SKYLIGHTS

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Astronomy news for the two weeks starting Friday, January 15, 2016.

Phone: (217) 333-8789
Prepared by Jim Kaler.

Clear skies and thanks to Skylights' blogger visitor reader.


Go to STARS for previous stars of the week. Access Skylights' Archive and photo gallery. From the Sun to the Stars: the OLLI Lectures provides a linked, illustrated introduction to astronomy.
The Constellations has a linked list with locations and brightest stars. Constellation Maps show the locations of the constellations. The 170 Brightest Stars lists them through magnitude 3.00. For more on stars and constellations, visit Stellar Stories.
Tour the Milky Way. Watch a total eclipse of the Moon and an annular eclipse of the Sun. Moon Light presents scenic photos of the Moon. Go to MoonScapes for labelled telescopic images of the Moon and other lunar information.
See the Moon move and pass just below Nu Virginis. Watch planets move against the background stars. See a classic proof of the curvature of the Earth with a "hull down" series. Visit Measuring the Sky to learn about the celestial sphere.
Admire sunsets, rainbows, and other sky phenomena in Sunlight. Read the illustrated Day Into Night on the phenomena of the sky See the The Aurora and the Midnight Sun. See and understand the ocean tides.
Enjoy Our Complex Universe: A Human Understanding through Art, with 12 illustrations. Advances in Astronomy, 1989-2011. Take a ride aboard Asteroid 17851 Kaler (1998 JK). Look for Books about the sky and stars.

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ASPSupport science literacy by joining the Astronomical Society of the Pacific, an international organization that is among the world's premier providers of astro education. Get Mercury and a variety of other benefits.


Presenting three audio courses with 70 to 100-page study guides, narrated and written by Jim Kaler.
Heavens Above: Stars, Constellations, and the Sky from Recorded Books. Astronomy: Earth, Sky, and Planets, is available from Recorded Books. Astronomy: Stars, Galaxies, and the Universe, is also now available from Recorded Books.
Astronomy: Earth, Sky, and Planets is published as Vault of the Heavens: Exploring the Solar System's Place in the Universe by Barnes and Noble.

Enjoy Our Complex Universe:A Human Understanding through Art, with 12 illustrations.

Read "Heaven's Touch: From Killer Stars to Seeds of Life, How We Are Connected to the Universe," Princeton University Press.

SSTo learn about stellar spectra, read STARS AND THEIR SPECTRA: An Introduction to the Spectral Sequence, Second Ed., with two new chapters and 140 new illustrations, Cambridge University Press (UK or North America), 2011.


Read From the Sun to the Stars: the OLLI Lectures, which provides a linked, illustrated introduction to astronomy.

SSNEWEST! FIRST MAGNITUDE: A Book of the Bright Sky, World Scientific, 2013. Read the interview with Jim Kaler.


NEW! Read Dangerous Waters in Stellar Stories.



The next skylights will appear January 29.

We begin as usual with the Moon, which on the night of Saturday, January 16, passes through its first quarter about the time of sunset in North America with the Moon high in the sky. The night before, on Friday the 15th, it will appear as a fat crescent, while after the 15th, the Moon runs through its waxing gibbous phase, which stops at full Moon on Saturday the 23rd, when it will appear down from Gemini's Pollux and Castor, the stars pointing more or less at it. We spend the remainder of the fortnight watching the waning gibbous phase, which does not terminate until Sunday the 31st.

As the Moon grows from the quarter, it approaches Aldebaran in Taurus. The night of Monday the 18th the Moon will be just to the west of the star, while the following night, that of Tuesday the 19th, sees it pass right over Aldebaran, occulting it. The time of the event depends on location. For the central US, the leading (eastern) edge of the Moon passes over the star around 8 PM CST, the star reappearing at the trailing edge a bit over an hour later, about 9 PM. The event occurs progressively later toward the east coast, earlier toward the west. Binoculars should provide a fine view. It's fun to see occulted stars wink suddenly in and out of sight, showing just how small the angular sizes really are. In the latter part of the fortnight, the Moon glides south of Leo. It will appear just below Regulus the night of Monday the 25th and will then make a fine passage below Jupiter the night of Wednesday the 27th as it heads towards Virgo.

Jupiter, dominating late evening skies, rises around 9:30 PM as our period begins, an hour earlier as it ends. Mars is next, coming up around 1 AM, and is followed by Saturn, which by the end of the month is up by 3 AM. Venus, rising later, brings up the rear just after 5 AM, the latter two planets and Antares making a nice triangle. Toward the end of January, even Mercury makes a brief appearance, by the end of our period coming up as dawn begins just behind Venus.

North of Orion look for the big pentagon that makes Auriga, the Charioteer, with its bright star Capella, the most northerly first magnitude star of the northern hemisphere Forty six degrees north of the celestial equator, Capella barely beats Deneb in Cygnus for the honor. The most southerly star is Acrux, Alpha Crucis, in the Southern Cross, which lies 63 degrees south of the equator. It's visible only south of 27 degrees north latitude. Southwest of Capella, which means the "She-Goat," is the narrow triangle that makes her "kids."

STAR OF THE WEEK: CHI AUR (Chi Aurigae). Located in a complex region in central Auriga four degrees north-northeast of Elnath (Beta Tauri, historically also known as Gamma Aurigae) and about the same angular distance north-northwest of the galactic anticenter (opposite the direction to the central black hole in Sagittarius), this fifth magnitude (4.76) class B (B5) mid-level supergiant (some say a B4 lesser supergiant) presents something of a mystery. The problem is that we have little idea of the star's distance. About all we know is that it's massive enough to blow up someday as a supernova. There is no measured parallax; what value exists is dwarfed by the statistical error, so it's useless. The distance is, however, great enough so that interstellar dust considerably reddens the star (dust absorption being more efficient at shorter optical wavelengths) to the point that instead of a gleaming blue white it takes on the more yellow-white color of a class F star, from which we estimate that the dust dims Chi Aur visually by as much as 1.7 magnitudes, a factor of five! Were the line of sight clear, Chi would shine at third magnitude (3.0), probably be part of the constellation outline, and maybe even have a proper name. Though the exact amount of dimming is controversial, it's enough that the star presents a good background with which to study the spectrum of the intervening interstellar medium, which includes the "diffuse interstellar bands" that have intrigued astronomers for decades. Even the temperature, which hovers around 14,100 Kelvin, is insecure. Chi Aurigae is listed as a member of the Auriga OB1 association of hot O and B stars, whose central distance is given as 4300 light years and which we might apply to the star, a dangerous procedure as OB associations are such vast assemblies. Straightforward calculations (which include the addition of ultraviolet radiation) get us a whopping luminosity of 230,000 times that of the Sun, which leads to a radius of 80 times solar, a rotation period (based on a projected equatorial speed of 40 kilometers per second) of under 100 days, a huge mass 25 times that of the Sun, and an age of 6.4 million years. There is a suggestion that the open cluster Messier 38 belongs to the association. If its distance of 3500 light years is used along with the minimum temperature and dust-dimming, we get a lesser luminosity of 90,000 Suns, 60 solar radii, a rotation period of under 75 days, and a mass of 18 Suns, still plenty enough for the star to blow. Playing around with the details does not buy us much. At the most extreme, we can drop the luminosity to 41,000 Suns and the mass to 14 Suns, making it still a supernova candidate. But the calculated age never comes up to that estimated for M 38, suggesting that the star and cluster are not part of the same set. For all the uncertainties, Chi Aur does have a well-observed spectroscopic companion with an orbital period of 1.79 years, which from Kepler's Laws gives an orbital radius in the neighborhood of four Astronomical Units. Even with all the interstellar obscuration, were the star to explode, it would be shine some 10 magnitudes (a factor of 10,000) brighter and could rival the light of a crescent Moon.


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