SKYLIGHTS

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Thundercloud

Photo of the Week.. Thundercloud.


Astronomy news for the two weeks starting Friday, January 13, 2017.

Prepared by Jim Kaler.

Clear skies and thanks to Skylights' blogger visitor reader.


Go to STARS for previous stars of the week. Last week's Skylights is still available. 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, now in Chinese translation.

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.


SSLive in town? Read FIRST MAGNITUDE: A Book of the Bright Sky, World Scientific, 2013. See the interview with Jim Kaler.


NEW! Read Dust to Dust in Stellar Stories.

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


IT'S HERE!, From the Sun to the Stars by Jim Kaler, World Scientific, 2016, a new book based on From the Sun to the Stars: the OLLI Lectures, which provides a linked, illustrated introduction to astronomy.


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Please Note

Skylights has been produced in various forms for 31 years, since 1985, and even before that as an annual bulletin. It's now time to simplify Skylights and bring part of it to a close. We will continue to list lunar phases, planetary passages, and other significant events for the coming two weeks on this website, but by bullet, not by prose text. Because there will no longer be any script, the telephone and emailing services have been dropped. The Star of the Week will continue as before, as will the Photo of the Week. Thanks all for your support.
Jim Kaler


The next Skylights will appear Friday, January 13.

The Sky

At mid-evening, Perseus, rescuer of Andromeda, rides high aboard his flying steed Pegasus. And he'd best move fast, as Auriga, the Charioteer is right behind him. The evening proceeds with bright Orion below Auriga. To find Auriga, just look for the brightest, most northerly, star in the area, Capella. Well to the west of upper Orion, featured by the bright star Betelgeuse, look for the roundish head of Cetus, the Sea Monster, and we are back to the Andromeda myth, as she was rescued from the jaws of the giant beast by Perseus.
Primary source: The Astronomical Almanac;

STAR OF THE WEEK

OMI AND (Omicron Andromedae)

Andromeda, the central character in the ancient Andromeda myth, has a strong representation in the sky in long strings of stars that come off the northeastern corner of the Great Square of Pegasus (Alpha Andromedae and Delta Pegasi being the same star.) The constellation boundaries spread far from the classic figure. At the far western boundary, 14 degrees north of the Great Square (and Beta Pegasi) lies rather bright fourth (3.62, nearly third) Omicron Andromedae, which, since it is just east of the border with Lynx, has the distinction of being Flamsteed's number 1 as well. At first look, it's a rather odd close orbiting double consisting of a fourth magnitude (3.73) class B (B6) emission line giant (a "Be" star) coupled with a sixth magnitude (6.03) class A (A2) "peculiar" (magnetic) star only a few hundredths of a second of arc apart, which at a distance of 687 light years (give or take 77) corresponds to an average separation of 12.8 AU (with a modest eccentricity) and an orbital period of 5.6 years. Adopting a small degree of interstellar dimming (0.19 magnitudes) and a temperature of 14,490 Kelvin for the Be star (from which we evaluate the degree of ultraviolet radiation), we find a hefty luminosity of 3945 times that of the Sun, a radius of 9.9 times solar, and a mass of 6.5 Suns, theory showing the star to be a subgiant that will shortly begin to fuse its core helium to carbon and oxygen. Interferometric measures of radius give a satisfyingly close 11.5 solar radii. A projected equatorial rotation velocity of 286 kilometers per second gives a rotation period under 1.7 days. A Be star is enclosed in a rotating disk that gives off hydrogen (and other) emission lines. The disks are variable, Omi And typically varying between around magnitude 3.5 and 3.65, in outbursts rather like Gamma Cas. In some stars the disks disappear and re-appear at will. The Be phenomenon is somehow related to high rotation velocity, but nobody knows quite how. Winds may well be involved. The class A companion should have a temperature around 9000 Kelvin (there is no measurement), which gives a much lower luminosity of 170 Suns, a radius of 1.3 solar, and a mass of 3 Suns, all of these numbers rather uncertain. The bright Be star (confusingly called Omi And A, rendering the A star Omi B) has a companion as well with a measured period of 117 days and an apparent mean separation of 62 AU. Both orbits (AbAa and AB) give highly unreasonable masses, so it's doubtful that that the orbits are accurate (not surprising for such a complex star). There is also a suggestion of a fourth member that was originally thought to orbit the A star (Omi B), but that might orbit the other. It's all surprisingly non-definitive for a star this observable and this bright.


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