HEROIC STARS
The 2004 Athenian Olympics allow our thoughts to drift back to
the lands and times of ancient Greece, to the greatest of Hellenic
heroes, Hercules, and to his feats and
labors. In the form of his grand constellation, he treads nearly
overhead across the northern sky between Arcturus and Vega,
holding within his mighty arms one of the most prominent of
celestial objects as well as a variety of other sights.
As part of his eleventh labor (the fetching of the Golden
Apples of the Hesperides for King Eurystheus), Hercules assumed Atlas's task of holding the Earth and
Sky. Appropriately, the constellation's most prominent asterism is
the quadrangle of stars called the "Keystone," an architectural
term for the central block that supports a stone arch. Extending
to the south is a conjoined quadrangle, and farther down, close to
Ophiuchus, is the Hero's Alpha star
(even though fifth brightest in the constellation), third magnitude
Rasalgethi, "the Kneeler's head."
As befits Hercules, Rasalgethi is one of the grand red giants
of the naked-eye sky. From 380 light years away, this cool (class
M5, 3300 Kelvin) star shines with the light of 17,000 Suns from a surface swollen to a diameter of nearly
four Astronomical Units (AU), larger than the orbit of Mars. Its
great size and internal construction make it somewhat variable in
brightness over periods of 50 - 100 or so days. Easily visible in
a small telescope is a companion, which itself is a close double
that is resolved only with the spectrograph, one that consists of
a G5 giant and F2 dwarf only 0.4 AU apart that orbit each other
every 52 days. While the spectrum lines of the orbiting stars
swing back and forth, there is another set of absorptions that do
NOT move and that come from the powerful wind of the bright M5
giant, which has encompassed the binary companion some 500 AU away.
This observation provided some of the first evidence that stars --
especially giants and supergiants -- vigorously lose mass.
Rasalgethi (which began life near 8 solar masses) is now in the
process of removing its outer layers as it prepares to become a
solar mass white dwarf
like Sirius B (a star shrunk to the size
of Earth with an average density of a ton in a sugar cube).
The constellation is best known, however, for the "Great
Cluster in Hercules," the globular cluster M 13, which vies with M 5 in Serpens for the title of visually
brightest in the northern hemisphere. To be barely visible to the
naked eye even though some 25,000 light years away, M 13 must shine
with the light of half a million Suns, the cluster's million stars
packed into a volume less than 40 light years across. Rare and
ancient, globulars were born before stellar winds (like those of
Rasalgethi) and explosions could enrich stellar birthclouds with
the by-products of nuclear fusion. Estimated to be just under 12
billion years old, M 13 has only five percent the iron content of
the Sun.
The 150 or so known globular clusters are members of the
Galaxy's extended halo. Concentrating toward the Galactic core,
most are in the direction of Sagittarius
and its environs. Hercules is fortunate then to contain not one,
but two of the best-known of the set, the other M 92. Somewhat
more distant, M 92 holds perhaps two-thirds the number of stars of
M 13, making it at best a binocular object. Nevertheless, M 92
tops M 13 in extremes. At 12.3 billion years (though note that
such ages are fraught with uncertainty), M 92 has a reputation as
being among the oldest of globulars, and is also one of the most
metal poor, its iron content well under one percent solar.
Two stars that lie across the Keystone from each other now
stand for attention. To the northwest is fifth magnitude 30 Herculis. About the same distance as
Rasalgethi, but slightly redder (class M6), cooler (3000 K), less
luminous (4000 Suns), and smaller ("only" about the size of Earth's
orbit), 30 Her is a noted "semi-regular variable" rather like its
bigger cousin, but with a well defined period of 89 days. "30" is
actually something of a Mira-type variable,
one that is brightening with a now-dead carbon core, and is also
preparing to pop its outer envelope to become a white
dwarf.
What comes between such giants and the white dwarfs? Witness
the "planetary nebulae" that, like
the famed Ring Nebula in Lyra (not far
away from Hercules), are made of compressed shells of expanding gas
illuminated by the ultraviolet light of extremely hot blue stars
within them. The central stars are actually the old nuclear-
burning cores of what were once giants, the shells the lost outer
stellar envelopes. While none is visible to the naked eye,
Hercules remarkably contains a naked-eye star that is in the
process of MAKING a planetary nebula, 89
Herculis. An almost-sixth magnitude mid-temperature class F
supergiant to the southeast of the Keystone, 89 is not only another
semi-regular variable (by about a tenth of a magnitude with a 70
day "period"), but is almost surely a "protoplanetary nebula."
Surrounded by a vast shell of its own making with an inner edge an
astonishing 100 AU across, it is apparently in the last stages of
mass loss. Slowly revealing its hot core, it represents what
Rasalgethi and 30 Her may well become. And you can admire it
without any optical aid at all.
The final step is the white dwarf, the fate of the Sun.
Though Hercules is filled with these faint stars (they are
everywhere), it does contain one truly grand example, one that gave
lasting public fame to the constellation. In 1934, near where
Hercules, Lyra, and Draco all join,
blazed forth a "new star," a "nova" that reached just shy of
first magnitude. Telescopic novae are common, while every couple
decades one hits within the top ranks of bright stars. Nova
Herculis 1934, which became know by the variable star name DQ
Herculis, paved the way for the understanding of novae when the
remnant star was discovered to be a close eclipsing binary with an
amazingly short orbital period of 4.65 hours. This star, and all
other novae, was thereby found to consist of a combination of a
low-mass ordinary dwarf and a compact, dead white dwarf. The
component stars of "pre-novae" are very close together, enough that
the ordinary dwarf is so distorted by tides that it passes fresh
hydrogen to the dense member. When the layer builds up enough
pressure and heat, it explodes as a natural hydrogen bomb, and yet
another nova blooms into the sky.
DQ plays the game well. The infalling mass, hitting to the
side, has caused the white dwarf to spin ever faster until it has
a rotation period of but 71 seconds, which causes the star to
flicker with the same period. Only 14th magnitude, DQ requires a
good-sized telescope to see. Nevertheless, perhaps while imagining
Hercules of ancient Greece, you might also look to the northern
part of his constellation, imagine the view of 1934, and appreciate
what transpired there, while at the same time appreciate the
stories of stellar life and death that the Hero holds on display.
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/December 2004 Newsletter of the Lowestoft and Great
Yarmouth Regional Astronomers, who are gratefully acknowledged.