SKYLIGHTS

PLEASE NOTE: SKYLIGHTS has moved to http://stars.astro.illinois.edu/skylights.html.

Skylights featured on Astronomy Picture of the Day

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

Photo of the Week. Sunset.

Astronomy news for the two weeks starting Friday, May 11, 2012.

Phone: (217) 333-8789
Prepared by Jim Kaler.

Clear skies and thanks to Skylights'

reader.

Go to STARS for previous stars of the week.	Last week's Skylights is still available.	Access Skylights' Archive and photo gallery.	Advances in Astronomy, 1989-2011.
The Constellations has a linked list with locations and brightest stars.	Constellation Maps show the locations of the constellations.	The 169 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	Enjoy Our Complex Universe: A Human Understanding through Art, with 12 illustrations.	See the The Aurora and the Midnight Sun.
	Take a ride aboard Asteroid 17851 Kaler (1998 JK).	Look for Books about the sky and stars.

Support 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.

NEW! 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.

NEW! Read Walking with Lions in Stellar Stories.

NEW! Advances in Astronomy for 2011.

The next Skylights will appear on Friday, May 25.

The Moon starts off our fortnight in its third quarter, the phase actually reached the afternoon of Saturday, May 12th, with the Moon out of sight. That morning will provide a fine view of the almost perfect phase. Look for it between the classical figures of Aquarius and Capricornus. The remainder of our first week sees the Moon fading in the east as a waning crescent, new Moon finally passed on Sunday the 20th. The Moon then switches into the west, allowing us to admire the waxing crescent. Be sure to look the evening of Tuesday the 22nd, when the thin crescent will appear just down and to the left of Venus. By the following evening, the thickening crescent will be well up and to the left of the planet. Look early! Earlier in our period, the Moon visits with Neptune on Sunday the 13th (the Moon six degrees to the north) and with Uranus on Wednesday the 16th.

This new Moon will produce an annular eclipse on the evening of Sunday the 20th. (In an annular eclipse, the Moon is a bit too far away to entirely cover the Sun, leaving a ring of bright sunlight. Apogee, where the Moon is farthest from Earth, takes place the day before.) Near sunset, the path of annularity goes from northern California through Nevada and Utah into New Mexico. Only those in the far west get to see all of the annular portion. A large part of western and central US and Canada, however, will witness a partial eclipse, but with the Sun only near or during its setting. Do not attempt to look at the Sun directly without a professionally made filter, or use pinhole projection (shining the sunlight through a pinhole in a piece of cardboard or paper onto a second sheet placed behind it).

Back to the planets. Working our way from inside out, Mercury rises in bright morning twilight and cannot really be seen. Venus, on the other hand, is still bright in the west. But you need now to look earlier, as the planet is preparing to swing between us and the Sun and is therefore setting notably earlier day by day, appearing to plunge toward the horizon. At the beginning of our two-week period, it is still setting around 11 PM Daylight Time. But by the end of it, Venus sets before 10 PM with twilight still lighting the sky. Up until now, Venus has been moving steadily to the east against the stars, more or less keeping pace with the Sun. On Tuesday the 15th, however, the planet reverses itself and begins westerly retrograde motion, which will cause our familiar evening companion to be gone by the end of the month.

Mars, however, is still nicely with us. Fading just a bit as Earth pulls away, the red planet is moving noticeably to the east against the background south of the classical figure of Leo to the southeast of Regulus. Now well past the meridian to the south as darkness falls, Mars sets around 2:30 AM or so Daylight Time. Much farther out, Jupiter makes invisible news by passing conjunction with the Sun on Sunday the 13th. As much as anything, though, the night belongs to Saturn. Distant and moving slowly, the planet continues to hang out to the northeast of Spica, transiting the meridian around 10:30 PM shortly after Venus sets. Saturn is then with us the rest of the night until it sets in dawn's light.

May belongs to the Big Dipper, which rides high in the hours before midnight. The asterism, and the constellation to which it belongs, Ursa Major, the Great Bear, is along with Orion among the sky's most beloved figures. Look at the second star in from the end of the Dipper's handle to find bright Mizar with dimmer Alcor next to it, the two making the Arab's Horse and Rider. Then follow the front bowl stars downward to Polaris, which is at the end of the handle of the Little Dipper and closely marks the north celestial pole. Finally, look to the southwest of the Dipper to find three unrelated pairs of stars that make both the Bear's feet and, in a quite different mythology, the Arab's "leaps" of the gazelle.

STAR OF THE WEEK: 72 LEO (72 Leonis). To the north of a line that extends between Leo's "Sickle" and the Lion's triangular hindquarters (that end in Denebola, the Tail), lies a fainter triangle of stars known by their Flamsteed numbers, 60 Leo at the southern apex (just three degrees west of Zosma, Delta Leo), 54 Leo (five degrees more or less north of 60, and our star, 72 Leonis (three degrees north of Delta). While 60 and 54 are made of white class A stars (54 an especially nice visual double), 72 decided to be a bit different, the fifth magnitude (but at 4.63, not by much) class M (M3) giant casting an orange-red glow, one that is rather obvious in binoculars. Somewhat on the luminous side, the star is actually classed in between the ordinary giants and the "bright giants," which are a stepping stone to the fainter supergiants. The farthest of the trio, 72 Leo lies at a rather whopping distance of 960 light years, far enough for a significant uncertainty of 180 light years, which gives us quite a range in parameters. In spite of its distance, the star's angular separation from the Milky Way results in no dimming by interstellar dust. An absolute visual magnitude of -2.77 (the magnitude the star would have at a distance of 32.6 light years) places it where the spectrum said it should be, between the giants and bright giants. A cool temperature of 3734 Kelvin plus distance give the star a rather high luminosity (which includes a lot of invisible infrared radiation) of 4570 times that of the Sun (with a rather large uncertainty that reflects that in distance), which again is consistent with its fairly bright giant class. Luminosity and temperature then conspire to give a radius of 162 times that of the Sun, or 0.75 Astronomical Units, which in our planetary system would take the star to the orbit of Venus. In spite of its distance, the size of 72 Leo's disk is accessible with sophisticated interferometers (which make use of the interfering properties of light waves that come from different parts of the star), from which we find a radius 179 times that of the Sun, just 10 percent larger, which given the various uncertainties, is fine agreement. 72 Leo is a fairly massive star, theory applied to luminosity and temperature giving us around 6 solar masses. It's hard to say just what evolutionary state the star is actually in. It could be brightening with a dead helium core or it could be on its "second ascent" among the giants after fusing its helium to carbon and oxygen, resulting in a dead carbon/oxygen core. Whatever the case, 72 Leo is slightly and irregularly variable (hence the variable star name FN Leo) with a range of magnitudes between 4.51 and 4.64, not surprising given the size of the star and its resulting instability. And whatever the case, 72 (once when new, a hot class B dwarf) will eventually slough off its outer hydrogen shell and finally die as a fairly massive white dwarf approaching a solar mass.

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.