Eclipse Eclipse

FIGURE 14-2. The eclipses of the Algol binary star system. Because the smaller star is much brighter than the larger, the primary eclipses cause the overall intensity to dip by a factor of three, while the secondary eclipses result in dimming by only about 10 percent.

are very different. Planets and their moons cannot produce light. All that Pluto and Charon (and all the other objects in the Solar System) do is reflect the light of the Sun back from their surfaces. During a Pluto—Charon eclipse the drop in the total intensity we receive is only 20 percent, depending solely on their comparative areas and albedos. In contrast, in a binary star system both components emit their own light, making possible much larger amplitudes in the variation of the total light received in our telescopes.

In most binary stars, the members are of differing types, and the intensities of the light each emits depend on the complexities of stellar evolution and their internal workings. It does not follow, therefore, that the larger of two stars must be the brightest, or even have the greater mass. If anything, the converse tends to be the case. A more massive star will have greater self-gravity, which condenses it. This makes it hotter and denser in its interior, promoting nuclear fusion (the energy generation within stars through fusion reactions was discussed in Chapter 5). As a result, its energy generation rate would be elevated. Smaller stars tend to be hot and thus white in color, larger ones cooler and redder. The amount of emitted light rises as the fourth power of the surface temperature, overpowering the influence of the greater surface area of big stars. It is like comparing a red-hot poker pulled from a fire with the filament in an electric light bulb: there is no doubt concerning which is the brighter. The tiny filament emits more light because it is much, much hotter.

In the case of Algol, then, the smaller member is hotter and brighter than its relatively dim companion. In the secondary eclipses (see Figure 14-2), at the phase when about half of the larger member is covered by the more brilliant, the total brightness of the system falls by only about 10 percent. In contrast, in the primary eclipses the dim star obscures more than half of its brighter companion, and the total intensity plummets by a factor of three. Separated by 69 hours, such eclipses last for 10 hours from start to end, the faintest part persisting for only an hour or so. During a long winter night Goodricke or others might have witnessed a complete eclipse in the Algol system and easily charted its relative magnitudes by comparison with other stars. Three nights later they could have seen the same thing, although gradually the eclipses would have fallen back until they occurred in daytime, because the cycle is not a multiple of 24 hours. Timing of the eclipses over a month or so, when visible, would have allowed the astronomers to determine the consistent 69-hour period governing the eclipses.

The fact that Algol's errant behavior is so obvious, and yet was ignored by the astronomical establishment for many decades, is a prime example of scientific conservatism. Scientists are some of the most conventional of creatures, the majority being totally unwilling to stick their necks out. Thinking back to the lead-up to the outermost planet's discovery, one might poke fun at Percival

Lowell and his beliefs about life on Mars, and the basis of the search that fortuitously turned up Pluto. Then again, I reckon that he derived more enjoyment from his astronomy than those who criticized him, before his death and after. "It takes all sorts to make the world turn," goes the old aphorism, and the thought may be extended to the entire universe, and our study of it. Without the radicals who will not listen to "conventional wisdom," scientific progress would be even slower than it is now.

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