SKYLIGHTS

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Hazy setting Sun

Photo of the Week.. The Sun sets peacefully through horizon haze.


Astronomy news for the two weeks starting Friday, June 16, 2017.

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.

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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, 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.

NEW! From the Sun to the Stars: the OLLI Lectures, which provides a linked, illustrated introduction to astronomy.


IT'S HERE!, 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.



NOTE: Sometimes the old ways really ARE the best ways. The "bullet" form of Skylights that I've been using since the beginning of the year did not work very well for me, and actually proved more difficult to put together. I also thought it was boring. Consequently, I'm returning to the narrative form and now have a script available to send via email. Thanks for your patience. It's good to be back, and thanks to those who wrote.

The next Skylights will appear July 7, 2017.

The last fortnight was the "bright run" with the Moon between first and third quarters lighting up the sky and hiding the stars. Though the full Moon is only a half-millionth as bright as the Sun, that is plenty enough light to make a lot of observing impossible. This fortnight, however, we encounter the flipside, the "dark run" between third quarter on Saturday the 17th near dawn and first quarter the morning of Friday, June 30, these two phases now centered on new Moon, which is passed on Friday the 23rd. In between third quarter and new, on Tuesday the 20th, the waning crescent Moon will make a fine sight with Venus to the left, while by the next morning the Moon will have slipped another 13 degrees along its path and will be down and to the left of the brilliant planet, which rises just as the first glimmer of dawn begins to light the eastern sky. Shifting your gaze to the west to watch the waxing crescent will yield a memorable sight when it is seen just south of Regulus in Leo. The Moon passesperigee, where it is closest to the Earth in its eccentric orbit, on Tuesday the 27.

Jupiter, the brightest body in the evening sky (the planet in Virgo northwest of Spica), sets about an hour after Saturn transits the meridian between Sagittarius and Scorpius. After another hour and a half, Venus rises, its luster putting all the other planets to shame. Going deeper into the Planetary System, Mercury goes through superior conjunction with the Sun (on the other side of it)on Wednesday the 21st.

The big event, though, involves Earth, when at 11:24 PM CDT on Tuesday the 20th, (12:24 EDT Wednesday the 21st), the Sun passes the Summer Solstice at the most northerly part of the ecliptic, 23.4 degrees north of the equator. On this longest day (and shortest night), the Sun will rise as far northeast (and set as far northwest) as possible, and pass overhead at the Tropic of Cancer (thus defining the Tropic's latitude). North of the Tropic, the Sun will also cross the meridian as far north as possible, giving us the maximum heating rate and, of course, the first day of astronomical Summer. Traditionally the Solstice lies in the constellation Gemini, but around 1930, the 26,000 wobble (precession) of the Earth's axis brought it across the border with Taurus established in 1930.

This time of year brings us a lovely parade of constellations high across the sky, beginning with kite-shaped Bootes (the Bear Driver) with bright Arcturus, the semi-circle of Corona Borealis (the Northern Crown), Hercules, Lyra (the Harp) with Vega, and finally Cygnus (the Swan) with Deneb. The constellations of autumn then begin to appear on the scene.

STAR OF THE WEEK: TAU CrB (Tau Coronae Borealis). Well north of the semi-circle of stars that defines Corona Borealis, the northern Crown (its southern cognate, Corona Australis, the Southern Crown, falling south of Sagittarius) lies fifth magnitude (but at 4.76 almost fourth) Tau Coronae Borealis, a class K (K1) giant-subgiant that's fairly close, 117 (give or take 5) light years away. Why bother with yet another orange K giant when there are so many others in the sky.? Part of the reason is just that, that that there ARE so many others (Arcturus, Aldebaran, and a vast slew of others) and the range of the class in luminosity it huge, so it takes a lot of stars to cover that space. Tau CrB is among the brighter ones of the K giants. With a temperature of 4860 Kelvin, it radiates at an impressive rate of 3840 times that of the Sun, which gives the star a radius 87.6 solar radii, 0.41 Astronomical Units, which is just over the mean orbital radius of the planet Mercury. If placed at the Sun, Tau would appear some 50 degrees across, around 1000 times the solar angular diameter. The theory of stellar structure and evolution gives Tau CrB a mass seven times that of the Sun with an age of about 350 million years. The star is most likely fusing its core helium into carbon and oxygen in preparation for becoming a carbon/oxygen white dwarf of about one solar mass similar to sirius.html Sirius B. First though, it must generate a powerful wind that will sweep away the residual exterior hydrogen envelope and present us with an ephemeral planetary nebula with the old stellar core at its center. Tau CrB has a couple other features to recommend it. It was at one time listed as a spectra.html#double">spectroscopic binary, but that seems not to have been confirmed. There is, however, a thirteenth magnitude visible companion now 2.2 seconds of arc away, which from its brightness would be a low-mass red dwarf. From a minimum physical separation of 79 Astronomical Units, it would take at least 250 years to make a complete circuit of Tau CrB itself. But since the separation changed by over a second of arc in 69 years, it's more likely that the little one is just in the line of sight and not physically connected. More interesting, the star is speeding along at some 60 kilometers per second relative to the Sun, some four times the usual, suggesting it is a visitor to our part of the Galaxy. . Nevertheless, its metal content appears to be quite normal.



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