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

Rain

Photo of the Week. Rainy sky.


Astronomy news for the two weeks starting Friday, June 19, 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 July 3, 2015.

We begin with the Moon in its waxing crescent phase as it heads toward first quarter the morning of Wednesday, June 24. By moonrise that evening our satellite will be just past the quarter as it begins its waxing gibbous phase. Full Moon is finally passed the evening of Wednesday, July 1, after which we see a bit of the waning gibbous. Look in the evening for the growing crescent to pass south of Venus and Jupiter the evenings of Friday the 19th and Saturday the 20th, the three making a lovely sight as the two planets draw together. The evening of Sunday the 28th the fat gibbous will glide just to the north of Saturn and (farther down) the classic figure of Scorpius. By the next night the Moon will be well east of Saturn and northeast of Antares. The Moon goes through apogee, where it is farthest from Earth (5.5 percent more distant than average) on Tuesday the 23rd.

All eyes are now on the two brightest of planets, Jupiter and Venus, as they approach each other early in our session, then pass only a mere third of a degree apart on Tuesday the 30th, formal conjunction taking place the next day. But it's not a sight of the moment, as the two planets, Venus by far the brighter (just short of maximum brilliance), stay near each other for a good week or so. To the east of them shines the Sickle of Leo that ends in bright Regulus. Look in early western twilight for this spectacular visitation, as the pair sets around 11 PM Daylight Time. Get ready too for UFO reports and invasions by space aliens.

Well to the west of the pair, Saturn remains northwest of Antares in Scorpius, transiting the meridian to the south around the end of twilight. In the morning sky Mercury makes a poor appearance in mid-dawn, going through greatest western elongation from the Sun on Wednesday the 24th when it is also in conjunction with, north of, Aldebaran in Taurus.

Our week of wonders is not yet done. On Saturday the 21st at 11:38 AM CDT (12:38 PM EDT, 10:38 AM MDT, 9:38 AM PDT), the Sun crosses the Summer Solstice in classical Gemini (technically just across the border in Taurus), marking the beginning of astronomical summer in the northern hemisphere (winter in the southern). On that day the Sun will rise and set as far to the northeast and northwest as possible, will cross the meridian as high as possible (thus rendering maximum heating), and we will have our longest day and shortest night. Our fortnight just misses the Earth going through aphelion, where it is farthest from the Sun (by 1.7 percent), thus clearly showing that the seasons have nothing to do with solar distance.

As evening falls, look to the northeast to see the only musical instrument in the sky, Lyra, the Harp or Lyre, brilliantly lit by the star Vega, just barely the second brightest star of the sky after Arcturus to the southwest of it. The body of the Lyre is formed mostly by a charming parallelogram of fainter stars to the southeast of Vega. Farther to the northeast rises Deneb, the luminary of Cygnus, the celestial Swan.

STAR OF THE WEEK: T LYR (T Lyrae). When one comes across a star like T Lyrae, it begs to be included in spite of being below naked-eye visibility. In Lyra a couple degrees south-southwest of Vega and at best just short of seventh magnitude, T Lyrae is a deep red carbon star that is quite visible with binoculars. Among the rarer gems of the sky, T Lyr has been referred to as "the Jewel in the Harp." It's nearly five magnitudes (a factor of 100) brighter at visual wavelengths than it is in the blue part of the spectrum. Carbon stars are highly evolved red giants with dead carbon cores in which carbon, made fresh from helium fusion, has been brought upward by convection from deep in the stellar interior to the surface. All carbon stars are cool, their spectra rich in carbon molecules (absorption by which in part causes the color) as well as in heavier elements made by neutron capture. There is a distinct evolutionary sequence going from giants in which oxygen dominates carbon (class M, like Mira) to those in which the elements are in roughly equal proportion (class S: Chi Cygni) to those in which carbon dominates (class C). Among the well-known carbon stars are 19 Piscium, Y Canum Venaticorum, and Hind's Crimson Star (R Leporis). None is visually bright. All vary to some degree, while some (like R Lep) are hugely-varying Mira (long-period) variables. Carbon stars go way back to their discovery in the middle of the nineteenth century by Father Angelo Secchi, one of the founders of spectral classification. Carbon stars are the source of much if not most of the carbon in the Universe.

T Lyr The red carbon star T Lyrae glows at the center of the picture, its color contrasting strongly with those of other stars. North is to the left. The bright star to the lower left is Vega, Alpha Lyrae. EpsilonLyrae (slightly elongated horizontally because of its duplicity) is to the lower left of Vega, Zeta Lyrae to the lower right. Photo by Jim Kaler.

More specifically, T Lyr is a spectrally advanced (C6.5 )carbon-star giant and irregular variable (hence the Roman-letter variable star name) that changes between roughly magnitudes 7.5 and 9.8. The distance is not well known. Parallax measures by the Hipparcos satellite place it at a distance of 2400 light years, but the uncertainties are such that it could likely fall anywhere between 1730 to 3600 light years away. The stellar "surfaces" of these giant stars are highly diffuse, and the temperature depends on where in the spectrum and during what part of the variability cycle one observes. Values for T Lyr range from 2400 to 3200 Kelvin, while the luminosity (most of which is in the infrared) is, from two sources, around 15,000 Suns. That combined with an average temperature of 2600 Kelvin yields a radius of 600 solar radii, 2.8 Astronomical Units, almost twice the size of the orbit of Mars. If the star is closer to the upper distance limit, the radius could be considerably larger. The original mass is not known, but is probably at least triple that of the Sun. Mass loss, now at a rate near a millionth of a solar mass per year (100 million times that of the Sun), has however reduced the mass, indeed will strip the star of its outer envelope, first creating a surrounding carbon-rich shroud, a "planetary nebula" ionized by the exposed core, then a fairly massive white dwarf. Moreover, T Lyrae is a "J star" (from a carbon-star sequence invented by Philip Keenan, 1908-2000) in which the abundance of the stable isotope carbon 13 is oddly enriched by a large factor to be close to that of normal carbon 12 for reasons not understood, adding to the mystery of the red star of the Lyre. (Thanks to Rachelle Leger, who suggested this star.)


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