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

Rainbow

Photo of the Week.. Double rainbow.


Astronomy news for the two weeks starting Friday, March 10, 2017.

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, 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.



Please Note

Skylights has been produced in various forms since 1985, for 31 years, and even before that as an annual bulletin. All the information is easily available elsewhere via various devices and apps, and it's time to simplify and bring part of Skylights to an end. We will continue to list lunar phases and other significant events on the website, but by bullet only, every two weeks. The Star of the Week will continue as before, as will the Photo of the Week. The telephone and e-mailing services will be dropped, as will most external links. Thanks to everyone who participated. It's been a joy, and it's not over yet.
Jim Kaler

The next Skylights will appear Friday, March 24. All times below are Daylight Savings Times.
Primary source: The Astronomical Almanac.

STAR OF THE WEEK

EZ CMA (EZ Canum Venaticorum=WR 6)

Among the strangest of stars are those discovered in the nineteenth century by the French astronomers Charles Wolf and Georges Rayet. Though roughly in the O-star temperature range (and higher), Wolf-Rayet (WR) stars fall outside of the normal classification scheme because of their unusual broad spectrum lines. They are characterized by wide emission lines that imply powerful high-speed winds (the Doppler effect widening the lines). There are two major kinds: those with strong emissions from helium and highly ionized nitrogen (the WN stars) and those whose spectra show helium but strong and wide lines of various ionization levels of carbon (the WC stars). Hydrogen is absent. (There is also a rarer variety characterized by oxygen.) Somewhat over 400 Wolf-Rayet stars are known, most of them residing toward the galactic center in the Milky Way and in our satellite galaxies, the Magellanic Clouds. They are truly rare. Their number seems high only because they are so incredibly luminous that we can see them over great distances. Only three are visible to the naked eye, and the light of the brightest, Gamma-2 Velorum (in Vela, the Sails), is entangled with that of a companion O star. Among the dozen or so brighter than magnitude 7.0, EZ Canis Majoris is the only one that carries a classical variable-star name. (It's also catalogued as WR 6.) Though at magnitude 6.95, EZ CMa is still among the "EZ-est" to find, falling just a few minutes of arc north of fourth magnitude Omicron-1 CMa, near the middle of Canis Major. W-R stars are born with masses above 40, maybe 60, Suns. As they evolve, they lose most or all of their their hydrogen envelopes through winds. When the hydrogen layers are gone, we begin to see the nitrogen that was been created through nuclear fusion reactions, and a WN star is born. Further mass loss eliminates the N-rich layer to expose the carbon-rich depths, and we see a WC star. By then the masses have been whittled down to 20 solar masses or less. The distance to EZ CMa is highly uncertain. The Hipparcos satellite gives 4500 light years, but the uncertainty is almost as big as the value itself. An earlier distance of 6900 light years is generally adopted from the distances of the stars surrounding it. Because of the strong winds, temperatures are also uncertain. EZ CMa is classed as a mid-temperature (relative to the set) WN4 star of 89,000 Kelvin with a radius of just 2.65 times that of the Sun, a luminosity of 400,000 Suns, and a current mass of 19 Suns. There is no trace of hydrogen. The mighty wind is still blowing at a rate of five hundred-thousandths of a solar mass per year, a billion times the flow rate of the solar wind. Wind speeds approach 2000 kilometers per second. The wind has created a surrounding "ring nebula" (a high-mass version of a planetary nebula) within which shock waves create X-rays. Eventually EZ should turn into a WC star and then blow up as a supernova. The variation in brightness is small, ranging from near zero to a couple tenths of a magnitude over a steady 3.61-day period. The existence of a companion to EX CMa is vigorously argued, however. But not the star's fate, as it will someday collapse and then tear itself apart. (Summary of properties from D.P.Huenemoerder, Astrophysical Journal 815:29 2015 December 10, 2015 and A. Flores, Rev. Mex A&A, 47, 261, 2011. Thanks to Margarita McElroy, who suggested this star.)


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