mardi 23 septembre 2008

Serpens 2

Aesculapius, we are told, was the first doctor of medicine, and his expertise led to his downfall in a very strange way. His career seems to have begun when one day while visiting a friend and saw a snake in the room and killed it. Then, to the great surprise of both, a second snake carrying an herb in its mouth crawled into the room. It gave the herb to the first snake, which immediately recovered. It was this herb, which Aesculapius took from the revived snake, that taught him the great powers certain herbs have over life and death. He traveled far and wide over the land, always learning more about the medicinal use of herbs, and before long his reputation as a saver of lives had become widely known. So expert had he become that Hades, God of the Underworld, complained to his brother Zeus that fewer and fewer souls were being sent down to the Underworld. Hades, of course, was worried about losing his important position. Aesculapius once is said to have brought Hippolytus back to life by "gluing" him back together. Hippolytus had been dragged to death and dismembered when his horses were frightened by a bull. Just as Aesculapius was about to bring the famous hunter, Orion, back to life after he had been accidentally shot with an arrow by his lover, Hades' patience ran out. He demanded that Zeus stop this wholesale restoring of life. After all, only the gods were immortal. If Aesculapius were permitted to increase his skill in bringing the dead back to life, mankind, too, would have attained immortality. Thus went Hades' argument to Zeus. Zeus agreed with his brother and hurled a thunderbolt at Aesculapius, killing him on the spot. But Zeus could not help but admire the skills of Aesculapius and so raised him among the stars as Ophiuchus, along with the serpent from which he had learned his skills. Aesculapius, as the God of Medicine, is always shown with a staff with a serpent wound around it. You have seen the symbol in hospitals and doctors' offices. Our words hygiene (meaning "the science of health") and panacea (meaning "a cure-all medicine") come from the names of two of Aesculapius' daughters, Hygeia and Panacea.

This region of the sky has been the scene of a number of nova stars, or stars that suddenly flare up into brightness for several days or weeks and then return to their former dimness. The first such nova was reported by the Greek astronomer Hipparchos in 134 B.C.; the second in A.D. 123; third in the year 1230; the one called Kepler's Star in 1604; and the fifth in 1848.

This is the second part of the Ophiuchus- Serpens group. The Serpent is being grasped in the hands of Ophiuchus the Serpent Holder. Thus the constellation wraps around Ophiuchus, and is divided into two parts: Serpens Caput (the head) and Serpens Cauda (the tail).

The constellation Serpens is spread across a greater part of the sky than is Ophiuchus, but it has far fewer features of interest. Still, there are several Messier objects and some very nice binaries.
There isn't any remarkable asterism in Serpens, and it might take some effort to decide just which stars belong to this constellation, and which belong to Ophiuchus. That is, the Bayer stars of Serpens compare in brilliance with those of Ophiuchus.
The brightest star, alpha Serpentis, is called Unukalhai, meaning "Neck of the Snake". It is 67 light years away, and is approximately ten times the size of the sun.

Double stars:
Serpens has three visual binaries of some interest, two of which are very attractive, and one which will test your observing skills.
Beta Serpentis (Struve 1970) is a wide visual yet difficult to observe due to the brightness of the primary compared to the faint companion: 3.0, 9.2; PA 265º, separation 30.8".
Theta Serpentis (Struve 2417) is a wonderful binary of two white stars: 4.0, 4.2; 103º, 22.2".
Struve 2375 is a superb pair: 6.2, 6.6; 116º, 2.4".

Variable stars:
R Serpentis is a long-period variable, 5.2-14.4, with a period of 356.41 days. In the year 2000 the star should have had a maximum on about 5 February.
The star is located 1.2 degrees ESE of beta Serpentis, nearly midway between beta and gamma Serpentis and very slightly south of a line drawn between them.

Deep Sky Objects:
There are two Messier objects in Serpens: M5 and M16; the first is found in the "head" of the serpent, the second in the "tail".
M5 (NGC 5904) is a spectacular globular cluster, containing a half a million stars. The cluster is quite compact and rather bright; it is about 25,000 light years away, and ten billion years old.
The cluster is found eight degrees SW of alpha Serpentis.
M16 (NGC 6611), "The Eagle Nebula", is a remarkable open star cluster surrounded by a huge nebula, very luminous with dark streaks of dust: a nursery of newly forming stars. Best seen in large scopes; a nebula filter might help.
The cluster is fifteen degrees south of eta Serpentis, but an easier way to find it may be to draw a line from eta Serpentis to xi Serpentis, to the SW. Now midway along that line is found the bright star nu Ophiuchi. Draw an imaginary perpendicular out from nu Ophiuchi, southeast. About seven degrees along this line is M16.
If this seems a bit complicated, first try locating M17, The Omega Nebula (or Swan Nebula), in Sagittarius. Two and half degrees north is M16.

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