SELECTED SOUTHERN DOUBLES and VARIABLES
R.A. 16 Hours

ι TrA/ Δ201 / Iota TrA (16280-6404) is an interesting pair of unequal brightness, at 5.3 and 10.3, respectively. Δ203 was discovered by Dunlop in 1826 and then 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-F4IV spectral type. In the last 160 years the separation has gradually decreased from 24.6 arcsec to merely 10.2 arcsec, while the PA has decreased from 25° to about 12°. (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 shows 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.

α Sco / GNT 1 / Alpha Scorpii / Antares / 21 Sco / PPM265579 / SAO 184415 (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 the scorpion.

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 M1.5Ia-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-variables 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-year 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 5°. An aperture as small as 7.5cm can easily see the companion. Its companion is so bright, that some observers have suggested that Antares is best split in broad daylight as to mainly to reduce the overall intensity of the light.

Δ207 (16444-4224) is a faintish yellow pair in far southern Scorpius some 1.9° WSW (PA 251°) from the brilliant open star cluster, NGC 6231 (16542-4150) or more directly 1.9°W (PA267°) from the mid-point (16543-4222) between 4.7 and 3.6 magnitude ζ¹ and ζ² Sco / Zeta (1,2) Scorpii. (The latter being the principle star of the Scorpion where the deadly tail bends almost at ninety degrees (90°) from the creature’s back. Placed in a field of many much fainter stars this 8.5v and 9.2v (9.13V & 9.75V) magnitude duo is actually among the slightly fainter pair that were discovered by Dunlop. Position angle has remained relatively fixed at 185° to 186° but the separation continues to increase - almost doubling from Dunlop’s estimated 6 arcsec to the recent properly measured result of 11.4 arcsec. This pair is visible in 7.5cm and might benefit slightly with larger apertures. This appear a nice pretty pair using low magnification in 20cm and impressive at moderate powers. Information of the G5III/IV pair’s attachment is rather uncertain, especially as both stars only appear in the Tycho catalogue - combined with very poor proper motion data. However it is possible that the could be attached but this is counteracted by only the seemingly slow widening of the components. Unless the orbit is edgewise from our perspective on Earth, the probability of this being the case certainly diminishes. An easy to find pair and attractive in small apertures.

α TrA / Alpha TrA / Atria / Alpha Trianguli Australis / HIP 82273 / SAO 253700 / PPM 362330/ HD 150798 (16487-6902) is placed as the brightest star in Triangulum Austrinus being the apex of the three stars making the Southern Triangle. It is α TrA that leads the procession of the bright stars in the most western portion of the Southern Milky Way. Atria is followed by Alpha and Beta Centauri, the constellations of Circinus, Musca and Crux, the Coal Sack and Acrux, then onto Southern Carina and finally finishing with the second brightest star in the sky, Canopus.

Having nothing to do with the same word that means courtyard or quadrangle, its orginal name comes from the clever combination of its Greek name and constellation. Its general acceptance likely derives from Elijah H. Burrett in 1833 when he placed the star in his popular astronomical work and star atlas, “Geography of the Heavens”. Yet it is possible that someone else had given this name prior to Burrett, but I could not find any other reference in the literature to support this. I presume it could have been named by some unknown American observer. Oddly, the name Atria does not feature in Richard Allen’s detailed 1900 classic “Star Names: Their Lore and Meaning”, and for me, he either left it out or just forgot to mention it. In both Burrett and in “Burnham’s Celestial Handbook ”(p.1906) is stated that the modern name is “Abraham’s Star”., “first suggested by the Dutch writer P. Caesius in the 17th Century”. Both say the proper name is based on the title of the book “The Three Patriarchs”, that refer to the three main stars of the Southern Triangle; α TrA, β TrA and γ TrA. Another feature is Allen’s odd descriptions of the origins of Triangulum Australe (See Allen; p.417) reads more like an author desperate for information to write about - making it strange why he would leave Atria out.

A better name, in my opinion, would perhaps be Atriaus, so as to not be confused with the other northern constellation, Triangulum. Even the shorter name of Atri also could not be used because is the Hindu name given for the star Delta (δ) Ursa Majoris.

From Sydney’s -34°S latitude, Atria itself is circumpolar but remains completely invisible to the majority of northern observers. Ranked as the 33rd brightest star of the naked-eye stars, this orange stellar gem is 1.88v / 1.91V / 2.95p magnitude - only 0.08 magnitudes behind the next brightest star β Car / Miaplacidus. B-V is 1.44 that reflects the K2 III spectral classification but some catalogues gives the addition luminosity sub-class of K2IIIa-IIIb - making it lie on the Giant Branch of the H-R Diagram.. Furthermore by using the given visual magnitudes finds an Mv -0.7. From the radial velocity, α TrA is presently travelling towards us at -3.6 kms^-1. Distance from the parallax of 7.85±0.63mas is 127±10pc. or 415±34 ly., but a derived spectroscopic distance made in 1981 (by using the spectral type and luminosity class) finds twice this far at 239.4pc., with the fainter Mv of +0.2. A diameter from this distance is about 20.8 million kilometres, a surface temperature of 4 900K and an overall mass of about 7.0 suns. (NOTE: Among other parts of Burnham’s text, he says of the star “...a yellow giant [K2III to K4III] some 80 light years distance”. Like many of the southern descriptions, this general information is now quite out of date.

COMMENT: I have been very interested in why Burrett was so keen to give the southern stars names to those that did not have them. Like Richard Allen, Burrett was a devote southern Baptist. No doubt, they were greatly moved by the words appearing in The Bible (Isaiah; 40, 26);

“Lift up your eyes on high; and see created these?
He who brings out their host by number,
Called them by name;
By the greatness of his might,
And because he is strong in power not one is missing.”

As God had presumably called these stars by name, and they were interested in astronomy, then their motivations were likely to glorify the bright stars by name. Those stars without any names would have seemed to them like an anathema that needed to be corrected.

Z TrA (16547-6512) is a Mira variable 1.5°S 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 II.e. 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.

Δ209 (16482-3653) lies some 1.4°NW (PA 325°) from the naked-eye blue stars of 3rd mag. μ¹ Sco / Mu (1) Scorpii (16519-3803) and 3½ mag. μ² Sco / Mu (2) Scorpii (16523-3801) OR 2.6° (PA 188°) from orange 2.3 magnitude ε Sco / Epsilon Scorpii / 26 Sco (16502-3418). Δ209 is a dual white pair is easy in small apertures for these 7.5v and 8.4v (7.51 & 8.37V) magnitude stars. When Dunlop discovered these stars in 1826, the distance apart was estimated at 14 arcsec and the PA was 150°. More recent data has shown the position angle continues to decrease to 139° while the separation has presumably increased from 14 to 23.6 arcsec. Some seventeen (17) measures have shown continued widening. Available proper motion quoted in the WDS84 and WDS03 are different between the PPM and Hipparcos results (I.e. PPM as A: +002 -012 B: +025 and -012 against HIP; A: -006 000 and B:+010 +007.) Either way this likely shows this as an optical pair. Spectral class for the stars is A5 IV/V.