May 25, 2008

Bootes

Today, I'd like to move a little bit North in the sky from the two constellations containing the Virgo Cluster of galaxies, Coma Berenices and Virgo, to a constellation with an a few possible origin stories and the 4th brightest star in the sky, the constellation Bootes (pronounced as three syllables with the emphasis usually on the second syllable, boh-OH-tease). Based on its position in the sky just "behind" the handle of the Big Dipper, Bootes is sometimes seen as an Ox driver with the Dipper as the wain or plough being driven. The pattern of stars in the sky is often referred to as the Kite asterism, as you can see in the star chart here, and the combination of Arcturus, Spica (in Virgo), Denebola (in Leo) and Cor Caroli (in Canes Venatici) forms the larger asterism known as the Spring Diamond.

In other legends, Bootes represents the hunter Arcas who was chasing after his mother Callisto, who he hadn't realized had been transformed into a bear. In this legend, the nearby constellation Canes Venatici represent the hunting dogs of Arcas. The origin of the name either comes from a Greek word meaning "noisy" (as the hunter is noisily yelling to his dogs while hunting) or another word meaning "ox driver". This constellation was also known by the name Arctophylax or Artophilaxe (from Allen's book), which means Bear-watcher or Bear-guard, and from this name comes the name for the brightest star in the constellation, Arcturus. Notice also the proximity of the word "arctic", which comes from a reference to the Bear constellations being in the North.

Still another legend associates these stars with Icarius, a grape grower who won the favor of Dionysus. This Greek god revealed to Icarius the secret of making wine, and so Icarius and his friends all got drunk. When they woke up the next day with severe hangovers, his friends assumed that Icarius had tricked them and tried to poison them, so they killed him. Dionysus that placed Icarius into the heavens to honor him. In this same legend, Icarius' daughter Erigone is placed in the sky below Icarius in the location of Virgo.

As I mentioned earlier, Arcturus, found by following the arc of the Big Dipper's handle, is the 4th brightest star in the sky, behind Alpha Centauri, Canopus and Sirius, and Arcturus is the brightest star in the North Celestial Hemisphere, outshining any of the stars in the Big Dipper or Orion. This is an orange K-class giant located 37 light years away, above us relative to the plane of the galaxy. It's diameter is about 25 times that of the Sun, and it radiates over 200 times as much energy as our Sun, most of it past the red end of the visible spectrum in the infrared.

The overall mass is not very well known (since it doesn't have a binary companion to help us "weigh" it), but it is thought to be between 1 and 1.5 solar masses. My guess is the low end of that range, because other indicators indicate an age much older than the Sun. For example, it is metal-poor relative to the Sun, and while composition is a very noisy indicator of age, it *is* an indicator (in this case, indicating an older age). Also, it has a very high proper motion relative to the Sun, a motion shared by about 50 or so other old stars, partly a slower orbital motion about the galactic center and partly a velocity more perpendicular to the disk.

This may indicate that it is part of the thick disk population, which is a body of stars orbiting the center of the galaxy like the disk but with a somewhat larger vertical velocity dispersion. That means as they orbit, they tend to bob up and down, passing through the disk frequently, sort of like the motion of a horse on a merry-go-round. Since very little gas and dust shares this sort of motion, this is thought to be an older population with no recent star formation. It could also be a population left over from a galaxy merger long ago.

The Hipparcos satellite, which measured not only the parallax distances but also the proper motions of over a million nearby stars, revealed many groups of stars with peculiar but similar proper motions to one another, and this has helped us reconstruct the history of several once-tight clusters and groups of stars that have broken up into otherwise unrecognizable groups, lost in the "noise" of the billions of stars in the galaxy. This is somewhat similar to our reconstruction of the asteroid belt in our own solar system. We do that by looking for groups or "families" of asteroids with similar compositions (they have completed way too many orbits over billions of years for us to be able to reconstruct their groupings dynamically).

Beta Bootis is Nekkar, which comes from the Arabic for "Ox Driver" and marks the head of the herdsman, the tip of the "kite" asterism that Bootes is famous for. It is an unremarkable yellow giant star a little over 200 light years away. It isn't even the 2nd brightest star in the constellation, far from it. The greek letter designations in this constellation begin with Arcturus as Alpha and then the rest are simply the bright stars ordered from the North toward the South.

Gamma Bootis is Seginus, and along with Delta Bootis (no proper name), these two are the 3rd magnitude left and right shoulders of Bootes, respectively. Seginus is a blue A star somewhat similar to the A stars that are common in the Big Dipper, but Seginus is well on its way out of that stage, slowly turning into a giant star as it ends the main sequence, Hydrogen-burning portion of its lifetime. Delta is a yellow giant like Nekkar and has a sunlike companion orbiting at a very great distance. The orbit takes about 120,000 years to complete, but we know the two stars are connected because their proper motions are identical and they are at the same distance from Earth.

Roughly halfway between Arcturus and Nekkar, about 10 degrees North-northeast from Arcturus, we find 2nd magnitude Izar (Epsilon Bootis), the 2nd brightest star in this constellation. The name is similar to Mizar in Ursa Major or Mirach in Andromeda, which all refer to "loin" or "girdle" or something in the middle part of the body. Burnham mentions that this is a very nice orange giant and blue main sequence star binary system that looks good in a 6-inch or larger telescope. Though these two are separated by about 200 AU (which is about four times the radius of Neptune's orbit), they are very close together on the sky (a few arcseconds) due to their large distance from Earth about about 200 light years. Here is a nice little picture of what Izar looks like through a small telescope.

The most prominent deep sky object in Bootes is a nice spiral near the Southern edge of the constellation, NGC 5248. This can be found by following a line from Izar through Arcturus about the same distance (15 degrees or so) again to the Southwest.

This galaxy is about 50 million light years away and is probably an outlying member of the Virgo Cluster of galaxies. What sets this grand design spiral apart from many others is that the spiral arm pattern so clearly seen out to great distances extends all the way to the central core of the galaxy, as seen in high resolution photos like the one I linked above. A lot of times the spiral pattern gets lost in the bulge or is disrupted by a ring or bar near the center of the galaxy. Knowing that it is possible for the pattern to extend this far helps us understand the density wave phenomenon that causes the arms in the first place.

10 degrees South of Arcturus and 10 degrees East of NGC 5248 is a fainter target, the 12th magnitude elliptical galaxy NGC 5532, also known as 3C 296 in a catalog of powerful radio sources. This is a very good example of a galaxy with a supermassive black hole in the center and a twin jet of material streaming from the region, each jet being over a million light years long. Closer to Arcturus, just a degree or so to the Southwest, is a triple galaxy highlighted by IC 983 (seen here in a very nice deep image). The little faint spiral in this trio is NGC 5490, which has had a couple of type Ia supernovae erupt in the past 20 years.

About 10 degrees North of Arcturus and a tiny bit West is the globular cluster NGC 5466 seen in a nice amateur image here. The interesting thing about this cluster is that it is so far out of the plane of the galaxy, making it very easy to see tidal tails trailing from the cluster that would normally be lost in the noise of the stars and gas in the disk. This cluster is slowly being pulled apart as it passes through the disk over and over, orbiting the center of the galaxy.

Posted by Observer at May 25, 2008 03:34 PM
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HD122563 whisper whisper prototype extremely metal poor star mutter grumble galactic chemical evolution grimace whisper [Fe/H] = -2.7 natter grommish rumble belch.

Posted by: Feff on May 29, 2008 06:36 PM