Gazing Toward Orion
we are looking edgewise
into the disk of the Milky Way galaxy. The center of the galaxy
is in just the opposite direction. So we are looking outward along
one of the spiral arms of the galaxy. There are fewer stars in
this direction, but those that are there are generally closer,
and many are brighter. Certainly there are a good number of interesting
stars and other deep-sky objects to consider.
Rigel and Betelgeuse are
the two brightest stars in Orion, which is the only constellation
within our vision to boast of two first-magnitude stars. The only
other such constellation is Centaurus, which can be seen only
from southern skies. In addition, Bellatrix, Alnilam, and Alnitak
(Gamma, Epsilon, and Zeta Orionis) shine at magnitudes between
1 and 2, with Saiph (Kappa Orionis) at magnitude 2.05 close behind
in the list of 51 brightest stars. Together, these stars form
one of the most distinctive of all constellations.
Betelgeuse is Alpha Orionis,
although Rigel (Beta) is a little brighter, on average. Betelgeuse
pulsates irregularly, varying from a brightest 0.4 to about 1.2.
The two stars are at about the same distance from us, about 1400
light years. The two are quite different, however. Rigel is a
hot blue supergiant star, burning its hydrogen fuel at a prodigious
rate. About 50 times the mass of the Sun, it is 57,000 times as
luminous. Betelgeuse is a red giant, so huge that its diameter
has been measured by direct interferometer means. It would encompass
the orbit of Mars if it were placed at our Sun's position.
The three stars that form
Orion's belt, prominent in the sky mythology of a number of cultures,
are Delta, Epsilon, and Zeta Orionis, from northwest to southeast.
Their respective names are Mintaka, Alnilam, and Alnitak. For
the Chumash Indians of California these stars were simply called
Three In a Row, and their location in the sky served as
a calendar to mark ritual events.
Of these three stars of
Orion's belt, Mintaka has the distinction of having a declination
of almost exactly zero; its rising marks the east on the horizon,
its setting the west. So it is not surprising that this star was
of special consideration to astrologers, who avowed that it meant
The Great Orion Nebula, M42, is
a huge interstellar gas cloud 20,000 times the diameter of our
solar system. Here stars like our Sun are being born; as they
ignite, their radiation causes the gas cloud to glow with different
colors. Photograph by R. S. Hoyle; 40-minute exposure on gas-hypered
Ektachrome P1600 film, using an 8-inch SCT; f/10.