Skylights featured on Astronom y 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

Greenland 12

Photo of the Week. Planet Earth: the ninth of twelve in the "Flight across Greenland," going from east to west above the fantastic glacier and a river of ice. See full resolution.

Astronomy news for the week starting Friday, April 4, 2014.

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 < i> 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 The Harp in Stellar Stories.

The Moon starts off our week as a fat waxing crescent as it heads toward its first quarter on the morning of Monday, April 7th, after moonset. The night of Sunday the 6th it will be seen just shy of the phase, the following night just past it. It then brightens in the waxing gibbous phase, full Moon not reached until Tuesday the 15th, when there will be a total eclipse visible throughout North America. Early in the week, the near-quarter Moon will play with Jupiter. The night of Saturday the 5th, the Moon will appear to the west of the giant planet, and then the evening of Sunday the 6th, the Moon will glide five degrees below Jupiter, the two making a fine sight against the stars of Gemini, which include bright Castor and Pollux. Even the evening of Monday the 7th it's worth a look to see the Moon moving off to the east of the far more slowly moving planet. The Moon reaches its apogee, the farthest point in its orbit from Earth (405,500 kilometers, 252,000 miles), on Tuesday the 8th.

Though night by night slowly shifting off to the west, Jupiter still rules the evening skies, at least until it sets just after 2 AM Daylight Time. It's really a week to celebrate Mars, however, as the red planet achieves opposition to the Sun on Tuesday the 8th, when it will rise at sunset, cross the meridian to the south at local midnight, and set at sunrise. With the Sun just east of the vernal equinox in Pisces, Mars must then be just east of the autumnal equinox in Virgo, where it is found a few degrees northwest of Spica. Since Mars's orbit is rather eccentric (by about nine percent, quite a bit more than any other planet except Mercury and Pluto, but the latter is no longer considered a planet and that's another story), closest approach to Earth (0.62 million Astronomical Units, 93 kilometers, 58 million miles) does not take place until next week, on Monday the 14th. Ah the oddities one finds. In contrast to Mars, the asteroid 3 Juno, third discovered, but with an average diameter of 240 km tenth largest (their irregularity making asteroids hard to rank in size), is in conjunction with the Sun on Thursday the 10th. Which means it's not visible at all. Saturn, though, certainly is, rising in Libra around 10 PM, an hour after twilight ends. Then just before the sky begins to lighten, near 5 AM, up comes Venus. Now in its role as "morning star," the planet's luster does not fade in southeastern skies until bright dawn.

In the evening we look now not at the Milky Way, but perpendicular to it, away from the pervasive obscuring dust of the Galaxy's disk, which allows us to see to great distances and into the swarms of other galaxies that surround us. The nearest large grouping, the Virgo Cluster of galaxies, lies "only" 55 million light years away, many of its members visible in a small telescope. Farther on, 330 million light years away, is the Coma Berenices cluster of galaxies (not to be confused with the nearby star cluster of the same name) south of the Big Dipper. Four galaxies are visible to the naked eye, in the northern hemisphere M31 in Andromeda and M33 in Triangulum (the latter just barely), and in the southern hemisphere the two Magellanic Clouds, the larger in Dorado, the smaller in Tucana.

STAR OF THE WEEK: THETA TUC (Theta Tucanae). Tucked (had to be said) into deep southern Tucana only 19 degrees from the South Celestial Pole, Theta Tuc is the most southerly star in the constellation with a Greek letter. It's hard to know what's more remarkable, Theta Tucanae's setting or its character. Dim to the naked eye (if visible at all), only sixth magnitude (6.13), it has a prominent place just to the west of the Small Magellanic Cloud. Easily visible though 200,000 light years away, the SMC is the second brightest of our two prominent satellite galaxies, the other (surprise) the Large Magellanic Cloud, which is somewhat closer (160,000 light years). Equally good, Theta Tuc is but a degree northeast of 47 Tucanae, one of the greatest globular clusters in our Galaxy. With a luminosity close to half a million Suns, containing maybe a million stars, 47 Tuc is exceeded only by Omega Centauri, which seems more to be the remains of a small galaxy that collided with us, rendering 47 Tuc on top. A class A7 subgiant 424 light years away, Theta Tuc has been the focus of intense scrutiny, as it's both a Delta Scuti variable and a close mass-transfer binary. Delta Sct stars are typically class A dwarfs, subgiants, or giants that vary by hundredths to tenths of a magnitude with multiple periods measured in hours. Theta Tuc fits right in, varying by a few hundredths of a magnitude with at least 10 (!) different periods and probably more, the main one at 1.183 hours, the second at 1.193 hours, and so on, none differing by very much. Through straightforward calculation, we find a luminosity for the star of 46 times that of the Sun and a mass of 2.1 to 2.3 Suns depending on its status as a near or true subgiant. But there's more! Theta Tuc has a spectroscopically observed companion that orbits every 7.04 days and seems to have an extraordinarily low mass of about 0.2 times that of the Sun. Not a red dwarf as might first be expected, Theta Tuc B apparently has a class near G0, a temperature of 7000 Kelvin, and a luminosity roughly similar to that of Theta-A, which cuts the luminosity of Theta A down to 23 Suns and the mass to 2.0, in agreement with that derived from oscillations. If not yet a true subgiant, it will be soon. The companion is probably in a "post red giant" state, having already gone through its giant phases. When in its swollen condition, it passed mass onto Theta Tuc A. The binary used to behave like Algol in Perseus and it has become what Algol has to look forward to. With a radius of 2.8 times that of the Sun and an equatorial spin speed of 81 kilometers per second, Theta A rotates every 1.7 days. The two stars are incredibly close together, just 0.09 Astronomical Units apart, 19 solar radii. When "A" begins to expand as a giant, it will pass mass back in the other direction onto "B," allowing weird things to happen. (Thanks to C. Serkin in Astronomy and Astrophysics, 325, 563, 1997, K. De Mey et al., A&A 336, 527, 1998, M. Tempelton et al, Astrophyical Journal, 528, 979, 2000, and M. Paparo and C. Serkin, A&A, 362, 245, 2000. Thanks especially to Cas Liber for suggesting this star.

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