People have been looking up at the night sky and making patterns out of the stars for millennium. These shapes are called asterisms, and one of the largest of these can be seen overhead in September’s evening sky, the Summer Triangle.
An asterism is an easily recognisable pattern of stars which can be within a constellation, e.g. the Big Dipper in Ursa Major, or it can be constituted of stars from more than one constellation, an example of which is the Summer Triangle.
Constellations are now defined by the International Astronomical Union as 88 areas of the sky. They usually contain the old star groups and many familiar patterns from earlier times.
All about Vega
The Summer Triangle is made up of one star each from Lyra, Cygnus and Aquila. Sir Patrick Moore popularised the name during his early Sky at Night TV programmes in the 1950s.
One vertex (point) of the asterism and one end of the base of the Triangle comes from the small constellation Lyra, the fifth brightest star in the sky, Vega. Vega is only about a tenth of the age of the Sun, but since it is 2.1 times as massive, its expected lifetime is also one tenth of that of the Sun; both stars are at present approaching the midpoint of their life expectancies.
The second star forming the base of the Triangle is Deneb, in the constellation of Cygnus. Deneb is the 19th brightest star in the sky and is also one of the most luminous nearby stars. Deneb’s mass is estimated at 20 solar masses. As a blue-white supergiant, its high mass and temperature mean that the star will have a short lifespan and will probably go supernova within a few million years. It has already stopped fusing hydrogen in its core.
The constellation contains the well known X-ray source Cygnus X-1, the first one to be accepted as a black hole candidate. It is a binary star system that includes a blue supergiant variable and a compact object, one that cannot be classified with certainty but is small for its mass, e.g. a white dwarf, a neutron star or a black hole.
The Swan
Cygnus, one of the most recognisable constellations of the northern summer and autumn, resembles a swan flying south along the Milky Way. The shape of the Swan is a prominent asterism known also as the Northern Cross (in contrast to the Southern Cross). Albireo, Beta Cyg, is at the “head” and Deneb, is at the “tail”.
Albireo appears to the naked eye to be a single star of magnitude 3 but through a telescope, even low magnification views resolve it into a double star. The brighter yellow star (actually itself a very close binary system) makes a striking colour contrast with its fainter blue companion star
Orpheus, the musician and poet, was said to have been transformed into a swan after his death at the hands of the Maenads. Zeus placed him in the sky, along with his lyre after Zeus had sent an eagle to retrieve it from the river into which it had been thrown. Lyra was often represented on older star maps as a vulture or an eagle carrying a lyre, either enclosed in its wings, or in its beak.
In another tale, Zeus transformed himself into a swan to seduce Leda, wife of King Tyndareus of Sparta, who later gave birth to Castor, Pollux, Helen (of Troy) and Clytemnestra.
The Chinese identify the constellation with Que Qiao, the “magpie bridge” that connects the lovers Niu Lang (“the cowherd”) and Zhi Nu (“the weaver girl”) once a year. It is said that the Goddess of Heaven found out that the two were married and separated them because Zhi Nu, a fairy, could not be with a mortal. Her husband then took their two children (Beta Aquilae and Gamma Aquilae) and went up to Heaven to be with his wife, but the Goddess would not allow this. She created a wide river in the sky with her hairpin to keep them separated. The river is the Milky Way between Altair (Niu Lang) and Vega (Zhi Nu). The story says that once a year, all the magpies in the world form a bridge so that the two can be together, and the constellation represents the celestial bridge.
Altair
The third vertex of the Triangle is Altair, the 12th brightest star in the night sky. Located 16.8 light-years from Earth it is one of the closest stars visible to the naked eye.
Altair possesses an extremely rapid rate of rotation; it has a rotational period of approximately nine hours. For comparison, the equator of the Sun requires just over 25 days for a complete rotation.
In classical Greek mythology, Aquila was identified as the eagle that carried the thunderbolts of Zeus and was sent by him to carry the shepherd boy Ganymede whom he desired, to Mount Olympus; the constellation of Aquarius is sometimes identified with Ganymede.
In illustrations of Aquila that represent it as an eagle, a nearly straight line of three stars symbolizes part of the wings. The centre and brightest of these three stars is Altair.
NASA’s Pioneer 11 space probe, which flew by Jupiter and Saturn in the 1970s, is expected to pass near the star Lambda Aquilae in about 4 million years.
Exoplanets have been found in all three constellations, and the Kepler space observatory is expected to find more. The spacecraft’s photometer points to a field in Cygnus, Lyra and Draco, which is well out of the ecliptic plane; so that sunlight never enters the photometer as the Kepler orbits the Sun. Cygnus is also a good choice to observe because it will never be obscured by Kuiper belt objects or the asteroid belt.
Astronomical society meeting
The Cayman Islands Astronomical Society meets once a month at Pedro Castle, and is planning to meet tonight, Friday, 2 September, at 7.30pm. All are welcome.
Related Videos
