At right is a UV picture of Betelgeuse taken by
the Hubble Space Telescope in March 1995.
It was the first image ever obtained that revealed
the spatial extent of a star other than the Sun.
Betelgeuse is a red supergiant.
The variability of its size and luminosity
explain why Betelgeuse appears in celestial maps as
a-Orionis
(at the right shoulder of Orion) although it's technically less
bright than the blue giant Rigel
(b-Orionis) which
also belongs to the constellation of Orion
(Rigel is at the "left foot" of Orion, the hunter).
According to Hipparcos parallax data, Betelgeuse
(HIP 27989)
is 427 light-years away (give or take 92 light-years).
However, the distance of Betelgeuse is still widely quoted to be
between 300 and 650
light-years.
Betelgeuse is one of the two stars with the largest apparent diameter
(besides the Sun, of course).
It's virtually tied with R-Doradus, a southern star with an apparent
optical diameter of 57 mas.
The apparent diameter of Betelgeuse is about 55 mas in the optical
spectrum (at 720 nm) but it's around 125 mas in the
near-UV spectrum and about 270 mas in the far ultra-violet.
The symbol "mas" stands for "milli-arcsecond", a unit of angular measure of
which there are 3600000 in a degree (or 1296000000 in a full turn).
1 mas is about 4.848 nrad ("nrad" = nanoradian).
In 1920,
Francis Gladheim Pease and Albert A. Michelson
used optical interferometry to
obtain the first determination of the size of a star.
They found the angular diameter of Betelgeuse to be
44 mas
(the average value of 55 mas is now commonly accepted).
The actual diameter of the star does vary by 60% or more,
as Betelgeuse shows an unstability indicative of its ripeness
to explode into a supernova (in a matter of mere centuries or millenia).
An angle of 55 mas
at 427 light-years corresponds to a linear distance of
7.2 astronomical units (au).
This translates into a radius of 3.6 au,
which is larger than the orbit of Mars (3.06 au).
Larger estimates for the distance of Betelgeuse and/or its angular diameter
would even make Betelgeuse's equator commensurate with the orbit of Jupiter
(5.2 au).
The mass of Betelgeuse cannot be much more 20 solar masses.
Therefore, its density is extremely low...
A ball whose radius is 3 au (650 times as big as the sun)
and whose mass is 20 solar masses has
an average density of only 0.0001 g/L.
This is just a rarefied gas, which is about ten thousand times less dense than air
(1.214 g/L).
The temperature of Betelgeuse has been estimated to be around 3900 K
(Tsuji, 1979).
