The Bright Stars of Orion
By Tim Hunter
Figure 1. Orion and its environs. Ten-minute exposure on Kodak
Ektachrome 800 slide film using a 50mm f/2 lens. Note NGC2024
and the Horsehead Nebula near Alnitak (Zeta Orionis) in Orion’s
belt, Barnard’s Loop in the eastern part of the constellation,
and the red Rosette Nebula in Monoceros to the east of Orion.
In the Northern Hemisphere, Orion typifies the essence of dark,
cold, clear nights with bright shimmering stars. It is the most
widely known constellation after the Big Dipper. It is also one
of the brightest constellations, and it contains two of the top
ten brightest stars in the sky (1-9). The contrast between the
orange red of Betelgeuse and the blue white of Rigel is striking
and one of the most evident contrasts of star colors in the
entire sky. A similar contrast in the Northern Hemisphere is
that of Spica and Arcturus.
Most of the stars in Orion are young, and the entire
constellation is embedded in a large nebulosity of dust and gas.
The constellation is a figment of mankind’s imagination. Its
appearance is only the chance happenstance of star arrangements
at this place and time, though most of the stars in the
constellation are associated with each other. If we were to look
at the constellation at another time and place, its appearance
would be radically different.
Figure two shows Orion with its brightest stars labeled. Three
bright stars, Betelgeuse, Meissa, and Bellatrix, make up the
upper extremities of Orion the Hunter, while three stars,
Alnitak, Alnilam, and Mintaka consist of his belt. The Sword of
Orion comprises both stars and the Great Orion Nebula (M42/43)
and other nebulosity, such as NGC1977 to the north of M42. The
Orion Nebula and its associated stars will not be discussed in
this essay. In the bottom of the constellation are two bright
stars Rigel and Saiph.
The eight brightest stars of Orion - Betelgeuse, Meissa,
Bellatrix, Alnitak, Alnilam, Mintaka, Rigel, and Saiph - will be
the subjects of this essay (Tables I and II). To start the
discussion, we will contrast Betelgeuse and Rigel, two giants
both in respect to physical size and brightness.
Betelgeuse vs. Rigel
Betelgeuse and Rigel have similarities and distinct differences.
They are both supergiant stars with thousands of times more
luminosity than the Sun. However, Betelgeuse is a red supergiant
with a low temperature, and Rigel is blue supergiant with a
relatively high surface temperature. Rigel is one of the most
intrinsically luminous stars known. A fascinating star similar
to Rigel is Deneb, which is also one of the most intrinsically
luminous stars known. A star similar to Betelgeuse is Antares.
The Macmillan Encyclopedia lists its magnitude as 0.45 and
describes it as a semi-regular pulsating variable star, varying
by + or – 0.15 magnitudes over ~ 6.4 years (6). It is a spectral
type M21b. This means it has a red-orange color, a temperature
between 2500 and 3900K and has strong titanium oxide and neutral
Ca lines in its spectrum. Betelgeuse has several faint companion
stars. Even though Betelgeuse is a relatively cool star, it has
luminosity 60,000 times that of the Sun. This means it has an
enormous size with a radius more than 600 times greater than the
Sun. Betelgeuse is the fist star other than the Sun to be
resolved as a disk.