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by
Jim Kaler
Hubble Space Telescope Captures First Direct Image of a Star. This is
the first direct image of a star other than the Sun. Called Alpha Orionis,
or Betelgeuse, the star is a red super giant, a Sun-like star nearing the
end of its life. The Hubble picture reveals a huge ultraviolet atmosphere
with a mysterious hot spot on the stellar behemoth's surface. The enormous
bright spot, more than 10 times the diameter of Earth, is at least 2,000
degrees Kelvin hotter than the star's surface. Credit: Andrea Dupree
(Harvard-Smithsonian CfA), Ronald Gilliland
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.
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