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Astronomy and Space articles
by Martin George of the Launceston Planetarium
22 February 2014
The Brightest Star in the Night Sky
Since writing recently about the first star to become visible in the evenings, which at this
time of the year is Sirius, I have had a few questions about that star, and why it is so
bright.
Sirius is a brilliant star, visible high in our northern evening sky. It is quite easily identified
by first looking for the three stars in a row in the constellation of Orion. These form
Orion's belt, but they are also well known as the base of 'The Saucepan', formed from
some of the stars of Orion.
If you extend a line from the belt stars upwards and to the right, you will come across
Sirius.
Of course, Sirius is not the brightest object in the current evening sky. When the Moon is
there, of course, it far outshines Sirius. Currently the giant planet Jupiter is visible much
lower down in the northern sky, and is also clearly brighter than the star.
Of course, Jupiter is a planet, not a star.
The name Sirius comes from the Greek Seirios, meaning 'scorching'. The name is
thought to have been applied not only because of Sirius' brightness, but also because of
it being in roughly the same direction as the Sun in the northern hemisphere summer. Its
prominence within the constellation of Canis Major, The Great Dog, led to it being called
the Dog Star, and this then led to the term dog days for midsummer days.
As with any star, Sirius' apparent brightness arises from a combination of its true
brightness and its distance. Stars really do have different brightnesses, and of course
they have a wide range of different distances from us.
To begin with, Sirius is more than twice as massive as the Sun, and is 26 times as bright.
It is also relatively nearby, being only 8.6 light years from us. This is only about twice as
far as Alpha Centauri, the closest star system to the Sun, which is the brighter of the two
'pointer' sitars to the Southern Cross.
The effect is easy to understand if you imagine holding a torch very close to your eyes. It
could be quite dazzling, but if the torch were held hundreds of metres away it would look
far, far less bright, and there would be a distance at which you would not be able to see
the light of the torch at all. However, a very bright torch at that great distance may still be
visible.
Astronomy and Space articles
by Martin George of the Launceston Planetarium
22 February 2014
Apart from its brilliance, Sirius is also famous for its companion star in orbit around it. It is
known as Sirius B, or The Pup. When spotted by Alvan Clark in January 1862, it became
the first white dwarf star to be discovered. Earlier observations of a 'wobble' in Sirius'
path across the sky had suggested the presence of an unseen companion but, up to that
point, it had eluded astronomers because of the brilliance of Sirius itself.
White dwarf stars represent one of the possible endpoints in the normal lifetime of a star;
indeed our Sun will become a white dwarf billions of years from now. They are very
small, but very dense, objects. A teaspoonful of material from Sirius B would have a
mass of several tonnes!
Article by Martin George, Launceston Planetarium, QVMAG.
Reproduced with permission of the Mercury newspaper.