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.

Introduction

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.

Betelgeuse:

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.

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