Andromeda to Bo¨ otes (The Herdsman)
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The vicinity of Capella is so rich with faint stars that in the past they were named Capella C, D, E, F, and G, but they are all close to their bright neighbor only when observed from Earth. Physically (gravitational) connected to Capella is a red dwarf, which was given the name Capella H. This is a faint magnitude 10 star (on the border of visibility in binoculars), which lies approximately 12 arcmin southeast from Capella (p. a. 1418, 1895). The actual distance between the Capella A–B system and Capella H is over 9,200 AU, or 0.15 light years. However, this is not all! Capella H is also a binary star with a separation of 3.3 arcsec (p. a. 1668, 1999). Stars with magnitudes 10.5 and 13 are seen and separated through larger amateur telescopes. The Capella system, therefore, includes at least four stars. If we create a model in which Capella A is a 10-cm big ball, Capella B measures 6 cm. The balls are 1 m apart. In this model, the stars in the Capella H system have a diameter of 6 mm each and are 43 m from each other. But the distance from pair A–B is over 11 km! And this wonderful system is gravitationally bound so that all stars move around a common center of mass! Gravity is truly an exceptional force. There is another interesting fact about Capella. This bright star is circumpolar for mid-northern observers (and those up further north). This means that we can see it all night and every night. In a dark and clear winter sky it glitters almost in the zenith, but on some summer nights, when the sky is clear and dark all the way down to the horizon, you can see Capella low above the northern horizon. Of course, it is not as bright as it is in winter, for its light is weakened by the thick layer of our atmosphere. At least once on a clear spring night, take some time and follow its path along the sky until the morning hours. Beta is the 41st brightest star in the sky. It is 82 light years from us. Its luminosity is 80 times that of the Sun. Theta is the 105th brightest star in the sky. Its light travels 174 years just to reach us. The star’s luminosity is that of 190 Suns. Delta and Iota shine in a beautiful orange color that is even more vivid when viewed through binoculars, especially if the image is slightly blurred. Iota is the 113th brightest star in the sky. It is over 510 light years away. The luminosity of the star surpasses our Sun by 1,600 times. Epsilon is a rather interesting eclipsing binary. One star is an extremely bright super giant, maybe as much as 18,000 times brighter than our Sun and over 100 times larger! Its companion is a mysterious object, so far never seen. We know of its existence, since it periodically passes across the super giant, and its shine is reduced by approximately 1 magnitude. The eclipse period is every 27 years (!), and the fall in brightness lasts for approximately 1 year. The last time this occurred was between January 1983 and January 1984. The next eclipse and fall of brightness is expected to take place around 2010. The nature of the companion star is still unknown. In the past, astronomers believed it was a very young contracting star. Then they assumed it was a black hole with an accretion disc surrounding it. Today, it seems most likely that the companion is a hot bluish star, surrounded by a big and cold cloud of nontransparent matter. Zeta is also an eclipsing binary star with a period of 972 days. The system is composed of an orange super giant, the color of which is evident through binoculars, and its hot, white blue companion has a luminosity that surpasses our Sun by 400 times. However, the drop in brightness is only 0.15 magnitude; thus it is almost impossible to notice visually. In the part of the Milky Way that meanders through the constellation, we can notice three splendid and easily found open clusters: M 36, M 37, and M 38. The best way to find them is to start your search at bright Theta. If the star is moved to the extreme northern edge of the field of view, the open cluster M 37 (6m2/240 ) will appear on the southern edge. It can be clearly seen through binoculars, but only as a big, rather bright spot of light, with some 20 individual stars of magnitude 9 and 10 shining through the haze. In perfect observing conditions and with averted vision the cluster is even more crumbled, and the number of stars increases to 80! However, if you wish to resolve the cluster completely, you need larger objective lenses and greater magnification.