Meissa at magnitude 3.39 is one of the faintest stars to carry a proper name. It is a double star that consists of a fourth magnitude O star (35,000K) and a sixth magnitude B star (27,000K) four arcseconds away. This pair is a beautiful double star for visual observers, and various persons have ascribed colors to the two stars, though they should be mainly white or blue-white in color (5). This double star system combined has luminosity approximately 70,000 times greater than the Sun. Meissa also illuminates a large surrounding ring of gas.

Saiph is at about the same distance as Rigel, though Saiph is two magnitudes fainter. This is because Saiph is probably not as intrinsically as luminous as Rigel, and because it is so much hotter than Rigel. Saiph has a temperature of 26,000K, and it radiates much of its light in the ultraviolet portion of the spectrum. Its spectrum should consist of neutral helium with some hydrogen absorption lines (8).

Discussion

The values for the bright stars of Orion in Table I were gleaned from a number of sources (4-6, 8-9). The luminosity and temperatures listed for these stars vary widely from one reference to another. For example, Professor Kaler’s excellent web site lists the luminosity of Rigel as 40,000 times that of the Sun, while Universe 6th Edition by Freedman and Kaufmann lists its luminosity as 700, 000 times that of the Sun (5,8). The program Guide lists the luminosity of Rigel as 40,000 ± 15,000 times that of the Sun (10). Guide derives its luminosity data from the Hipparcos data, but Guide does not explicitly state the luminosities it derives are visual luminosities.

If one looks at the apparent brightness of a star and compares this with the star’s estimated distance and absolute (visual) magnitude, it is possible to calculate a visual luminosity for the star. This is what Guide has done with the Hipparcos data. However, the luminosities listed by Professor Kaler and Universe are apparently the luminosities for all wavelengths emitted by a given star. I presume these latter values were calculated based on a star’s apparent brightness, its spectral class (temperature), and its distance. Unfortunately, neither Professor Kaler nor Universe 6th Edition provide any specific references for their luminosity data.

With regard to the bright stars of Orion, it is apparent their large distances produce considerable imprecision in the parallax estimates for them, despite the superb results from the Hipparcos mission (Table I). Thus, while we have a fair understanding of their size, their temperature, and their luminosity, the imprecision in their distance measurements means our estimates for their size and other physical characteristics need much further refinement. Also, various experts disagree on how much weight to apply to a star’s measured distance and its spectral class in calculating its physical parameters. “…the discrepancies do not reflect errors, only points of view…” (9).

Conclusions

Despite the imprecision for the estimated physical parameters of the bright stars of Orion, we have a reasonably good understanding of them. The bright stars of Orion are no ordinary stars. They are all spectacularly luminous and are giants or supergiants. They will have short lives and end their existences in supernova explosions.

To me, Betelgeuse is orange in color, and Rigel has a metallic blue-white color. The other stars appear white. All are easily visible from my backyard in Tucson, Arizona, a city of 1,000,,000 persons (figure 2). It would be fascinating for us if they were closer. We could study them more fully and enjoy the magnificent stars and nebulosity of Orion better if they were not so distant.

However, the bright stars of Orion give off tremendous illumination, much of it in the ultraviolet region of the spectrum. We certainly do not want them nearby, for they would cause light pollution similar to the Moon, and if one of them should decide to explode as a supernova, we would be obliterated. Some consider Betelgeuse the most likely candidate for a supernova explosion in our galaxy, possibly in our lifetime or in the expected lifetime of our civilization, whatever that may be. How close could we be to these bright stars and still be out of danger from a supernova explosion? That is an interesting question, and it would be a great subject for a future essay.

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