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Hercules, a Constellation for July

Contributed by Paul Rodmell, Southland Astronomical Society.

HERCULES, pronounced HUR-kyuh-leez

Chart showing the constellation.

Hercules, the Roman name for the Greek mythological ahero, famous for his twelve labours, is the fifth constellation in order of size. Earlier visualisations were of an anonymous kneeling man, with one foot on the adjacent celestial Dragon, Draco. Some legends identify the constellation with the ancient Sumerian superman, Gilgamesh. The brightest stars are third magnitude and it lies low in the Northern sky for New Zealand observers.

To find the constellation, look low down in the north evening sky to find the bright stars Arcturus and Vega. Locate Hercules using the map. On the chart the horizon is marked for Auckland. Places south of Auckland will see less of Hercules. M 92 lies on the horizon for Invercargill.

Chart showing Hercules as seen to the north at about 11 pm on July 1 or 9 pm on August 1.

Hercules Constellation Serpens Constellation Ophiuchus Corona Borealis Constellation Bootes Constellation Virgo Constellation Lyra Constellation Cygnus Constellation Vulpecula Constellation Sagitta Constellation Aquila

Some stars and interesting objects in the Constellation

α Herculis (Ras Algethi, kneeler's head) is a red super-giant star about 600 times the Sun's diameter. Like most red giants it is an erratic variable fluctuating between magnitudes 3 and 4. It is actually a double star with a magnitude 5.4 blue-green companion, visible in small telescopes.

β Her (Kornephoros) is a magnitude 3.8 white star 140 light years away. It consists of a close pair of stars orbiting one another in about 14 months. A wide 10th magnitude companion is probably not connected with the other two stars.

γ Her is a magnitude 3.7 star nearly 200 light years away. It has a 9.9 magnitude companion 42" from it.

δ Her (Sarin) is a magnitude 3.1 white star 91 light years away. Small telescopes show a magnitude 8.8 star nearby, which is physically unrelated, making this a fine example of an optical double.

ζ Her is a magnitude 3.1 white star 31 light years away, with a close magnitude 5.6 red companion orbiting the primary every 34.5 years. The stars are closest in 2001, but were widest in 1990. Since William Herschel first measured the binary star, more than six complete revolutions have taken place.

κ Her (Marfak) is a magnitude 5.0 yellow giant with a magnitude 6.3 companion easily seen in small telescopes. The pair lie about 280 light years away.

ρ Her is a blue-white magnitude 4.5 star with a magnitude 5.5 companion lying 170 light years away. This is a fine binary for small apertures. The primary is a very close interferometric pair McA 48. 95 Her is a famous pair of stars lying 470 light years away, suitable for small telescopes. A great many double star observers in the 19th Century estimated the colours from "apple green and cherry red" of Piazzi Smith to both pure white. Colour estimates with older refracting telescopes were rather unreliable and the stars appear pale and deep yellow.

95 Her is a pair of 5th magnitude stars with a separation of 6.3". They were first measured by F Struve in 1829. During the 19th century there were reports of fluctating colours, but now they are seen as an identical coloured yellow pair.

M 13 (NGC 6025) is the finest globular cluster available to northern hemisphere viewers. Even at the low altitude southern hemisphere the Great Hercules Cluster is very effective, though not in the same league as omega Centauri or 47 Tuc. The dense broad centre is very bright and sparkling, and the outlying stars cover a diameter greater than the full Moon. It was found by chance by Edmond Halley of comet fame in 1714, and rediscovered by Messier in 1764, although he could see no stars in it, which is a comment on the quality of his telescope. Distance is estimated as 24,000 light years.

M 92 (NGC 6341) is a globular cluster only slightly inferior to its more famous neighbour M 13. It is easily seen in binoculars, though being more condensed, needs a greater aperture than M 13 to resolve its stars. It was discovered in 1777 by J. E. Bode and rediscovered by Messier in 1781, who saw no stars in it. Its distance is estimated as 26,000 light years. NGC 6210 is a 10th magnitude beautiful bluish planetary nebula in an attractive contrast with a number of field stars. It is round and bright with well defined edges. This excellent object for small apertures is enhanced by all types of nebula filters.

NGC 6210 is a planetary nebula, magnitude 9.7 with a bluish disc. It is slightly elliptical,, 20" by 16" with a 12th magnitude central star.

Visibility

Hercules is a large Northern hemisphere constellation, extending from a little north of the celestial equator to just over 50° north. As a result it is never very high from the Southern hemisphere, and the more northerly parts are not visible from the South Island of New Zealand.

α Her is in the more southerly part of the constellation and reaches an altitude of between about 30° and 40° from southern and northern New Zealand respectively. It and δ Her, about 10° lower, are due north and at their highest point in the sky about 11 pm on July 1 and 9 pm on August 1.


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