Skylights, University of Illinois Department of Astronomy. Astronomy News for the week starting Friday, October 5, 2001. Phone (217) 333-8789. Prepared by Jim Kaler. Find Skylights on the Web at http://www.astro.uiuc.edu/~kaler/skylights.html, and Stars (Stars of the Week) with constellation photographs at http://www.astro.uiuc.edu/~kaler/sow/sow.html.
The Moon descends in phase and brightness toward its third quarter, the phase reached the night of Tuesday, October 9, shortly before Moonrise in the Americas. The Moon will then begin its waning crescent phase as it moves through Cancer and then into Leo. In the early part of the week the Moon will make a fine passage between the giant planets Saturn and Jupiter. The night of Saturday the 6th, the Moon will approach Saturn. The next two nights, Sunday the 7th and Monday the 8th, it will be between the pair, and then before moonrise the night of Tuesday the 9th will pass north of Jupiter, appearing to the east of the planet upon its rising. Mars hangs low in the southwestern sky, where it will be for the rest of the year as (while trying to keep up with faster Earth) it travels easterly along the ecliptic from its current low position within the constellation Sagittarius. As it moves, it will noticeably dim as the Earth pulls away from it. Our evening attention now is slowly being displaced from Mars to Saturn, which is now rising around 9:30 PM Daylight Time, and then to Jupiter, which this week rises as Mars sets, around 11:30 PM. These two great planets are then high in the sky near dawn when brilliant Venus hovers over the eastern horizon. Early October evenings, especially those with no Moonlight, provide a fine time to view the remaining summer stars, the Summer Triangle of Vega, Deneb, and Altair high in the sky, the northern two of the stars nearly overhead in mid-latitudes. As the Big Dipper falls in the northwest, The "W" of Cassiopeia rises in the northeast, followed by Perseus and bright Capella in Auriga. From out of Perseus flows the Milky Way through Cassiopeia and then through dim Cepheus and into Cygnus, from which it falls through Aquila and down to Sagittarius past Mars. As the evening progresses, watch for the passage of the lonely first magnitude star Fomalhaut, which in mid-northern latitudes appears to glide slowly across the far southern sky, the star the luminary of Piscis Austrinus, the "Southern Fish." Both Fomalhaut and high brilliant Vega are surrounded by dusty disks of matter that may hold some kind of planetary system, though no planets have ever actually been detected. To the northwest of Fomalhaut is rather dim Capricornus, the Zodiac's "water goat," while to the northeast lies Aquarius, the "water bearer" and then farther to the northeast Pisces, the classic "fishes." The whole area represents a "wet quarter" of the sky that once signalled a rainy season in some ancient land. STAR OF THE WEEK. ETA AQL (Eta Aquilae). Lying almost exactly one degree north of the celestial equator, this quite-wonderful mid- fourth magnitude (nominally 3.90) star in Aquila (the Eagle) glides across the sky just 8 degrees to the south of much brighter first magnitude Altair. Unfortunately given no proper name by the ancients, it is now known principally as Eta Aquilae, Eta the seventh letter of the Greek alphabet. Near one of the Eagle's talons, Eta also represents the head of the now-defunct no-longer- recognized constellation Antinous, who was honored in the sky by the Roman emperor Hadrian, and depicted as being carried by Aquila. The star seems relatively dim only because it is so far away, an uncertain 1200 light years. It remains quite visible to us only because it is a luminous yellow-white class F (nominally F6) supergiant that shines 3100 times more brightly than the Sun. While such brilliance may pale behind that of the blue hydrogen- fusing dwarfs or the great red supergiants, the star's moderate temperature of 5600 Kelvin assures that nearly all of its radiation pours out in the visual where we can see it rather than hidden in the invisible ultraviolet (as it is for hot stars) or the infrared (cool stars). The luminosity and temperature conspire to give a radius 65 times that of the Sun, while direct measures of angular diameter give a close 59 times. But these characteristics are only an aside compared with the star's status as one of the sky's most prominent Cepheid variables (the variability discovered long ago, in 1784), Eta Aquilae comparable to the prototype Delta Cephei (the name "Cepheid" taken from the constellation "Cepheus"), Mekbuda (Zeta Geminorum), and the southern hemisphere's Beta Doradus and W Sagittarii. Polaris is actually the brightest of all Cepheids, but its small variations are not sensible to the eye. The variations of Eta Aquilae, however (as they are for the others listed here), are obvious, the star changing its brightness from magnitude 3.6 to 4.4 and back again over a precisely determined period of 7.176641 days (7d 4h 14m 22s). As Eta dims, it dips to spectral class G, the temperature falling from a high of 6200 Kelvin to 5300. Like all Cepheids, Eta Aquilae is in the process of dying. Having given up core hydrogen fusion, it is probably fusing helium in its deep core. In its current structure, it has become unstable, a deep layer that valves radiation making it pulsate and change its surface temperature and radius. Cepheid variables are among the most important stars of the sky, as their variation periods are strictly related to their luminosities. Once the period of a Cepheid is found, we thus know the luminosity, which through comparison with apparent brightness gives the distance. Cepheids are so luminous they are easily seen in other galaxies, and are thus the chief means by which we can determine their distances. Eta's luminosity and general characteristics lead to a mass about seven times that of the Sun. As it evolves, it will someday cease its pulsations, lose most of its outer envelope, and die as a tiny white dwarf smaller than the Earth. **************************************************************** Jim Kaler Professor of Astronomy Phone: (217) 333-9382 University of Illinois Fax: (217) 244-7638 Department of Astronomy email: [EMAIL PROTECTED] 103 Astronomy Bldg. web: http://www.astro.uiuc.edu/~kaler/ 1002 West Green St. Urbana, IL 61801 USA Visit: http://www.astro.uiuc.edu/~kaler/ for links to: Skylights (Weekly Sky News updated each Friday) Stars (Portraits of Stars and the Constellations) Astronomy! A Brief Edition (links and updates) ***************************************************************** -- This is the ISTA-talk mailing list. 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