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The northern constellation of Bootes, one of the 88 modern constellations recognized by the IAU. Credit: smokymtnastro.org


13 Oct , 2008



Bootes is one of the 88 modern constellations and was also one of the 48 constellations listed by Ptolemy. It is frequently called the “Watcher of the Bear”, guarding over the northern constellations of both Ursa Major and Ursa Minor, the Greater and Lesser Bears. It also contains the third brightest star in the night sky – Arcturus – whose Greek meaning of Arktos also means bear and is associated with all things northern – including the aurora. It is bordered by Canes Venatici, Coma Berenices, Corona Borealis, Draco, Hercules, Serpens Caput, Virgo and Ursa Major.

In mythology, it is possible that Bootes may have represented a ploughman, and the Big Dipper the northern plough. His dogs, Chara and Asterion were represented by the constellation of Canes Venetici (the Hunting Dogs) who drove the oxen on and kept the wheels of the sky turning. It is also possible (mythologically speaking) that Bootes may have invented the plough. This is said to have greatly pleased Ceres, the goddess of agriculture who asked Jupiter to give Bootes a permanent stellar home in the heavens as a reward. There are also version where Bootes represents a form of an Atlas, who holds up the weight of the world as it turns on its axis, or yet another of Hercules’ labors. No matter what its background, Bootes is a sprawling constellation, and home to a great many stellar gems.

Bootes can be found south of Ursa Major the Great Bear, off the handle of the Big Dipper. Bootes’ brightest star, Arcturus, is part of a mnemonic device used to orient people to the night sky. Because the Big Dipper constellation is easy for most observers to find, the handle of the dipper is used to point to other important stars. The saying is, “Arc to Arcturus, speed on to Spica.” This means you as follow the curve in the Dipper’s handle away from Ursa Major you will run into a bright star – this is Arcturus in Bootes. The other star mentioned in the saying is Spica – a part of neighboring Virgo.

Arcturus star is quite important as a star, too. It is a type K1.5 IIIpe red giant star — the letters “pe” stand for “peculiar emission,” which indicates the spectrum of the star is unusual and full of emission lines. Not uncommon in red giants, but Arcturus is particularly strong. It’s about 110 times more luminous than our nearest star, but this underestimates its strength as much of the “light” it gives off is in the infrared. The total power output is about 180 times that of the Sun! The lower output in visible light is due to decreased efficiency from a lower surface temperature. Arcturus is also notable for its high proper motion, larger than any first magnitude star in the stellar neighborhood other than Alpha Centauri. It is now almost at its closest and is moving rapidly (122 km/s) relative to the solar system. Arcturus is also thought to be an old disk star, and appears to be moving with a group of 52 others of its type. Its mass is hard to exactly determine, perhaps the same as Sol, or perhaps about 50% larger. Arcturus is probably also older than the Sun, and much like what the Sun will be in its red giant phase. Arcturus achieved fame when its light was used to open the 1933 Chicago World’s Fair. The star was chosen as it was thought that light from Arcturus had started its journey at about the time of the previous Chicago World’s Fair in 1893. Technically the star is 36.7 light years away so the light would have started its journey in 1896, but why should that spoil the fun? Arcturus’ light was still focused onto a cell that powered the switch for the lights that eventually light polluted the skies to the point where you couldn’t see Arcturus!

For binocular observing, check out Tau Bootis, a yellow-white dwarf approximately 51 light-years from Earth. It is a binary star system, with the secondary star being a red dwarf. In 1999, an extrasolar planet was confirmed to be orbiting the primary star! The planet was discovered by a team of astronomers lead by Geoff Marcy and R. Paul Butler. Maybe you’d like to look at long term variable star R Boötis? It ranges from 6.2 to 13.1 every 223.4 days.

For the telescope we have plenty of excellent binary star systems. Pi Boötis is located approximately 317 light years from our solar system and the primary component, P¹ Boötis, is a blue-white B-type main sequence dwarf with an apparent magnitude of +4.49. It’s companion, P² Boötis, is a white A-type main sequence dwarf with an apparent magnitude of +5.88. The stars are separated by 5.6 arc seconds on the sky. Now try 21.8 light years distant Xi Boötis. The primary star in this system, Xi Boötis A, is a BY Draconis variable with an apparent magnitude that varies from +4.52 to +4.67 with a period just over 10 days long, and is classified as a yellow G-type main sequence dwarf. Small velocity changes in the orbit of the companion star, ? Boötis B, an orange K-type main sequence dwarf, indicate the presence of a small companion that is less than nine times the mass of Jupiter. The AB binary can be resolved even through smaller telescopes. The primary star (A) has been identified as a candidate for possessing a Kuiper-like belt, based on infrared observations. The estimated minimum mass of this dust disk is 2.4 times the mass of the Earth’s Moon.

Still more? Then head off to triple system, Mu Boötis. The primary component, Mu¹ Boötis, is a yellow-white F-type sub giant with an apparent magnitude of +4.31. Separated from the primary by 108 arc seconds on the sky is the binary star Mu² Boötis, which has a combined spectral type of G1V and a combined brightness of +6.51 magnitudes. The components of Mu² Boötis have apparent magnitudes of +7.2 and +7.8 and are separated by 2.2 arc seconds. They complete one orbit about their common centre of mass every 260 years. How about colorful yellow and blue Kappa Boötis? Kappa2 Boötis is classified as a Delta Scuti type variable star and its brightness varies from magnitude +4.50 to +4.58 with a period of 1.83 hours. The companion star, Kappa¹ Boötis, has magnitude +6.58 and spectral class F1V.

For deep sky observers with large telescopes, why not challenge yourself to globular cluster NGC 5466 about a fist width north of Arcturus? This class XII, 9th magnitude globular was discovered in 1784 by Sir William Herschel and presents an nice challenge. Or try compact spiral galaxy NGC 5248. It’s about a fist width south of Arcturus and about a finger width southwest. It’s part of the Virgo cluster of galaxies and could be as far as 50 million light years away. It’s another great grand design spiral which shows spiral galaxy structure in long exposure photographs. You can mark it on your list as Caldwell 45!

bsg-viper-09If you’d just like to have some fun, then why not try picking out an asterism created by dedicated SkyWatcher, Ed Murray? It’s called the “Colonial Viper and Launch Tube”! If you’re a longstanding Battlestar Galactica fan, then you’ll recognize this ultra-cool spaceship as it sits in its triangular shaped launch tube, just waiting to be unleased on the Cylons. Because this is a “pattern” of stars, it doesn’t take any special equipment – just some imagination! Here’s how to find it…


To Find The Colonial Viper and launch tube draw a line between Arcturus, Alphekka (Alpha Corona Borealis) and Gamma Bootis which make up the “Launch Tube” and Izar (Epsilon Bootes) is the Viper. But this just isn’t any ordinary star… Izar is actually a binary star, consisting of a bright orange giant and a smaller main sequence star. The orange giant is a fairly late-stage star well into its stellar evolution, having already exhausted its supply of hydrogen fuel. By the time the smaller main sequence star reaches this point in its evolution, the primary star will have lost much of its mass in a planetary nebula and will have evolved into a white dwarf, and the pair will have essentially changed roles – the primary star becoming the dim dwarf, while the lesser A star will shine as a bright orange giant!

(Just in case you were wondering – it’s pronounced buh-OH-tays.)
Source: Chandra
Constellation Chart Provided by Your Sky

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