"Let your soul stand cool and composed before a million universes" Walt Whitman
Sierra NightSky for the period starting September 25, 2015 by Jim Kaler
The next skylights will appear October 9, 2015.
Sometimes everything seems to happen all at once. The Moon begins our fortnight late in its waxing gibbous phase and then hits full on Sunday, September 27, when it goes through the Earth's shadow to present us with a marvelous early evening total lunar eclipse that's visible throughout the Americas. The partial phase, wherein the Moon first encounters the full shadow of the Earth, begins at 8:07 PM Central Daylight Time. Total eclipse, with the Moon fully immersed in umbral shadow, starts at 9:11 PM, with mid-eclipse taking place at 9:48 PM, when the Moon is just to the south of the central part of the shadow. The Moon starts to leave full shadow at 10:23 PM and the partial portion is over at 11:27. Add an hour for EDT, subtract one for MDT, two for PDT.
In the far west, the Moon will rise during the partial portion before totality, while in Alaska the eclipse will be even farther along. The penumbral phases, during which some direct sunlight falls on the Moon, are here ignored. Even during totality, the Moon will be visible as a dull reddish orb as a result of sunlight scattered and refracted into the Earth's shadow. The brightness of the totally eclipsed Moon depends on atmospheric transparency, most critically on prior terrestrial volcanic activity. Since the eclipse is not exactly central, at mid-eclipse the northern part of the Moon will be darker than the southern.
More remarkably, the Moon passes perigee, where it is closest to Earth, and therefore of maximum angular size, just an hour before maximum eclipse. Even without the eclipse, this full Moon is famed as the "harvest Moon." It's not just a name. This time of year the evening ecliptic (the solar path that the Moon closely follows) lies at a shallow angle to the horizon. As a result, near full phase, the delay in moonrise from one night to the next is minimized, giving us lots of moonlight in the early evening to help bring in the harvest. This eclipse also ends a "tetrad" of four total eclipses in a row, each separated by about half a year. It can't better than this, except of course for having clear skies. The display during totality is beautiful and quite colorful as seen through binoculars or a telescope.
After the eclipse, the Moon passes through the waning gibbous phase, which ends at third quarter on Sunday, October 4, following which it wanes as a crescent. A day after the eclipse, the fat gibbous passes just south of Uranus.
The night of Thursday the 1st, the fading gibbous Moon will be seen west of Aldebaran in Taurus, while the following night it will fall to the east of the star with the Pleiades floating north of them both. The Moon actually occults the star during the day in North America. The thinning crescent will then present us with more delightful sights.
The morning of Thursday the 8th, it will lie just above Venus, while the following morning it will be between Venus and Jupiter with Mars to the left, all to the south of the rising classical figure of Leo. As seen above, the morning planets present us with quite a line-up, with Venus, Mars, and Jupiter falling in a ragged line toward the pre-dawn horizon, the configuration of the trio continuously changing. Venus far outshines them all, Jupiter next, Mars relatively faint. Early in our period Mars will lie just to the north of somewhat brighter Regulus, while by the end of the fortnight, it will be Venus's turn, the brilliant planet now passing to the south of the Leo's luminary.
The night of Thursday the 8th and the morning of Friday the 9th be on the lookout for the Draconid meteor shower, the product of short-period comet (6.6 years) Giacobini-Zinner, whose leavings appear to come out of the northern constellation Draco. While it usually does not amount to much, in 1933 and 1946 the Draconids put on spectacular displays.
With the Moon fading away as a crescent, or even during the total part of the eclipse, if in a dark location take time to admire the evening Milky Way as it comes out of Cassiopeia in the northeast, then passes through two celestial birds, first Cygnus (the Swan) with bright Deneb, then across the celestial equator through Aquila (the Eagle) with first magnitude Altair. To the northeast of Aquila and south of Deneb, look for Delphinus, the Dolphin, which looks like small hand with a finger pointing to the southwest.
STARS OF THE WEEK: KAPPA AQL (Kappa Aquilae) with a look at neighboring U AQL (U Aquilae)
Massive stars attract us, and unassuming Kappa Aquilae does not disappoint. The southernmost Greek-lettered star in Aquila (the Eagle), just five degrees north of the border with Sagittarius, Kappa Aql is a hot (26,400 Kelvin) class B (B0.5) "giant" (but see below) 1700 light years away, give or take a couple hundred.
With such a distance within the Milky Way, the star is dimmed by as much as 0.93 magnitudes by interstellar dust. With a clear line of sight, Kappa would brighten to magnitude 4.02. The amount of dimming is contended, however, another study giving 0.69 magnitudes. We'll go with the heavier obscuration. Factoring in a lot of ultraviolet radiation, Kappa Aql shines with the light of 55,500 Suns, which gives a radius of 11.3 solar.
As are most stars of its class, Kappa Aql is a very fast rotator, spinning at the equator by at least 267 kilometers per second. Yet there seems to be no circumstellar disk that would make it into a "B-emission" (Be) star like Gamma Cas or Zeta Tauri. Theory suggests a mass of 17 solar masses if the star is a dwarf near the end of its hydrogen-fusing lifetime of 10 million years, or slightly lower, 15 Suns, if it has already run out of hydrogen fuel in its core and has become a subgiant (such stars commonly spectrally classed as "giants").
The lesser dimming gives a luminosity of 44,000 Suns but pretty much the same masses. With a mass clearly above the theoretical limit of 8 to 10 solar masses for eventual explosion, Kappa Aql does not have very long to live before it blows up as a grand supernova. The star may be unassuming now, but if it were to go off tonight, it would shine at magnitude -8, roughly 15 times the brightness of Venus. So keep your eye on the Eagle: you never know.
While Kappa Aql is a single star, it does provide a guide to another of some interest. A pair of degrees due west lies the supergiant (or bright giant) Cepheid variable U Aquilae. Cepheids are highly evolved, pulsating bright giants and supergiants that typically vary by a magnitude or so over a period of a few days. U Aql changes from magnitude 6.08 to 6.86 and back every 7.024 days, while the class goes from F7 to G1.
Classical Cepheids such as Delta Cephei, Eta Aquilae, Zeta Geminorum, and our "U" are crucial to the calibration of the period-luminosity relation, from which the distances of other galaxies (and the nature of the Universe) can be derived. U Aql, however, is not very good at it.
Direct parallax gives a distance of 899 light years, while the period-luminosity relation (including 1.3 magnitudes of dimming by interstellar dust) gives some 2000. The parallax is probably the culprit, but nobody knows. If at the distance given by the pulsation period, U Aquilae, with a mean temperature of about 5700 Kelvin, would shine with the light of 2800 Suns and carry a mass of around six Suns.
U Aql, however, is not single. A 7.6 magnitude companion hovers at a close but unknown separation. Subtraction of its light increases the period-luminosity distance to 2500 light years and makes the discrepancy with parallax even bigger. More obvious is 11th magnitude U Cephei B 1.6 seconds away.
From its brightness, it's probably a class A dwarf. Given a mass of two Suns, it's at least 122 Astronomical Units away from U proper and must take at least 14,000 years to make a full orbit. Half a minute of arc away is 14th magnitude U Aquilae C, which is just an aligned coincidence, gladly removing it from the already-complex analysis of the star.
Do you have a favorite star or one you would like to see highlighted on the Star of the Week? Send a suggestion to Jim Kaler.
Sierra NightSky thanks to Jim Kaler.
Check out his site for more