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 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 is facing you, his
right shoulder). One of the sky's two first magnitude supergiants (the other Antares of northern summer), Betelgeuse is
one of the larger stars that can be seen, indeed one of the larger
stars to be found anywhere. Typically shining at magnitude 0.7,
this class M (M1.5) red supergiant (with a temperature of about 3600
Kelvin) is a semi-regular variable that changes between magnitude
0.2 and 1.5 over multiple periods between roughly half a year and
6 years. At its most likely distance of 425 light years, its
measured angular diameter yields a radius 600 times that of the
Sun, 2.8 Astronomical Units. If placed at the Sun, the star would go 55% of the way to the
orbit of the planet Jupiter. From its size and temperature,
allowing for its infrared radiation, Betelgeuse shines an amazing
60,000 times brighter than our Sun, which coupled with the
temperature also gives a radius of 2.8 AU. However, the star is
ejecting part of itself through a strong wind, and is surrounded by
a huge shell of dust of its own making. That, an extended
atmosphere, and the pulsations make it difficult to locate an
actual "surface" and to tell just how large the star actually is.
Even the distance is subject to uncertainty, the luminosity ranging
from 40,000 solar to 100,000 solar. 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 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 between 12 and about 17 times that of the Sun
which suggests that the core will fuse elements through neon,
magnesium, sodium, and silicon all the way to iron. It will then
collapse, and Betelgeuse will 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 crescent Moon, would
cast strong shadows on the ground, and would be seen easily in full
daylight.
Update 2008: A new parallax derived from optical
and radio observations now place Betelgeuse at a much greater
distance of 640 light years, which gives a luminosity of 135,000
times that of the Sun and a radius of 4.4 AU
(950 times solar). The optical angular diameter yields a closely
agreeable 4.6 AU (996 times solar). The star is 87 percent the
size of Jupiter's orbit. Infrared observations at a wavelength 20
times that of visual light show Betelgeuse to be even bigger, 5.3
AU (just bigger than Jupiter's orbit), the result of a diffuse
surface and circumstellar matter. From the luminosity and
temperature, Betelgeuse carries a mass of 20 Suns. With an age of
8.5 million years, it is a prime candidate to explode as a supernova. 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.
Written by Jim Kaler 11/13/1998; updated
5/20/05; last updated 8/01/08. Return to STARS.