SKYLIGHTS

Skylights featured on Astronomy Picture of the Day

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Skylights featured nine times on Earth Science Picture of the Day: 1 2 3 4 5 6 7 8 9

Colorful sunset

Photo of the Week.. Colorful sunset.


Astronomy news for the two weeks starting Friday, January 27, 2017.

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, 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, February 10.

The Sky

Orion, with his three-star belt dominates the stellar sky, Betelgeuse to the upper left, Rigel to the lower right. Just up and to the right of Rigel, is Cursa, Beta Eridani, which begins Eridanus the River. The River then meanders off to the south and west, ending in bright Achernar. Sirius, the sky's brightest star, lies southeast of Orion. If you live in the far southern US, you might spot Canopus, the second brightest. Then see the Big Dipper rise in the northeast as the stars of the Andromeda myth fall to the northeast.
Primary source: The Astronomical Almanac.

STAR OF THE WEEK

RHO AND (Rho Andromedae)

RHO AND (Rho Andromedae). If you look about 5 degrees southwest of the Andromeda Galaxy (Andromeda's greatest feature), roughly between it and the Great Square of Pegasus, you'll find a small triangle of fifth magnitude stars a couple degrees across. At magnitude 5.18 the faintest of the three, Rho Andromedae is also the most easterly, while brighter Theta is at the northern apex and Sigma is at the southern. Classed as an F3 giant, the first impression would be that it is well along in its evolutionary track on the HR diagram (a plot of brightness vs, temperature, wherein absolute magnitude and spectral class are used as proxies). Andromeda is rather well off the Milky Way, so dimming by interstellar dust is not much of a problem and we'll ignore it for now. Rho's temperature is rather well-defined by a number of observations at around 6720 Kelvin, so most of the light is in the visual spectrum and we need make little correction for either infrared or ultraviolet light. At a distance of 158 light years (give or take 2 and in between Theta and Sigma), the star shines with the light of 16.6 Suns, which with temperature gives it a radius of only 3.0 times that of the Sun, not much for a so-called "giant." The projected equatorial rotation velocity of 42 kilometers per second gives a rotation period of less than 3.6 days. In spite of the relatively small size, the angular diameter has been measured by interferometry at 0.600 thousandths of a second of arc to one percent precision. Given the distance, the star then has a radius of 3.13 times that of the Sun, just four percent higher than that calculated from temperature and luminosity, not a bad fit at all. A little juggling of parameters cold bring the two into exact agreement. While Rho And's spectrum may be that of an F3 giant, the theory of stellar structure and evolution shows that the star is really a 1.75 (Or a bit higher) solar mass subgiant that has just taken on the role and is more a dwarf that has just given up core hydrogen fusion and is some 1.8 billion years old. The expected color of a dwarf is just what we see, so there indeed appears to be no need for correction by interstellar dimming, not that it makes too much difference. Rho And appears to be all alone with no companion to witness its expansion to a real red giant, which will happen before long, though the pace of evolution is so slow (barring explosions) that we will not see it. Nowhere nearly massive enough to blow up as a supernova (at least 8 or 9 Suns is required), Rho And will slough off its outer envelope, maybe produce an ephemeral planetary nebula, and die as a white dwarf of about 0.6 solar masses. The star teaches a nice lesson in that there is not a one-to-one relation between spectral class and actual evolutionary class, the dichotomies more noticeable among the class B stars.


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