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Like a number of other constellations, the Hydra concerns the story of
Heracles' labours. This time it is the Second Labour of Heracles.
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Having killed the Nemean Lion, and wrapping the protective pelt around
himself, Heracles set off to find the Hydra. This was a gigantic
dog-like beast with many heads. Some say it had seven, others as many as
fifty. Euripides, with characteristic exaggeration, gave it ten thousand heads.
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Seven is usually the accepted number, possibly because of the seven rivers in this part of the Peloponnesian peninsula, tributaries of the river Amymone.
At any rate, one of these heads was immortal, which meant the beast was
immortal. Its breath was deadly and if one even smelled its tracks, he
would die.
With the help of Athene, Heracles locates the monster's lair and soon
he is in a life and death struggle with the beast. Heracles slices off one of the monster's heads, only to see a couple more grow in its place.
Heracles calls out to his chariotter Iolaus for help. Iolaus sets the
grove on fire and then begins cauterizing the bleeding stumps. With the
blood flow stopped the Hydra's heads cease to multiply.
Finally Heracles cuts off the immortal head and - as it is still furiously hissing away - he promptly buries it under a huge rock. He cuts up the rest of the Hydra's body and dips all his arrows in the dead monster's gall. The slightest scratch from one of these arrows would bring instant death.
While one might be tempted to say "well done", Eurystheus (who had assigned these labours to Heracles) was not satisfied. Heracles had cheated, he said, since he had needed Iolaus' assistance.
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One might wonder just why Heracles had been forced to do all these labours. Before either he or his twin were born, Hera decided that Eurystheus rather than Heracles should become the king of Mycenae. So she hastened his birth; Eurystheus was born two months premature. And Heracles, being the younger of the two, became subservient to Eurystheus.
Thus born premature, this king of Mycenae was weakly and timorous. He
commanded Heracles to perform twelve labours in twelve years in order to
keep him away from his own kingdom, afraid Heracles would take his
throne. Eventually he would be killed by either Heracles' son (Hyllus)
or by his charioteer, Iolaus.
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As a constellation Hydra sinuously winds down from
the northern hemisphere, bordering Cancer, to as far in the southern
hemisphere as Centaurus, stretching out to around one hundred degrees in the
process. At one point its body is actually cut off by another constellation
(the southwestern tip of Crater).
Although the constellation is best seen in March and April, its brightest
star, alpha Hydrae, transits on 12 February. Slowly over the next few
months the Hydra then slithers over the skies earlier and earlier.
To find Hydra, locate Regulus (alpha Leonis). Now drop straight down
twenty degrees, passing through the uninspiring constellation of
Sextans. The bright star to the west is Alphard (alpha Hydrae).
You've just found the "heart" of the Hydra. To find its head,
look for the compact stars to the northwest. It is easiest to do this with
the naked eye, or with binoculars. These stars are not quite as far north as
Regulus.
The tail of the monster stretches far to the east, even past Spica (alpha
Virginis). You'll have to wait until the small hours of the morning to see
these stars, or until later in the spring when they appear at a more reasonable
hour.
There are a number of interesting deep sky objects, including two Messier
objects, and several very nice double stars. All in all, Hydra's
stars are of average brilliance, except for
Alphard (alpha Hydrae).
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Alphard is a bright giant, about 25 times the Sun's diameter. Its distance
is 89 light years, and it has a luminosity of 95 Suns.
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The Arabs called the star "Al Fard al Shuja" (The Solitary One in the
Serpent) while later on Tycho Brahe named it "Cor Hydrae" (The Hydra's
Heart).
Beta Hydrae is the southern-most star of any importance. Quite
dim for its name, the star is a visual binary (see below).
Double stars in Hydra:
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Beta Hydrae is a pair of nearly equal stars (4.7, 5.5) at PA 37º and a separation of 0.7".
Epsilon Hydrae is a multiple binary; four stars can be seen and
another has been calculated to exist.
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Components A and B form a rapid binary with a period of 15.05 years; its
orbit is nearly circular.
Component C is much easier to resolve, with a period of 990 years. At present it can be found at a PA of 304º and separation 2.7".
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Chi1 Hydrae is a binary of two similar stars (5.8, 5.8) with an even
more rapid orbit. Its period of 7.4 years means
an exceedingly difficult binary to resolve.
Struve 1474 is a fixed binary forming a wonderful triple. AB: 6.7, 7.0;
28º and separation 67.8"; C: 6.7.6, 28º, 73.3" separation.
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To find the binary, locate nu Hydrae then move one degree northwest.
(Just north half a degree is the nearly attractive Struve 1473.)
For more details on these and other binaries see the Binocular Section link below.
Variable stars in Hydra:
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R Hydrae ranges from a visual magnitude of a fairly bright 3.5 to
a faint 10.9 every 388.87 days. However this star has shown a decrease in
its period. (In 1920 it had a 404 day period.)
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This star is one of the earliest variables to be catalogued, having been
discovered in 1704. Only Mira (omicron Ceti), beta Persei, and chi
Cygni predate R Hydrae.
Burnham gives a finder's chart. Note that the brightest star in the
region, SS Hydrae, is also a variable, so don't base R Hydrae's visual
magnitude on this star.
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T Hydrae has a shorter period, 298.7 days, and a range of 6.7-13.5.
To find T Hydrae, star hop westward from Alphard. First, due west of
alpha Hydrae you find 24 Hydrae, then 20 and 19 (which is the brighter
of the two). Now just about the same distance that you've taken to get
to 19 Hydrae you'll find T Hydrae, due south of 17 Hydrae one and a half
degrees.
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This star, 17 Hydrae, is the brightest star in the region, and a visual
binary as well (7, 7; 359º, 4.3"). The star is useful to determine
the visual magnitude of T Hydrae to the south.
Deep Sky Objects in Hydra:
Hydra has three Messier objects: M48, M68, and M83.
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M48 (NGC 2548). Messier actually gave the wrong location for this
star cluster, putting it four degrees north of the current position. But
by his description this seems to be the right object.
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Not terribly spectacular, this is a group of fifty stars, the brightest
of which is about 8.8 visual magnitude. The cluster is thought to be
about 1700 light years away, and is easily seen in binoculars or small
telescope.
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M68 (NGC 4590) is a much richer globular cluster of over a
hundred thousand stars, resolved in medium-sized telescopes.
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The cluster lies in a desolate part of the sky. Locate gamma Hydrae then
move west to beta Corvi. Now drop down three degrees to the brightest star
in this region, a fifth-magnitude star (this is the binary B230: 5.5, 12;
170 degrees, separation 1.3"). M68 is about a half degree to the northeast.
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M83 (NGC 5236) is a spiral galaxy sitting on the Hydra-Centaurus
border farther to the east, nearly twenty degrees south of Spica (alpha
Virginis).
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While Burnham says this is considered one of the brightest galaxies with
a visual magnitude of about 8, other references give it only a 10. And
since it is very diffuse, it may be difficult for those living in
northern latitudes.
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NGC 5694 is an extremely compact globular star cluster, thought
to be in the region of 100,000 light years away.
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The cluster sits just east of the mid-way point between pi Hydrae
and sigma Librae, at the border with Hydra. From pi Hydrae
move east until you encounter a group of five magnitude stars lined up
roughly north-south. These are 54, 55, 56, and 57 Hydrae. NGC 5694 lies
one degree west of 57 Hydrae.
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NGC 3242 clearly deserves to be a Messier object. Small but bright,
at a visual magnitude of 8.6, this planetary nebula is often called The
Ghost of Jupiter because of its slight resemblance (?) to that planet.
Also at times called The Eye Nebula, perhaps a closer description.
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The nebula gives off a soft bluish colour, sometimes described as a "glow",
that is clearly visible even in small scopes. The central star may be
difficult to resolve: this is an 11.4m star, a blue dwarf considered to
be as hot as 60,000 kelvin.
The nebula is one of the easiest to find. Just locate mu Hydrae
then move south two degrees.
Trying to resolve the inner ring could prove difficult. Large telescopes
should show the object as resembling an eye, with the central star the pupil.
The greenish-blue colour adds to this intriguing sight.
For a more detailed appreciation of Hydra, visit the Binocular Section.
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