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Each month, we identify the best constellations to be seen from the UK between 9pm and 10pm.
This month we'll be looking for the autumn constellations of Aquarius, Cepheus,
which can be found close to an imaginary line running North to South
in the October night sky.
Aquarius - is named after the Latin word for a 'Water-bearer', and is the
10th largest of the 88 modern constellations. It is also one of the 12 constellations through
which the ecliptic passes and is therefore one of the signs of the zodiac. The ecliptic
is an imaginary line across the night sky that the planets never stray far from during their
orbits of the Sun. It marks out the plane of our
Solar System, or alternatively, can be used to indicate the direction of its rotation axis (at
right angles). Despite being large, Aquarius only has 2 bright stars, with the brightest being
Sadalsuud. One of the more notable objects in the constellation is the recently
discovered Gliese 876 planetary system, which is known to have at least 3 planets
orbiting a red dwarf star. One of these is a terrestrial planet around 6-8 times the
mass of Earth.
Cepheus - is named after Cepheus, King of Aethiopia in Greek mythology, and
is considered to represent a king. It is the 27th largest of the 88 modern constellations and
can be identified by looking close to Polaris (the North Star) for a box shape with a
triangle on top. Many observers think this shape looks somewhat like a royal crown. Cepheus
contains just one bright star, called Alderamin, but enjoys a number of other interesting
objects, such as the Fireworks Galaxy (NGC6946). NGC6946 is a spiral galaxy in which
eight supernovae have been observed, more
than in any other galaxy. The constellation is also home to a binary star system, γ
Cephei, which is just 50 light years
from Earth and marks out the point of the crown shape. Due to the precession of the Earth (its
rotation axis changing direction - a bit like a spinning top), this system will become the
North pole star between 3000 A.D. and 5200 A.D. - with the closest approach being around 4000 A.D.
Cepheus also contains the prototype Cepheid variable star, named δ Cephei, which
was discovered in 1784 and found to periodically double in brightness (and then dim again) over
the course of 5.4 days. The variation has continued over hundreds of years and is thought to
be caused by unstable layers in the outer atmosphere of stars in the
latter stages of their lifetime.
Pegasus - is named after the mythological winged horse of the gods, Pegasus.
It is the 7th largest of the 88 modern constellations and contains 5 bright stars, the brightest
of which is Enif (see above), which is said to represent the nose of the horse. The easiest
way to find Pegasus is by looking for four bright stars that make up the shape of a box, known as
the Great Square of Pegasus. The four stars of the square are Sirrah (top-left), Scheat
(top-right), Algenib (bottom-left) and Markab.
The constellation has become more studied in recent years because it is home to the first extra-solar
planet ever discovered, which orbits the star 51 Pegasus every 4.2 days (Mercury orbits every
88 days). The word extra-solar means that the planet is outside our own Solar System. Such a short
period means that the planet orbits very close to its parent star, which was totally unexpected by
astronomers. The planet is now known to be a gas-giant planet, like Jupiter or Saturn, with an
atmospheric temperature well over 1000°C. Another planetary system in Pegasus, HD209458, has
provided the first evidence that water vapour exists in the atmosphere of a planet outside of our
own Solar System.