||Sun Angle ?
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 summer constellations of Draco, Hercules
which can be found towards the south in the July night sky.
Draco - is named after the Latin word for 'Dragon', and is a far northern
constellation that is circumpolar for most northern hemisphere observers - i.e.
it closely circles around the North Star during the night and can also be seen throughout
the year. Draco is the 8th largest of the 88 modern constellations and contains just 3 bright stars,
the brightest of which is γ Draco or Eltanin. The most notable object in
Draco is thought to be NGC6543, a planetary nebula more commonly referred to as the
Cat's Eye Nebula. A planetary nebula is a spherical cloud of gas and dust, which
used to be the outer atmosphere of a star (similar to our Sun) that has recently come
to the end of its lifetime. The
faint star generally found in the middle of planetary nebula are known as white dwarfs, and are the hot leftover cores of the original star.
Hercules - is the 5th largest of the 88 modern constellations and was
named after the Roman name (Hercules) of the Greek mythological hero Heracles. The
fact that the constellation contains no bright stars can make it a little difficult
to locate in the night sky, but this task is made easier by the presence nearby of
the Summer Triangle - see below. Hercules contains two classic examples of
globular clusters: M13,
which is the brightest globular cluster in the northern hemisphere, and M92. These
are spherical collections of many tens of thousands of stars that are tightly bound
(tied) together by gravity. The stars are all orbiting around the middle of the cluster,
and often remind people of bees swarming around their hive.
Ophiuchus - is the ancient Greek word for 'serpent-holder' and is often
depicted as a man grasping a serpent (snake) in both hands. This idea is reinforced by
the presence of the two-part constellation of Serpens Caput (serpent head) and
Serpens Cauda (serpent tail) on either side of Ophiuchus. Note that Serpens is
regarded as one constellation despite being split into two halves. Ophiuchus is the
11th largest of the 88 modern constellations and contains 5 bright stars, the brightest
of which is Ras Alhague. The constellation is also famous for containing the
star with the fastest apparent motion, a red dwarf called Barnard's Star. This
cool star is the second closest star system after Alpha Centauri and is located just
under 6 light years away. Its
fast movement, however, means that it will one day become the closest star to the Sun
Summer Triangle - is not actually a constellation, but is three bright
stars called Vega, Altair and Deneb, which make up a triangular shape
during the summer months and can help us to identify less obvious constellations in the
vicinity. Because the stars all look about the same brightness, it's easy to think that
they are about the same distance away. However, Deneb is actually 100 times more distant
than Altair, which is only 17 light years away. For
Deneb to appear almost as bright as Altair in our sky means that Deneb must be far brighter
than Altair in terms of true light output. In fact astronomers estimate that it pumps out
around 60,000 times more light than our Sun, while Altair outshines the Sun by just 13 times.
Vega, by comparison, is 25 light-years away and some 50 times brighter than the Sun. The reason
Deneb is so bright is because it's a type of star known as blue supergiant, whose size is around
200 times larger than the Sun. Because supergiants are in the last stages of their lives, it's
likely that Deneb will become a supernova in the next couple
of million years - which could cause significant problems for us still stuck on Earth when it goes off.