Sierra NightSky

Sierra NightSky for the period starting Friday, May 19, 2017 by Jim Kaler.

Note: Sometimes the old ways really ARE the best ways. The “bullet” form of Skylights that I’ve been using since the beginning of the year did not work very well for me , and actually proved more difficult to put together. I also thought it was boring. Consequently, I’m returning to the narrative form, which I much prefer. Thanks for your patience.

The next skylights will appear June 2, 2017.

With last quarter passed on Thursday, May 18, we begin this two-week round with the Moon just barely in its waning crescent phase as it thins and heads towards new Moon on Thursday the 25th. On the morning of Monday the 22nd, the rising Moon will make a fine sight just down and to the right of Venus. The following morning, we will get our last look at the thinning crescent as it stands a bit up and rather well to the right of Mercury. Our companion then flips to the western side of the sky, making its first appearance as a very thin waxing crescent in early twilight the evening of Friday the 26th, when it appears down and to the left of Mars. It then makes its way upward against the stars of Taurus, Gemini, and Leo. The evening of Sunday the 28th, it will be down and to the left of Pollux and Castor, on the evening of Tuesday, May 30, down and to the right of Regulus in Leo, while the following night it will be up and to the left of the star, which it occults on Wednesday the 31st as seen from the southern hemisphere. The crescent phase will end at first quarter on Thursday, June 1. The Moon passes perigee, when it is closest to Earth, on Friday, the 25th, the night of the new Moon, when the alignment of the Earth, Moon, and Sun will produce especially high and low tides at the coasts.

As augured above, Mercury and Venus are both morning planets, though Mercury, rising in bright eastern twilight, will be tough to see. Venus is vastly better, rising brilliantly just as dawn begins to light the sky around 3:30 Daylight Time. It’s hard to miss. The planet will be our “morning star” until November. The outer two ancient planets are also paired. Jupiter, due north of spica.html”>Spica in Virgo, dominates the sky the first half of the night, not setting until about 3 AM, shortly before Venus rises and just after Saturn transits the meridian low to the south between the classic outlines of Scorpius and Sagittarius. In the “for what it’s worth” department, the Moon occults Neptune on Friday the 19th as seen from Madagascar, and Venus slides 1.8 degrees south of Uranus on Friday the 2nd.

The winter constellations of Gemini, Auriga, and the like are mere ghosts of themselves as they fall to the west in late spring twilight. Those of spring and summer now hold sway. In the early evening find Spica north of Jupiter and then continue northward to Arcturus, who drives the Great Bear, Ursa Major (with the Big Dipper), around the North Celestial Pole, which is marked by Polaris, the luminary of Ursa Minor, the Smaller Bear. South of the curve of the Big Dipper’s handle, bark the two stars that make the modern constellation Canes Venatici, the Hunting Dogs.


Fifth magnitude (4.86, almost fourth) Mu Aurigae sits in somewhat lonely splendor in western Auriga within the confines of the main pentagonal figure two or three degrees south-southeast of the Kids. Perhaps “splendor” is too strong a word for the star. It does dominate a relatively dim area of sky, however, and is thereby quite easy to locate, which is worthwhile as the star has a number of things that recommend it. A white, class A (A4?) hydrogen-fusing dwarf with one cited temperature of 7560 Kelvin, Mu Aur’s major claim to fame lies in its surface chemistry. It’s a “metallic-line” star of the sort within class A whose atmospheres are greatly enriched in copper and zinc, even more in the “rare earths” such as europium, but depleted in calcium. The cause is elemental separation, wherein some heavy atoms fall under the force of gravity, while other kinds are raised up by the powerful radiation emitted by the star. The phenomenon works only when the outer stellar gases are relatively quiet and unstirred by circulation caused by rotation. And, ignoring axial tilt, Mu Aur accommodates by spinning at its equator by only 89 kilometers per second. While this may seem high as compared with 2 km/s for the Sun, it’s nothing for an A dwarf, whose spin rates can go much higher. The odd metallicities make it difficult to classify such stars, as the classes are based upon strengths of various absorption features in the spectra. Standard classes are tied to solar chemical abundances, and when they go awry, you don’t get a unique class. Mu Aur has thus been classed anywhere between A1 and F1 depending on which absorption lines are used. The temperature, low for A4, suggests that a cooler class might be more appropriate. Though only 153 light years away (give or take 6), there seems to be a small amount of dimming by interstellar dust, which might be a result of an erroneous spectral class. Nevertheless we adopt 0.22 magnitudes of interstellar absorption. In a way, that would not be all that surprising, as the star is almost at the anticenter of the Galaxy, with a galactic latitude just 0.3 degrees south of the galactic equator (which is based on the adopted centerline of the Milky Way). The galactic equator then goes northwest right between the Kids’ Zeta and Eta Aurigae. With virtually no correction for infrared or ultraviolet radiation, Mu Aur shines with the light of 23.8 Suns, which yields a radius of 2.85 solar. From the projected equatorial rotation velocity, the star spins in under 1.6 days. Application of theory yields a modest mass twice that of the Sun, and indeed shows the star to be a dwarf that is probably nearing the end of its hydrogen-fusing lifetime of 1.1 billion years (a tenth that of the Sun’s, showing how sensitive lifetime is to mass). In 1986 a tiny companion was found 0.1 seconds of arc away from the main star. If it exists, the true separation is at least 4.7 Astronomical Units and the period more than 7.2 years.


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