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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 summer constellations of Bootes, Libra
and Ursa Minor,
which can be found towards the south in the June night sky.
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Bootes - is generally referred to as the 'Bear Watcher' of 'Herdsman'
because it slowly circles the two bears (Ursa Major and Minor) over the course of
a year - thus keeping them penned in. Bootes is the 13th largest of the 88 modern
constellations and contains just 3 bright stars, the brightest of which is a red
giant star called Arcturus, which also happens to be the third brightest
star in the night sky. Red Giants are stars in the final stages of their
lifetimes that have swollen to many times
their original size. Although Arcturus is roughly the same mass
as the Sun, it has grown to 25 times its width,
occupies around 2,000 times the volume and pumps out over 100 times as much power. If
Arcturus was in our own Solar System, its surface
would extend beyond the orbit of Mars and will
thus have engulfed the Earth. Our Sun will undergo the same fate as Arcturus, but
thankfully not for another 5,000 million years.
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Libra - is named after the Latin word for 'balance' and is said to represent
the scales of justice. It is the 29th largest of the 88 modern constellations, but contains
just two bright stars, none of which are particularly well known. 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, indicates the direction
of its rotation axis. The most interesting object within the constellation is the recently
discovered extrasolar planet, Gliese 581c, which is the
first Earth-like exoplanet to be found within its parent star's
habitable zone, raising the question of whether it could possibly support
life.
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Ursa Minor - takes its name from the Latin words for 'Little Bear' and is
home to one of the most famous stars in the night sky, Polaris, which is more commonly
known as the North Star (or Pole Star). Ursa Minor is not a particularly large
constellation and comes in at just 56th largest out of 88. It has two bright stars, the
brightest of which, not surprisingly, is Polaris. Because Polaris is sat directly above the
rotation axis of the Earth, it doesn't move during the night, and if you were to look at it
for any length of time you would see other stars circling around it. This means that on a
clear night we can always find out where North is by looking for this famous star. That said,
the direction of Earth's rotation axis is very slowly shifting over time due to a physical
process knwon as precession. This means that Polaris will not always be the North Star,
but will become so again in around 25,000 years.
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