The Rival of Betelgeuse

Let's turn at last to the matter of what Antares is like as a sun. We've already discussed the other 1st-magnitude star that is a red giant, Betelgeuse. Perhaps the best way to begin our profile of the physical nature of Antares is therefore by comparisons to Betelgeuse.

Until recent years, such comparisons would not have been flattering to Antares. Betelgeuse was regarded as the more luminous, larger, and cooler star (which is more impressive at this cool end of the range of spectral types). Betelgeuse is almost always a star of brighter apparent magnitude than Antares (both stars are variable in brightness—more on this in a moment). But which star has the greater true brightness—and true diameter—depends on how far they are from Earth. That remains poorly known because these vast, diffuse-edged, pulsating stars are difficult to get accurate enough positions for to allow good parallax measurements. But in recent years, most authorities seem to estimate Antares as being considerably farther from us than Betelgeuse—and therefore having greater true brightness and size. For instance, one source has Betelgeuse as 520 light-years away and therefore an average absolute magnitude of about -5.0, but places Antares 600 light-years away with an average absolute magnitude of -5.8. Another source, older and perhaps less reliable, lists the distance to Betelgeuse as only 350 light-years (a mere 40 light-years farther than Cano-pus) and to Antares as 450 light-years. If these latter, closer distances to us are true than Betelgeuse would have an absolute magnitude of -4.9 and Antares, -4.8.

James Kaler emphasizes the uncertainty in the distances of the two stars but opts to give a distance of about 425 light-years for Betelgeuse and 600 for Antares. The diameter of Betelgeuse in visible light, he gives as about 2.8 AU, and he lists two different methods for determining Antares's diameter—one that gives a figure of about 3 AU; and the other, about 3.8 AU. And yet he seems to list the same luminosity—60,000 times solar if we include the large percentage of infra-red light—for both stars. Could this equality be due to the fact that Betelgeuse is a subtype M2 star and Antares an M1.5, so that Betelgeuse, though smaller and less luminous in visible light, is cooler and produces more infrared radiation than Antares? Actually, among M-type stars, scientists think that surface temperature does not have a strong correlation with subtype—an M2 star is not necessarily cooler than an M1.5. Kaler, in fact, lists the surface temperatures of Betelgeuse and Antares as both being approximately 3,600 K. So on this point, too, the jury is out.

Betelgeuse and Antares are both luminosity class Iab supergiants. Kaler suggests that the initial mass of Antares was about fifteen to eighteen times that of the Sun, whereas that of Betelgeuse was about twelve to seventeen solar masses. Both stars are almost certainly destined to end up as supernovae. Both emit radio energy; both appear to have vast sunspots and bright regions upon them. And both vary considerably in both size and brightness.

One category in which Betelgeuse does seem to exceed Antares is in its range of variability. Betelgeuse fluctuates in brightness by perhaps about 0.4 magnitude commonly, and has a rare more extreme range of perhaps more than 1.5 magnitudes. The variations of Antares are rarely more than from about magnitude 0.86 to 1.06, and the star apparently never gets brighter than this most typical range. But an extreme minimum of 1.8 has been reported for Antares. At this point it would be only the second (or, if Delta Scorpii were in its bright state, third) brightest star in Scorpius. Could Antares get even dimmer on rare occasions? Eratosthenes said that Beta

Librae was the brightest star in the combined Scorpion (Scorpius) and its claws (Libra). Ptolemy held that Beta Librae and Antares were of the same brightness. Today, Beta Librae shines at only magnitude 2.6, dimmer than several stars of Scorpius. So if we correctly understand those ancient authorities, and if they were right—two big ifs—then Beta Librae must have been considerably brighter than it is now, not just Antares dimmer.

The main brightness variation of Antares has a semiregular periodicity of approximately 1,733 days, or 4.75 years.

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