A Selection of Southern Doubles, Bright Doubles and Variable Stars

   

U Sge / U Sagittae (19188+1937) has one of the deepest eclipses than most of the known EA eclipsing binaries, which is easily observable even in binoculars. It can be found 2.8^oW of the centre of the bright star group called Collinder 399, that contains the three brightest stars of 4, 5 and 7 Vulpeculae. It is also some 0.7^oNE from the yellowish 5th magnitude star of 9 Vulpeculae. U Sge's brightness varies between 6.58v and 9.1v over 3.380626 days (3 days 09 h 08.1m). Its deepest or primary eclipse drops significantly because of the presence of a much cooler companion. The primary eclipse lasts about 3.5 hours with the magnitude changing twenty minutes either side of main eclipse falling near the rate of about 0.15 magnitudes per minute! Primary total eclipse lasts about 1.6 hours, before the brightness again begins to increase. Visual observations when compared before the primary eclipse may notice some colour change in the star. U Sge's secondary 'occultation' eclipse is relatively minor spanning only 0.1 magnitudes.
U Sge from the latest analysed light curve and spectra reveals two stars with respective masses of 7.4 M and 2.2 M - a mass ratio of about 3:1. Previous calculations had yielded smaller values for each star. The literature, Ie. Plavic (1973) continues to state that U Sge is an unusual system, because it defies many known assumptions associated with eclipsing binaries. The uncertain masses give us the warning of the problems associated with all photometric systems.
We calculate actual distance between the two stars as 14 million kilometres, with each star having the respective diameters of 5 million and 7 million kilometres. The primary has the luminosity 140 L with the secondary only 11 L. Unusually, there is some evidence from the small fluctuations seen in the light curve, that the primary has a cloud of gas and dust orbiting in a circumstellar disk.
Spectral class for the primary sub-giant is B8 III while the secondary, which originally was stated as sub-giant G2 IV-III , though the recent literature tends towards perhaps a 'K' class star. Temperatures of the two stars are estimated to be 10 240K and 4 720K, respectively. Distance is presently estimated as 720 ly.
The well separated pair Σ2504 lies some 0.7^o away, as stated in Bumham's Celestial Handbook Vol. 3, pg.1528. Both make good comparison stars, having respective magnitudes of 7.0 and 8.7 and both about the U Sge's variability. Separation of the pair is 8.9"arcsec, and this has only slightly increased since F.G. Struve discovery in 1830.

Alpha (α) Cygni / Deneb / Arided / 50 Cyg / H 73 (20414+4517) appears as the brightest star in the 'Northern Cross' of Cygnus has Deneb ends its tail. Rising to a maximum height of only about 10^o from the Sydney (Australia) skyline. The ancient natural philosopher Eratosthenes termed the constellation as a hen, though it was Aratos that first named it as a “quickly flying swan”. Deneb's name is a corruption of the Arabic name Al Dhanab, which literally means the “hen's tail”. The German observer, Rosemund in the 15th Century named it Uropygium - meaning the "Pope's nose" or "Parson's nose" part of the roast chicken eaten at the dinner table - depending if your religion was either Catholic or Protestant! This shimmering pure white star is the 19th brightest in the nighttime sky - having the visual magnitude of 1.25. In true brightness it is extremely luminous. The liberate energy is more than a million times that of the Sun and is only outshone by a few stars - second only to Rigel. For many years it distance was given as 1 500ly., though the latest data suggests it maybe as close as 1 000ly.
Deneb is also the wide pair H 73 which was discovered by William Herschel. Magnitudes are 1.2 and 11.7, respectfully, while the separation is a wide 75.4"arcsec along PA 106^o. Both stars have remained relatively fixed since discovery, and it is likely that this star is merely just in the field.
Deneb is the brightest example of the ACYG's and this is followed by others examples like Mu Normae, Mu Sagittarii, Epsilon (2) Orionis and Omicron (2) Centauri. All variables in the category are true monsters, that some have referred not as supergiants but "hypergiants". (All are luminosity class Ia.) These variable supergiants have spectral types between B0.5 and A3, and all display characteristic emission spectra. They are among the brightest stars in the galaxy, whose absolute magnitudes commonly exceed -7.5! Some fifty-two are known (1998), with another nine being suspected ACYG's. Primary periods for the very poorly observed light curves range between 8 days and 46 days, though the longest suspected period may be last about 180 days.
This first-magnitude star has the mass somwhere between 12 M to 14 M - being about half the value of the earliest estimates from the 1960's. Like most heavyweights, the general stellar parameters are equally mind-boggling. Its diameter seventy times the diameter of the Sun (14 R), equivalent to 97 million kilometres and is slightly smaller than the distance between Venus and the Sun! Deneb's effective temperature is 9 160K, while the absolute magnitude is -8, equivalent to a candle-power several million times more luminous than our Sun! The spectral class is A2Iae, whose "supergiant" luminosity class was first decided by Keenan and Morgan in 1951. Closest in kinship is Orion's first-magnitude blue Rigel, which is slightly less luminous but higher surface temperature. The spectral class of Rigel being B8I.

“One of the more intriguing results [Hayashi and Cameron (1962). that during the late stages of helium-burning the star moved rapidly [moves] across the Hertzsprung gap, changing from early type A to M in some 20 000 years, averaging [a loss of] 0.3K per year. It is entirely possible the examination of spectrograms... over a long time, evolutionary changes could be detected...”

“Perhaps the most attractive hypothesis is that, like the Sun, Deneb has a definite cycle of activity with the period of at least one hundred years and, more probably, several hundred years. To differentiate between these... will require many more years' of observations .... [the] B-V colour changes at a rate of 0.3K. It is entirely possible produce detectable changes in ten or twenty years.”

Positions are given as ; Ie. (02583-4018). This is Right Ascension 02h 58.3m and Declination -40^o 18', following the WDS Conventions.