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

earthlight and flowers

Photo of the Week. Crescent with earthlight (light reflected from Earth onto Moon's nighttime side) and Venus.


Astronomy news for the two weeks starting Friday, April 10, 2015.

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 Bearly North in Stellar Stories.



The next skylights will appear April 24, 2015. Skylights' current two-week period is the result of travel and medical issues. I hope to go back to weekly reports. Thanks for your patience.

We begin the week with the Moon in its late waning gibbous phase. Last quarter is passed the night of Saturday April 11, after which the Moon fades as a waning crescent. With your last view of the crescent to the east the morning of Friday the 17th, the phase ends at new Moon on Saturday the 18th, after which the Moon switches to western evening skies as a waxing crescent. The twilit evening of Sunday the 19th, the ultraslim crescent will appear to the left of Mars and Mercury, while the following evening, the crescent will appear well below Venus. The best sight will be the evening of Tuesday the 21st when the growing Moon will make a nice pairing to the left of the brilliant planet, with the Hyades and Aldebaran of Taurus below. The Moon passes perigee, where it is closest to Earth, the night of Thursday the 16th.

Until a couple hours past the end of twilight, western skies are completely dominated by lustrous Venus, which is behind only the Moon and Sun in celestial brightness. Look for Venus to the left of the Pleiades cluster the evening of Saturday the 11th. Just after sunset, number four in brilliance, Jupiter, still to the west of Leo's Regulus, crosses the meridian to the south, while a couple hours later, Saturn rises just north of Antares in Scorpius. Not that we will actually witness the event, but dim Pluto begins retrograde motion in Sagittarius on Friday the 17th.

Among the better-known meteor showers, the Lyrids (which seem to emanate from the constellation Lyra), will reach a peak the night of Wednesday the 22nd, rather the morning of Thursday the 23rd, when Lyra is high in the sky and the Moon is out of the way. The shower typically produces 15 to 20 meteors an hour. The Lyrids are the debris of Comet Thatcher, which has a long period of 415 years.

As the Sun climbs north along the ecliptic, the constellations of the "wet quarter," Capricornus (the Water Goat) to the west, Aquarius (the Water Bearer) in the middle, and Pisces (the Fishes) to the east, are being left behind to appear in morning skies. Taurus and the rest of the winter gang will soon follow to be taken over by evening twilight. Look then for the figures of summer, Lyra with Vega (as noted above), followed to the east by Cygnus, the Swan. Upside down, Cygnus becomes the Northern Cross, which rises on its side with Deneb at the top, the star representing the tail of the graceful bird.

STARS OF THE WEEK: RHO AQR (Rho Aquarii), with a nod toward THETA AQR (Theta Aquarii). Within Aquarius (the Water Bearer) what comes most to mind is his "Water Jar," from which he pours the streaming liquid into the mouth of the Southern Fish, Piscis Austrinus. We might also admire the two great supergiants off to the west, Sadalmelik (Alpha Aquarii) and Sadalsuud (Beta). The numerous stars to the south are less appreciated. Among them, some eight degrees below the Water Jar, is a pair a bit over three-fourths of a degree apart. The more westerly and brighter of the two, fourth magnitude Theta Aquarii, is an ordinary class G (G8 giant-subgiant) that serves as a guide star to more interesting, though fainter, fifth magnitude (5.37) Rho Aqr to the east, a class B (B8) "giant" (but see below). Though proximate on the sky, they have nothing to do with each other. At a distance of 882 light years (give or take 91), Rho is almost five times farther than Theta (187 l-y, plus/minus under 3). The distance plus a well-defined temperature of 4970 Kelvin (needed to allow for infrared radiation) yields a modest luminosity for Theta Aqr of just 16.6 Suns, from which we find radius of 11.8 solar. Theory suggests that it's a helium-fusing giant of perhaps two solar masses. Even though Theta is just over two degrees north of the ecliptic, there seems to be no observation of radius via lunar occultation, which tells how ignored the star really is. But then there is Rho. It's an underobserved "mercury-manganese" (HgMn) star, one with huge overabundances of these chemical elements as well as enrichments of rare earths such as europium, alongside depletions of some others, exemplified by calcium. For relatively slow rotators, the outer stellar layers are fairly peaceful and unstirred. As a result, some elements are lofted upward by radiation pressure, while others sink under the force of gravity. With a projected equatorial spin-speed of 69 kilometers per second rotation (which translates to a rotation period of at most 4.4 days), Rho apparently qualifies. The HgMn stars are higher temperature versions of class A metallic line stars like Sirius. The odd abundances make classification problematic. With a temperature of 12,450 Kelvin (needed for evaluation of ultraviolet light) and a 0.1 magnitude correction for dimming by interstellar dust, Rho radiates at a rate of 773 Suns, the radius 6.6 solar. Theory reveals that the star is either a subgiant with a dead helium core or a dwarf that is close to becoming one with a mass of 4.3 to 4.5 Suns, its age some 112 million years. Slight Doppler shifts in the spectrum suggest a companion with a 220-day period, which yields a separation for low companion-mass of 1.2 Astronomical Units. As different as they are now, the two stars have similar fates. Both are destined to lose their outer layers and die as white dwarfs with respective masses of 0.63 and 0.82 times that of the Sun. Because of its higher mass, Rho goes first, not that Theta gives a hoot.


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