Extraterrestrial Civilizations - Isaac Asimov [51]
This would be a return to a form of catastrophism, even though the passing of the Sun through a cloud of gas is not nearly so violent an event as the collision or near collision of two stars. It is still an accidental event and would necessarily result in relatively few planetary systems.
On the other hand, if it turned out that the evidence clearly indicated that a great many stars happened to have planets, then we could not possibly expect this to happen in any catastrophic way. Some version of the nebular hypothesis with the automatic or near-automatic formation of planets along with a star would have to be correct.
The trouble is, though, that we can’t see whether any stars have planets in attendance. Even at the distance of the nearest star (Alpha Centauri, which is 4.3 light-years from us) there would be no way of actually seeing even a large planet the size of Jupiter or greater. Such a planet would be too small to see by the reflected light of its star. Even if a telescope were invented that could make out that dim flicker of reflected light, the nearness of the much greater light of its star would utterly drown it out.
We must give up hope of direct sighting then, at least for now, and resort to indirect means.
Consider our own Sun, which is a star that certainly has a planetary system. The remarkable thing about the Sun is that it rotates so slowly on its axis that 98 percent of the angular momentum of the system resides in the insignificant mass of its planets.
If angular momentum passed from the Sun to its planets when those planets were formed (by any mechanism), then it is reasonable to suppose that angular momentum might pass from any star to its planets. If, then, a star has a planetary system, we would expect it to spin on its axis relatively slowly; if it does not, we would expect it to spin relatively rapidly.
But how does one go about measuring the rate at which a star spins when even in our best telescopes it appears as only a point of light?
Actually, there is much that can be deduced from starlight even if the star itself is but a point of light. Starlight is a mixture of light of all wavelengths. The light can be spread out in order of wavelength from the short waves of violet light to the long waves of red light, and the result is a “spectrum.” The instrument by which the spectrum is produced is the “spectroscope.”
The spectrum was first demonstrated in the case of sunlight by Isaac Newton in 1665. In 1814, the German physicist Joseph von Fraunhofer (1787–1826) showed that the Solar spectrum was crossed by numerous dark lines, which, it was eventually realized, represented missing wavelengths. They were wavelengths of light that were absorbed by atoms in the Sun’s atmosphere before they could reach the Earth.
In 1859, the German physicist Gustav Robert Kirchhoff (1824–1887) showed that the dark lines in the spectrum were “fingerprints” of the various elements, since the atoms of each element emitted or absorbed particular wavelengths that the atoms of no other element emitted or absorbed. Not only could spectroscopy be used to analyze minerals on Earth, but it could be used to analyze the chemical makeup of the Sun.
Meanwhile, the art of spectroscopy had been refined to the point where the light of stars, though much dimmer than the light of the Sun, could also be spread out into spectra.
From the dark lines in the stellar spectra much could be worked out. If, for instance, the dark lines in the spectrum of a particular star were slightly displaced toward the red end, then the star would be receding from us at a speed that could be calculated from the extent of the displacement. If the dark lines were displaced toward the violet end of the spectrum, the star would be approaching us.
The significance of this “red shift” or “violet shift” was quite evident from work that