity has contracted the core into a hotter and denser state to fuse helium into carbon and oxygen and produce enough radiation to swell out its outer envelopes of hydrogen and helium. The star is relatively rich in nitrogen compared to a less evolved star like Sol (Lambert et al, 1984 ). Its extreme luminosity is probably being generated by a mass so large (e.g., 15 to 17 Solar-masses), so that eventually the star will fuse higher elements all the way to iron and collapse and explode as a supernova (James Kaler, 2002). If Betelgeuse actually has only 12 to 14 Solar-masses, however, it may become one of the exceedingly rare neon-oxygen white dwarfs (rather than the more common ones made mostly of carbon and oxygen nuclei). Useful catalogue numbers and designations for the star include: Alp or Alf Ori, 58 Ori, HR 2061*, HIP 27989, HD 39801, BD+07 1055, SAO 113271, FK5 224, and ADS 4506 AP.

An Irregularly Pulsating, Red Supergiant

Betelgeuse is an irregular pulsating variable that exhibit changes in magnitude from 0.2 to 1.2, and in diameter from 550 to 920 times the diameter of the Sun (University of Leicester Astronomy Society). In 2000, astronomers announced the detection of statistically significant cycles in "longwave" (1900-3200 Å) ultraviolet light radiated by the star at 0.531 years (just over six months) and 3.46 years Rinehart et al, 2000). The star's variability may also be related to periods of mass ejection and the formation of dust grains (Leo Goldberg, 1984).

Andrea Dupree, Ronald Gilliland,
CfA, STScI, NASA, ESA


Larger ultraviolet image.



Betelgeuse pulsates, swells
and shrinks asymmetrically
(CfA press release).

The light variations of Betelgeuse are not completely understood. Astronomers think that the outer layers of the star expand slowly for several years and then shrink again, so that the star's surface area alternately increases and decreases, the temperature rises and falls, and the star brightens and dims. Red supergiants pulsate this way because their atmospheres are not quite stable. When the star is smallest, the atmosphere absorbs a bit too much of the energy passing through it, so the atmosphere heats and expands. As it expands, it becomes thinner. Energy then passes through the outer layers more easily so that its gases cool, and the star shrinks again (AAVSO page on Betelgeuse). In 2000, however, the pulsations of the star were found to beat assymetrically, possibly because of hot spots in its chromosphere from a few, unusually large convection cells (CfA press release).

In 1995, Betelgeuse became the target of the Hubble telescope's first direct image of a star which revealed an enormous hot spot in its atmosphere. Instead of lots of little sunspots as on most main sequence stars, astronomers found an enormous bright area more than 2,000 °K hotter than the surrounding surface of the star. Turbulence below the star's surface may also have been driving a 420-day oscillation (whereby the star "rang" like a bell) and may cause changes in the observed hot spot (more from STScI press release, Astronomy Picture of the Day, and Gilliland and Dupree, 1996).

Jeremy Lim, Chris Carilli, Stephen White,
Anthony Beasley, and Ralph Marson
VLA, NRAO, NSF, NASA





Larger radio image.





Propelled by convective forces,
the star's atmosphere can expand
beyond the orbit of Saturn (more).

In 1998, a team of astronomers announced that observations with the National Science Foundation's Very Large Array (VLA) radio telescope showed that the star has giant convective plumes which propel gas from its surface (photosphere) up into its atmosphere. Previous observations with the Hubble Space Telescope 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. (NRAO press release). Such refinements in the images of Betelgeuse's atmosphere has been helping to 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. More recent observations suggest that the relatively cool environment at lower levels allows dust grains to condense effectively, from which they can be strongly propelled in to deep space by intense radiation that carries gas with them.