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


Photo of the Week. A complex lightning display rips the sky from a thunderhead 60 miles away. The curve of stars up and to the right is Corona Australis, the Southern Crown. See full resolution.

Astronomy news for the two weeks starting Friday, September 9, 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.


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.

Coming in October, 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.

The next skylights will appear September 23.

The Moon begins our session in its first quarter on Friday the 9th to the northeast of Saturn and above Mars , leaving the planets behind as it grows in the waxing gibbous phase that terminates at full Moon on Friday the 16th, when it will undergo another minimal penumbral eclipse that in any case is nowhere visible from the Americas. Of more significance, this full Moon is the famed Harvest Moon. At this time of year, the evening ecliptic in the east bears its most shallow angle to the horizon with the result that the intervals between successive Moonrises from one night to the next are minimized, giving us lots of early evening moonlight for outdoor activities, such as harvesting. Two days after full, the Moon passes perigee, where it is closest to Earth, making it even brighter, though not really noticeably so. The Moon then gibbously wanes, terminating our fortnight at third quarter on the morning of Friday the 23rd with the moon high in the sky. Look the night of Tuesday the 20th to see the Moon to the west of Aldebaran. After occulting the star during the day, the Moon will be just east of the star the following night.

The big event is the passage of the Sun across the autumnal equinox in Virgo at 9:21 AM Central Daylight Time (10:21 EDT, 8:21 MDT, 7:21 PDT) on Thursday the 22nd, which begins autumn in the northern hemisphere, spring in the southern. Discounting the effects of atmospheric refraction and the finite angular diameter of the Sun, at that time the Sun rises due east, sets due west, days and nights are of equal length, and the Sun rises at the north pole and sets at the south pole. Moving south against the constellations of the Zodiac, it will bottom out 24.4 degrees south of the celestial equator on December 21, the first day of winter in the northern hemisphere.

Mars and Saturn are quickly separating as Mars moves to the east. Both still in the southwest in early evening, they set just after 11 PM at the beginning of our fortnight, while by the time of the equinox Saturn (still north of Antares in Scorpius) sets about 10 PM, while Mars lingers for about an hour. With a good horizon, Venus is visible in western twilight, while Mercury disappears, as it goes through inferior conjunction with the Sun (on this side of the Sun) on Monday the 12th, but then pops up in eastern twilight at the end of our session.

Ever so slowly, the stars of summer are being replaced by those of autumn. As the Big Dipper glides off into the northwest, the "W" of Cassiopeia climbs the northeastern sky. In mid-evening look about halfway up the sky to find bright Altair, the luminary of Aquila the Eagle, which is nicely set within the Milky Way. While rarely so represented, Altair with its two flanking stars (Tarazed to the north, Alshain) look much like a bird in flight, and have been taken for an airplane with wing lights. Up and to the left of Altair, find the compact and delightful Delphinus, the Dolphin, which looks rather lie a hand with its finger pointing south.

STARS OF THE WEEK: OMEGA-1 AND OMEGA-2 SC0 (Omega-1 and Omega-2 Scorpii), another two-for-one special. In Scorpius, seven and a half degrees northwest of Antares and just one degree south-southeast of Beta Sco (the northernmost star in the trio that makes the Scorpion's head), lies a prominent duo of stars, fourth magnitude (3.96) Omega-1 Scorpii (the western of the pair) and fainter-fourth magnitude (4.32) Omega-2. Only a quarter of a degree apart, at first they seem like a naked-eye double. But once again the eye is fooled, as they have no relation to each other except chance alignment, as Omega-1 is 471 light years away (give or take 18), Omega-2 60 percent as far, at 291 light years (plus or minus 8). They are radically different physically as well. Omega-2 is a yellowish class G (G3, though classed as cool as G8) ageing giant, whereas Omega-1 is a blue, hot class B (B1) youthful dwarf. Unfortunately Omega-1's true color does not come through as a result of interstellar dust absorption and reddening that makes it appear more white. If the path to the star were clear, Omega-1 would be just over a magnitude brighter, shine at a nice third magnitude of 2.94, and perhaps even have a proper name. Distance and a temperature of 26,530 Kelvin (from which we calculate a lot of ultraviolet radiation) gives a whopping luminosity of 11,700 Suns, a radius 5.13 time that of the Sun, and a mass of just under 12 Suns (which agrees well with a published value of 11.1), the star roughly 80 to 100 million years old, well along to the cessation of core hydrogen fusion. A projected equatorial rotation speed of 105 kilometers per second (nowhere near the limit) yields a rotation period of under 2.5 days. It may be a subtle "Beta Cephei" variable with a period of 0.667 days but of unknown amplitude. Omega-1 Sco is part of the expanding Scorpius OB association of young luminous stars, specifically belonging to the "Upper Scorpius" subgroup along with Antares, Sigma, Pi, and Nu Scorpii, Chi and Rho Ophiuchi, and 48 Librae. The average distance to the subgroup is 145 light years, which places Omega-1 pretty much in the middle. Its mass of a dozen Suns is enough to someday send it over the edge. An iron core derived from advanced nuclear fusion will collapse to a neutron star, blowing the outer envelope away in a grand supernova, the event creating all the elements of the periodic table, which will mix with the gases of interstellar space from which new generations of stars will be born.

Nowhere near as dramatic, Omega-2 is an ordinary helium fusing giant. Including some infrared radiation from its 5300 Kelvin surface, the star radiates at a rate of 141 Suns, which gives a radius 14.1 times solar and a mass of 3 Suns. There is little if any intervening interstellar dust. An interferometer measure of radius in the infrared gives 15.6 times solar, which is decent agreement given that such large stars do not have sharply defined surfaces. Instead of exploding, Omega-2 will slough off its outer envelope. The inner portion will turn into a planetary nebula illuminated by the exposed core, which in turn will cool forever as a white dwarf of around 0.7 solar masses.

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