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.
“Southern Astronomical Delights”
© (2009) |
LAST UPDATED
10 Mar 2009 |
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