Extraterrestrial Civilizations - Isaac Asimov [52]
Suppose, now, that a star is rotating and that it is so situated in space that neither of its poles is facing us, but that each pole is located at or near the sides of the star as we view it. In that case, at one side of the star between the poles the surface is coming toward us, and on the opposite side it is receding from us. The light from one side causes the dark lines to shift slightly toward the violet, the light from the other causes them to shift slightly toward the red. The dark lines, shifting perforce in both directions, grow wider than normal. The more rapidly the star rotates, the wider the dark lines in the spectrum.
This was first suggested in 1877 by the English astronomer William de Wiveleslie Abney (1843–1920); and the first actual discovery of broad lines produced by rotation came in 1909 through the work of the American astronomer Frank Schlesinger (1871–1943). It was only in the mid-1920s, however, that studies on the rotation of stars began to be common and the Russian-American astronomer Otto Struve (1897–1963) was particularly active here.
It was indeed found that some stars do rotate slowly. A spot on the Sun’s equator travels only about 2 kilometers (1¼ miles) per second as the Sun makes its slow rotation on its axis, and many stars rotate with that equatorial speed or not very much more. On the other hand, some stars whirl so rapidly on their axis as to attain equatorial speeds of anywhere from 250 to 500 kilometers (165 to 330 miles) per second.
It is tempting to assume that the slow-rotators have planets and have lost angular momentum to them, while the fast-rotators do not have planets and have retained all, or almost all, their original angular momentum.
That is not all that can be learned in this way, however. When stellar spectra were first studied, it was clear that while some had spectra resembling that of the Sun, others did not. In fact, stellar spectra differed from each other widely and, as early as 1867, Secchi (the astronomer who had anticipated Schiaparelli’s discovery of the Martian canals) suggested that the spectra be divided into classes.
This was done, and eventually the various attempts to label the classes ended in the spectra being listed as O, B, A, F, G, K, and M, with O representing the most massive, the hottest, and the most luminous stars known; B was next, A next, and so on down to M, which included the least massive, the coolest, and the dimmest stars. Our Sun is of spectral class G and is thus intermediate in the list.
As stellar spectra were more and more closely studied, each spectral class could be divided into ten subclasses: B0, B1 … B9; A0, A1 … A9; and so on. Our Sun is of spectral class G2.
The American astronomer Christian Thomas Elvey (1899–), working with Struve, found by 1931 that the more massive a star, the more liable it was to be a fast-rotator. The stars of spectral classes O, B, and A, together with the larger F-stars, from F0 to F2, were very likely to be fast-rotators.
The stars of spectral classes F2-F9, G, K, and M were virtually all slow-rotators.
Half the spectral classes, then, are fast-rotators and half are slow-rotators, but that doesn’t translate into an equal division of stars. The smaller stars are more numerous than the larger ones, so that there are more stars, by far, that are spectral class G or smaller than are spectral class F or larger. In fact, only 7 percent of all the stars are included in spectral classes 0 to F2.
In other words, there are not more than 7 percent of the stars that are fast-rotators and fully 93 percent of the stars that are slow-rotators. This would make it seem that at least 93 percent of the stars have planetary systems.
In fact, we might not even be truly able to eliminate the 7 percent of the fast-rotators. They happen to include the particularly massive stars, which are likely to have a