vendredi 19 septembre 2008

Hercules 5

Unless you are an avid stargazer, you might not be sure just where to look for Hercules. While the fifth largest constellation, it isn't very obvious.
And yet Hercules boasts one of the finest collection of binary stars, and two Messier objects as well.
We will make a fine distinction here: the constellation name is Hercules, while the Greek hero is Heracles.
Heracles was named after the greatest of Greek goddesses, Hera. Her name means "Lady" and she was the daughter of Cronus, and sister of Zeus (they were twins). Zeus later changed into a cuckoo and seduced his sister (he had that kind of reputation), and the two were married.
Hera became the Queen of the Heavens: goddess of childbirth, marriage, and of women, she was the most widely beloved of goddesses in antiquity. It would only be natural that the greatest of Greek heroes would be named after her: Heracles means "the glory (or honour) of Hera".
Although named after Hera, Heracles didn't have her immediate respect. Heracles was the son of Zeus and a mortal woman (Alcmene). Hera resented Zeus' philandering nature, and tried to have the child killed. She sent two monstrous snakes to his crib, but the infant strangled them both with his pudgy little hands.
Heracles became a favourite with the gods. Apollo made his bow and arrows; Athene gave him a magnificent robe; Hermes provided him with a sword, and Castor (the greatest warrior) taught him how to use it. Hephaestus, the smithy of the gods, made a golden breastplate for Heracles. Thus armed and protected, Heracles paraded through Greek mythology, performing eight heroic deeds and the Twelve Labours.
In fact, the very word "hero" has links with the names Hera and Heracles. The Romans would change his name to Hercules (and hers to Juno, and Zeus to Jupiter).
"Hercules" came to Italy in his tenth labour. He would later be given credit for abolishing human sacrifice in the land.

The constellation was originally represented as a kneeling man, with a foot on the neighbouring dragon (Draco). Some star names reflect this earlier association.

Is a sprawling constellation just to the west of Lyra. From Vega (alpha Lyrae) swing to the west-southwest eight degrees. This is theta Herculis, a 3.86 magnitude star - which is about typical brightness for the main stars of this constellation.
The principal stars are found farther south. Star hop from theta over to pi Herculis, and then to the southwest (about the same distance from pi Herculis to Vega) is beta Herculis, which is actually the brightest star in the constellation.
Now look southeast and you will come across alpha Ophiuchi (Ras Alhague), at 2.1 magnitude, the brightest star of the region. Alpha Herculis is northwest of this star.
Alpha Herculis is better known as Ras Algethi: The kneeler's head. It is estimated to be from 430 to about 650 light years. Some authorities believe the star to be as large as 400 solar diameters.
This is a fine double: a red (or orange) supergiant and a blue-green giant (see below). The primary is also an irregular variable (see below).

Double stars in Hercules:
Hercules has several binaries with contrasting colours, as well as several close binaries, challenging those with larger telescopes.
Alpha Herculis is a visual binary with a very long period, something like 3600 years. 3.2, 5.4; PA 104, separation 4.6".
Zeta Herculis is a rapid binary with colour contrast, a yellow primary and red companion with a period of 34.45 years: 2.9, 5.5. The 2000 values: PA 12º degrees, and the separation 0.7".
Kappa Herculis is an easily resolved binary: 5.3, 6.5; PA 12 degrees, separation 28.4".
Rho Herculis: two white stars which make a lovely double. 4.6, 5.6; PA 326, separation 4.1".
95 Herculis is a very attractive double with contrasting colours, often described as gold and silver (although you may disagree): 5.0, 5.1; PA 258 degrees, separation 6.3".
99 Herculis is a very close rapid binary: 5.1, 8.4; currently the PA is 92 degrees and the separation 0.3".
100 Herculis is another gorgeous binary of two equal white stars easily resolved. 5.9; 5.9; PA 183 degrees, separation 14.2"
Struve 2319. This is a very beautiful binary of two rather faint stars: 7.2, 7.6; PA 191 degrees, separation 5.4".
Variable stars in Hercules:
Alpha Herculis is an irregular variable with a range from 2.7 to 4.0, with a period of roughly three months.
S Herculis is the brightest long-period variable in Hercules, with a visual magnitude range of 6.4-13.8 every 318.14 days. The maximum for the year 2000 should occur in mid July.

Deep Sky Objects in Hercules:
There are two Messier objects in Hercules: M13 and M92.
M13 (NGC 6205) is a spectacular globular cluster sometimes known as "The Hercules Cluster". It is universally acclaimed as the best globular in the northern hemisphere.
This is a very compact cluster of over a million stars. It is also very old - at an estimated age of ten billion years. It's around 25,000-30,000 light years away.
M13 lies on a line between eta Herculis and zeta Herculis, due west of pi Herculis.
M92 (NGC 6341) is also a globular cluster, located nine degrees northeast of M13, and six degrees directly north of pi Herculis.
M92 is also very striking and worthy of consideration, even if considerably overshadowed by M13.

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