RA : 16h to 18h
SOUTHERN VARIABLES : Z TrA
BRIGHT STARS : Antares : α Scorpii
OTHER DOUBLES and VARIABLES : ZZ Her / u Her / 68 Her / OΣ328 ,
Iota (ι) TrA /
Δ201 / SAO 253555 (16280-6404) is an
interesting pair of unequal brightness, at 5.3 and 10.3,
respectfully. The object was discovered by Dunlop in 1826
and later measured by Sir John Herschel in late August
1836. On the 13th July 1871, H.C.Russell saw the colour as
light yellow and bluish but the primary to me is a distinct
yellow colour, reflected by the F3-F4 IV spectral type. In
the last 160 years the separation has gradually decreased
from 24.6" to merely 10.2"arc sec, while the PA has
decreased from 25o to about
12o. (Last measured in May 1997.)
There is some uncertainty on whether the pair is actually
physically connected. Some consider that the proper motions
favour that the stars are in chance alignment. If it is a
binary, the period is likely to be very long, which is
unlikely to be found out even in the next century!
The 'A' component is a known spectroscopic binary that has small component velocities around ~6 kms-1. In the 1920's, the period was determined to be 39.8880 days, first published by Jones in 1928. Orbital information has deduced an average separation of about 20 million kilometres. Little data about the Aa system has been made since Jones's observations.
Alpha (α) Scorpii /
Antares / GNT 1 / 21 Sco / PPM265579 / SAO
184415 / HD18478 (16294-2626) is the brightest star is the
constellation of Scorpius, whose name likely originates to
literally mean the "rival to mars". Undeniably, this is
entirely appropriate because the star orange-red colour
certainly matches that of Mars. Antares' position is
central to the constellation, as it marks the very heart of
Hipparcos (HIP 80763) measures find the parallax as 5.40±1.68 mas. and if this is true, then the distance is 185±63pc, or about 640ly. The Tycho values for the parallax are exactly twice the Hipparcos ones, set at 11.80±0.014mas. If this distance is true, the value is closer to 84pc. or 276 ly.
Antares spectrum is commonly listed as either M0 or M1, though some have listed it as M1.5 Ia-Ib. The companion is a hot blue star whose spectrum is B2.5V whose B-V value is 1.865±0.014. Compared to our Sun, Antares is huge, that current estimates place somewhere in the order of the size of Jupiter's orbit - some 1.7 billion kilometres across! Antares is nowhere near the biggest star known - though it still remains a true colossus. It is also listed as an LC-type variable star, fluctuating anywhere between 0.88 and 1.16 magnitudes, which is just detectable to the naked-eye by comparing it to the surrounding bright stars. LC- variable are notorious for being totally unpredictable changes in brightness. Their light curves overtime seem more random than in following any predictable pattern.
The Notes in the WDS 2003 say;
The primary is variable: irregular or semi-regular with a trace of a 5.8 yr period. This duo has respective magnitude of 1.0 and 5.4, respectively and has a composite spectrum of M1.5Iab-Ib+B4Ve.
Antares is also the double star GNT 1, whose companion is seen by many observers as green or greenish in colour. As there are no true green stars, as such, the colour seen is thought to be due to contrast effects, and not that the star is truly green. Although an early orbit has been calculated, its accuracy can be add can be best described as indeterminate. Since first observed in 1847, the separation has slowly diminished from 3.3" to 2.8" arc seconds, while the PA has increased by merely 5o. An aperture as small as 7.5cm can easily see the companion. The companion is so bright, that some observers have suggested that Antares is best split in broad daylight, suggested mainly to reduce the intensity of the light.
Z TrA (16547-6512) is a Mira variable 1.5oS of Iota TrA The magnitude range is between 9.8 and maybe as low as 12.8 (JD 2429710) the period is 150.55 days. The spectrum is M3e - M5 IIe. A 10cm will show the deep red colour of the star, and under good conditions observers could see the entire 3.0 magnitude cycle. Having a 150-day period, the whole cycle can be seen during a single year. As the entire constellation is circumpolar, the star can be monitored over the entire year if you have a clear southern horizon. The period of observation is worst between November and March, as it is close to the horizon. The field is devoid of bright stars.
ZZ Her / u Herculis / 68
Her / OΣ328 (17173+3306) is the second brightest
northern eclipsing binary - second only to the eclipsing
binary Beta Persei or Algol. ZZ Her is easily located by
the naked eye some 5.5oS of the
bright orange 3.16 magnitude star Pi (π) Herculis (67
Her) and varies between 4.6p and 5.3p magnitude over 2.0510
days. Burnham's Celestial Handbook Vol.2 pg.956 listed the
star as 68 Her, but gives the magnitude range as slightly
brighter 4.7v to 5.4v. Spectral classes have been found to
be B1.5 Vp and B5 III, respectively.
Both stars are separated by 10.2 million kilometres, each having the individual diameters of 3.2 and 3.1 million kilometres. By luminosity, the primary is 1 159 times the solar output, while the secondary is 295 times. Both are B-spectral type stars, with respective temperatures of 15 780 and 11 450K. Masses of the stars are 7.2 M and 2.9 M.
There is some indication that the secondary has exceeded its Roche Lobe in the past, and has already bestowed most of its mass to the primary. This is a clear example of the so-called Algol paradox, with the primary being the smaller in mass although it is the most evolved component in the system. The shape of the secondary is still likely a teardrop, with the primary is only slightly distorted by the gravitational field. Distance is 720 ly.
u Herculis is also the double star OΣ328 / STT 328, whose 10th magnitude blue-white to greenish companion is separated by 4.4"arcsec along PA 59o. Little has changed in the positions since Otto Struve's discovery in 1843. It is likely that the stars are physically associated with the close eclipsing binary.
Southern Astronomical Delights © Andrew James 2002 Sydney, Australia