BETELGEUSE. The great star Betelgeuse is one
of the two that dominate mighty Orion of winter, the other Rigel,
the pair respectively called Alpha and Beta Orionis. One of the sky's two
first magnitude supergiants (the other Antares of summer), Betelgeuse
is one of the larger stars that can be seen, indeed one of the larger stars
to be found anywhere. At its most likely distance of 425 light years, its
measured angular diameter yields a radius 630 times that of the Sun, 2.9
astronomical units. If placed at the Sun, the star would go 55% of the
way to the orbit of the planet Jupiter. The star is so large that it is
the first ever actually directly imaged as a disk from Earth (by the Hubble
Space Telescope). From its size and temperature, allowing for its infrared
radiation, Betelgeuse shines an amazing 60,000 times brighter than
our Sun. The distance, however, is so great to be subject to some uncertainty,
the possible radius ranging from 45% to 70% of Jupiter's orbit, the luminosity
from 40,000 solar to 100,000 solar.
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.
Most likely, the star is now in the process of fusing helium into carbon
and oxygen in its core. Betelgeuse is variable over long time periods,
is ejecting part of itself through a strong wind, and is surrounded by
a huge shell of dust of its own making. The wind and variability are perhaps
related to huge hot spots on the star's surface, one of which was seen
by Hubble.
Betelgeuse is also surrounded by some controversy.
From theory, its initial mass should have fallen somewhere between 12 and
about 17 times that of the Sun. If at the high end, 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. If the star is near or under the lower end of the range,
then Betelgeuse may eventually become a shrunken and dense white
dwarf about the size of Earth. Even then, however, it will be different.
Most white dwarfs are made of carbon and oxygen, whereas Betelgeuse
has enough mass to become one of the exceedingly rare neon-oxygen white
dwarfs. The only way we will really know is to wait a few million years.
Fitting of the flux distribution using Kurucz models
was not atrempted. Red crosses in the IUE spectra indicate bad-quality
points.