Sierra NightSky

Sierra NightSky for the period starting Friday, July 14, 2017 by Jim Kaler.

The next Skylights will be presented Friday, July 28.

Our fortnight pretty much fits between the quarters of the lunar phases, with third quarter taking place on Sunday, July 16, and first quarter on Sunday the 30th. In between, the Moon is in the waning crescent phase in the eastern morning sky from Sunday the 16th until Sunday the 30th, when it graces the western evening sky as a waxing crescent. New Moon, of course, is right in the middle of the time-period, on Sunday the 23rd. During the first week, the Moon plays with Taurus. The morning of Wednesday the 19th, it will rise west of the Hyades and Aldebaran (with the Pleiades above), while the following night the Moon will have flipped to the other side of the stars. Even better, the crescent will take up a near-classic position just down and to the right of brilliant Venus the morning of Thursday the 20th. After new Moon, on the western side of the sky the slim waxing crescent will appear down and to the right of Mercury and the star Regulus on Monday the 24th, while the following evening it will lie up and to the left of the pair. You’ll need a flat horizon to see the trio in bright twilight. Then it’s Jupiter‘s turn, the Moon just above the giant planet he evening of the 28th. In the middle of our session, on Friday the 21st, the Moon will pass through perigee, where it is closest to Earth (by about five percent).

The planets have their own business to mind. In the middle of the second week, as presaged above, you can find Venus (by far the brighter) to the east of Aldebaran in Taurus. In the evening of the second week, Jupiter (in Virgo to the northwest of the bright star Spica), dominates the western sky, not setting until local midnight, shortly after Saturn transits the meridian well to the south between Scorpius and Sagittarius (technically within the confines of southern Ophiuchus). Mercury sets about as late as possible during this round (but still in twilight), while in the morning, Venus (impossible to miss) rises about as early as possible, around 3 AM Daylight Time. On Wednesday the 26th, Mars is finally in conjunction with the Sun.

Prepare now for what may be the event not just of the year but of the century, when the Moon eclipses the Sun on August 21, the path of totality going from the American northwest to the southeast across the US. Do NOT look at the bright part of the Sun. Heavily filtered eclipse glasses are readily available from the Web or from your local planetarium.

We finally made it to the dead of the Summer sky as Arcturus begins to slide down the southwestern sky, Antares shines far to the south, and Vega is nearly overhead. With the Moon out of the way, the sky just becomes alive with stars. To the southwest of Antares is a fine loop of stars that represents Lupus, the Wolf, while to the south of Scorpius’s tail lies Ara , the Altar.


Touring the Milky Way, the observer’s eye quickly goes to the striking figure that makes up Scorpius, the celestial scorpion, with its bright red supergiant Antares and long curved stellar tail. By comparison, the next-door neighbor, the loop of bright stars to the southeast of Antares that make Lupus, the Wolf, tends then to be a bit ignored. And too bad, as it’s filled with hot massive blue stars that give the Milky Way much of its sparkle. Among the fainter of the stars near the outline of this lush constellation is a fourth magnitude (4.42) class B dwarf, which after some consideration is most likely subclassed B1/B2. Sigma Lupi, which lies toward Lupus’s southern end some 3.5 degrees south-southwest of the constellation’s second magnitude luminary Kakkab (Alpha Lupi). A temperature of 24,500 Kelvin coupled with a distance of 575 light years (give or take 19) yield a luminosity 3700 times that of the Sun. There is no correction for interstellar dust as the line of sight seems quite clear. In fact the star is slightly bluer than one might expect given the spectral class, which is something of an oddity. Luminosity and temperature conspire to give a radius of 3.4 times that of the Sun, which with a projected equatorial rotation velocity of 75 kilometers per second, gives a rotation period of less than 2.3 days. Theory suggests a whopping mass of 9.5 Suns. A more sophisticated analysis gives a radius of 4.8 times solar and a mass of 9.3 Suns. The star seems quite young, as it is still close to its birth luminosity and temperature.

Sigma Lup The visual light curve of Sigma Lupi is very subtle. The scale on the left is of relative magnitudes against a comparison star, that on the bottom the fraction of the variation period. (HF Henrichs et al., Astronomy and Astrohysics, 545, A119, 25, 2012.)

Sigma Lupi is slightly variable, with an amplitude of about a hundredth of a magnitude and a period of 3.02 days. The origin of the variation is unknown. One suggestion is that the star is an “ellipsoidal variable” like Spica. Rotation and tides distort the shape of the star into an ellipsoid and, as it orbits an unseen companion, presents a face of varying angular size to the observer. Unfortunately, such an arrangement has never been confirmed and now seems unlikely. Sigma Lupi has a weak magnetic field roughly 100 times the strength of Earth’s. Variations in magnetically-sensitive ultraviolet spectrum lines give a period of 3.02 days, the same as determined visually, which must be the true rotation period of the star, Given the uncertainty of the input parameters, the limit given above is not all that bad. From mid-class F and hotter we commonly see chemical abundance variations that are produced by separation of elements. At the cooler end of class B we find the “helium-weak” stars, which are converted to “He-strong” stars toward B2, which includes Sigma Lup. Sigma Lupi is right at the edge of the limit of 8-10 solar masses above which stars develop iron cores and explode as supernovae. Indeed local supernovae have cleared out the volume of space surrounding the Sun, putting the Sun into what is called the “local bubble.” (Some data and discussion taken from a paper by H.F.Henrichs et. al, Astronomy and Astrophysics, vol. 545, A119, 2012.)


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