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


Sierra NightSky for the period starting January 29, 2016 by Jim Kaler

The next skylights will appear February 12.

The Moon starts us off near the end of its waning gibbous phase, which ends at third quarter on Sunday, January 31, shortly before moonrise in North America. It then enters the waning crescent phase, which slims until the Moon disappears from the sky at new Moon the morning of Monday, February 8. Your last chance to see the ultra-thin crescent will be on the twilit morning of Sunday the 7th. The Moon then flips to the other side of the sky, appearing as a thin waxing crescent in western twilight the evening of Tuesday the 9th.

The waning crescent will drive through a wonderful array of planets. The morning of Monday the 1st, look for the crescent Moon to rise just three degrees north of Mars. Then on Wednesday the 3rd, it passes a similar distance above Saturn, when they will make a nice triangle with the red supergiant Antares below. Two mornings later the crescent will lie well above brilliant Venus. The Moon finishes by making a beautiful triangle with Venus and Mercury on Saturday the 6th, Venus to the right of the Moon, Mercury below. Sights like this are treasures that do not come along very often. However the three do not rise until the beginning of twilight and will require a clear southeastern horizon.

During our fortnight the Moon runs from apogee (where it is farthest from Earth) on Saturday, January 30, to perigee (closest) on Wednesday, February 10, during which the angular diameter increases by about 10 percent, the effect really not noticeable to the eye.

Jupiter now seriously encroaches upon evening skies, rising brightly in southern Leo around 8:30 PM at the start of our session, closer to 7:30 at the end. It's followed by Mars northwest of its rival-in-color, Antares, around 1 AM and then by Saturn (to the northeast of Antares) at 3:30 (late January) to 2:30 (mid-February).

Orion still dominates the early evening. Look to the south of him for a boxlike set of stars that represents his prey, Lepus, the Hare, and then farther down for the flat triangle that makes Columba, the Dove. Immediately east of him is the modern constellation of Monoceros, the Unicorn. While faint, Monoceros contains some of the great treasures of the dim winter Milky Way, including the delightful Rosette Nebula.

STAR OF THE WEEK: GAMMA COL (Gamma Columbae)

Dropping south of Orion (or if in the southern hemisphere, climbing up from) we run into the Hunter's prey, Lepus (the Hare) and then into the pretty triangle that makes the most prominent part of the modern constellation, Columba (the Dove), composed of the three brightest stars, Alpha, Beta, and Epsilon Columbae. Gamma Col, shoveled off 1.5 or so degrees east-northeast of the third magnitude (2.64) luminary Alpha, oddly ranks not third in the constellation, but sixth. Indeed, at fourth magnitude (4.36) it just barely beats out Kappa (4.37).

But it's still the Gamma star, so we should take a look at it. And as a quite-hot class B (B2.5) subgiant, it's certainly worthy of attention. And it's received it, as the temperature of 17,100 Kelvin is quite well known. The hefty distance of 870 light years (plus or minus 42) is not badly determined either. Rather well to the west of the Milky Way, the star is subject to only 0.19 magnitudes of dimming by interstellar dust. Adding in a lot of ultraviolet radiation, Gamma Col shines with a magnificent luminosity of 5010 times that of the Sun, which leads to a radius of 8.1 times the solar value.

The only statement of projected equatorial rotation velocity is an uncertain 96 kilometers per second, which loosely defines a rotation period of under 4.2 days. The theory of stellar structure and evolution gives a mass of 7.0 Suns and shows that at an age or 43 million years the star has, or will shortly, run out of hydrogen fuel in its core. (As much as 7.2 Suns is possible depending on the exact state of ageing.) It will soon expand and brighten to become a giant star as it prepares to ignite its internal helium (the ash of the original hydrogen fusion) into a mix of carbon and oxygen, the star eventually dying as a white dwarf of about a solar mass, its outer layers having been ejected back into interstellar space (in the process forming glowing planetary nebula).

That might be the end of it. But Gamma Col has a dim 13th magnitude (12.7) companion of considerable interest. At an angular separation of 34 seconds of arc, over the past century and a half little Gamma B has tracked Gamma A pretty well and seems to be really connected. If so it is somewhat dimmer than the Sun, a G7 or so dwarf with a luminosity two-thirds solar. But if it truly belongs to the brighter star, then it must also be very young (42 million years old).

As dwarfs age, they slowly brighten. When the Sun was young its luminosity was about 70 percent what it is today (giving us the "young Sun paradox": how could life have developed with a frozen Earth?). Gamma Col B thus carries about a solar mass, a star so young that it is considered a "post T-Tauri star," one that has recently (or still in the act of) settling down onto the hydrogen-fusing "main sequence" of dwarfs. So we have a double in which one massive star is starting to die while at the same time the other is still frolicking away in exuberant youth. At Gamma Col's distance, the pair must be separated by at least 9000 Astronomical Units and from Kepler's Laws must take at least 300,000 years to orbit each other. If the physical separation is correct, then from the bright star, little Gamma B would shine with the light of four times that of our Venus, while From B, A would appear as bright as eight full moons.

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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