July 02, 2008

Hercules

Travelling high through the zenith on summer evenings is the constellation Hercules, a constellation so old that even the Greeks in their day couldn't really remember how it originated, so we have a series of educated guesses. On the one hand, the figure was Engonasin, a kneeling figure, but Eratosthenes later linked this figure with Draco and so the legend of Hercules slaying the dragon guarding the golden apples came into being in the sky.

The mythological story of Hercules has a few different versions, so I'm only giving you one of the most well-known. Zeus fathered a child on the mortal woman Alcmene, and he laid the infant at the breast of his wife, the goddess Hera, while Hera slept so that having drunk the milk of a goddess, Hercules became immortal. Hera vowed revenge, promising to make the life of Hercules horrible, and she did. Ultimately, the Oracle at Delphi directed Hercules, whose name was originally Alcides but changed now to mean 'glory of Hera,' to serve the king of Mycenae for 12 years, and that king gave Hercules a series of 10 tasks or labors to perform.

When Hercules successfully completed the tasks, the king wasn't satisfied and gave him two more. Among the tasks were stories I've talked about in many other constellations, including Hydra, Corvus, Crater, Cancer, Leo and Draco. Though it is the fifth largest constellation in the sky, it is one of the harder constellations to spot since it has no stars of 2nd magnitude or brighter. It is most easily found by spotting the four-star trapezoid shape known as the keystone, 20 degrees or so West of the bright summer triangle star Vega and about a third of the way along a line to the bright star Arcturus.

Though the major stars of the constellation Hercules are somewhat faint, there are plenty of interesting targets there. I'll use the keystone of Hercules as a guide to help find the stars. The stars designated alpha through delta are all found South of the keystone. Beta and Delta make up the southernmost two stars in the crooked hourglass that I usually look for when looking for Hercules. Delta Her (Sarin) is 12 degrees due south of Pi Herculis, which makes up the northeastern (upper left if you are facing South) star in the keystone, and Alpha Herculis (Ras Algethi) is another 10 degrees due South of that.

Ras Algethi translates as the head of the kneeling one, but it isn't clear where this name comes from for Hercules. Perhaps he was kneeling while stringing his bow. Or he may have been kneeling in prayer during a famous battle, a prayer his father Zeus answered in helping Hercules to win the battle. Ras Algethi is one of the largest red supergiants visible to us from the Earth, at a distance of about 380 light years, and it is usually about 2nd magnitude, though its brightness varies between a magnitude of 1.5 and 2.5 with a period of 128 days. This star also has a 5th magnitude companion, itself a binary of sun-like main sequence stars, and the companion(s) are close enough that they are enveloped by a shell of material being lost from Ras Algethi, a common trait among highly evolved supergiant stars.

Beta Herculis (Kornephoros, which means club bearer) is about as far West of Delta Her as Ras Algethi is South, and so the three stars make up a rough equilateral triangle of roughly equal brightness. In some traditions, this star is known as Rutilicus, perhaps a translation of a different kind of weapon sometimes carried by Hercules. This is a yellow giant star about half as distant as Ras Algethi, the closest easily visible analogue on the sky being Capella in the constellation Auriga, part of the Winter Triangle. Gamma Herculis, another yellow giant, is not properly named and is found a few degrees Southwest of Kornephoros and about a magnitude fainter.

Delta Herculis (Sarin) has a location I mentioned just above, and it is an unremarkable multiple, with several stars very close to the line of sight but only two of them connected physically. The main pair of stars are two bright main sequence stars a mere 0.06 arcseconds apart and orbiting one another with a period of less than a year at a distance from Earth of about 80 light years.

Now for the stars in the keystone of the constellation Hercules, starting in the Northeast corner, closest to Vega, with Pi Herculis. This is a 3rd magnitude K-class (orange) giant star about 300 light years away. As is typical with most giant stars we see, this star is a few times more massive than our Sun and large enough that it would almost engulf the orbit of Mercury if it were placed in the middle of our own solar system. In the northeast corner is Eta Herculis, an unremarkable (barely) 3rd magnitude double star that is best known as a finder star on the way to locating the famous globular cluster Messier 13.

Passing through M13 on the way to the Southeast corner, we find Zeta Herculis, a 3rd magnitude subgiant star with an orbiting companion. Kaler notes that this star shares a common proper motion with many other stars in the sky, making it the brightest star in a moving group similar to what we've seen elsewhere in some of the stars of Ursa Major. Most of the stars in this group are near the South Pole. Though Zeta is far from these stars in the sky, it is moving very quickly along our line of sight in the same direction. This is a moving group that is probably surrounding us.

Finally in the southwest corner, we have Epsilon Herculis, a very closely separated binary system consisting of two bluish main sequence stars about 2.5 solar masses each. This one is so close that we call it a spectroscopic binary. That means it is only detectable because of the Doppler shifting of the spectral line groups of each star moving independently over time, splitting and merging as the stars orbit a common center of mass and move along or across our line of sight.

To finish off the significant stars in the constellation Hercules, I'd like to talk about a pair of famous and archetypical variable stars. Both of them are in a little patch of sky about 7-10 degrees North and 1-3 degrees West of the bright star Vega, in the Northeast corner of Hercules. Both stars are cataclysmic variables, which are binaries in which one star is a white dwarf and the other a red giant losing mass to the white dwarf. As these stars orbit around one another, their light curves vary somewhat due to our changing perspective as different parts of the system are obscured or revealed.

Occasionally, though, these systems undergo cataclysmic outbursts when big lumps of matter from the unstable accretion disk dump onto the white dwarf, sometimes igniting the Hydrogen on the surface of the white dwarf in a brief outburst of fusion. So they are a type of nova, and there are many different subclasses. The two stars I want to discuss today are magnetic CV's, which mean the white dwarf has a moderate (DQ Her) or intense (AM Her) magnetic field. AM Her is the prototype of the strongly magnetic CV's.

Normally, when matter falls off of a red giant en route to accretion onto a white dwarf, its angular momentum spirals it out into a disk. When AM Her stars, the magnetic field is so strong on the white dwarf that the matter stream simply falls directly onto the magnetic poles of the white dwarf. The magnetic field is so strong that the binary system is tidally locked, so there are fewer degrees of freedom here compared to a normal accreting binary, but still lots of high-energy radiation and variability due to instability in the matter stream. The magnetic fields we are talking about here are many millions of times Earth's field.

DQ Her stars have a weaker magnetic field, and so a little bit of a disk can form but then gets disrupted close in to the white dwarf. Also, the stars are not synchronized. Both kinds of stars flicker constantly thanks to the instabilities in the matter accretion process, and the light that comes from the stars is strongly affected by the magnetism present, making it a neat little laboratory for a study of basic Physics. Of course, these systems are also supernova candidates. As the white dwarf accretes matter over time, it heats up, and some ultimately cross a temperature/density limit that causes explosive fusion throughout the star, like lighting a match in a room full of natural gas. The Type I supernova that results is one of the brightest objects in the known Universe, at least for a brief time.

Now the deep sky objects in the constellation Hercules, and any such discussion must begin with the famous Hercules Globular Cluster, also known as Messier 13. This is one of the rare Messier objects visible to the naked eye under ideal conditions at magnitude 5.8 (the limit is around 6.0). It is about 25000 light years away from the Earth, and it is located on the boundary of the keystone, about one third of the distance from Eta (Northeast corner) and Zeta (Southeast corner).

In a small telescope, this hazy globe of light appears to be about 10 arcminutes in diameter, one third that of the full moon, but the true extent is easily doubled with a larger telescope. Burnham quotes Mary Proctor's 1924 book about the heavens: "It is the finest of all the clusters in the northern skies, and is just visible to the unaided eye on a dark night. ... By means of photography, it has been possible to obtain a close-up view, as it were, of what may be termed literally a ball composed of thousands of suns, with outlying streamers curving outward as though wafted by a celestial breeze. ... The cluster is a mass of glittering starlight."

The actual star count is difficult to determine due to the very dense nature of the core, but it is surely well over a million stars. The cluster is one of the oldest in the galaxy, with an age roughly equal to the oldest objects ever observed at around 12 billion years. Though the center seems dense from our perspective, a little scaling experiment is suggested by Burnham. If we take the dense central core of the cluster and assume a million stars in the innermost regions, this is equivalent to taking a million grains of sand about 0.03 inch in diameter and sprinkling them throughout a spherical volume of space about 300 miles in diameter. Even here most stars are separated from their nearest companion by a mile or more, and this is one of the densest regions in the galaxy!

Of course, the view from one of these stars would be amazing. Imagine thousands of stars in the sky with brightnesses somewhere between that of Venus and the full moon. Inhabitants of the cluster would be hard-pressed to see beyond the local neighborhood due to the immense sky brightness and would have no idea of the galaxy or indeed the rest of the Universe that exists outside of the cluster. There is some very interesting raw material for speculative fiction authors, with one of the best being Asimov's famous novella "Nightfall", about a planet continually bathed in light from surrounding stars experiencing a brief time of darkness.

The less famous globular cluster in this part of the sky is Messier 92, about seven degrees due North from Pi Herculis (the Northwest corner of the keystone). This is a little less bright and a little bit less extended than Messier 13 but still a very nice target for a small telescope, which can easily resolve some of the hundreds of stars in the outer fringes of the cluster. Messier 92 is a little bit younger than M13 and a little bit further away. Like M13, it has very few variable stars, which has hindered our distance determination efforts (variable stars are the most commonly used and most reliable standard candles for distant clusters).

Some of the better images on the web clearly show the red giants in this cluster standing out against the general background. Since both M13 and M92 are quite old, it is worthwhile to look at this age determination since it is cosmologically significant. For a few decades, our best guesses as to the ages of these clusters came from an analysis of the red giants. We look for the least massive red giant in the cluster, and we work on the assumption that it has just left the main sequence. We know that stellar main sequence lifetimes depend on their masses, so by determining the mass of these newly minted red giants, we could then use their lifetimes as a lower limit on the age of the cluster (we assume all the stars in the cluster formed at roughly the same time).

Well, these ages were upwards of 16 billion years in some estimates, which contradicted was cosmologists were telling us (around 10-12 billion years). Since that time, cosmological age estimates have increased due to analysis of galaxy motions and the cosmic background. At the same time, new distance estimates to these clusters have come about thanks to the Hipparcos parallax measuring satellite. The result is the clusters are further away and therefore brighter than we thought. That increases our mass estimates of the red giants, which in turn reduces their lifetime and thus the age of the cluster, bringing them into good agreement. The other globular in Hercules, NGC 6229, is much further away and smaller than the two Messier objects.

Also present in Hercules is NGC 6210, about halfway between Sarin and Kornephoros in the southern half of Hercules. This little planetary nebula vaguely resembles a turtle as you can see here in this image obtained by the Hubble Space Telescope. Though it is about 9th magnitude, it is pretty small and hard to see much detail with a small telescope. The other planetary nebula here is way down in the Southeast corner of the constellation. If you follow a line from Pi to Epsilon (the Western half of the keystone) about three times further than the separation between these two stars, you'll wind up very close to IC 4593, also imaged here with the Hubble Space Telescope.

Posted by Observer at July 2, 2008 10:17 PM
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