Sierra NightSky

Sierra NightSky for the period starting Friday, July 28, 2017 by Jim Kaler.

The sky seems to be storing up its energy for one the most spectacular eclipses to take place for many years. So reserve August 21. The Moon’s orbit is tilted relative to the Earth’s by about 5 degrees. The Moon crosses the ecliptic twice a month at the “nodes.” If the full Moon is close enough to a node it will hit the shadow of the Earth and we’ll see a lunar eclipse, while if it’s at a node at new Moon we’ll get a solar.

If conditions are right for a solar eclipse, they are close to being right for a lunar eclipse. And during this phase cycle there are no exceptions, as we’ll get one when the Moon passes full phase near a node on August 7. It’s not much an eclipse though. The Moon only barely skirts the dark umbral shadow for a minimal lunar eclipse. Worse, it will be visible only from the eastern hemisphere. We’ll see nothing from the Americas at all. What makes things a bit tricky is that the lunar orbit precesses (wobbles) over an 18.6-year period, so the nodes continually move backwards, and so do the dates of the eclipses.

On Friday the 28th, the waxing crescent Moon will present a fine sight just above Jupiter with the star Spica to the left. From Tuesday August 1 through Friday the 4th, the waxing gibbous will glide above the main figures of Scorpius and Sagittarius. Look for the Moon above Antares the night of Tuesday, August 1. We close out our period with a fat waning gibbous Moon. The Moon passes apogee, where it is farthest from Earth on Wednesday the second.

On Sunday the 30th Mercury reaches greatest eastern elongation of 27 degrees. Nevertheless it remains low in western twilight. Other than the Moon the first thing to be seen in western twilight is Jupiter, which sets about an hour and a half before local midnight an hour after Saturn crosses the meridian far to the south, Next up is”>Venus, which rises around 3 AM Daylight Time, then Uranus, which enters retrograde motion (westerly against the stars) on Wednesday the 2nd. You might also see a few strays from then Perseid meteor shower, which peaks the night of Friday the 12th.

This is prime time to see the Milky Way, which runs from Cygnus to Sagittarius and beyond, cutting diagonally across the sky. Below Cygnus and the bright star Vega lies Altair, in the heart of the Milky stream, which is made from the combined light of the stars in the disk of our Galaxy.

STAR OF THE WEEK: DELTA TRA: (Delta Trianguli Australe)

If you were in charge of making up the constellations and you got stuck for a name or configuration, there is always the Triangle. Indeed, there are enough of them to make Euclid happy. Within the set of “official” constellations accepted by the International Union there are two: plain old Triangulum in the northern hemisphere and Triangulum Australe in the southern. The northern one is a fine, small isosceles triangle between Andromeda to the north and Aries to the south that contains one of the great spiral galaxies of the sky, M 33. The southern triangle is very far south indeed, its bright stars falling from 63 to 70 degrees south of the equator. You have to be below 27 degrees north latitude to see it all, but at least you get a glimpse of the second magnitude luminary, Alpha TrA (the obviously made-up “Atria.”) Hevelius (1611-1687) invented “Triangulum Minor” near our accepted Triangulum, making the latterTriangulum Major,” but it did not last (at least officially: it’s still there), and so we are back to the northern and southern triangles. And if you wish you can add larger figures that cut across constellation boundaries, the “Summer Triangle of Vega, Deneb, and Altair, and the Winter Triangle of Betelgeuse, Procyon, and Sirius. Small Triangulum Australe does not have much to offer except three nice second magnitude stars: Alpha (1.92), Beta (2.85), and Gamma (2.89), the list properly-ordered by brightness then jumping down to Delta (3.85) and Epsilon (4.10). The best sight is probably the deep red carbon star and long-period variable, X TrA, which can make it to naked-eye brightness.
The first of fainter set, Delta TrA, is an unusually luminous class G (G2: the same as the Sun) lesser supergiant at a distance of 607 light years (give or take 19) with a temperature of 4970 Kelvin, which is lower than the 5680-Kelvin Sun because of the giant’s lower surface density. Factoring in a fair bit of infrared radiation, Delta TrA glows with the light of 1064 Suns, which with temperature yields a radius 44 times that of the Sun, or 0.21 Astronomical Units. With a hefty mass of 6.3 Suns, it looks as if it has just started fusing its core helium into carbon and oxygen. Only 80 million years old, when done with helium-burning, it will swell again, slough off what remains of its outer envelope to produce a planetary nebula, the core evolving into a white dwarf of 0.88 solar masses, in the meantime becoming a carbon star like X TrA. Delta has long been thought to have a twelfth magnitude partner some 30 seconds of arc away, but it is moving too fast for it to belong to Delta, which now seems all alone. Of greater significance, Delta TrA is in the realm of the “hybrid stars” which have magnetic fields and produce fast, hot magnetically-driven winds as well as slower, cooler winds.


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