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

Wispy clouds

Photo of the Week. Blowing clouds.

Astronomy news for the two weeks starting Friday, February 12, 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. 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.


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 February 26.

Happy Valentine's Day, and a belated Happy Groundhog Day, the latter marking the halfway point in winter on our way to spring.

The Moon begins our fortnight in the fat waxing crescent phase, which ends at first quarter the night of Sunday, February 14, shortly after moonset in North America. It thereafter enters the waxing gibbous phase as it heads towards full Moon on Monday the 22nd around noon. The Moon will thus be just shy of full the evening of Sunday the 21st and just past the phase the night of the 22nd. It then concludes our period in the waning gibbous phase. During our two weeks, the Moon steadily moves away from us toward apogee, where on Friday the 26th it will be farthest from Earth, a change of only 5.5 percent from the average distance.

Watch the evening of Monday the 15th as the quarter Moon plows through the southern part of the Hyades cluster in Taurus, covering stars as it goes, then finally meeting up with Aldebaran in the morning, the occultation of the star visible only from the west coast. Look for the Pleiades to the northwest of the Moon. The night of Sunday the 21st, the near-full Moon will appear west of the star Regulus in Leo, while the following evening it will lie between Regulus and bright Jupiter. Then the night of Tuesday the 23rd we are in for a treat as the Moon passes just a couple degrees south of the bright planet.

Given Jupiter's angular proximity to the near-full Moon, it obviously rises early, by 7:30 PM at the start of our fortnight (just after the end of twilight), an hour earlier at the end of it. Next up is Mars, which rises just after midnight and shortly before Jupiter transits the meridian, Mars well to the west-northwest of Antares. Having a similar color, the star's name means "rival of Mars." At about the time Antares rises, so does Saturn, 2:30 AM as we begin, 1:30 AM as we end, Mars and Saturn bracketing northern Scorpius and forming a nice triangle with Antares. Finally, if you have a good southeastern horizon you might spot Venus glimmering up in morning twilight.

Orion rides high to the south in early evening, Gemini (with Castor and Pollux) to the northeast, Auriga (with Capella) above. To the southeast of the Hunter glows white Sirius, the brightest star of the sky and in winter nights the champion twinkler as its light is jittered about by our irregular atmosphere. The planets on the other hand shine with a steadier light, the result of their being extended disks that average out the twinkling suffered by the near-points of light that are the stars.

STAR OF THE WEEK: OMEGA GEM (Omega Geminorum). Bayer sometimes could not run through the entire Greek alphabet within each of the constellations. There had to be enough bright stars, which confined the "Omegas" to the larger figures and those nearer the Milky Way. Gemini has a good one, a fifth magnitude (5.18) yellow-white class G (G5) supergiant (though a lesser one, perhaps even a "bright giant"). Omega Gem's relative faintness is the result of its great distance, 1489 light years (with a significant uncertainty of 177). Even though the star is close to the path of the Milky Way, there is little dimming by interstellar dust. Distance and a temperature of 5064 Kelvin, from which we calculate a bit of infrared radiation, yield a luminosity of 1820 Suns and thus a radius 56 times solar. The theory of stellar structure and evolution gives a mass of 5.5 Suns and shows that the star is contentedly fusing helium to carbon and oxygen in its deep core. With an age of around 65 million years, it will before long lose its outer envelope to produce a planetary nebula, the carbon/oxygen core destined to become a white dwarf with a mass of around 90 percent solar. The star is similar to Psi Andromedae, though slightly more massive and without obvious companions.

All seems straightforward. Hardly. First, Omega Gem teaches a modest lesson about classification and thoroughly researching the literature. In 1977 the star was declared a Cepheid variable with a small variation of about 0.09 magnitudes and a period of 0.7282 days. How interesting, since Omega Gem is just 22 minutes of arc north of one of the great classical Cepheids of the sky, Mekbuda, Zeta Geminorum. Eleven years later, however, a dedicated study at three different observatories found no variation in Omega Gem whatsoever, the star steady as the proverbial rock. But the label stuck, and Omega Gem is listed as a Cepheid to this day. Second, a measurement of the angular diameter suggests a physical radius of 72 times that of the Sun, 29 percent higher than given by theory. If we push the uncertainties in distance and angular diameter to the limits, we close some, but not all, of the gap. However, the angular diameter was measured in the infrared part of the spectrum, and given the diffuse "surfaces" of supergiants, we can hardly expect the sizes in different wavelength bands to be the same. Third, Omega Gem is listed as a "barium star," one that's been contaminated with freshly-made elements during the evolution of a more-massive companion that is now a white dwarf. However there is no confirmation of the designation and neither is there evidence for a white dwarf in orbit about the bigger star. Not that one does not exist, as detection would be difficult. So we'll have to admire Gemini's star at the end of the alphabet for what it is, a distant supergiant, massive to be sure, but one well under the limit of 8 to 10 Suns above which stars explode as supernovae.

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