RA : 18h to 21h
SOUTHERN DOUBLES :
SOUTHERN VARIABLES :
BRIGHT STARS : DENEB : α Cyg
OTHER DOUBLES and VARIABLES: U Sge / U Sagittae
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 2.8oW 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
0.7oNE 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 0.7o 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.
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 10o
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 106o. 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.
Deneb varies between 1.21 and 1.29 magnitudes over numerous
semi-periods averaging some 11.7 days. Some have even
considered this star as the prototype example of the ACYG
Class. Remarkably, other changes have been observed with
Deneb since its variability was discovered in 1887. This
includes a slow and continuous rise in the brightness of the
Hydrogen-Epsilon (Hε) line when compared to the
nearby Calcium II line some 0.16nm away. This particular
observation also appears in "Burnham's Celestial
Handbook"; Vol.2 , p.757, and much of the discussion on
this page by Burnham is taken from the paper by Liller, M.H.
and Liller, W., "Spectral Changes in Deneb,
1887-1964."; AJ.,142, p.1028-1032 (1964) It
seems that this trend is continuing, and a more recent
detailed discussion on this can be found in Albayak B."A
Spectral Analysis of Deneb (A2 Iab)"; PASP,
111, 917 (1999)) Deneb, like V370 Car, maybe
illuminating the majority of the nearby nebulae. In Deneb's
case, this is the grand NGC 7000 "North American
Nebula" (20580:+4429:), IC 5067-70 (20510:+4400:)
some 3.5oE, and the IC 5068
Our view of supergiants is pictured as just the scaled-up version of ordinary stars, but this view is in someway flawed. In fact, all supergiants are quite unstable and nearly all of them display either variations in brightness, radial velocity or colour. As early as 1951, Buscombe, and later Groth in 1960, established the existence of significant atmospheric motions, which Huang and Struve (1960) interpreted as solar like prominences appearing from Deneb's photosphere. To explain these variations remains uncertain, but more recent explanations for the pulsation mechanism are likely to be strong shock waves traversing across the stellar photosphere. What we maybe observing are "starquakes" rumbling across the surface.
This picture is quite different to the solar photosphere, which is more or less a surface more akin to a saucepan of boiling water, with the solar wind radiating outward from the surface. Lucy, L.B. (Astrophy. J., 206, 499 (1976)) was first to suggest the supergiant's variability could originate from a system of "multiple non-radial pulsations" One prediction was that at least sixteen separate components were hidden in Deneb's spectrum, causing the surface to bob up and down in amplitudes anywhere between the velocity of ±0.29 and ±1.02 kms-1 - something akin to a cork bobbing up and down in the "ocean" of the star's photosphere. Furthermore, the transverse waves are travelling about 15 kms-1 (54 000 kph) across Deneb's surface, faster in fact than the local velocity of sound of 13.7kms-1. So violent are the shock waves, that significant mass loss is being induced away from the star.
Astronomers biggest woes are that all ACYG's are uniformly quite distant, with Deneb likely being the closest at 490pc. (1 600 ly). Recent studies of GP Vel and Mu Sgr have been investigated because they are eclipsing binaries, allowing improved estimations of mass, but overall, the available data remains poor.
The most extraordinary thing with these stars is that we maybe seeing real stellar evolution in action.
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...More exactly;
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.Other ACYG variables are V369 Car, which is some 45'NW from the second, V370 Car that is 20'E from another bright open cluster NGC 3293. Together these three star are in fact highly luminous and are interesting in that they are associated with prominent nebula or open clusters.
Southern Astronomical Delights © Andrew James 2002 Sydney, Australia