Sierra NightSky for the period starting Friday, June 16, 2017 by Jim Kaler.
The next Skylights will appear July 7, 2017.
The last fortnight was the “bright run” with the Moon between first and third quarters lighting up the sky and hiding the stars. Though the full Moon is only a half-millionth as bright as the Sun, that is plenty enough light to make a lot of observing impossible. This fortnight, however, we encounter the flipside, the “dark run” between third quarter on Saturday the 17th near dawn and first quarter the morning of Friday, June 30, these two phases now centered on new Moon, which is passed on Friday the 23rd. In between third quarter and new, on Tuesday the 20th, the waning crescent Moon will make a fine sight with Venus to the left, while by the next morning the Moon will have slipped another 13 degrees along its path and will be down and to the left of the brilliant planet, which rises just as the first glimmer of dawn begins to light the eastern sky. Shifting your gaze to the west to watch the waxing crescent will yield a memorable sight when it is seen just south of Regulus in Leo. The Moon passesperigee, where it is closest to the Earth in its eccentric orbit, on Tuesday the 27.
Jupiter, the brightest body in the evening sky (the planet in Virgo northwest of Spica), sets about an hour after Saturn transits the meridian between Sagittarius and Scorpius. After another hour and a half, Venus rises, its luster putting all the other planets to shame. Going deeper into the Planetary System, Mercury goes through superior conjunction with the Sun (on the other side of it)on Wednesday the 21st.
The big event, though, involves Earth, when at 9:24 PM on Tuesday the 20th, (12:24 EDT Wednesday the 21st), the Sun passes the Summer Solstice at the most northerly part of the ecliptic, 23.4 degrees north of the equator. On this longest day (and shortest night), the Sun will rise as far northeast (and set as far northwest) as possible, and pass overhead at the Tropic of Cancer (thus defining the Tropic’s latitude). North of the Tropic, the Sun will also cross the meridian as far north as possible, giving us the maximum heating rate and, of course, the first day of astronomical Summer. Traditionally the Solstice lies in the constellation Gemini, but around 1930, the 26,000 wobble (precession) of the Earth’s axis brought it across the border with Taurus established in 1930.
This time of year brings us a lovely parade of constellations high across the sky, beginning with kite-shaped Bootes (the Bear Driver) with bright Arcturus, the semi-circle of Corona Borealis (the Northern Crown), Hercules, Lyra (the Harp) with Vega, and finally Cygnus (the Swan) with Deneb. The constellations of autumn then begin to appear on the scene.
STAR OF THE WEEK: TAU CrB (Tau Coronae Borealis)
Well north of the semi-circle of stars that defines Corona Borealis, the northern Crown (its southern cognate, Corona Australis, the Southern Crown, falling south of Sagittarius) lies fifth magnitude (but at 4.76 almost fourth) Tau Coronae Borealis, a class K (K1) giant-subgiant that’s fairly close, 117 (give or take 5) light years away. Why bother with yet another orange K giant when there are so many others in the sky.? Part of the reason is just that, that that there ARE so many others (Arcturus, Aldebaran, and a vast slew of others) and the range of the class in luminosity it huge, so it takes a lot of stars to cover that space. Tau CrB is among the brighter ones of the K giants. With a temperature of 4860 Kelvin, it radiates at an impressive rate of 3840 times that of the Sun, which gives the star a radius 87.6 solar radii, 0.41 Astronomical Units, which is just over the mean orbital radius of the planet Mercury. If placed at the Sun, Tau would appear some 50 degrees across, around 1000 times the solar angular diameter. The theory of stellar structure and evolution gives Tau CrB a mass seven times that of the Sun with an age of about 350 million years. The star is most likely fusing its core helium into carbon and oxygen in preparation for becoming a carbon/oxygen white dwarf of about one solar mass similar to sirius.html Sirius B. First though, it must generate a powerful wind that will sweep away the residual exterior hydrogen envelope and present us with an ephemeral planetary nebula with the old stellar core at its center. Tau CrB has a couple other features to recommend it. It was at one time listed as a spectra.html#double”>spectroscopic binary, but that seems not to have been confirmed. There is, however, a thirteenth magnitude visible companion now 2.2 seconds of arc away, which from its brightness would be a low-mass red dwarf. From a minimum physical separation of 79 Astronomical Units, it would take at least 250 years to make a complete circuit of Tau CrB itself. But since the separation changed by over a second of arc in 69 years, it’s more likely that the little one is just in the line of sight and not physically connected. More interesting, the star is speeding along at some 60 kilometers per second relative to the Sun, some four times the usual, suggesting it is a visitor to our part of the Galaxy. Nevertheless, its metal content appears to be quite normal.