Sierra NightSky for the period starting Friday, September 8, 2017 by Jim Kaler.

The next Skylights will be presented Friday, September 22, 2017.

We begin with the Moon barely past its full phase opposite the Sun as it heads towards its third quarter the night of Tuesday, September 12, about the time of Moonrise in North America. On that Tuesday morning the Moon will occult Aldebaran in Taurus, the event seen best in the far west where it will still be dark. The night of the 13th find the Moon just north of Orion. We then get to see the full display of the waning crescent, which terminates at new Moon on Wednesday the night of Tuesday the 19th. Look the morning of Saturday the 16th to see the crescent standing at the top of a stack of stars and planets, from Venus to Regulus then Mars and Mercury. Over the next couple days we will see the slimming Moon moving down among them, creating a most unusual sight. The morning of Saturday the 16th, the latter two will be less than half a degree apart. The Moon passes perigee, where it is closest to Earth by about five percent on Wednesday the 13th.

Looking beyond the Moon, on the morning of Sunday the 10th, Mercury passes barely south of Regulus just a couple days before the little planet goes through its greatest western elongation relative to the Sun, when it rises just as dawn commences. Back in the evening, Jupiter sets as twilight ends, Saturn following in the southwest about an hour before local midnight still between Sagittarius and Scorpius.

The biggest event, though,, is scheduled for Earth and Sun on the morning of Friday the 22nd, when the Sun crosses the Autumnal Equinox in Virgo, at 3:02 PM CDT (4:02 PM EDT, 1:02 PM PDT), marking the beginning of astronomical autumn in the northern hemisphere, spring in the southern. Except for some technical details (atmospheric refraction, which lofts the Sun upward and the half-degree size of the Sun), the Sun will rise due east, set due west, lengths of daylight and night will both be 12 hours long, and the Sun will set at the north pole and rise at the south pole.

‘Tis a beautiful time for the southern reaches of the Zodiac with Sagittarius sitting 30 degrees south of the celestial equator. In mythology Sagittarius is a centaur, half-man, half horse. But the constellation is better known for its smaller asterisms, one an upside-down five-star figure called the “Little Milk Dipper” because of its placement in the heart of the Milky Way, the other an old-fashioned Teapot. Beneath that figure is the graceful curve of stars that forms Corona Australis, the Southern Crown.

STAR OF THE WEEK: OMI SGR (Omicron Sagittarii)

A small configuration of modestly bright stars in Sagittarius about five degrees north of the “Little Milk Dipper” (and just barely north of the ecliptic) makes the lesser-known “Teaspoon,” which consists largely of Pi Sgr(“Albaldah“), at magnitude 2.89 the luminary, fourth magnitude (3.77) Omicron, which is about a degree and a half southwest of Pi, Xi-1 Sagittarii at 2000 light years the most distant of the set), and Xi-2 Sagittarii along with a small bunch of fainter Flamsteed stars. In spite of its location within the Milky Way, Omicron Sagittarii is close enough (just 142 light years, give or take 2) that there is little interstellar dust absorption along the line of sight, and we’ll ignore it. A class G (G8) giant with a temperature of 4844 Kelvin (thus producing some infrared light that must be accounted for), Omi Sgr shines with the light of 66 Suns, really rather modest for a “giant,” especially when noting that the star’s radius is but 11.6 times that of the Sun. Theory suggests a mass of about 2.7 Suns, though oddly other studies yield a smaller mass of 2.0. Otherwise the star is a normal “clump” giant (so-called because its luminosity and temperature are similar to so many others of its kind. As a result, clump stars actually make a good distance indicator for other galaxies. But back to Omicron itself. It has a purported fourteenth magnitude companion 38.4 or so seconds of arc away. Given the density of faint Milky Way stars, a physical relation seems unlikely and the pairing is probably just a line of sight coincidence. If not, however, the “companion” would be at least 1700 AU from the bright star and take at least 42,000 years to make a full revolution around Omi Sgr A. It would also be a red dwarf of little consequence down toward the bottom of the “main sequence” of stars that don’t get hot enough inside to run thermonuclear fusion. As feeble as they are such red dwarfs lead to even lower mass “brown dwarfs” and then on to planets. Stars are usually thought of as being made from “the top down”, from collapse of dense bloblets in interstelllar clouds, while planets are commonly defined as being built from the bottom up, by accumulation of dust in circumstellar disks. Is there an overlap? Probably, but no one yet knows.

Sierra NightSky thanks to Jim Kaler. Read Dust to Dust in Stellar Stories.
Check out his site for more.