Vega - the Summer Sapphire Star Content from the guide to life, the universe and everything

Vega - the Summer Sapphire Star

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Vega is the fifth brightest star in the night sky, after Sirius, Canopus, alpha Centauri and Arcturus. To most people in the world, who live in mid-northern latitudes, it is the third brightest: Canopus and alpha Centauri are either too far south to be visible or only appear low on the horizon and are dimmed by the atmosphere. For people in the Northern Hemisphere, Vega hangs high in the sky in the summer months and shines with a clear blue colour.

Blue Star

All stars have a colour but it is usually very hard to see. Stars are very faint and the human eye's colour vision doesn't work well at low light levels. This is why things look grey by moonlight. Vega is bright enough for its colour to be clearly visible.

The colour of a star is an indication of the temperature of its surface. Although we think of blue as a cool colour compared with red, in stars blue is one of the hottest colours. When things glow bluish-white, it is because they are very, very hot. Think of an oxyacetylene torch or a flash of lightning. These are both much hotter than the red of a glowing coal, or even the white-hot glow of molten iron as it is poured from a crucible into a mould. Vega is blue because it has a surface temperature of 9,600°C, which is considerably hotter than the surface of our own Sun.


Vega, the Alpha star of Lyra

Located at RA=18h36m56s Dec=38°47'01"N, Vega is in the constellation of Lyra, the lyre. It is the brightest star by far in this constellation so it gets the designation alpha Lyrae (the First of the Lyre).

Vega's brightness is magnitude 0.0. While this might seem like a coincidence, in fact Vega was used as the standard from which other magnitudes were measured by comparison, so it was defined as 0.0. In recent times, a more accurate way of measuring stars' brightness was devised, and by this method, Vega was found to have a magnitude of 0.03.

Vega's great brightness comes from two things. Firstly, it is very close to us, only about 25 light years1 away. Secondly, it is intrinsically a very bright star anyway, being about 40 times as luminous as the Sun.

Vega is more massive than the Sun as well - it weighs in at 2.1 times the mass of Sun. This mass causes the gas in it to be more compressed at the centre, giving rise to a much more vigorous nuclear reaction, and a hotter surface, which is the explanation for it being brighter, as well as bluer, than the Sun.

Vega is a young star compared with the Sun. It is 450 million years old, which sounds like a lot, but in geological terms it's not. At the time when Vega first started shining, the Sun and the Earth were both already about four billion years old. Life had developed on Earth and was at the stage of arthropods (insects, crabs, spiders etc) crawling out of the oceans onto the land. It's unlikely any of them noticed the new star in the sky.

With stars, the motto is 'Live Large, Die Young'. The bigger the star the faster it burns, so Vega, although only one tenth the age of Sun, is already at the same middle-aged stage in its life, and will reach the end much sooner. It has probably another 450 million years to go before it moves into its next much briefer phase, as a red giant star.

Record Holder

Vega was the first star in the night sky to be photographed, on the night of 16/17 July, 1850, by George Phillips Bond and John Adams Whipple using the Harvard 15-inch refractor telescope. The photograph used the daguerreotype process and needed an exposure of 100 seconds.

In 1872, Vega became the first star other than the Sun to have its spectrum measured, by Henry Draper. The spectrum is an indication of the frequencies of light that the star emits, and can provide information on the chemical composition of the star. Vega has an unusually simple chemical composition, with almost no elements other than hydrogen and helium. These two make up 99.5% of the star.

Vega's Name

In ancient times, Vega was called Lyra. It had the same name as the constellation in which it is the main star. This was recorded by Ptolemy in his star catalogue in the 2nd Century AD. The lyre was a musical instrument similar to a small harp. The constellation was said to represent the lyre of Orpheus, the legendary Greek hero who travelled to the underworld to recover his lost love, Eurydicë.

Most of the star names we use now come from the Arabs, who were avid star watchers. They identified two stars in the sky as eagles. One was called Al Taïr, the soaring one, while the other was Al Waki, the falling one. The Arabic word 'al' means 'the'. The name Al Taïr became Altair, retaining the 'al' - Altair is the brightest star in our Eagle constellation, Aquila. The name Al Waki dropped the 'al' and gradually changed to Wega, which later was written Vega.

You can read about the mythology of Vega and its appearances in popular fiction along with those of other stars in a separate Entry.

Fat Star

When the radius of Vega was measured, it turned out to be much bigger than expected considering its brightness and the type of star it is. Eventually it was discovered that the star is rotating very rapidly, and this causes it to bulge at its equator. It is thought to turn once on its axis every 12.5 hours, which is about 50 times as fast as the Sun. This means that the equatorial radius is up to 19% bigger than the polar radius. We're looking down on the pole of Vega, so we see the widest possible radius.

Vega's rotation speed is so fast that if it was only 16% faster, it would start falling apart.

Pole Star

As the Earth turns on its axis, the axis points in a constant direction, to a point in the sky called the celestial pole. This point moves slowly through the constellations, tracing out a full circle every 26,500 years. When a star is at or near the celestial pole, it seems to hang in the sky without moving. The star Polaris is in this position at the moment, and is a valuable aid to navigation. About 14,000 years ago, Vega was the pole star. Although it wasn't particularly close to the pole, its great magnitude compared with any other pole star must have made it very impressive. It will return to its pole position in another 12,000 years.

Long Term View

While Vega's position varies over a 26,000 year cycle, if we take a longer term view we find that the Sun is getting slowly closer to Vega. In about 210,000 years it will close enough that it will be brighter than Sirius, making Vega the brightest star in the night sky.

Dusty Neighbourhood

The Earth's atmosphere absorbs infra-red light, so if you want to see how the stars look in infra-red, you have to either go to the top of a very high mountain where the air is thinner, or better still into space. In January 1983, the Infra-Red Astronomical Satellite (IRAS) was launched. It provided a real surprise - when observed in infra-red, Vega looks much wider than it should. Instead of a disc the size of the star, the infra-red cameras see a disc the size of a solar system. The star is surrounded by a giant disc of dust. This is heated by the star and gives off enough heat itself to be detected by the infra-red cameras.

Initially it was thought that this must be a protoplanetary disc, lumps of rock in orbit which are in the process of forming into planets. (Astronomer and TV presenter Sir Patrick Moore expressed such a view in his 1987 book Astronomers' Stars.) Many more observations have been performed on this disc; it is now thought that it is made up of very fine dust particles and has a hole in the middle, like the hole in a Compact Disc, with Vega sitting in the centre of the hole. It's likely that there are lumps of rock in orbit around Vega, but they're in the 'hole' in the dust disc and are not detectable. When they crash into each other, they give off dust and this is pushed outward into the disc by the stream of radiation coming out from Vega.

So does Vega have planets? We don't know. Huge planets would disturb the smoothness of the dust disc, so we're pretty sure there are none of those, but there might be small Earth-like planets which have not yet been detected.

Celestial Sights near Vega

Vega forms a very striking triangle in the sky with two other bright stars - Deneb in the constellation of Cygnus, and Altair in the constellation of Aquila. This pattern is known as the Summer Triangle, and was popularised by Patrick Moore. Because the pattern is made of stars from different constellations, it is known as an asterism rather than a constellation.

The Summer Triangle is a very useful pattern to watch out for, as it will lead you to the three constellations at the three corners, the tiny constellation of Sagitta the arrow in the middle of the triangle, and the nearby faint-but-cute Delphinus the dolphin. The faint band of the Milky Way is visible 'flowing' through the triangle at Deneb and separating Vega from Altair.

Closer to Vega, the constellation of Lyra itself has two things to look out for, but you'll need binoculars or a telescope to see them:

  • Binoculars will show that the star Epsilon Lyrae, very near to Vega, is two identical stars very close together. Such a group is called a binary star. A strong telescope will reveal that each of the two stars is itself a binary star, so there are actually four stars here.

  • The Ring Nebula in Lyra is a faint grey circular smudge. A powerful telescope will show it to be a circular ring of dust. This is classified as Messier 57 (M57). This type of object is called a 'planetary nebula'. Nebula means cloud, and it was called planetary because early astronomers on the lookout for new planets thought it could be easily mistaken for a planet. It is a giant shell of gas and dust given off by a dying star. The remnant of the star is visible to very high powered telescopes at the centre of the ring.

1A light year is the distance that light travels in a year, approximately 10 trillion kilometres.

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