BETELGEUSE (Alpha Orionis). The great star Betelgeuse is one of
the two that dominate mighty Orion of
northern winter, the other Rigel, the pair
respectively also called Alpha and Beta
Orionis. The name Betelgeuse is a corruption of the Arabic "yad al
jauza," which means the "hand of al-jauza," al-jauza the ancient
Arabs' "Central One," a mysterious woman. For us, it marks the
upper left hand corner of the figure of the Greek's ancient hunter
(and since he depicted is facing you, his right shoulder). One of
the sky's two first magnitude supergiants (the other Antares), Betelgeuse is one of the larger
stars that can be seen, indeed one of the larger stars to be found
anywhere. Typically shining at visual magnitude 0.7 (ranking 11th
in the sky), this class M (M1.5) red supergiant (with a temperature
of about 3650 Kelvin) is a semi-regular variable that changes
between magnitude 0.3 and 1.1 over multiple periods between roughly
half a year and 6 years -- and possibly longer (and of course
changing its rank). That may explain why Betelgeuse is the Alpha
star even though it is generally fainter than Rigel. (On the other
hand Bayer frequently also used position
in lettering his stars.) The star's distance is a problem and a
puzzle (true for all the other parameters as well). Direct parallax measures from space,
using the most modern results, give 495 light years, whereas the
parallax using the star's natural radio emission gives 640 light
years. At a compromise distance of 570 light years, and allowing
for a lot of infrared radiation and for absorption of light by
circumstellar dust, the luminosity comes in at 85,000 times that of
the Sun, considerably more than comes out of
Antares. At the larger distance, luminosity boosts up to 105,000
Suns. From these and the temperature, we derive respective radii
of 3.1 and 3.4 Astronomical Units, more than double the size of the
Martian orbit. The star is so big that its angular diameter is
easily measured. Indeed it was the first to have such a measure,
of 0.047 seconds of arc, from which we find a true radius of
between 4.1 (compromise distance) and 4.6 (larger distance) AU,
considerably greater. However, the star is surrounded by a huge
complex pattern of nested dust and gas shells, the result of aeons
of mass loss, that extends nearly 20,000 AU away (Betelgeuse so far
having lost over a solar mass). That, an extended atmosphere, and
the pulsations make it difficult to locate an actual "surface" to
tell just how large the star actually is. Moreover, because of
changes in gaseous transparency, the "size" of the star depends on
the color of observation. Long-wave
infrared measures give a
vastly larger radius of up to 5 AU and greater, as big as the orbit
of Jupiter, while shorter-wave infrared light gives as small as 3
AU. Moreover still, infrared measures reveal Betelgeuse to be
shrinking (by some 15 percent over about 20 years), and other
measures show that the star is not even round, but somewhat oval.
Hubble observation also shows hot spots. It's no wonder that we
find the various disagreements. It's more surprising that all
agree as well as they DO. Whatever the actual numbers, Betelgeuse
is clearly a highly evolved star, one whose central hydrogen fuel
supply has run out. As a result, the core has contracted into a
hot dense state, and the outer portions swelled outward. We do not
really know the star's condition at the moment, but the odds are
that it is now in the process of fusing helium into carbon and
oxygen in its core. From theory, its initial mass should have
fallen somewhere around 18 or 19 times that of the Sun. Starting
life as hot, blue, class O star only around 10 million years ago,
Betelgeuse will fuse elements through neon, magnesium, sodium, and
silicon all the way to iron. The core will then collapse, causing
the star to blow up as a supernova, most likely
leaving a compact neutron
star about the size of a small town behind. If it were to
explode today, it would become as bright as a gibbous Moon, would cast strong
shadows on the ground, and would be seen easily in full daylight.
Birthplace was far away. The star's motion shows it to be a
runaway member of the Orion OB1
association, particularly the subgroup that involves the stars
up and to the right of the Belt.
New observations show that Betelgeuse is producing a shock wave as
it plows through an interstellar cloud at 30 km/s, double the speed
of its wind. The star is also violently convective, non-spherical,
and irregularly losing mass at a rate of some five hundred thousandths
of a solar mass a year.
Written by Jim Kaler 2/20/98. Last updated
1/25/11. Return to STARS.