Dr. Whitney Shane, MIRA's Charles Hitchcock Adams Fellow
Vega is one of the two brightest stars in the northern sky, and we all use it and the equally bright Arcturus as beacons with which we orient ourselves during the summer nights. Although Vega is by far the brightest star in the small constellation Lyra, it is by no means the only one deserving of our attention. We recognize Lyra by the presence of Vega and, to the southeast, a parallelogram of fainter stars. The brightest of these, forming the southwest corner point, is Sheliak, known to all astronomers as Beta Lyrae.
It has long been known that Beta Lyrae was a variable star. It is an eclipsing binary with an amplitude of about 1 magnitude and a period of 12 days. It happens also to be a visual double star with a faint companion at about 45 arc seconds distance. This star has been the subject of intensive study for many years. Recently we found in the MIRA library a long treatise (in Latin) on the subject by F. W. Argelander, the creator of the BD Catalog. The star was made famous by the work of Otto Struve and Gerard Kuiper during the 1930s and 1940s.
An eclipsing binary is a pair of stars revolving about a common center of gravity and so oriented that the Sun lies almost in the plane of revolution. In this way each star appears periodically in front of the other, causing the brightness to decrease during eclipse. We can learn much from this information, such as the sizes of the stars relative to their separation and even the distribution of light over their surfaces. From the spectrum we can measure the line-of-sight velocity of the brightest star or, if the brightness difference is not too great, of both stars. This gives us a complete description of the orbit, including the true separation of the stars, information that is seldom available from any other source.
Otto Struve, 1897-1963
As Struve observed, the spectrum of Beta Lyrae is much more complicated than expected from the description above. There is evidence of gas streaming around and between the two stars and being expelled from the system. The latter occurs, not because the gas is being driven off by stellar activity as in a planetary nebula, but, as suggested by Kuiper, by the dynamics of the system, where angular momentum from the binary is being transferred to the gas stream. This forms an expanding disk around the system, which we observe in the spectrum.
This kind of process has now been identified in many binary stars, some of which show very complicated and exciting properties. As just one example, imagine what would happen if one of the stars in the binary were a neutron star or even a black hole. Any gas in the system captured by such a star would release an enormous amount of gravitational energy as it fell inward. This is the favored explanation of the many X-ray sources that are currently being discovered in large numbers.
At the beginning of the quarter Mercury is still visible in the northeast morning sky, but it will soon be lost in the twilight. It will not be observable again from the northern hemisphere until October.
Venus will still be visible in the southwest for northern observers, but with increasing difficulty as it is lost in the evening twilight.
Mars will be in conjunction with the Sun on August 10, so it will not be visible until the very end of the quarter, when it will appear in the northeast during morning twilight.
Jupiter will be in conjunction with the Sun on July 20, but it will reappear in the northeastern morning sky during August and will be easily observable in the morning hours by the end of the quarter.
Saturn will become visible in the northeastern morning sky during July and will become better observable in the morning hours as the quarter progresses.
Uranus and Neptune are still close together and both will reach opposition during August. They will be in, or very near, Capricornus, but the observer will need a good finding chart.
After the remarkable alignment of the naked eye planets in the evening sky during May, it is not surprising that they will be visible with difficulty or not at all at the beginning of Summer.
The summer quarter is always dominated by thePerseid shower, and this year will be particularly favorable because the peak, on August 12, falls only a few days after new Moon. Because of the location of the radiant, the morning hours are the most favorable. During the 1990s multiple peaks were observed, as is common when the parent comet is close to perihelion. These outbursts are now disappearing as comet Swift-Tuttle departs for the outer Solar System.
Among the several minor showers, the most interesting appears to be the delta-Aurigids. This difficult shower is observable during most of September, with a predicted peak on September 8, very close to new Moon. It is said to be badly in need of additional observations.
Comet C/2002 C1 (Ikeya-Zhang), discovered as the Spring Newsletter went to press, is now fading from view, but should be visible with a telescope early in the quarter.
Comet C/2001 N2 (LINEAR) will reach perihelion at 2.7 A.U. on August 19. Before that time it should be observable as a 13 or 14 magnitude object. An equally faint comet, C/2001 RX14 (LINEAR), passed perihelion at 2 A.U. in January, but it will continue to brighten until the end of the year. In August it should be magnitude 13.5.
Periodic comet 46P/Wirtanen will reach perihelion of 1.06 A.U. on August 26 when it may reach magnitude 10.5. However, it will be poorly placed for observation, low in the morning sky and less than 45 degrees from the Sun.
Except for the recent partial solar eclipse, this has been a bad year for eclipses, and there will be none of any sort during the summer quarter.
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Last updated 12/15/02 DMC