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Sirius - The Dog-Star

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Sirius is the brightest star in the firmament, and one of the night sky's most impressive sights. It's so bright that it can sometimes be seen in the day, just before sunset, if you know exactly where to look. Sirius is more than twice as bright as the next brightest star1, and it outshines all the planets except Venus and Jupiter. It is bright for two reasons: firstly it is intrinsically a very bright object, giving off 25 times as much light and heat as our Sun. Secondly, at only 8.60 light years2 away, Sirius is the seventh closest star to the Sun, after the three stars of the Alpha Centauri system and three invisible red dwarf stars you may not have heard of: Barnard's Star, Wolf 359 and Lalande 21185.

If you're in the Northern Hemisphere, like most of the population of the world, then Sirius is a winter star - in summer it is on the same side of the sky as the Sun, so it is hidden in the glare of daylight, but in the winter it dominates the sky, even though it can be quite low down. You'll find Sirius below and to the left of the great constellation of Orion. You can find it by drawing a line to the left through the three stars of Orion's belt, and then looking for the brightest star you've ever seen. In the Southern Hemisphere, on the other hand, Sirius will be much higher in the sky and will be most prominent in the summer, above and to the right of Orion.

The true colour of Sirius is a slightly bluish-white. This is clearly visible to Southern Hemisphere observers who see the star high in their sky. But when Sirius is low down near the horizon, as it normally is for Europeans and North Americans, it twinkles in great flashes of colour - you'll see green, blue and red, as well as the white which is the star's actual colour. This twinkling is technically known as 'scintillation'. All the stars do it, but it's more noticeable with Sirius because of its great brightness. The star itself is shining with a steady light; the twinkling and flashing is an effect of the Earth's atmosphere.

Sirius is part of the constellation of Canis Major, the Great Dog, supposedly one of the hunter Orion's dogs. It is often known as 'the Dog-Star' for this reason. But it is so much brighter than all the other stars in the constellation that they tend to get ignored. Sirius is also part of the asterism known as the Winter Triangle - this cross-constellational grouping consists of the stars Sirius, Procyon in Canis Minor and Betelgeuse in Orion.

Some Facts About Sirius

Bayer designation:Alpha Canis Majoris
Location in Sky (as of 2000):06h 45m, -16.7°
Spectral Type:A1V
Absolute Magnitude:+1.45
Distance:8.60 light years
Surface Temperature:9,880K
Diameter (compared to Sun):1.75
Mass (compared to Sun):2.12
Proper motion:1.324 arcsec/year
Proper motion direction:204° (SSW)
Velocity Radial:-8km/s (towards us)
Velocity Radical:19 km/s

Sirius Rising

As already mentioned, in the summer, Sirius is on the same side of the sky as the Sun. Due to the rotation of the Earth and its orbit around the Sun, the stars rise four minutes earlier each day, so there comes a day when Sirius rises at exactly the same time as the Sun. A week or so later, it will rise about half an hour before the Sun, which means it will be visible briefly in the dawn sky just before the Sun comes up. For a given location, this will be the same day every year (although the exact date moves slightly over centuries). This phenomenon of rising just before the Sun is known as 'heliacal rising' from the Greek word for Sun, Helios.

The heliacal rising of Sirius was considered very important in ancient Egypt, making Sirius by far the most important star. They noticed, probably in about 2800 BC, that the rising of Sirius coincided with two natural phenomena: the beginning of the hot weather of summer, and the annual flooding of the Nile. They used the appearance of Sirius to predict the coming of the floods, and declared the first day that Sirius was visible before the Sun to be the first day of their new year. At that time at the latitude of Egypt, the date of the heliacal rising of Sirius was about 1 July, although it has moved to later in the year during the intervening thousands of years.

Double Star

We're used to there being just one sun in the sky, so we tend to think of stars as being single entities. But in fact many, if not most, of the stars in the sky are binary stars, two stars orbiting a common point, or even multiple stars, with anything from three to six stars orbiting in complicated arrangements. Sirius is no exception - what we see as one bright star is actually two stars very close together. They are known as Sirius A and Sirius B. Sirius A is the bright star we see, but mixed in with it is the light of the much fainter Sirius B. The two stars are about 20 AU apart (that's the distance from our Sun to Uranus).

The hidden companion of Sirius was first deduced by the German mathematician and astronomer, Friedrich W Bessel. Because it is so close to us, Sirius moves slowly across our sky relative to the other, more distant stars, which don't appear to move. By careful observation, Bessel plotted the exact position of Sirius over the course of about ten years and saw that it wobbled in its motion. He deduced the existence of a large object orbiting around Sirius once every 50 years. Bessel reckoned that it would have to have a mass similar to our own Sun to cause the wobble, which meant that it would be a star rather than a planet; he was puzzled by the fact that he couldn't see anything there.

The companion star, officially labelled Sirius B but affectionately known as 'the Pup', was first spotted many years later by AG Clark in 1862. It was found to be very small and dim. But when the spectrum of the star was finally measured, it led to great excitement. The star is incredibly hot with a surface temperature of about 25,000°C, far hotter than Sirius itself. It is a 'white dwarf', only the second such object ever discovered3.

Sirius B is very, very small - slightly less than the size of the Earth, which is tiny for a star, but has a mass about the same as our Sun. To pack that amount of mass into such a small volume, it must be incredibly dense - it is estimated that the matter is 125,000 times as heavy as water, or 11,000 times as heavy as lead. Normal matter cannot be that dense. This must be a special sort of matter in which the intense gravity has collapsed the atoms down, so that there are no gaps between the electrons and their nuclei. Imagine a giant hall, with some balloons in it. Inside each balloon there is a tiny peppercorn. The balloons represent atoms, and the peppercorn is the nucleus of the atom. The balloons are free to bounce around, as this is a gas. Now pack the balloons so closely together that they touch each other. This is a normal solid. Squash the balloons down so that they burst and they will fit into a much smaller space. This is the type of matter in a white dwarf. It is known as 'degenerate matter'. A cubic centimetre of it would have a mass of 125kg. That's 275 pounds or almost 20 stone!

Stellar Evolution

Both stars of Sirius formed out of a cloud of hydrogen gas, relatively recently in geological times, about 225-250 million years ago. At that time, Earth had just moved into the Triassic period and frogs and other amphibians walked the land while turtles swam in the seas. Sirius B was the bigger and hotter of the two stars, having a mass about five times that of our Sun. Both stars burned brightly, Sirius A blue-white as it is now, and Sirius B with a blue light, as they orbited around each other.

Because B was bigger, it was compressed by gravity more, forcing the hydrogen down into the central furnace, so it burned faster and brighter. Eventually, it used up all the hydrogen in its core. This caused the star to collapse; this in turn generated even greater pressure in the centre, making it hot enough to burn helium in a more elaborate reaction which produced much more heat than before. This caused Sirius B to expand into a giant star. Although it was now producing more light and heat than ever, it was spread over a much greater surface area, so the surface cooled somewhat, turning the star red - Sirius B had become a red giant, similar to Aldebaran.

In the meantime, Sirius A went on burning hydrogen, giving off a steady white light. It continues to do so, and will shine for many millions of years to come.

Eventually, the helium in the core of Sirius B started to run out. The star underwent a series of dramatic upheavals in which it blew off its outer parts into space. It lost four fifths of its mass in this way. What was left collapsed again, compressing down into an ultra hot ball. This ball is the white dwarf which we can see with strong telescopes - it no longer supports nuclear reactions - it has finished burning. But it is still very hot; it glows white with heat, the way a piece of metal glows white when it is heated up in a very hot fire, and continues to glow for a time after it is removed from the fire. The fire in Sirius B was so intense that it will continue to glow for another trillion years at least, although it is no longer burning fuel. It is in effect the glowing coal of a dead star.

So the Sirius system now consists of Sirius A, a white star which is still in its hydrogen-burning phase, and Sirius B, a hot glowing ultra dense ball.

Sometime in the future, Sirius A will run out of hydrogen, and will follow the same path as Sirius B, turning first into a red giant and then a white dwarf. Our own Sun is on a similar career path - it will reach the red giant stage in about five billion years' time.

The Colour Mystery

In ancient times, many authors described Sirius as 'red' or 'reddish'4. Some writers described it as redder than Mars. Yet astrophysicists say that both Sirius A and Sirius B are stable. By current theories they should not have changed colour any time in the last million years.

Some rather unlikely explanations have been put forward, such as that there is a third as yet undetected star in the Sirius system which is red and variable, or that there was a cloud of dust which got between us and Sirius, reddening the light. But the first would make Sirius considerably brighter than it is now, and the other would make it dimmer, and ancient reports do not mention any difference in brightness. The most likely explanation is that we've misinterpreted the ancient texts and that what the authors were referring to was the 'red flashes' of Sirius's scintillation.

Name and Mythology

In ancient Egypt, Sirius was called Sopdet. Its hieroglyph was a tall triangle beside a star, with a small semicircle (the 't' sound) over the star. Sirius was associated over the years with a number of different gods: Anubis the jackal, Thoth the ibis, and most recently Hathor the cow.

The Greeks themselves saw Sirius below and to the left of the most distinctive constellation in the sky, the giant hunter Orion, so they imagined Sirius and the stars around it to be Orion's dog. Both the star and the group of stars were known simply as 'Kyôn' which means literally 'the dog', although today we consider them to be Canis Major, the Big Dog. The hot days of summer which were signalled by Sirius rising just before the Sun were called the 'dog days' for this reason.

The Greeks also occasionally used a different name for the star, Seirios, which means scorching. This may have been because of its association with the hot days of summer, or it may have been because of its flashing colours. This name changed in spelling to our 'Sirius' over time.

Not surprisingly given its position as brightest star, many other cultures have had their own myths about Sirius. It is curious that many of them also considered Sirius to be a dog or associated with dogs in some way. The Phoenicians called it Hannabeah, the Barker. The Chinese called it Tianlang, 'heavenly wolf'. Native American/First Nations People called it names such as Dog Face, Coyote Star, Moon Dog and Wolf Star.

Some cultures saw Sirius in different ways. In ancient Sanskrit, it was Murgavyadha, the Deer Slayer, a representation of the god Shiva. In Norse mythology, it was Loki's Torch. Australian Aborigines saw it as an eagle. Sirius also attracts descriptive names in many languages, such as the Arabic Barakish, meaning 'of a thousand colours', an obvious reference to its flashing appearance.

Sirius in Popular Culture

  • In the Hitchhiker's Guide to the Galaxy works of Douglas Adams, the Sirius Cybernetics Corporation was the company responsible for the production of the spaceship Heart of Gold and the depressed robot Marvin. They specialised in human/machine interfaces that were sickeningly condescending. Their slogan was 'Share and Enjoy'.

  • In the children's book Dogsbody by Diana Wynne Jones, the soul of the star Sirius is accused of committing a murder and is punished by being embodied as a small dog on planet Earth. He must prove himself honourable and trustworthy in order to be reinstated as a star.

  • Sirius Black is a character in the Harry Potter series of books by JK Rowling. His name reflects the nature of his character.

  • In the short anime  Voices of a Distant Star, humans travel to the Sirius star system in pursuit of some aliens. One of the main characters makes a home on the fictional planet Agartha in orbit around Sirius.

1The second brightest star in the night sky is Canopus, alpha Carinae.2A light year is the distance light travels in one year, roughly 5.88 trillion miles or 9.46 trillion kms.3The first white dwarf was 40 Eridani B, discovered by Russell, Pickering and Fleming in 1910.4The Greek word 'hypokirros' literally means yellowish, but is used to describe other notable red stars such as Betelgeuse and Aldebaran.

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