27 July 2017 by Brian Jones
Corona Australis (the southern crown)
is a small broken circlet of stars found to the east of Scorpius (the scorpion)
and to the north of the faint constellation Telescopium (the telescope). As its
name suggests, Corona Australis is the southern counterpart of Corona Borealis
(the northern crown) and was one of the 48 constellations drawn up by the Greek
astronomer Ptolemy during the 2nd century.
Although there appears to be no
legend associated with Corona Australis, the Greeks saw Corona Australis as a
wreath rather than a crown, and its distinctive shape has led to other cultures
likening it to a host of different objects, including an ostrich nest, a turtle
and even a tent. The stars forming this group are not particularly prominent, but
its shape is distinctive in spite of this.
This chart shows the stars and
constellations on view during evenings in or around July and August, and you
can use it to locate Corona Australis by following a line due south from the
brilliant star Vega in the tiny constellation Lyra. The whole of Lyra can be
seen from latitudes anywhere north of 42°S, and ramblers who may be holidaying
in central South America, South Africa or Australia will be able to see Vega
low down in the northern part of the sky.
The stars Alpha, Delta1,2,
Epsilon and Zeta (all in Telescopium) are included on both charts for
reference. The whole of Corona Australis is observable from the central United
States, southern Europe and from latitudes anywhere south of these regions.
The two brightest stars in Corona
Australis have almost identical magnitudes. The brighter of the two is Alfecca Meridiana which, at
magnitude 4.10, shines from a distance of 125 light years. The first part of
this name is derived from the Arabic for break, alluding to the fact that the
constellation as a whole takes the form of a broken circle of stars. The second
part of the name is loosely derived from the Latin meridiem, meaning south, perhaps as a contrast
to the similarly named Alphecca in this constellation’s northern counterpart
Marginally dimmer than Alfecca
Meridiana is Beta, a magnitude 4.11 orange giant - you can find out more about star colours on Starlight Nights.
Beta lies at a distance of 510 light years, setting off on its journey towards
us at around the time Henry VIII came to the throne!
Just to the south of Beta is
Delta, another orange giant star, whose light set off towards us around 180
years ago, roughly at the time Queen Victoria came to the throne. Much further
away from us is Theta, a yellow giant star shining from a distance of around
550 light years.
Lying between Delta and Theta are
the two white stars Eta1 and Eta2 which form a wide optical
double star that can just be resolved with the naked eye. Another optical
double is Kappa, which lies a little to the north of Theta and is made up of
two stars of magnitudes 5.65 and 6.30 that can be resolved in a small
telescope. As with all optical doubles, the stars forming these two pairs
actually lie at different distances from the Earth and only appear to be close
to each other when we see them in the sky. This is particularly true of the two
stars forming the Theta pair. The fainter of the two lies almost 500 light
years away, although the brighter star shines from a distance of around 1,700
light years. This means that the light we are seeing from this star today
actually set off towards us while the Romans were still occupying Britain! It
is a sobering thought that, although these distances are huge by everyday
standards, the stars in question are regarded as being among our closest
neighbours in space.
Magazine of the Ramblers