Ara the Altar lies in a dense part of the Milky Way immediately south of the hook of stars that make up the tail of Scorpius the Scorpion. This means that it is a summer constellation. It is invisible from Britain but has been known since ancient times, remember that many of these constellations were created in Greece or by peoples living near the Mediterranean. This means that from these areas this constellation will be seen much higher in the sky.
Although a relatively minor group with no named stars in
Greek mythology, Ara represented the altar used by Zeus and other Greek gods to
swear a vow of allegiance before they went to war against Cronus and the
Titans. According to legends, it is believed the Cyclops originally built the
altar as a place to sacrifice to the Olympian gods. It was also upon this altar
that Centaurus the centaur sacrificed Lupus the wolf.
In Arabia it was Al Mijarah or a censer which means it had
no shape before the introduction of Greek mythology. However in Chinese
astronomy the stars that form Ara are part of the Azure dragon of the East,
this is one of the dragon gods .
Ara has a fairly distinctive shape, its brightest stars are:-
Alpha , the second brightest star can be found below the
bright star theta in the tail of the
Scorpion. Alpha has a magnitude of 2.9 and is 270 light years away. It
is a B class star with a surface temperature of around 17, 700 degrees compared
to 5,800 degrees of our Sun.
A line drawn down from alpha leads to beta, the brightest
star in Ara which has a magnitude of 2.8 and is 650 light years distant. It is
a K class super giant star with a surface temperature of 3,800 degrees
indicating it is cooler than our Sun.
Just underneath beta is gamma, located 1110 light-years from
Earth, and is a B1 class supergiant with a whopping surface temperature of
21,200 degrees with a magnitude of 3.3.
Continuing a line
further down we reach delta with a magnitude of 3.6 it’s a B 8 class star
indicating that it is much hotter than the Sun. Delta lies 198 light-years from
Earth.
A line drawn from beta to the right will lead you to zeta
the third brightest star in Ara with a magnitude of 3.1. Zeta is a K3 giant
star with a surface temperature cooler than the Sun, it lies 490 light years
away. As zeta is the 5th letter of the Greek alphabet it should be
the 5th brightest star here. It isn’t which again shows that the
Bayer system of using Greek letters in constellations does not always work.
From zeta a line drawn down and slightly to the right leads
to eta which is about 299 light years away, eta is somewhat fainter than other
stars in Ara with a magnitude of 3.8.
Another K class star.
Since Ara contains part of the Milky Way to the south of
Scorpius, it therefore has rich in clusters of stars. There are no messier
objects in Ara this might be because the constellation was too far south for
him to easily observe it.
Ara gives us the chance to view the two types of clusters we
see in the sky both are just visible to the naked eye if the sky conditions are
very good. Open Clusters are groups of very young stars which are loosely held
together by gravity and will over time drift apart over a period of several
million years. Globular Clusters on the other hand are formed from very old
stars.
Both clusters have NGC numbers which stand for the New
General Catalogue (NGC) which was produced in 1888 and was an addition to the
General Catalogue and Nebulae and Clusters that had been produced in the first
half of the 19th century by William Herschel who had discovered the planet
Uranus in 1781 and his sister Caroline and William’s son John Herschel.
NGC 6193 is open cluster which can just be seen with the
naked eye, although binoculars will make it more easily visible. There are 27
stars, it’s about 3 million years old and is about 3,800 light years away. It was discovered by the Scottish astronomer
James Dunlop 1793-1848 in 1826 while observing in Australia. Under the very
beat of conditions, it can be just glimpsed with the naked eye but can be seen
much easier with binoculars.
NGC 6397, is a globular cluster and is located about 7,200
light-years from Earth, making it one of the two nearest globular clusters to
Earth The cluster contains around
400,000 stars, and like NGC 6193 can be seen with the naked eye under good
observing conditions but can easily been through binoculars. It was discovered
by the French astronomer Nicolas Louis de Lacaille 1713-1762 in 1752 while observing from South Africa.
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