By Jim Kaler

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.