July 24, 2008

Ophiuchus

The next constellation I'd like to talk about is Ophiuchus the serpent holder. Ophiuchus is found at its most easily visible during the summer months, about 45 degrees above the Southern horizon, and he splits the constellation Serpens into two parts, holding the serpent with both hands. I'll cover Serpens next on the list since these two constellations are so closely linked. Ophiuchus is positioned on the sky between the two halves of Serpens, but it also borders Scorpius and Sagittarius.

From an astrological standpoint, it is a wonder that Ophiuchus isn't included in the standard zodiac since the Sun does spend a fair amount of time in this constellation, more so than when it briefly passes through Scorpius. Since Ptolemy first codified the constellations over 2000 years ago, Ophiuchus has straddled the ecliptic, and in the present day, the Sun is in Ophiuchus from November 30 through December 17. Apparently, this inconvenience is ignored by astrologers, who divide the sky into 12 equal sections, each 30 degrees wide.

The mythology of Ophiuchus is a mixed bag. Originally, this constellation represented the healer Asclepius, who learned so much that he was not only able to heal the living, but also raise the dead and make people immortal. This was too much for the gods, so they struck him down, but in honor of his achievements placed him in the sky holding the body of a dead snake which he had used to heal one of his patients. Snakes are still used as a symbol of healing because they shed their skin occasionally and are "reborn". The constellation has also been known as Serpentinarius (snake charmer), another healer noted for his skill in curing snake bites.

For the bright stars here, I'll start with Alpha Ophiuchi, Rasalhague. This one is a bit tough to find, but it is the brightest (2nd magnitude) star in its little region of the sky. If you can find the summer triangle (Vega, Altair and Deneb), Rasalhague holds a mirror image position compared to Deneb on the opposite side of the line connecting Vega and Altair. The name translates to the head of the serpent charmer, Ras al Hawwa. It is a Sirius-like star, about 50 light years away, and Burnham notes that it shares the same motion as the Ursa Major moving group, which seems to just about surround us.

About eight degrees South and a little bit East is Beta Ophiuchi, Cebalrai, which means "the shepherd's dog", originating from a different asterism (a pasture) that was once in this part of the sky. It is an orange giant, a bit more massive and much more advanced in its evolution compared to our Sun. It is also very close on the sky to a nice little star cluster known as IC 4665, about a degree to the Northeast. The left hand of Ophichus is represented by two stars, Yed Prior (front of the hand) and Yed Posterior (back of the hand). Though close together in the sky, these two stars are not physically connected as Yed Prior is almost twice as far away from us. Both stars are giants.

While going through some of these bright stars, it may strike you that so many of them just happen to be giants or in an unusually rare stage in their evolution. After all, stars spend 90% of their lives on the main sequence, so why aren't 90% of the stars we see in the night sky main sequence stars? This is a selection effect, actually. If you want a truly representative sample of stars, you must select them based on some property than does not affect their intrinsic qualities. For example, if you pick only stars above a certain minimum brightness threshold (i.e. stars visible to the naked eye), these stars are going to either be very close to us, intrinsically very bright, or both.

Just because a star is close to us, this doesn't make it unusual. If we were to create a sample of the nearest 10,000 stars or so, that would be perfectly fine. Probably about 90% of them would be main sequence stars. But we are studying a sample of the visible stars, which means we tend to be biased toward stars that are intrinsically very bright. Since each star tends to have its own little quirks, we tend to study stars on a statistical basis, looking for broad patterns when we can, rather than just focusing on one star and assuming it represents all similar stars. When we study stars like this, we must take care not to fool ourselves, so we must select our sample carefully if it is to be truly representative.

Going from Yed Prior through Yed Posterior and down along that line toward the southeast, we run into the 3rd magnitude star Zeta Ophiuchi. This is a rare, hot main sequence O-class star, among the more massive stars in the sky (20 solar masses, though not very large because it hasn't exhausted its core hydrogen fuel and begun expanding yet, it is only about eight times the size of the Sun). Under normal circumstances at a distance of about 460 light years, it would appear at least as bright as 1st magnitude, but it is shrouded in a thick dust cloud that knocks its light down by two magnitudes (about a factor of 7).

Zeta Ophiuchi lights up this surrounding dust cloud, exciting the Hydrogen and inducing it to emit one of its most common emission lines, the red Balmer-alpha line at 656.3 nm. Zeta also has a very high space velocity, having recently been ejected from a binary companion that went supernova and is now a neutron star in the nearby Scorpius-Centaurus Association shooting off in the opposite direction.

Next in line is Sabik, a (barely) 2nd magnitude star, actually the combined light of two A-class (Sirius-like) stars that are both 3rd magnitude but only an arcsecond or less apart, depending upon where they are relative to one another as they orbit. Last but not least among the significant stars in this constellation is the famous Rho Ophiuchi, which isn't so famous for its qualities but instead for the way this beautiful binary system lights up the dusty surrounding nebula, just a couple of degrees north of the bright star Antares in Scorpius. You can see reflection and emission of several different colors in this nebula, and there is a nice cluster nearby as well, making for famous photographs.

Before covering a couple of historic novae in Ophiuchus, I would like to spend some time talking about the 2nd closest star to our Sun, Barnard's Star. This 10th magnitude star is very tough to find without good finder charts. Kaler provides a chart that shows the star near the tip of one of the horns of a very small, faint asterism known as Poniatowski's Bull, a V-shaped feature of five separate 4th or 5th magnitude stars. You can find this by following a line from Rasalhague (Alpha Ophiuchi) almost due South about 10 degrees to Cebalrai (Beta Ophiuchi) and then a few degrees south of east for about 3 degrees to the V-shape that opens toward the direction north-northeast.

This star was discovered by astronomer E. E. Barnard in 1916, who noted its very high proper motion. The star moves about 10 arcseconds per year, which doesn't seem like much, but it takes only about 200 years to move an angular distance equivalent to the full moon in the sky. The actual space velocity of Barnard's Star is not remarkable. There are several other stars moving much more quickly (usually ejected from explosive systems or multiple-star systems), but the angular motion across the sky is greatly magnified by the fact that this star is so close to the Sun. From our perspective, it is only about 10th magnitude, but if we were able to somehow survive on a planet orbiting Barnard's Star, looking back at our Sun, it would be a very bright star, about the equivalent of Pollux in Gemini, in the eastern part of the constellation Monoceros.

You can see an animation of its proper motion here. For a while back in the 1960's, it was thought that Barnard's Star had an orbiting planetary companion. This discovery was due to apparent wobbles in its proper motion through space, but closer observations later disproved this idea. For now, we have not detected the telltale wobble that would indicate the presence of a planet, either a transverse wobble or a Doppler wobble along our line of sight, but our measurements are somewhat crude compared with the kind of precise and systematic observations scheduled for the near future by satellites.

Barnard's star *does* appear to wobble through space if you take a series of photographs over the course of a year, but that's due to its parallax. As Earth orbits the Sun, from our perspective, Barnard's star seems to shift back and forth relative to the background stars. We correct for this, of course, before analyzing motion for possible planetary companions.
Barnard's Star has a component of its velocity in our direction, and in about 10,000 years, it will have the distinction of being the closest star to our Sun, at a distance of about 3.8 light years, holding that record briefly before receding away from us. It will be a bit brighter then, too, but only about a magnitude of 8.5.

The first of the variable stars in this constellation is RS Ophiuchi, a rare recurrent nova, and even more remarkable in that it reaches a brightness visible to the naked eye during its outburst. This is a binary system consisting of a red giant dumping mass onto its white dwarf companion, and the system is about 5000 light years away with a quiescent magnitude of 12.5. During outbursts (most recently in 2006 and historically spaced apart by about 20 years since first seen in 1898), it reaches a magnitude of somewhere between 4.5 and 5.5, and most recently it was studied in some detail by satellite observatories.

What happens in these systems is that Hydrogen from the envelope of the red giant accumulates on the white dwarf, lost from the red giant due to its overextended, expanding nature. As the Hydrogen accumulates, its temperature and density rise until it reaches a point where fusion can take place. Once fusion ignites, its like lighting a match in a building with a leaky gas pipe. BOOM! The system will brighten by as much as a factor of 500 or so in a very short time thanks to the bright, expanding debris cloud. Right around maximum, the star appears very red thanks to emission from all of the excited Hydrogen that has been blown off the white dwarf. Because this star is relatively bright and easy to find and because it has such a bright maximum, it is one of the most carefully monitored stars in the sky, with observers looking for a sign of a new outburst or clues as to its behavior before and after.

The other object now sort of classifies as a star and a deep sky object: it is the remnant of the famous 1604 event known as Kepler's Supernova and also known as V843 Ophiuchi or SN 1604). On October 9, 1604, there was an interesting conjunction in the sky of Mars, Jupiter and Saturn near Ophiuchus. Kepler wasn't the discoverer, but he was notified of the discovery and studied it intently until it faded from view in early 1606. It originally reached a peak magnitude comparable to Jupiter in opposition and slowly faded with a light curve most closely resembling a Type I supernova (originating from a white dwarf rather than a massive core-collapse star), similar to Tycho's supernova in 1572. Today, it appears in this nice Hubble photo as a wispy expanding remnant about 13,000 light years distant. No supernova explosion has occurred in the Milky Way since Kepler's.

Now for some of the deep sky objects in the constellation Ophichus. I'll start with what is generally considered to be one of the nicest NGC objects easily visible to a small telescope, the 5th magnitude small cluster known as NGC 6633. This cluster of about 30 blue stars around 1000 light years distant is roughly the same angular size as the full moon and can be found about five degrees South of the midpoint of a line connecting Rasalhague and Altair, one of the three stars of the summer triangle. Moving north to south through the constellation, the next stop is IC 4665, a little open cluster very close to Beta Ophiuchus on the sky, as seen in this image. It is similar to NGC 6633 but about 4 magnitudes fainter (400 light years more distant and obscured by interstellar dust) and a little more loosely bound (twice the angular size).

Next are four globular clusters in the torso of Ophichus, starting seven degrees due South of Beta Oph. The globular cluster here is known as Messier 14. This is one of a swarm of clusters circling about the center of our galaxy (located in the direction of nearby Sagittarius), 8th magnitude and about 30,000 light years distant. The diameter of this cluster is only about 3 arcminutes, 10 times smaller than the relatively sparse NGC 6633. There are easily 100,000 stars in this tiny little region of the sky, shining with a combined luminosity about 400,000 times that of our Sun. With 10-inch or smaller telescopes, this resembles a fuzzy blob like an elliptical galaxy. The individual stars in the outer reaches of the cluster are extremely difficult to pick out without a professional research-grade telescope.

About 3 degrees Southwest of Messier 14 is NGC 6366, seen here very close in the sky to a non-descript 5th magnitude star which outshines the integrated light from the cluster by a factor of about 40. This cluster is closer to us, only about 12,000 light years away, but with far fewer stars than any of the three bright Messier clusters in this part of Ophichus. The next stop is Messier 10, about 10 degrees due West from Messier 14 and a degree or so South. Somewhat fainter than Messier 14, it is nonetheless one of the finer clusters in the sky, seen in this brilliant colorful image.

The last cluster in this part of the sky is about 3 degrees to the Northwest, and that's Messier 12, very similar in size and distance to Messier 10. In recent research on this cluster, astronomers have discovered that its population of low mass stars is underdeveloped. This probably means that a lot of the low mass starts were stripped the last time this cluster swung through the bulge of the Milky Way, much closer to the center than it is now. Indeed, deep images show that the center of this cluster is far less crowded than most other clusters, and it has apparently left these stars behind in a lengthy tidal tail stretching along its orbital path.

The next object to see is about three degrees south by southwest of Zeta Ophiuchi, and that is the globular cluster Messier 107, a relatively open globular cluster at a distance of about 21000 light years. In photos, you can see a few relatively dark regions near the center. This is an 8th magnitude object anywhere from 3 to 15 arcminutes in diameter depending upon your light gathering power, not the best globular in this part of the sky but certainly a different look.

Moving further south, about 3 degrees Southeast of Sabik (which recall is at the end of the line from Yed Prior to Zeta to Sabik), is the globular cluster Messier 9, another 8th magnitude object a little under 26000 light years away. Like most such Messier objects, this was originally discovered by Charles Messier, who classified it as a nebula, then a couple of decades later with a much better telescope, William Herschel was able to resolve some of the stars in the outskirts and therefore properly classify it as a cluster. In photos, you can tell there is an obscuring dust cloud, the edge of which is nearly along the line of sight to this cluster. Look to the lower left of this image and note how the star counts are much less. Such obscuration can make accurate distance determination very difficult.

Further south, near the southern border of the constellation, we find the last two Messier objects, the first being about 10 degrees due South from Sabik and 8 degrees East from Antares in the constellation Scorpius, and that is Messier 19, a 7th magnitude globular about 28000 light years distant. This one is unusual in that it is the most oblate known globular, somewhat elliptical in shape, perhaps the result of a more disturbed formation or history.

Three degrees due South is our last object, the globular cluster Messier 62, a 6th magnitude cluster about 22000 light years away. All of these clusters are rather close in the sky to the galactic bulge and orbiting around it quickly as they are currently passing close in their elliptical orbits about the galactic center. Like Messier 19, this one is slightly deformed in shape, perhaps the result of tidal forces from the galactic center. This is one of the closest clusters to the galactic center, so that's no surprise.

Posted by Observer at July 24, 2008 06:09 PM
Comments

Comments on entries can only be made in pop-up windows while those entries are still on the main index page. Sorry for the inconvenience this causes, but this blocks about 99.99% of the spam the blog receives.