mardi 23 septembre 2008


Equuleus is the second smallest constellation, at 72 square degrees. The only constellation that is smaller is Crux. It is not a particularly conspicuous constellation; its brightest star is α Equ (Kitalpha), at magnitude 3.92m.
There are few variable stars in Equuleus. Only around 25 are known, most of which are obscure. γ Equ is an alpha CVn star, ranging between magnitudes 4.58m and 4.77m over a period of around 12½ minutes. R Equ is a Mira variable that ranges between magnitudes 8.0m and 15.7m over nearly 261 days.
Equuleus contains some double stars of interest. γ Equ consists of a primary star with a magnitude around 4.7m (slightly variable) and a secondary star of magnitude 11.6, separated by 2 arcseconds. ε Equ consists of four components. The brightest three are of magnitudes 6.0m, 6.3m, and 7.2m. δ Equ is a binary star with an orbital period of 5.7 years, which at one time was the shortest known orbital period for an optical binary. The two components of the system are never more than 0.35 arcseconds apart.

This insignificant constellation, second-smallest in the sky, first appeared among the 48 constellations listed by the Greek astronomer Ptolemy in the second century AD. It was unknown to Aratus 400 years earlier. The actual inventor is unknown; it may have been Ptolemy himself, or one of his predecessors such as Hipparchus in the second century BC.

Equuleus consists merely of a few stars of fourth magnitude and fainter forming the head of a horse, next to the head of the much better-known horse Pegasus. The early mythologists such as Eratosthenes and Hyginus never mentioned this little horse, but perhaps Ptolemy had in mind the story of Hippe and her daughter Melanippe, sometimes told for Pegasus but which seems more appropriate for Equuleus.
Hippe, daughter of Chiron the centaur, one day was seduced by Aeolus, grandson of Deucalion. To hide the secret of her pregnancy from Chiron she fled into the mountains, where she gave birth to Melanippe. When her father came looking for her, Hippe appealed to the gods who changed her into a mare. Artemis placed the image of Hippe among the stars, where she still hides from Chiron (represented by the constellation Centaurus), with only her head showing.
The fourth-magnitude star Alpha Equulei is called Kitalpha from the Arabic meaning ‘the section of the horse’, in reference to the whole constellation.

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