Skylights featured on Astronomy Picture of the Day

Scout Report Selection Webivore Selection SpaceCareers Selection

Skylights featured nine times on Earth Science Picture of the Day: 1 2 3 4 5 6 7 8 9

Lunar corona Lunar corona

Photo(s) of the Week.. At left is an especially lovely and wide "lunar corona," a diffraction ring around a nearly-full moon caused by the mutual interference of light waves as they pass through high clouds made of small water droplets or ice crystals. (You see the same thing looking at a light through fogged eyeglasses.) At right is a longer exposure that over-exposes the Moon but reveals a rare second-order ring around the first one. Both were easily visible with the naked eye. A third ring was elusive.

Astronomy news for the two weeks starting Friday, June 3, 2016.

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.

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 Queen in Stellar Stories.

The next skylights will appear June 17.

With the Moon pretty much out of the way during the first half of the fortnight, the night sky darkens, allowing the stars to shine through. We begin appropriately with the beginning Moon, the new Moon, which takes places on Saturday, June 4. With a good horizon, by the evening of Sunday the 5th you should see the ultrathin waxing crescent in western twilight. By the following night, the growing crescent should be obvious as the Moon heads towards its first quarter the night of Saturday the 11th shortly after moonset in North America. That evening the Moon will fall to the east of bright Jupiter (while appearing to the west of it the previous night). The Moon then fattens through its waxing gibbous phase, not hitting full until next week, on June 20. The night of Thursday the 16th, our companion will shine northeast of Mars and Saturn. The fortnight begins with the Moon at perigee (closest to the Earth). Our companion then moves away from us until it hits apogee on Wednesday the 15th, where it is around 11 percent farther than it was at perigee.

As presaged above, a glorious trio of planets awaits the most casual of observers. Jupiter, to the southeast of Regulus in Leo, leads the parade. Riding down the western sky, the giant planet does not set until around midnight. We then switch our sight to the southeast, where in early evening we find reddish Mars (almost as bright as Jupiter) and, following, dimmer (though still quite bright) Saturn. To the south of the pair shines the red supergiant Antares in Scorpius, the trio crossing the sky and not setting in the southwest until dawn. Inside Earth's orbit, Mercury is barely visible in eastern morning twilight, passing greatest western elongation (24 degrees west of the Sun) on Sunday the 5th, while Venus is completely invisible as it goes through superior conjunction with the Sun (on the other side of the Sun) the following day. It will become visible in western twilight next autumn. To complete the planetary act, Neptune, in northern Aquarius near the border with Pisces (and visible in binoculars to the east of Lambda Aquarii), begins retrograde motion on Tuesday the 14th.

Mars and Jupiter together are a guide to an amazing concentration of hot massive stars in Scorpius (to the south of the planets), Lupus (farther south), and Centaurus (to the southwest). Here we find loose associations and sub-associations that have given us generations of supernovae and have cleared the local interstellar region of much of the usual gas and dust, leaving us within a "local bubble." Well to the west of Scorpius, on the other side of Libra, slithers the tail of Hydra, the Water Serpent.

STAR OF THE WEEK: CHI-2 HYA (Chi-2 Hydrae). Given that sixth magnitude (5.71) Chi-2 Hydrae is only 8.4 minutes of arc east of Chi-1 (such numbers generally running from west to east), we might expect them to be a naked-eye binary. But even though Chi-2 is catalogued as Chi-1 Hydrae D, as if they actually composed a true double, the two Chi's are not at all connected except by accidental alignment. Chi-1 is 141 light years away, while Chi-2, at 692 light years (give or take 40), is nearly five times farther (which tells us that Chi-2 is by far the more luminous: more about that below). What the two stars do oddly share is confusion regarding their memberships. Chi-1 has a putative third star called Chi-1 C that does not exist (though it lives on through Chi-2 being called Chi-1 D). But Chi-2's problem goes deeper. It's an eclipsing binary made of two class B stars (a B8 giant- subgiant coupled with a B8.5 dwarf) in which one star periodically gets in front of the other, which drops combined brightness, in Chi-2's case by about 0.3 magnitudes in the visual band (making the eclipse detectable by eye through the telescope, especially since it's so close to Chi-1, which can be used as a reference).

Chi-2 Hydrae In mutual orbit, the two stars of Chi-2 Hydrae partially eclipse each other every 1.134 days. At left, the dimmer secondary component covers part of the primary, while at right the primary covers part of the secondary. The actual orbital period is thus twice the interval between the eclipses, or 2.267 days. The light curve (magnitude vs. time as compared with a nearby non-variable star) is presented in three colors or wavelength bands: standard blue (B), shorter-wave blue (b), and ultraviolet (v). If the eclipses were total, the minima would have flat bottoms as one star disappears behind the other. Instead, the recovery is instantaneous. The variation outside of eclipse is the result of mutual tidal distortion, the orbiting stars thus continuously presenting different angular areas to the observer. (Light curves from H. Mauder and U. Koehler in Astronomy and Astrophysics, 1, 147, 1969.)

The problem is with the orbital period, the time it takes the stars to go around their common center of mass. Three periods are given in various sources: 1.134 days, 2.267 days, and 16.54 days. The last makes it look as there is a third star going around a double with a shorter period. But like Chi-1 Hydrae C, the longer period is a chimera; the responsible star does not exist. The shortest period listed, 1.134 days, is exactly half that of 2.267 days, and is clearly the interval between primary and secondary eclipses (star A getting in front of star B to star B getting in front of A), understandable since the two eclipses are difficult to distinguish from each other. So once again we are left with a simple binary with a period of 2.267 days. Like Algol, the eclipses are partial, from our point of view neither star fully covering the other. Moreover, the binary varies even outside of eclipse as a result of considerable tidally-induced oblateness. Solution from all the data, including the light curve (variation with time), gives respective temperatures for the two of 11,750 and 11,100 Kelvin, luminosities of 344 and 66 Suns, radii of 4.48 and 2.21 solar radii, and masses of 3.61 and 2.63 Suns. The separation between the two is only 13.4 solar radii, less than four times the size of the larger star, making tidal distortion inevitable. With an age of about 700 million years, the secondary is still fusing hydrogen in its core. The more massive primary however is ready to give it up if it has not done so already. As the primary evolves, it will expand toward the secondary and eventually pass mass toward it. As the secondary expands with its new-found mass, it will eventually send matter back toward the current primary, and the two will come into contact looking something like a three-dimensional figure-8. Eventually they may merge into one star with serious loss of mass, though the theory is not yet good enough to tell us exactly what will happen. (Stellar parameters from Y.Meng and Q.S.Zhang in the Astrophysical Journal, 787:127, 2014.)

Valid HTML 4.0! Copyright © James B. Kaler, all rights reserved. The written contents and (unless otherwise specified) the photograph are the property of the author and may not be reproduced in whole or in part without the author's consent except in fair use for educational purposes.