SKYLIGHTS

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. Cygnus, the Swan, departs for warmer climes.

Astronomy news for the week starting Friday, November 14, 2008.

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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.	Find out what happened in astronomy at Astronomy Updates.
The Constellations has a linked list with locations and brightest stars.	Constellation Maps show the locations of the constellations.	The 151 Brightest Stars lists through magnitude 2.90.	For more on stars and constellations, visit Stellar Stories.
Tour the Milky Way as seen from the northern hemisphere.	Watch a total eclipse of the Moon and an annular eclipse of the Sun.	Moon Light presents photos of the Moon.	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.
Go from Day Into Night, with 83 linked 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.

NEW! Heavens Above: Stars, Constellations, and the Sky from Recorded Books.

View Jim Kaler's "From Pluto to Planets: What Other Stars are Telling us" at the World AstroCast 2008 archive, brought to you by the Astronomy Section of the Northampton Natural History Society, England.

NEW! Read Tea With the Scorpion in Stellar Stories.

Skylights was featured on the Earth Science Picture of the Day for October 2, 2008.

After passing its full phase last week, we pick up the Moon going through its waning gibbous phase as it heads toward third quarter, the phase reached during the day on Wednesday, November 19, after which we can watch a little bit of the waning crescent.

The morning of Monday the 17th finds the Moon in a lovely setting in northern Gemini just to the south of Castor and Pollux. Then two days later it takes a bead on Leo and Saturn. The morning of Wednesday the 19th, just a hair before it hits third quarter, the Moon can be found to the east of Regulus and to the left of Leo's "Sickle," while the following morning it will have flipped to the other side of the star. As our week draws to a close, on the morning of Friday the 21st, the waning crescent will be just to the southwest of Saturn, the two making a fine pairing.

The sky is currently bookended by Saturn and the Venus/Jupiter duo. Look to the southwest in early evening in twilight for a magnificent view of brilliant Venus, which, though low above the horizon, can hardly be missed. Up and to the left is bright Jupiter, the two bracketing Sagittarius, Venus in the western part of the constellation, Jupiter in the east and still up and to the left of the Little Milk Dipper. Moving easterly against the stars faster than Jupiter, Venus is closing in the giant planet. Watch as they appear to get closer and closer. The juxtaposition, is however, only in the line of sight, as when they cross paths at the end of November, Jupiter will be 5.7 Astronomical Units away from us (the AU the average distance between Earth and Sun), Venus just 1.0 AU distant, Jupiter nearly 6 times farther away. We lose Venus first, the planet setting around 7 PM, shortly after the end of twilight. Jupiter follows around 8:30 PM. Then we have only Uranus and Neptune to kick around until Saturn lofts itself above the eastern horizon at 1:30 or so.

The week features one of the most famed of meteor showers, the Leonids, which peak the morning of Monday the 17th. But don't expect much. We are well past the maximum (which we hit in a 33 year cycle), and the Moon will take out much of the leavings.

Once the Moon is out of the way, you might try admiring the Great Square of Pegasus, a large box of stars that crosses the meridian fairly high to the south at around 8 PM. Below it, find the Circlet of Pisces, and to the right the "Y"-shaped Water Jar of Aquarius. Pegasus is home to a 6th magnitude star called HR 8799, around which three planets have been directly observed, each of which is considerably more massive than Jupiter. Fomalhaut, in Piscis Austrinus, has another one buried in its dusty disk. Astronomers have been waiting for decades for just such discoveries. You can see Fomalhaut far to the south in early evening.

STAR OF THE WEEK. NU AQUARII. Some stars are made well-known by the cultures that found them important, others by their physical natures, still others by position. Here we have a little bit of the first, not much of the second, and a whopping lot of the third. Nu, Mu, and Epsilon Aquarii in southwestrn Aquarius north of Capricornus are part of an Arabic trio called Albali, "The Swallower" (of unknown origin). The name eventually was applied to Epsilon alone, leaving Mu and Nu with their Greek letters. As a just-barely-fifth magnitude (4.51) class G (G8) giant star, Nu Aquarii pretty normal. A well-determined temperature of 4980 Kelvin (to account for infrared radiation) and a distance of 164 light years leads to a luminosity 42 times that of the Sun and a modest radius (for a giant) of 8.7 solar. A slow projected equatorial rotation speed of 2.8 kilometers per second then leads to a rotation period of under 155 days, which is not very restrictive. Application of theory yields a mass of 2.0 times that of the Sun, and shows it to be a 1.25 billion-year-old helium-fusing "clump star," so-called because it is one of many of its kind. And it seems to be all alone, with no evidence of any companion. It's Nu Aquarii's location that got the star into the history books, as it was given as a guide by William Herschel (the discoverer of Uranus) to the first of the "planetary nebulae," an object we now know as the Saturn Nebula, or simply NGC (for New General Catalogue) 7009. (The Ring Nebula in Lyra was already known, but was not put into that category until later.) Planetary nebulae, having nothing to do with planets, are expanding gas shells surrounding very hot stars. The shells are the ejected envelopes of advanced giant stars like Mira, and the resulting central stars are the ex-nuclear-fusing cores that once powered them and that are now turning into white dwarfs. Hear Herschel's own words in the Philosophical Transactions of 1785, from an article entitled "On the Construction of the Heavens:"

"I shall conclude this paper with an account of a few heavenly bodies, that from their singular appearance leave me almost in doubt of where to class them. The first precedes Nu Aquarii 5'.4 minutes in time (5.4 minutes, 1.25 degrees), and is I' (minute of arc) more north ... The planetary appearance of the first two is so remarkable, that we can hardly suppose them to be nebulae; their light is so uniform, as well as vivid, the diameters so small and well defined, as to make it almost improbable that they should belong to that species of bodies."

NGC 7009 displays a pair of jug-handle-like "ansae" that give it the name "The Saturn Nebula." On the left is a drawing made early in the twentieth century from a series of early phtographs, the gaseous nebula perhaps appearing as its discoverer William Herschel may have seen it. On the right is a Hubble Space Telescope image made 80 years later, showing the vast improvement in astronomical imagery as well as the immense complexity of the nebula. The hot (90,000 Kelvin), blue, 13th magnitude star at the center is the old nuclear-burning core of what was once an extended giant star, while the surrounding nebula is the inner part of the star's lost envelope that has been structured by the remaining star's hot wind. The distance is not known. If at 2000 light years, the nebula is 3/4 of a light year long. The 13th magnitude central star seems faint only because of its distance and because the vast majority of its radiation comes out as energetic ultraviolet light. In reality, the star is thousands of times more luminous than the Sun. It will eventually fade to become one of the many white dwarfs that dot the cosmos. Left: Studies of the Nebulae, Lick Observarory Publications, vol XIII, 1918. Right: B. Balick, J. Alexander (U. Wash.), A. Hajian (USNO), Y. Terzian (Cornell U.), M/ Perinotto (U. Florence), P. Patriarchy (Arcetry Obs.), and NASA.

Thus was born the oft-confusing term, by which Herschel merely meant "disk-like." After all, who had a better right than the discoverer of the first planet since ancient times? After tens of thousands of years, the cloud surrounding the blue central star will dissipate, leaving behind the hot, cooling white dwarf -- a ball of carbon and oxygen about the size of Earth. Find it by setting a telescope on Nu Aquarii. Wait a bit over five minutes, and lo, the Saturn Nebula drifts into view, and you might experience the mystery that Herschel himself must have felt.

For more on the sky, visit the Earth and Sky Skywatching and General Science pages.

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.