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


Photo of the Week.. A cloudbank moves off to the east at sunset.

Astronomy news for the two weeks starting Friday, June 2, 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.


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 June 16, 2017.

'Tis the week of the Moon, or the "bright run" in astrospeak. We begin with the Moon just past its first quarter phase, then sail through waxing gibbous to full Moon on Friday, June 9, just after Moonset, and after the waning gibbous finish with the third quarter the morning of Saturday the 17th with the Moon high in the sky. The night of Saturday the 3rd, the Moon can be seen just a couple degrees north of Spica well to the south. Look well to the south of the two to find the box of stars that make the constellation Corvus (the Crow or Raven), whose top two stars point leftward back at the Spica. They may be a bit difficult to find with the Moon so bright and close. The following evening finds the rising Moon to the left of the bright planet, with Jupiter, Spica, and Corvus all in a line. The evening of Friday the 9th, the Moon will rise a few degrees northeast of Saturn, with the star Antares to the right (the star passed the previous night).

The Moon's orbit is tilted by about 5 degrees to the plane of the ecliptic (the apparent path of the Sun), and is now oriented such that the "nodes," where it crosses the ecliptic, are this month roughly marked by the quarter Moons. Full Moon then appears a few degrees north of the solar path, and thus farther from Antares than average. The Moon is at apogee, where it is farthest from Earth by about five percent, on Thursday the 8th, near full phase, which will have the effect of diminishing the highs and lows of the tides at the coasts.

Jupiter (in Virgo) rules the mid-evening, the planet crossing the meridian to the south around sunset (so it will be a bit west of it as the sky darkens), while Venus rules the morning, rising about 4 AM just as twilight begins. the giant planet ceases retrograde motion on Friday the 9th, and will now resume normal easterly motion against the stellar background. Saturn is between, transiting just before Jupiter sets. The ringed planet lies north of the line between the classic figures of Sagittarius and Scorpius but within the boundaries of southern Ophiuchus, the Serpent Bearer, the constellation sometimes referred to as the "13th constellation of the Zodiac." Saturn is in opposition to the Sun on Thursday the 15th, when it rises at susnset, crosses the meridian at midnight, and sets at dawn. It's almost as far south as it can get, 22 degrees below the celestial equator. Mars is still in the evening sky but is essentially invisible as it sets in bright twilight. Mercury plays the same role with dawn in the morning sky. At about the same time Saturn goes through opposition, Neptune begins retrograde motion, westerly against the stars.

This month provides a superb view of the southern sky, with this fortnight marred by the brightness of the near-full Moon. Ah, but wait 'till the next one when the sky turns dark and the Milky Way glows, coming out of Cygnus to the northeast, then cascading through Aquila with bright Altair (the southern anchor of the Summer Triangle, Scutum (the Shield), Sagittarius, and Scorpius, until it disappears below the southern horizon.

STAR OF THE WEEK: VV CRV (VV Corvi). Modestly glowing in northeastern Corvus (the Crow), fifth magnitude (5.08) VV Corvi's name is as much a tangle as its physical nature. Through the telescope, we see a visual binary made of similar mid-F (probably F5) sixth magnitude dwarfs (magnitudes 5.84, 5.82) separated by 5.1 seconds of arc. The magnitudes are uncertain, but the leader is clearly somewhat the fainter of the two. They have no singular common name, neither Greek letter nor Flamsteed number. But they are distinctive enough for each to carry its own number from the Bright Star Catalogue, HR 4821 and HR 4822. They have in fact been called the "A-B pair." There are insufficient data on motion to compute an orbit, but at a distance of 278 (give or take around 9) light years, they are estimated to go around each other every 4500 years, which (from the masses below) implies a separation of some 800 Astronomical Units. Each of these stars is a spectroscopic binary, with respective periods of 1.46 days (for "A") and 44.1 days. The first was later found by photometry to be an "alias" for the true period of 3.1445 days: a false period that fit the then-available data. Photometry also revealed the fainter western star "A" to be a totally-eclipsing binary with a maximum variation of 0.15 magnitudes. Since the star is variable it was assigned the variable-star name VV Corvi, which has confusingly at times been applied to the whole system.
VV Crv The light curve of the eclipsing binary VV Corvi (HR 4821) is scaled in magnitudes in yellow light on the left and in blue light on the right. The "phase" on the lower axis is the fraction of time covered over the full orbital period of 3.144536 days. Each star eclipses the other once over the full period, yielding two minima of a couple tenths of a magnitude. With velocity data we can determine a wide range of stellar properties. (Light curve from a paper by F.C.Fekel, G.W.Henry, and J.R.Sowell in the Astrophysical Journal, vol. 146, December 2013.)
With brightness and velocity variation known we can calculate most all of the parameters of the western eclipsing star's (HR 4821's) parameters, including masses (1.978 and 1.513 Suns) and radii (3.375 and 1.650 solar times solar). They are separated by only 0.064 AU(with an 8.5 percent eccentricity) or about 14 solar radii, values not much bigger than the stars themselves but still "detached," one not feeding mass on to the other. with projected equatorial rotation velocities of 81 and 24 kilometers per second, the primary components spins in 2.1 days (less than orbital synchronicity) the other in 3.5 days (more than synchronicity). Given the period of 44.1 days the companion to the eastern star should orbit at a distance of roughly 0.35 AU. So we have a double-double of nearly identical stars, a bit like Castor, Mizar, or Epsilon Lyrae except that one pair eclipses. But wait, there's more. Nearly a minute of arc (59.5") away is a 10th magnitude (10.3) neighbor that was long thought to be "optical," that is, just in the line of sight. Apparently, though, it isn't. It's now believed to belong to the system. If so, it is roughly a solar-type star at least 5000 AU away from the inner quartet and would take around 150,000 years to make a full orbit. Moreover, there is just the barest suggestion in the data of yet another star with a period of 9 years, making poor mis-named VV Corvi (which seemingly can be applied to either the actual eclipsing binary or to the whole system) sextuple star. And who knows what red dwarfs may lurk within. Probably not many, as gravitational interactions would have thrown them out. It's rather hard to figure even where a 9-year-period member would fit in. Ignoring the small companions, the temperatures of the two bright F stars are very similar at 6570 Kelvin, while the luminosities are 18.2 and 147 times that of the Sun. The brighter component is an evolving subgiant, while the lesser star is still on the hydrogen-fusing main sequence. The VV Crv system is speeding along at 48 kilometers per second relative to the Sun, about three times normal. Its motion may be related to the metal content (relative to hydrogen), which appears to be almost double that of the Sun, that is, the star is something of an interloper into our part of the Galaxy. (Much of this discussion was taken from a paper by F.C.Fekel, G.W.Henry, and J.R.Sowell in the Astrophysical Journal, vol. 146, December 2013, with thanks.)

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