INTERLUDE 20-2 Learning Astronomy from History
Sirius A, the brighter of the two objects shown in Figure 20.14, appears twice as luminous as any other visible star, excluding the Sun. Its absolute brightness is not very great, but because its distance from us is small (less than 3 pc), its apparent brightness is very large. Sirius has been prominent in the nighttime sky since the beginning of recorded history. Cuneiform texts of the ancient Babylonians refer to the star as far back as 1000 b.c., and historians know that the star strongly influenced the agriculture and religion of the Egyptians of 3000 b.c.

Even though a star's evolution takes such a long time, we might have a chance to detect a slight change in Sirius because the recorded observations of this star go back several thousand years. The chances for success are improved in this case because Sirius A is so bright that even the naked-eye observations of the ancients should be reasonably accurate. Interestingly, recorded history does suggest that Sirius A has changed in appearance, but the observations are confusing. Every piece of information about Sirius recorded between the years 100 b.c. and a.d. 200 claims that this star was red. (No earlier records of its color are known.) In contrast, modern observations now show it to be white or bluish white—definitely not red.

 

 If these reports are accurate, then Sirius has apparently changed from red to blue-white in the intervening years. But according to the theory of stellar evolution, no star should be able to change its color in this way in such a short time. A color change such as this should take at least several tens of thousands of years, and perhaps a lot longer. It should also leave some evidence of its occurrence.

Astronomers have offered several explanations for the rather sudden change in Sirius A. These include the suggestions that (l) some ancient observers were wrong and other scribes copied them; (2) a Galactic dust cloud passed between Sirius A and Earth some 2000 years ago, reddening the star much as Earth's dusty atmosphere often reddens our Sun at dusk; and (3) the companion to Sirius A, Sirius B, was a red giant and the dominant star of this double-star system 2000 years ago but has since expelled its planetary nebular shell to reveal the white-dwarf star that we now observe.

Each of these explanations presents problems. How could the color of the sky's brightest star be incorrectly recorded for hundreds of years? Where is the intervening Galactic cloud now? Where is the shell of the former red giant? We are left with the uneasy feeling that the sky's brightest star doesn't fit particularly well into the currently accepted scenario of stellar evolution.