Monthly Sky Notes November 2006 (1 of 3)

The Summer Triangle
Comet C/2006 M (SWAN)
Leonids
Double Stars
Deep Sky Objects

The Summer Triangle

chart created in Stellarium www.stellarium.org

Vega - Alpha Lyrae One of the most famed stars of the sky, Vega is the luminary of the dim but exquisite constellation Lyra, the Lyre, which represents the harp of the great mythical musician Orpheus. Its name derives from an Arabic phrase that means "the swooping eagle." Vega is one of three brilliant stars that divide the northern heavens into thirds, the others Arcturus and Capella, and with Altair and Deneb forms the great Summer Triangle, lying at its northwestern apex. At magnitude zero, it is the sky's fifth brightest star, falling just behind Arcturus and just ahead of Capella. It is also one of the closer stars to the Earth, lying just 25 light years away. Though its proximity helps make it bright in our skies, it is also inherently luminous, 54 times brighter than our Sun. Vega is a classic white main sequence star, like the Sun quietly running off the nuclear fusion of hydrogen deep in its core, with a surface temperature of 9600 degrees Kelvin. Its color and apparent brightness made it the basic standard against which the apparent magnitudes of all other stars are ultimately compared. Because it is 2.5 times as massive as the Sun, it uses its internal fuel much faster and will burn out in less than a billion years, less than 10 percent of the solar lifetime. Vega was one of the first stars to be discovered with a large luminous infrared-radiating halo that suggests a circumstellar cloud of warm dust. Since Vega seems to be rotating with its pole directed toward the Earth, the dust cloud probably represents a face-on disk that may not be unlike the disk surrounding the Sun and that contains the planets. Several other stars similar to Vega (Fomalhaut, Denebola, Merak, for example) possess similar disks, and astronomers speculate that they may indicate the existence of planetary systems, though no planets have ever been detected. Even if they exist, it seems unlikely that life would have developed to any degree because of the short lifetimes of these hot stars.

Deneb - Alpha Cygni One of the truly great stars of our Galaxy, Deneb serves a three-fold role among the constellations. Its very name tells the first. "Deneb" is from an Arabic word meaning "tail," as this first magnitude (1.25mv) star, the 19th brightest as it appears in our sky, represents the tail of Cygnus the Swan, a classical figure seen flying perpetually to the south along the route of the Milky Way. As the constellation's luminary, the star is also Alpha Cygni. The reversal of Cygnus makes the asterism of the Northern Cross, with Deneb now at the top, the cross seen rising on its side in early northern summer, standing upright in the west in early northern winter evenings. Deneb also makes the western apex of the famed Summer Triangle, which also incorporates Vega and Altair. All three of these white class A stars (Deneb a spectral class A2 supergiant) have similar surface temperatures, Vega, at 9500 Kelvin, the warmest, Deneb radiating at 8400 Kelvin. Though Vega and Altair are really quite luminous, they are first magnitude primarily because they are close to us, averaging only 25 light years away. Deneb, on the other hand, may be as far as 2600 light years. Based on that distance, its awesome luminosity of 160,000 Suns makes it about the intrinsically brightest star of its kind (that is, in its temperature or spectral class) in the entire Galaxy. If placed at the distance of Vega, Deneb would shine as bright as a well- developed crescent Moon. Deneb is a true supergiant, its diameter, calculated from its temperature and luminosity, is 200 times that of the Sun. Direct measurement of its tiny angular diameter (a mere 0.002 seconds of arc) gives a very similar value of 180 solar. If it were placed at the center of our Solar System, Deneb would extend to the orbit of the Earth. While far from the largest star in the Galaxy, Deneb is one of the biggest of its kind. It is evolving and has stopped fusing hydrogen in its core. Just what it is doing, however, we do not know. Having begun its life as a star of some 25 solar masses, its fate is almost certainly to explode sometime within the next couple of million years. The star is constant in its light, but its spectrum, its light as seen when stretched into a rainbow, is slightly variable. Blowing from its surface is a wind that causes the star to lose mass at a rate of 0.8 millionths of a solar mass per year, a hundred thousand times the flow rate from the Sun. Deneb is among the most magnificent stars you can see with the unaided eye.

Altair - Alpha Aquilae Aquila the Eagle, is also the southern anchor of the famed Summer Triangle, which it makes with Vega and Deneb. The Arabic name "Altair," reflective of the constellation itself, comes from a phrase meaning "the flying eagle." Though the constellation does not look much like its name, Altair itself is flanked by a pair of stars (the Beta and Gamma stars Alshain and Tarazed) that really do remind the sky- gazer of a bird with outstretched wings. The trio of stars has in fact been taken for an airplane with wing lights slowly flying across the sky. Though three of the stars of the Summer Triangle are all white in color and hotter than the Sun, all are also individuals. A class A hydrogen-fusing dwarf with a temperature of 7550 degrees Kelvin, Altair is the coolest of the three (with Vega and Deneb warmer at 9500K and 8400K respectively). Altair is also the least luminous. From its distance of 16.8 light years, we find it to be 10.6 times brighter than the Sun, as opposed to 50 times for Vega and an astounding 160,000 or so for much more distant Deneb. Like the Sun and Vega, Altair is "on the main sequence" of stars, fusing hydrogen into helium in its core, its mass falling between 1.7 and 1.8 solar. Though seemingly ordinary, the star is not without its own striking characteristics. It is moving across the sky against the background of distant stars more quickly than most, and will displace itself by as much as a degree in only 5000 years. Altair is also a very rapid rotator. Its equatorial spin speed, while certainly not a record, is still an astonishing 210 kilometers per second (and may be greater, since the axial tilt is not known), as compared with the Sun's 2 kilometers per second. With a radius 1.8 times that of the Sun (confirmed by direct angular diameter measures, which give 1.7), the star has a rotation period of at most only 10 hours, as opposed to nearly a month for our ponderously spinning Sun. Altair's high speed has even caused it to become distorted. Observation with a sophisticated interferometer, from which the angular size of the star is measured, reveals a 14% oblateness. Even with its high rotational velocity, however, Altair is far from its rotational breakup speed of 450 kilometers per second. Altair has also recently been identified as a subtle "Delta Scuti" variable, the brightest in the sky, the star flickering by a few thousandths of a magnitude with nine different periods that range from 50 minutes to 9 hours.

Over a period of 18 hours on November 5, 1999, Altair varied by a few thousandths of a magnitude (millimagnitudes, mmag, on the vertical axis), showing it to be Delta Scuti star that varies with multiple periods. The observations were made in the visual part of the spectrum by the star camera on the sensitive Wide Field Infrared Explorer satellite. The horizontal axis is the Julian Day number, a running count of dates beginning on January 1, 4713 BC of the Julian calendar. (From an article in the February 1, 2005 Astrophysical Journal by D. L. Buzasi et al.)

Read Vega, Deneb and Altair's entries in Wikipedia.

Comet C/2006 M4 ( SWAN )

Discovery Date
June 20, 2006
Magnitude
12 mag
Discoverer
R. D. Matson (Irvine, CA), M. Mattiazzo (Adelaide, S. Australia)



Orbital Elements
Epoch 2006 Sept. 22.0 TT = JDT 2454000.5
T 2006 Sept. 28.72820 TT MPC
q 0.7830401 (2000.0) P Q
z -0.0003384 Peri. 62.59160 -0.22213602 +0.84758993
+/-0.0000717 Node 148.72683 +0.15020218 -0.45862084
e 1.0002650 Incl. 111.82213 +0.96337682 +0.26694238
From 75 observations 2006 July 12-Sept. 29, mean residual 0".60.







Magnitudes Graph
m1 = 7.2 + 5 log d + 12.0 log r

http://www.aerith.net/comet/catalog/2006M4/2006M4.html

LEONIDS (maximum November 19; 4:45 UT)  
Moon: New 20th (no interference)

Best viewing window:  European observers should have the best view, during the predawn hours of Sunday, November 19.  North American observers (East Coast strongly favored) should start watching just before midnight on Saturday evening, November 18, and continue watching through the morning hours of November 19.

Recommended for: Anyone interested in meteor observing; just keep in mind uncertainties in predictions and don't expect to see any Leonids if you're watching before the radiant rises (11pm-midnight for most mid-northern locations).

Some years ago, when the teams of David Asher and Robert McNaught were making their groundbreaking predictions of Leonid storms and outbursts for the years 1999-2002, they also noted a possible outburst for 2006.  This November, we'll find out whether that prediction comes true.  On November 19, the Earth is due to pass through a trail of debris left by the Leonids' parent comet on one of its previous returns.  A sharp peak of perhaps 100 Leonids/hour is expected, although there is a bit of uncertainty.  If it occurs very near the predicted time of 04:45 UT, Europe and Western Africa will see the display during the favored morning hours.  The East Coast of North America will see a bit of the display (maybe up to 25 per hour) as earthgrazing Leonids starting when the radiant rises at around 11pm.  Depending on just how short and sharp this peak is, most of North America may be out of luck. 

The Leonids are very fast meteors.  Most of the meteors seen during this outburst are expected to be faint, so dark skies will be very helpful.  Even if you miss the November 19 outburst, the shower is active at a low "background" level for about a week from November 14-21.  Expect about 10 meteors per hour on the mornings of November 17 and 18 due to this activity (best during the couple of hours just before the beginning of local morning twilight).  A roughly equal number of sporadic meteors should be visible, along with a few late Taurids.

ref: http://skytour.homestead.com/met2006.html#anchor_12