By Dr. Whitney Shane, MIRA's Charles Hitchcock Adams Fellow
Algol (or beta Persei) is one of the brightest and best-known variable stars. The arabic name, assigned many centuries ago, means the demon, and the name was given because of the very peculiar behavior of the star. Every 2.9 days the brightness decreases briefly by a factor of more than 2. Any patient observer, even one without a telescope, can check this behavior for himself.
Algol is an example of an eclipsing binary, in which the light variation is caused by blocking of the light of one component by the other. Since eclipses can take place only when the system is seen nearly edge on, or the Earth is close to the plane of the orbit, most binary stars are not eclipsing. Visual binaries are never eclipsing because of their wide separation and the very small chance that the Earth is close enough to the orbital plane, although some eclipsing binaries can be resolved using long baseline interferometers. On the other hand, almost all eclipsing binaries show velocity variations when observed with a spectrograph, and they are thus classified as spectroscopic binaries as well. Since the inclination of the orbit is known for these stars, this gives an absolute scale to the orbit. When the spectra of both stars are observed, the ratio of the masses of the stars can be determined.
The light curve of Algol. The deep minima (dips in brightness) occur when the fainter star blocks the light of the brighter.
The light variation is also very revealing. The depths of the minima during eclipse tell us about the surface brightness of the stars, the duration of the eclipses gives us the sizes of the stars, and the shapes of the minima tell us something about the light distribution over the stellar disks. With all this information available it is no wonder that astronomers find the study of eclipsing binaries very rewarding.
As it turns out, Algol is even more demonic than the ancient Arabs realized. The unusual depth of the primary eclipse, along with a little information about the colors, tells us that the fainter of the two stars is also the larger. This is a surprising result, because it seems to contradict our understanding of stellar evolution. We may assume that two stars so close together formed about the same time. We know from stellar evolution calculations that stars, after they form, spend much of their lives on the main sequence, where the mass determines the temperature, size, and thus luminosity. The more massive a star the larger, brighter and hotter it is. Only after most of the hydrogen has been converted to helium does it begin to cool and expand, becoming somewhat brighter in the process. The more massive the star the sooner this happens, because these stars burn their hydrogen much faster. Thus a large cool star which is comparatively faint must be quite old, whereas a compact bright star must be much younger. The fact that Algol, along with a number of other binaries, contradicts this expectation is known as the Algol paradox.
If the story stopped here it would just be one of the many unsolved puzzles of astronomy, and not so very instructive. Fortunately, we now think we know how Algol got to be the way it is. The fainter of the two stars was originally the more massive. It evolved quickly, converting the hydrogen in its center into helium, and soon began to expand. In doing so, its outer layers got so close to the smaller companion that they were captured by its gravitational attraction. As the massive star lost mass to the companion, this gravitational effect became stronger and the mass transfer accelerated. Only when the whole outer part of the star, containing most of the unburned hydrogen, had been lost, did this process stop. What was left was a faint star in an advanced stage of evolution, with most of the hydrogen burned, and a brighter star which is full of hydrogen and just starting its evolutionary path. The mass transfer is probably still going on, as the faint star slowly expands, but it is no longer the runaway process that it was in the early stages. We seldom see stars in this early stage because it lasts only a short time, but it is possible that beta Lyrae, which we discussed last time, may be such a star.
Venus passes inferior conjunction at the end of October, but quickly becomes visible in the morning sky and is favorably located for observation from the middle of November until the end of the year.
Mars is observable with difficulty in the eastern morning sky at the beginning of the quarter, but its visibility will improve as the quarter progresses. It will appear close to Venus during most of December. A lunar occultation on December 30 will not be visible from our region.
At the beginning of the quarter Jupiter rises at midnight, rising earlier as the end of the year approaches. It will be stationary, in Leo, on December 4.
Saturn will be visible during most of the night as it moves from Orion into Taurus. It will be at opposition on December 17.
The Draconids, which occasionally appear in great numbers but are hard to predict, should be observable between October 6 and 10. In view of the favorable phase of the Moon, this would be a good opportunity to look for these elusive objects.
The Taurids, which peak in early November, are not very numerous, but they are said to produce occasional spectacular fireballs, so it may be worth watching for these. Again, the Moon will be favorable.
Your correspondent has achieved some degree of fame through his inaccurate predictions of the Leonids. Peaks are predicted on November 17 and 19, the latter being the more favorable for North America. These meteors are best observed after midnight, and the Moon will be a problem during most of the night.
The Geminids will peak during the night of December 13-14. They should be quite numerous. The Moon will set shortly after midnight, making way for the best part of the shower.
The brightest predicted comet during the last quarter of 2002 appears to be C/2001 RX14 (LINEAR) which should reach magnitude 10.5 by the end of the year and be well observable from the northern hemisphere.
A penumbral lunar eclipse will be visible in the early evening of November 19, although it will not be very dramatic. The eclipse will have begun at moonrise in our region, but maximum will occur after moonrise.
A total solar eclipse on December 4 will be observable almost exclusively from the southern hemisphere.
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Last updated 12/15/02 DMC