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


Sierra NightSky for the period starting June 17, 2016 by Jim Kaler

The next skylights will appear July 1.

The Moon starts off toward the end of its waxing gibbous phase, which abruptly ends at full Moon the morning of Monday, June 20, about the time of Moonset in North America. It then reverses itself in the waning gibbous phase, which terminates at last quarter on Monday the 27th, after which we see it as a waning crescent. The night of Friday the 17th, the Moon will shine to the northeast of Mars and to the northwest of Saturn, above the line that connects the two planets and above Antares in Scorpius. The following night the Moon will appear just to the left of the ringed planet, the Moon, Saturn, and Antares falling into a nice line.

Jupiter, well to the west as the sky darkens, sets shortly before 1 AM Daylight Time as we open our fortnight and about midnight as we conclude. The Big Show is off to the south, where in mid-evening we can see astonishingly bright Mars (though fading, still nearly as bright as Jupiter) followed by Saturn to the east and Antares down below. It's a remarkable sight, especially in a dark sky, which allows one to see Scorpius and the Milky Way. Be sure to note the strong color contrast between the yellow-white Jupiter/Saturn pair and reddish Mars. The color of the two giant planets comes from reflection of sunlight off ammonia clouds floating in a hydrogen-helium atmosphere loaded with hydrocarbons, whereas Mars's color comes largely from iron oxide: rust. The group begins to set around 3 AM.

At 5:34 PM CDT (6:34 EDT, 4:34 MDT, 3:34 PDT) on Monday, June 20, the Sun passes the Summer Solstice, when and where it is as far to the north as it can get, 23.4 degrees north of the celestial equator. On that day the Sun rises and sets as far to the north of east and west as possible, is overhead at the Tropic of Cancer, and the midnight Sun stretches as far to the south of the North Pole as the Arctic Circle. In the northern hemisphere, daylight hours are longest and night is shortest. The Sun thereafter begins its slow trek to the south toward the Autumnal Equinox, which it will pass on the first day of autumn.

With the Sun at the Summer Solstice, the Winter Solstice will cross the celestial meridian to the south at local midnight, the main figure of Sagittarius to the east (the Solstice within its border) and Scorpius to the west. North of Scorpius stands great Ophiuchus, the Serpent Bearer, while farther north we find Hercules, who needs no introduction and who stands between orange Arcturus (to the south as the sky darkens) and almost-as-bright Vega to the northeast. To the right of the Hero is the charming semi-circular pattern of Corona Borealis, the Northern Crown.

STAR OF THE WEEK: HR 6535 SCO (HR 6535 Scorpii), or just HR 6535, as HR numbers in the Yale Bright Star Catalogue are sequential from west to east (counting from the Vernal Equinox) and ignore the constellations. A seemingly innocuous sixth magnitude star (though at 5.70 on the bright side) buried in the Milky Way, HR 6535 was clearly too faint to merit a Greek letter and at 33 degrees south of the celestial equator really too far down in Scorpius for Flamsteed. A closer look reveals some remarkable properties. First, HR 6535 is almost exactly on the Galactic Equator, the great circle that divides the Milky Way exactly in two (or as exact as we can get it). The star has a galactic latitude of just 0.0493 degrees (2 minutes, 57.5 seconds) north. For completeness, with a galactic longitude of 355.667 degrees, it's not that far off the galactic center either, defined by Sagittarius A*, the supermassive black hole and source of radio emission at the Galaxy's core. Second, like 10 Monocerotis HR 6535 sits squarely on, and is the most luminous member of, an open cluster, NGC 6383.

NGC 6383 The luminous binary class O star (both O7 dwarfs) HR 6535 overwhelms the rest of the stars in the rather sparse open cluster NGC 6383. (STScI Digitized Sky Survey.)


Third, it's not just one star, but two nearly identical ones in close orbit and, to top it off, they are not just any old stars, but two of rare class O dwarfs, both originally thought to be 06, but now widely accepted as O7 (which is not that much of a difference). The distance is a real problem as the uncertainty in the parallax is almost as big as the parallax itself, which for what it's worth yields a distance of 4240 light years. The better-determined distance of the cluster considered as a whole, however, is a somewhat supportive 3200 light years, so for further argument take an average of 3750 light years. (It really doesn't matter all that much.) At that distance and in the star's central direction, we might expect significant dimming by interstellar dust, and we are not disappointed, as the visual extinction of light is (calculated from the star color, which is that of a class A2 star) 1.22 magnitudes, which raises the visual magnitude (if the view were clear) to a nice fourth magnitude (4.48). Assuming the stars to be equal yields a visual magnitude for each of 5.23. The temperatures are given as 39,000 and 38,000 Kelvin, which we'll use even though inconsistent with equality (the other uncertainties certainly worse).

Adding in the amounts of ultraviolet light calculated from the temperatures, we find luminosities of 260,000 and 220,000 Suns, radii of 11.2 and 11.4 times that of the Sun, and masses of 38 and 35 times solar, similar to other analyses. Still fusing hydrogen in their cores, the stars are only two to three million years old, as is the cluster. No matter what the details, these are two whopping stars.

Taking an average projected equatorial rotation velocity of 155 kilometers per second, we get a rotation period of under 3.66 days, which is the same as the measured orbital period of 3.66767 days, showing the pair to be in synchronous rotation, the same sides of each perpetually facing each other, and the orbital plane to be nearly in the line of sight. Kepler's third law then gives a separation of 0.19 Astronomical Units, half Mercury's distance from the Sun, 40 solar radii, 3.5 stellar radii. Mutual tides and rotation somewhat distort the stars, making the pair into a non-eclipsing "ellipsoidal variable" that changes over the cycle by a few hundredths of a magnitude. Winds that blow from each star at a speed of some 2900 kilometers per second and at a rate of three hundred-thousandth of a solar mass per year, 600 million times that of the solar wind, collide to make the star a relatively strong source of X-rays. Several small companions are listed for HR 6535, but that is not surprising either given that it's a member of a cluster. The future of the stars is unclear. As they evolve, they will encroach upon each other, perhaps merge, perhaps not. We can be assured of at least one supernova explosion and perhaps a resulting black hole. Maybe even two of them that will eventually merge to create gravity waves like those just discovered. (Stellar data are taken in part from M. De Becker et al. in Astronomy and Astrophysics, 416, 233, 2004.)

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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