What Can You See in the Night Sky This Week? - Sirius!

28 March 2012 by Anonymous (not verified)

An image of Sirius A and Sirius B acquired with the Hubble Space Telescope. Sirius A is the big, bright star in the middle, whilst Sirius B is the small dot visible towards the lower left.

Throughout the following days, look towards the South in the evening and you will see a sparkling gem - a bright star called Sirius. In fact, it happens to be the brightest* star you can see in the sky all year round. It is no wonder then that its name is derived from the ancient Greek for "glowing". If you are still not sure which star I am referring to, proceed by first finding the easily identifiable three stars that form Orion's belt (still looking towards the South), then imagine a line running through these and extrapolate it to your left. You will inevitably end up spotting the dazzling Sirius, writes Joseph Caruana

Before I describe to you some of the really interesting things which we have learnt about Sirius in the modern age, let me first recount some stories from the past. Being such a bright star, it stirred excitement even amongst very early civilizations. In ancient Egypt, it just so happened that around the time the river Nile would flood, Sirius would rise right before the sun in the early morning (shortly before the Summer solstice), after 70 days of being absent in the skies. This occurrence of a star rising in the morning just before the sun is called Heliacal rising. The flooding of the Nile being such an important event in the life of ancient Egypt, Sirius ended up being the star on which the Egyptian calendar was based during the Middle Kingdom (roughly between 2055 BC and 1650 BC). The ancient Greeks also observed that the Heliacal rising of Sirius came before the advent of Summer, and since Sirius lies in the constellation of Canis Major, the hot summer days acquired the name "dog days".

Sirius: If you are unsure where to find Sirius, locate the easily identifiable three stars that form Orion's belt and extrapolate a line to the lower left.

So what do we know about Sirius nowadays? Well, we have learnt that it is actually two stars, not one! (In astronomical terminology, it is a binary star system.) The first person to deduce that there must be a companion was the German astronomer Friedrich Bessel. Whilst he could not actually see the companion star, in 1844 he figured out from the proper motion of Sirius that there had to be an invisible companion. (If you are interested in the original communication of this discovery, see: this paper.) This companion was then observed for the first time by Alvan Graham Clark (of Alvan Clark & Sons fame) in 1862, while testing out an 18.5-inch refracting telescope.

The main larger star, dubbed Sirius A, is about twice as massive as the Sun. The smaller companion is a white dwarf (In one of the accompanying pictures, taken by the Hubble Space Telescope, you can see it as a tiny dot lying to the lower left of the bright star, Sirius A). It is about as massive as our own sun, but all its mass is packed in a much smaller space - in fact, Sirius B has been determined to be about as big as our own planet Earth. The distance between the two stars is about 20 times the distance between the Earth and the Sun and it takes them more than 50 years to complete one orbit around each other. Sirius A is destined to share the same fate as Sirius B; once it exhausts the hydrogen fuel that powers it, it will become a red giant star, after which it will become a white dwarf too.

So the next time you look up at the sky and see Sirius shining brilliantly, keep in mind that you are actually looking at a faraway binary star system, whose light has taken about 8 years and 7 months to reach you! That means that the light we'll get to see over the coming few days left sirius in the summer of 2003. What were you doing then?

Sirius: In this picture, Sirius is visible above Christ Church Dining Hall, Oxford.

*Note that actually, at the moment you will be able to see two even brighter "stars" lying fairly close to each other towards the West - but these are not stars - they are the planets Venus and Jupiter.

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