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Sierra NightSky

"Let your soul stand cool and composed before a million universes" Walt Whitman

Sierra NightSky for the period starting July 15, 2016 by Jim Kaler

The next skylights will appear July 29.

As we open up the fortnight, the Moon is in its waxing gibbous phase, heading toward full the evening of Tuesday, July 19, when (known as the Thunder or Hay Moon) it will rise just barely after sunset in North America.

The evening of Friday the 15th, the Moon will lie above Saturn with Mars to the right and Antares in Scorpius down below, while the following night it will shine to the left of the ringed planet. After passing third quarter the night of Tuesday the 26th (before moonrise in North America) the Moon turns into a waning crescent, the phase ending at new Moon on August 2.

The morning of Thursday the 28th, the crescent will make a fine triangle with the Pleiades up and to the left and the star Aldebaran in Taurus below the famed cluster of the Seven Sisters.

The morning of Friday the 29th, the thinning crescent Moon will appear tight against the star Aldebaran, and will actually cross over or occult it as seen from the southern and eastern US, though for most sites in twilight. The event takes place at roughly 5 AM CDT, but depends considerably on the exact location.

The Moon passes perigee, where it is closest to Earth, on Wednesday the 27th, the event not really sensible to the eye.

As evening falls, Jupiter still dominates the deep early-evening western sky, though by the end of our two-week term it will set about at the end of twilight, when the sky becomes fully dark. So we look to the other planets that grace the sky, Mars then Saturn as augured above.

Only slowly falling behind the orbiting Earth, Mars sets around 1 AM Daylight Time, Saturn following roughly an hour later. Now moving to the east against the background stars, Mars is slowly catching up with the ringed planet, the two coming into conjunction with each other on August 25, Mars's motion obvious almost from night to night.

Toward the end of July the Delta Aquarid meteor shower comes to its vague peak, producing a few meteors per hour while in competition with other minor showers going on at the same time. Meteor showers are named after their apparent points of origin, which is a perspective effect, the meteoroids actually travelling on parallel paths as they strike Earth's atmosphere.

Saturn is directly above Scorpius, which along with Sagittarius to the east (recognizable by its upside-down "Little Milk Dipper, both in the heart of the Milky Way ) are the southernmost constellations of the Zodiac, the figures that embrace the apparent path of the Sun.

To the west of Scorpius lies Libra, whose two most prominent stars (Zubelelgenubi and Zubeneschamali) are the Scorpion's outstretched claws. Farther west, as the sky darkens find bright Spica, the luminary of sprawling Virgo, which holds the autumnal equinox, where we find the Sun on the first day of Fall.

STAR OF THE WEEK: SIGMA VIR (Sigma Virginis, actually more commonly known by its Flamsteed number, 60 Vir).

Cool class M stars, whether dwarfs, giants, or supergiants, are usually of interest, especially those that seem isolated. Sitting in a relatively blank area of Virgo (the Maiden), between Vindemiatrix and Heze (Epsilon and Zeta Vir), fifth magnitude (4.80) Sigma Virginis, a class M (M2) giant, seems to fit the bill. A slight variable it can dim to 4.85, which is not noticeable to the naked eye.

Rather far away, 675 light years (plus or minus 27), with a temperature of 3800 Kelvin, Sigma Vir shines with the light of 1600 Suns, from which we derive a radius of 93 times that of the Sun (0.43 Astronomical Units, making it a true giant for sure).

Sigma Vir is used as a calibrator for measures of angular diameter, the accepted value leading to a significantly higher radius of 138 times solar.

In spite of the star's position off the Milky Way to the southeast of the giant Virgo cluster of galaxies, Sigma Virginis seems to suffer from a slight degree of dimming by interstellar (or perhaps circumstellar) dust, as much as 0.22 magnitudes. If so, and it's certainly the extreme limit, the luminosity jumps to 1960 Suns and the radius to 102 times solar.

Theory fits the undimmed version with a mass of 3.0 Suns, the star most likely rising as a giant for the second time with a dead carbon and oxygen core.

If that be the case, Sigma Vir is 450 or so million years old, and gave up core hydrogen fusion around 90 million years ago. (If we apply the correction for interstellar dust, the mass increases to 3.5 Suns).

If the star follows the usual path, and there is little reason to think that it would not, it will brighten and expand eventually to become a Mira-type Long Period Variable. It will slough off its outer layers, probably pop an ephemeral planetary nebula, then die as cooling white dwarf with a mass of around 0.7 solar masses. Along with a variety of class M giants, Sigma Vir has been examined to see how many of them have spectroscopic companions.

An orbiting neighbor would pull the bright star back and forth, resulting in Doppler shifts in the absorption lines, alternately toward shorter then longer wavelengths.

Sigma failed the test but gave a significant lesson, as convective, up and down motions in its atmosphere seemed to mimic orbital motions. But they apparent movements are not quite enough to declare a companion, so the star is sadly most likely single.

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



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