vendredi 19 septembre 2008

Lyra 5


The lyre is one of the most ancient of musical instruments. For example, in the royal city of Ur (circa 3000 BC) musicians played the lyre for royalty, according to excavated artifacts.
In Greek mythology, the lyre was invented by Hermes. When only a child, he pulled a cow-gut across a tortoise shell, and thereby created the lyre. Hermes gave this lyre to his half-brother Apollo (both were fathered by Zeus). As the god of music, Apollo became associated with the instrument.
Orpheus was given the instrument by Apollo when only a child, and the Muses taught him to use it. Even Nature herself would stop to listen, enraptured by his music.
When Eurydice, the wife of Orpheus, died from a snake bite and was taken to the Underworld, Orpheus followed in hopes of bringing her back. His playing convinced Hades to release Eurydice, providing Orpheus didn't look back at her during the journey home - but just as he emerged into the sunlight Orpheus turned and gazed upon his wife, and lost her forever.
There are several versions about the death of Orpheus. In the most widespread version Dionysus invades Thrace, home of Orpheus, and the female followers of Dionysus (the Maenads) tear Orpheus from limb to limb. His head is thrown into the river Hebrus, where it floats to Lesbos, singing the entire time.
The lyre of Orpheus is also thrown into the river, and it too floats to Lesbos, beached near the temple of Apollo. Apollo then convinces Zeus that the instrument should become a constellation. Zeus agrees, and places the lyre of Orpheus between Hercules and Cygnus.


The constellation is small and rather faint, but it is home to the fifth brightest star, Vega. The asterism resembles some multi-legged creature more than it does a lyre, with Vega at the head.
The constellation hasn't the full complement of Bayer stars, and only three stars are brighter than fourth-magnitude. Still, there are some very fine objects to view.


Vega, "Falling Eagle" or "The Harp Star", is only the fifth brightest star, but it dominates the summer skies in the northern hemisphere, with a transit date of 1 July.
About 12,000 years ago Vega served as the Pole Star, and it will again in another 12,000 years. Beta Lyrae, sometimes known as "Sheliak" (Tortoise), is a prototype of a variable star in which a close companion is transferring matter to its gigantic primary. In Beta Lyrae's case, the transference is occurring very rapidly. Eventually the system will become an Algol variable. (See below for its values.)


Double stars in Lyra:
Delta2-Delta1 Lyrae form a wide binary that may be gravitationally bound despite the great distance. The two have a nice colour contrast, orange and blue. Note that delta2 is the primary: 4.3, while delta1 has a visual magnitude of 5.6.
Beta Lyrae is a fixed multiple binary, with a primary of 3.5. AB: 3.5. 8.6; 149º, 46"; AE: 9.9, 318º, 67"; AF: 9.9, 19º, 85".
Epsilon1-Epsilon2 Lyrae: the famous "Double- Double". All four stars are fifth-magnitude. The two principal stars form a very wide binary: PA 173º, separation 208".
Each star is itself a double:
Epsilon1A-Epsilon1B is a slow binary with 1165 year orbit: 5.0, 6.1; PA 350º and separation 2.6".
Epsilon2C-Epsilon2D is about twice as fast, with a period of 585 years: 5.2, 5.5; PA 83.5º, separation 2.3".
Zeta Lyrae is another relatively fixed multiple. The brightest component is D: AD 4.3, 5.9; 150º, 43.7". The other components are fourteen magnitude.
Struve 2470 and Struve 2474 form another fine double-double, that some say is equal to epsilon1,2.
Struve 2470: 6.6, 8.6; 271 degrees, 13.4" and Struve 2474: 6.5, 8.6; 261º, 16.4".
The two binaries are found two and a half degrees NE of gamma Lyrae, which is the brightest star in the region. Or, if you can find iota Lyrae, drop south one and a half degrees. It's a sight well worth the detour!


Variable stars in Lyra:
Alpha Lyrae (Vega) is a delta Scuti type variable, fluctuating from -0.02 to 0.07 every four hours 33.6 minutes.
Beta Lyrae is an EB variable: 3.25 to 4.4 with a period of 12h22m.
RR Lyrae is a prototype for a pulsating type of variable with short periods, usually less than twenty-four hours. RR Lyrae's period is 13h36m, and it changes in magnitude from 7.1 to 8.1.


Deep Sky Objects in Lyra:There are two Messier objects in Lyra: M56 and M57.
M56 (NGC 6729) is a globular cluster, very condensed. It is found eight degrees due south of theta Lyrae.
M57 (NGC 6720) known as the Ring Nebula, is the finest planetary nebula in the skies. The ring itself should be clearly visible in medium scopes, while the fourteen magnitude central star may take a little longer. Burnham gives an excellent discussion on this object.
It is located between beta and gamma Lyrae (slightly closer to beta), and is about 4000 light years distant.

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