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Photo of the Week.Summer storm departing.


Astronomy news for the week starting Friday, July 11, 2014.

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.

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 Harp in Stellar Stories.



We start our week quite nicely on Friday, July 11, with the Moon rising just short of its full phase, which is reached the morning of Saturday the 12th about the time of Moonset in North America. Early risers will see it as a glowing globe in the southwest as it pursues its track just north of the ecliptic. Since the Sun has recently passed the Summer Solstice in classical Gemini, the full Moon will fall just to the east of the Winter Solstice in Sagittarius, and be a pretty sight indeed. From there, the Moon wanes in its gibbous phase, which quits at third quarter on Friday the 18th shortly before Moonrise. There are no significant planetary passages unless you want to count those with Neptune on Tuesday, July 15, and Uranus on the morning of Friday the 18th, the Moon over just over a degree north of the distant planet, which orbits twice as far from the Sun as Saturn. The Moon passes perigee , where it is closest to the Earth, the morning of Sunday the 13th.

It's a real treat to have two bright planets so close in the evening sky. Though fading as the Earth pulls away from it, at magnitude zero, Mars still dominates the early southwestern sky. Just look for the brightest starlike object you can see. In rapid direct easterly motion against the background stars, reddish Mars has been closing in on Spica in Virgo, and finally passes just 1.4 degrees north of the blue-white star on Saturday the 12th, the pair making a fine color contrast. Not far to the east, Saturn sits in the pans of Libra, the Scales, to the northeast of Zubenelgenubi, the "Southern Claw" of Scorpius. Mars finally sets shortly after midnight Daylight time, Saturn about an hour later. The morning sky hosts the two inner planets. Venus rises right at the start of dawn, just before 3 AM Daylight Time. Mercury then comes up half an hour later, the little planet reaching greatest elongation to the west of the Sun (by 21 degrees) on Saturday the 12th.

The upside-down Big Dipper is now starting to fall into the northwest. It's the tail and hindquarters of Ursa Major, the Great Bear. The snout is out in front of the Dipper, while south and west of it are three pairs of unrelated stars that represent three of the Bear's feet. The ancient Arabs called the trio "the leaps of the gazelle," which have nothing to do with a bear at all. The population of the sky all depends on local culture and the state of imagination.

STAR OF THE WEEK: GMB 1830 UMA (Groombridge 1830 Ursae Majoris, HR 4550 in the Bright Star Catalogue), which lies in southern Ursa Major roughly 17 degrees south of the bowl of the Big Dipper and eight or so degrees southwest of Chara (Beta Canum Venaticorum) in Canes Venatici. It's named after Steven Groombridge (1755-1832), who compiled a catalogue of accurate positions of circumpolar stars as seen from England. Just barely sixth magnitude (6.45, not quite seventh), this remarkable class G (G8) hydrogen-fusing dwarf ranks third in the list of high "proper motions" (angular speeds across the line of sight). Moving at a rate of 7.06 seconds of arc per year against the distant stellar background, it's behind only Barnard's Star in Ophiuchus (10.4 seconds) and Kapteyn's Star in Pictor (8.67). The rapid motion was discovered by the German astronomer F. W. Argelander (1799-1875), father one of the great star catalogues, the Bonner Durchmusterung (Bonn Survey), "Argelander's Star" also BD38 2285 (the star falling between 38 and 39 degrees north of the celestial equator). One might expect that, like Barnard's, it must be quite nearby to be zipping along so fast, but at a distance of 29.6 light years (good to 0.1) it doesn't even make the list of the nearest 100 stars. Instead, it really IS moving fast, at an amazing 305 kilometers per second across the line of sight relative to the Sun. When combined with a speed toward us (the "radial velocity") of 98 km/s, we find the star to be clipping along at 320 km/s, some 20 times normal, making it among the fastest stars known. Equally remarkable is the chemical composition, which is very low in "metals" (to an astronomer everything but hydrogen and helium), the ratio of iron to hydrogen a mere four or five percent what it is in the Sun. The combination of speed and composition immediately tell that the star is a visitor from the Galaxy's extended and sparsely populated halo, which surrounds the Galactic disk that makes our Milky Way. Disk stars like the Sun have roughly circular orbits about the Galactic center, which gives them low relative velocities. Halo stars on the other hand are in highly elliptical orbits similar to those of the great globular clusters. Like its mates, Gmb 1830 is "falling through" the disk on its own eccentric path while the Sun moves past it in a different direction. Stars like this one help give us a look at how the Galaxy is structured.

With a temperature of 4985 Kelvin, from which we get the amount of infrared radiation, Gmb 1830 shines with only 23 percent the luminosity of the Sun, which gives it a radius of 0.64 times solar. Direct measure of angular size via interferometry gives 0.66 solar radii. More detailed studies that include a mass of 0.61 Suns yield similar results. The low metal content comes from age that, like the other stars of the halo, is some 12-13 billion years. Back then, there had not yet been enough time for supernovae and other evolving stars to build up the metal content of stellar birthplaces with their explosions and winds. When plotted on the HR diagram of absolute magnitude vs. spectral class, the low metal content makes Gmb 1830 fall to the left of, or below, ordinary dwarfs, turning it into a "subdwarf" ("sd"). (Do not confuse such with hot subdwarfs, which are highly evolved stars preparing to become white dwarfs.) Gmb 1830 is the closest of its kind. In 1936, it blasted a "superflare" that brightened it for 18 minutes by up to 0.6 magnitudes (70 percent) in blue light, much like the one that that lit 5 Serpentis and some others. By comparison, even the brightest of solar flares would not be visible to an observer at stellar distances. Given Gmb 1830's age, it's surprising that it can generate so much magnetic energy. The flare was so powerful that it apparently gave rise to a "false companion." Given the luminosity, the "habitable zone" for a planet around Gmb 1830 might be in range of 0.38 to 0.83 Astronomical units. But low metallicity may preclude planets and in any case, given the flares, you sure would not want to live there.


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