DANGEROUS WATERS
Long lines of stars seemed to intrigue the ancients. Draco the Dragon wraps itself around the
northern pole, Eridanus (the River) drains the sky from Orion's foot to its end in the southern hemisphere at brilliant blue Achernar, and Hydra, the Water Serpent, slithers to the east-
southeast from its scary head below
Cancer nearly to Libra, making it not only the longest constellation in the sky but also the
largest, just barely beating out Virgo for the dubious honor. As if a Water Serpent
had not been enough, early explorers of the southern hemisphere
invented Hydrus, the Water SNAKE, which
Bayer included in his Uranometria of 1603. While the pairing of
the two can be confusing, Ian Ridpath points out that they fall
into the "major-minor" tradition that pairs big and small bears,
dogs, triangles, horses, as well as northern and southern
couplets that include crowns, triangles, and flies (the Minor
Triangle and Northern Fly no longer recognized). Hydra is
peculiar in that it has two ancient constellations riding on its
back, Corvus the Crow (which perhaps
makes some sense) and dim Crater, the Cup
(which doesn't), dividing it into crude thirds. Hydrus is
peculiar in that it winds so as to avoid the two Magellanic
Clouds, our brightest satellite galaxies.
While celestial treasures within the two constellations are not
numerous, the ones that are there grab attention. Hydra's
luminary, Alphard, leads the naked-
eye list. At the heart of the Serpent, the name from Arabic
means "the solitary one," the second magnitude (1.98) star
shining in lonely splendor to the southeast of Regulus in Leo.
While a relatively common orange helium-fusing K giant 180 light years away,
Alphard has an unusually high mass 4.5 times that of the Sun and radiates at a rate of nearly 1000
solar luminosities. It has the distinction of being a mild sort
of "barium star" that was contaminated by heavy elements from
the wind of a more massive companion that is now a white dwarf. No
degenerate neighbor, however, has ever been found, and in fact
the star might not be contaminated at all. Stellar oddities
include a fourth magnitude class A0 dwarf near Hydra's extreme
western edge that goes by both "c" Hydrae (from Bayer) and 30
Monocerotis (from Flamsteed), the star once
included in Monoceros (the Unicorn) but
orphaned by modern boundaries. While no planet
has been discovered, 30 Mon is
surrounded by a dusty disk similar to that around Vega in which one or more planets may well
orbit. Hydra also fools us, as the second brightest star is not
Beta Hya (an interesting fourth
magnitude binary 310 light
years away with a magnetic main component), but third magnitude
Gamma, another common G8 helium-
fusing giant 134 light years away. Follow Corvus's two western
stars downward to find Beta and the bottom two eastward to
locate Gamma, which lies below Spica.
The most interesting Greek-
lettered star in the huge constellation may well be the
northernmost in Hydra's evil-looking head, Epsilon, a multiple system 129 light years
away with at least four components and a rarity in having not
one, but TWO determined orbits. Separated by only a few tenths
of a second of arc, Epsilon Hydrae A (a class G giant) and B (an
A8 dwarf) revolve around each other every 15 years at an average
distance between them of 17 Astronomical Units. Then Epsilon C
(an F7 dwarf) orbits the inner AB pair with a period of 590
years 130 AU apart, the visual double of AB and C separated by a
couple seconds of arc and accessible to the amateur's telescope.
The spectrum then reveals that Epsilon Hya A (the G giant) has a
much closer low mass companion that orbits in just 9.9 days.
Given that "A" has a mass of around 2.5 Suns, the little one
goes around just 0.13 AU from its giant master and close to its
surface. The tight double radiates X-rays, so one or both are
magnetically active rather like the Sun but aided by the
duplicity. Way outside these are D, E, and F. All three,
however, are just line of sight coincidences, so we can quit at
four.
Hydra's most famed star may be the classic Mira variable R
Hydrae, a red M6-M8 giant located just a couple degrees east
of Gamma. A dying star 400 light years away, radiating in the
tens of thousands of Suns, R Hya has become highly unstable,
varying between a readily-visible fifth magnitude (or brighter)
and ninth or tenth over a decreasing period of around 388 days.
Though visually much fainter, V Hydrae takes a notable position
as one of the weirder stars, not just of Hydra but of anywhere.
It's a very red carbon-rich
giant that varies by 1.5 magnitudes over 130 days upon which
is superimposed another variation twice that of 17 years,
probably as a result of ejection of carbon-loaded dust shells.
A by-product of helium fusion, the star's carbon has been
brought up by convection from below. Most of the carbon in the
Universe, including that here on Earth, was created by such
stars.
As Mira and related variables lose their outer envelopes, they
expose their hot cores, which will eventually become carbon
white dwarfs. A hot wind blowing from the core shovels the
fleeing envelope into a complex shell, which in turn is ionized
and illuminated by the hot core's ultraviolet radiation,
birthing an ephemeral planetary nebula.
Just west of Crater, Hydra holds one of the real classics,
bright NGC 3242, the "Ghost of
Jupiter." It's a prototypical double-shell nebula with bipolar
jets of the kind best seen in Herschel's discovery nebula, NGC 7009 in Aquarius. Around 1600 light years away and half a
light year wide, NGC 3242's twelfth magnitude central star
shines at around 90,000 degrees Kelvin with a luminosity of 1000
Suns, almost all in the invisible ultraviolet. Far less known
is NGC 2610 in western Hydra. It's a particularly high
excitation planetary nebula with a central star so hot that
standard temperature determinations do not work, but at least
show that it tops 100,000 Kelvin.
Hydra is home to three very different sorts of Messier objects.
In far western Hydra find the near-naked-eye sprawling open
cluster M 48. Nearly a degree
across, 2500 light years away, almost half a billion years old,
M 48 is a nice sight in binoculars. Next, pointed downward to
by Corvus's eastern edge, M 68 is a tiny globular cluster, one of the
few in the celestial hemisphere that lies opposite the Galaxy's center. Even more
special, it has one of the lowest metal contents known for any
of its kind, its iron-to-hydrogen ratio just 0.38 percent of
that found in the Sun. The cluster is a true halo object formed
as the Galaxy was being born. Third is M 83, a prominent barred
spiral galaxy called the "southern pinwheel." Eight degrees
south-southeast of Gamma Hydrae, it lies 15 million light years
away and recedes from us at a speed of 515 kilometers per
second.
Far to the south, Hydrus serves up three rather diverse third
magnitude stars. At magnitude 2.80, Beta
Hyi just barely tops the short list. As a G2 subgiant just 24 light
years away rather similar to the Sun, it's clearly the most
interesting as well, though no planets have been found. The
closest bright star to the South
Celestial Pole, Beta Hyi served as the southern pole star
around 150 BC, but has now moved away thanks to the 26,000-year
wobble of the Earth's
rotational axis. Magnitude 2.86
Alpha Hyi, an F0 dwarf, so
served around 2900 BC. At magnitude 3.24, Gamma Hyi provides contrast as a class
M2 red giant around 215 light years away.
While Hydrus manages to miss the two Magellanic Clouds (the
Small Cloud 60,000 light years away in neighboring Tucana, the Large Cloud 50,000 light years away in
Dorado), it still manages to get a tiny
piece in the form of a relatively young globular cluster, NGC
643, in the SMC's outer halo. Beta Hyi however provides a
beautiful guide to one of the greatest of all globulars, 47
Tucanae five degrees to the north, which helps maintain the old
(paraphrased) adage that the Good Lord put most of the
astronomers in the north but most of the good stuff in the
south.
Copyright © James B. Kaler, all rights reserved.
These contents are the property of the author and may not be
reproduced in whole or in part without the author's consent
except in fair use for educational purposes. First published in
the August 2015 Newsletter of the Lowestoft and Great
Yarmouth Regional Astronomers, who are gratefully acknowledged.