During summer evenings, and indeed all through the hours of darkness, the sky is dominated by three brilliant stars \u2013 Vega, Deneb and Altair.<\/p>\n
The stars of the Summer Triangle are not officially associated, and are not in the same constellation. They are:<\/p>\n
- Vega<\/a><\/strong> in Lyra (the Harp)<\/li>\n
- Deneb<\/strong> in Cygnus (the Swan)<\/li>\n
- Altair<\/strong><\/a> in Aquila (the Eagle).<\/li>\n<\/ul>
Yet they make up a large triangle \u2013 an asterism<\/a> \u2013 that cannot possibly be overlooked even by the most myopic observer.<\/p>\n
They are also among the brightest stars in the sky<\/a>; indeed, Vega is surpassed only by Sirius, Canopus, alpha (\u03b1<\/span>) Centauri and Arcturus.<\/p>\n
Of these, only Arcturus is north of the celestial equator, while Canopus and alpha (<\/span>\u03b1<\/span>) Centauri never rise over the British horizon.<\/span><\/p>\n
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<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>A star chart showing the location of the Summer Triangle in the night sky. Credit: Pete Lawrence<\/span><\/figcaption><\/div>\n An asterism is a pattern made up of stars that are not genuinely associated.<\/p>\n
The Summer Triangle is an outstanding example of an asterism, and during a Sky at Night<\/i> television programme I casually christened it, never imagining that the name would become widely accepted!<\/p>\n
Strictly speaking, a constellation may be regarded as a kind of asterism; for example, the five stars making up the cross of Cygnus<\/a> do not have any real connection with each other.<\/p>\n
How bright are the Summer Triangle stars?<\/strong><\/h2>\n
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<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>Milky Way and Summer Triangle on a Somerset Summer Night, by Graham King, Winscombe, Somerset, UK.<\/span><\/figcaption><\/div>\n Vega is of apparent magnitude +0.0, Altair +0.8 and Deneb +1.2, so that Vega is much the most brilliant, well over a magnitude brighter than Deneb.<\/p>\n
Of the brightest stars in the sky, Vega comes fifth, Altair 12th and Deneb 19th<\/span><\/p>\n
Yet as so often in astronomy, appearances are deceptive.<\/p>\n
Vega and Altair are ordinary main sequence stars, classed as dwarfs, while Deneb is an immensely remote, powerful supergiant.<\/p>\n
How to see the Summer Triangle<\/strong><\/h2>\n
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<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>The Summer Triangle. Credit: Pete Lawrence<\/span><\/figcaption><\/div>\n From Britain, Vega passes almost overhead during summer evenings, and it stands out particularly well because there are no other bright stars in the immediate area: the constellation of Lyra contains no other star above the third magnitude.<\/p>\n
Vega is decidedly bluish in colour, and in binoculars or a telescope it is strikingly beautiful.<\/p>\n
According to an old Chinese legend, the star was a weaving girl and Altair was a shepherd boy.<\/p>\n
They were so engrossed with each other that they neglected their heavenly duties, and so were\u00a0<\/span>placed on opposite sides of the barrier of the Milky Way, and were allowed to meet only on special occasions.<\/p>\n
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<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>Star Capella. Credit: Pete Lawrence<\/span><\/figcaption><\/div>\n It is worth noting that the only other brilliant star to reach the zenith from British latitudes is Capella<\/a>, which does so on winter evenings \u2013 but Capella is yellowish.<\/p>\n
It and Vega are on opposite sides of the north celestial pole, and about the same distance from it \u2013 so that when Capella is high up, Vega is low down, and vice versa.<\/p>\n
Owing to the precession of the equinoxes, Vega will be the Pole Star by about the year 14,000 AD.<\/p>\n
Science of the Summer Triangle stars<\/b><\/h2>\n
Vega (alpha (<\/b>\u03b1<\/span>) Lyrae)<\/b><\/h3>\n
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<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>Vega and the surrounding stars through a telescope; unguided, the scene looks blurry and indistinct. Compare this to the one taken with a guided setup at the top of the article. Credit: Ade Ashford.<\/span><\/figcaption><\/div>\n - Apparent magnitude:<\/strong> +0.03<\/span><\/li>\n
- Absolute magnitude:<\/span><\/strong> +0.58<\/li>\n
- Type:<\/strong> A0 Va<\/li>\n
- Distance:<\/strong> 25.3 lightyears<\/li>\n
- Mass:<\/strong> 2.6 times the Sun<\/li>\n
- Luminosity:<\/strong> 52 times the Sun<\/li>\n
- RA:<\/strong> 18h 36m 56.3s<\/li>\n
- Dec.:<\/strong> 38\u00ba 47\u2019 18\u201d<\/li>\n<\/ul>
Vega is 25 lightyears away, and 52 times as luminous as the Sun; it is two and a half times as massive, and considerably hotter; its age is thought to be in the order of 200 million years.<\/p>\n
When it was examined in 1983 with the then-new Infra-Red Astronomical Satellite, IRAS, it was found to have what was described at the time as \u201ca huge infrared excess\u201d, due to a cloud of cool particles surrounding the star.<\/p>\n
It seems that Vega is associated with a debris disc, and irregularities in it may possibly indicate the presence of a planet \u2013 though this is obviously very uncertain.<\/p>\n
Vega has a tenth magnitude companion at a separation of just under an arcminute, but there\u2019s no connection; this is an optical pair, not a binary system.<\/p>\n
Altair (alpha (<\/b>\u03b1<\/span>) Aquilae)<\/b><\/h3>\n
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<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>\n- Apparent magnitude:<\/strong> +0.77<\/span><\/li>\n
- Absolute magnitude:<\/span><\/strong> +2.22<\/li>\n
- Type:<\/strong> A7<\/li>\n
- Distance:<\/strong> 16.8 lightyears<\/li>\n
- Mass:<\/strong> 1.7 times the Sun<\/li>\n
- Luminosity:<\/strong> 10 times the Sun<\/li>\n
- RA:<\/strong> 19h 50m 47.0<\/li>\n
- Dec.:<\/strong> +8 52\u00ba 6\u2019<\/li>\n<\/ul>
Altair, one of our nearer neighbours, at a distance of less than 17 lightyears, is a mere 10 times as powerful as the Sun, and less than twice as massive.<\/p>\n
Its spectral lines are broad, because Altair spins so quickly. The rotation period is probably no more than 9 hours, so that the star is flattened.<\/p>\n
Its equatorial diameter is about 15% greater than its polar diameter. Any planets would have a curious sun, but there are no signs that Altair is the centre of a planetary system.<\/p>\n
Neither are there any optical or binary companions.<\/p>\n
Deneb (alpha (<\/b>\u03b1<\/span>) Cygni)<\/b><\/h3>\n
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<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>Deneb in the Milky Way by Paul Holland, Dronfield, Derbyshire, UK.<\/span><\/figcaption><\/div>\n - Apparent magnitude:<\/strong> +1.25<\/li>\n
- Absolute magnitude:<\/strong> -8.7<\/li>\n
- Distance:<\/strong> 3,000 lightyears<\/li>\n
- Type:<\/strong> A2 Ia<\/li>\n
- Mass:<\/strong> 25 times the Sun<\/span><\/li>\n
- Luminosity:<\/strong> 250,000 times<\/span> the Sun<\/li>\n
- RA:<\/strong> 20h 41m 25.9s<\/li>\n
- Dec.:<\/strong> +45\u00ba 16\u2019 49\u201d<\/li>\n<\/ul>
Deneb, the third member of the Summer Triangle, is so remote that its distance is difficult to measure with any real accuracy.<\/p>\n
Probably the best estimate is 3,000 lightyears, in which case it is about 260,000 times as luminous as the Sun \u2013 the most powerful of all the first magnitude stars \u2013 and perhaps 25 times as massive as the Sun.<\/p>\n
This makes it a true cosmic heavyweight. If it were as close to us as Altair, it would look much brighter than the full Moon<\/a>.<\/p>\n
Its diameter is probably greater than that of the Earth\u2019s orbit round the Sun.<\/p>\n
In one version of the Chinese legend, Deneb acts as chaperone when weaving girl and shepherd boy cross the Milky Way bridge to meet.<\/p>\n
Even for a supergiant, it\u2019s plain that this must have been quite a difficult task!<\/p>\n
Constellations of the Summer Triangle<\/strong><\/h2>\n
Once you\u2019ve found the Summer Triangle, take the time to explore its neighbouring constellations.<\/p>\n
Lyra<\/strong><\/h3>\n
- Absolute magnitude:<\/strong> -8.7<\/li>\n
- Absolute magnitude:<\/span><\/strong> +2.22<\/li>\n
- Absolute magnitude:<\/span><\/strong> +0.58<\/li>\n
- Deneb<\/strong> in Cygnus (the Swan)<\/li>\n
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