The Bootes Constellation

The Bootes Constellation.

Boötes is a constellation in the northern sky, located between 0° and +60° declination, and 13 and 16 hours of right ascension on the celestial sphere, covering 907 square degrees. It is best seen in June at 9.00pm between latitudes 90 and -50 degrees. One of the 48 constellations described by the 2nd-century astronomer Ptolemy, Boötes is now one of the 88 modern constellations. Exactly whom Boötes is supposed to represent in Greek mythology is not clear. According to one version, he was a son of Demeter, Philomenus, twin brother of Plutus, a ploughman who drove the oxen in the constellation Ursa Major. This is corroborated by the constellation’s name, which itself means “ox-driver” or “herdsman.” Boötes is bordered by Virgo to the south, Coma Berenices and Canes Venatici to the west, Ursa Major to the northwest, Draco to the northeast, and Hercules, Corona Borealis and Serpens Caput to the east. The official constellation boundaries, as set by Eugène Delporte in 1930, are defined by a polygon of 16 segments. Colloquially, its pattern of stars has been likened to a kite or ice cream cone.

In his Uranometria, Johann Bayer used the Greek letters Alpha through to Omega and then A to k to label what he saw as the most prominent 35 stars in the constellation, with subsequent astronomers splitting Kappa, Mu, Nu and Pi as two stars each. John Flamsteed numbered 54 stars for the constellation. Located 36.7 light-years from Earth, Arcturus, or Alpha Boötis, is the brightest star in Boötes and the fourth-brightest star in the sky at an apparent magnitude of −0.05. It is also the brightest star north of the celestial equator, just shading out Vega and Capella. An orange giant of spectral class K1.5III, Arcturus is an ageing star that has exhausted its core supply of hydrogen and cooled and expanded to a diameter of 27 solar diameters, equivalent to approximately 32 million kilometres. Though its mass is approximately one solar mass (M_☉), Arcturus shines with 133 times the luminosity of the Sun (L_☉). Boötes is home to many other bright stars, including eight above the fourth magnitude and an additional 21 above the fifth magnitude, making a total of 29 stars easily visible to the naked eye. Nearby Eta Boötis, or Muphrid, is the uppermost star denoting the left leg. It is a 2.68-magnitude star 37 light-years distant with a spectral class of G0IV, indicating it has just exhausted its core hydrogen and is beginning to expand and cool. It is 9 times as luminous as the Sun and has 2.7 times its diameter. Marking the herdsman’s head is Beta Boötis, or Nekkar, a yellow giant of magnitude 3.5 and spectral type G8IIIa, and is 219 light-years away.

Mu Boötis, known as Alkalurops, is a triple star with an overall magnitude of 4.3, and is 121 light-years away. Its name is from the Arabic phrase for “club” or “staff.” The primary appears to be of magnitude 4.3 and is blue-white. The secondary appears to be of magnitude 6.5, but is actually a close double star itself with a primary of magnitude 7.0 and a secondary of magnitude 7.6. The secondary and tertiary stars have an orbital period of 260 years. Epsilon Boötis, also known as Izar or Pulcherrima, is a close triple star and the most prominent binary star in Boötes. The primary is a yellow- or orange-hued magnitude 2.5 giant star, the secondary is a magnitude 4.6 blue-hued main-sequence star, and the tertiary is a magnitude 12.0 star. The system is 210 light-years away. Besides Pulcherrima and Alkalurops, there are several other binary stars in Boötes.

Extrasolar planets have been discovered encircling ten stars in Boötes as of 2012. Tau Boötis is orbited by a large planet, discovered in 1999. A companion, GJ527B, orbits at a distance of 240 AU. Tau Boötis b, the sole planet discovered in the system, orbits at a distance of 0.046 AU every 3.31 days. It has a mass of 5.95 Jupiter masses (M_J). Like Tau Boötis b, HAT-P-4 b is also a hot Jupiter. Discovered in 2007, HAT-P-4 b has a mass of 0.68 M_J and a radius of 1.27 R_J. It orbits every 3.05 days at a distance of 0.04 AU. Boötes is also home to multiple-planet systems. HD 128311 is the host star for a two-planet system, consisting of HD 128311 b and HD 128311 c, discovered in 2002 and 2005, respectively. There are several single-planet systems in Boötes.

Boötes is in a part of the celestial sphere facing away from the plane of our home Milky Way galaxy, and so does not have open clusters or nebulae. Instead, it has one bright globular cluster and many faint galaxies. The globular cluster NGC 5466 has an overall magnitude of 9.1 and a diameter of 11 arcminutes. It is a very loose globular cluster with fairly few stars and is classified as a Shapley-Sawyer Concentration Class 12 cluster, reflecting its sparsity. Its fairly large diameter means that it has a low surface brightness, so it appears far dimmer than the catalogued magnitude of 9.1, with only 12 stars. Boötes has two bright galaxies. NGC 5248 (Caldwell 45) is a type Sc galaxy (a variety of spiral galaxy) of magnitude 10.2. It measures 6.5 by 4.9 arcminutes and fifty million light-years from Earth. NGC 5676 is another type Sc galaxy of magnitude 10.9. It measures 3.9 by 2.0 arcminutes. Other galaxies include NGC 5008, a type Sc emission-line galaxy, NGC 5548, a type S Seyfert galaxy, NGC 5653, a type S HII galaxy, NGC 5778 (also classified as NGC 5825), a type E galaxy that is the brightest of its cluster, NGC 5886, and NGC 5888, a type SBb galaxy. NGC 5698 is a barred spiral galaxy, notable for being the host of the 2005 supernova SN 2005bc, which peaked at magnitude 15.3.

Further away lies the 250-million-light-year-diameter Boötes void, a huge space largely empty of galaxies. Discovered by Robert Kirshner and colleagues in 1981, it is roughly 700 million light-years from Earth. Beyond it and within the bounds of the constellation, lie two superclusters at around 830 million and 1 billion light-years distant. The Hercules–Corona Borealis Great Wall, the largest known structure in the Universe, covers a significant part of Boötes.

Boötes is home to the Quadrantid meteor shower, the most prolific annual meteor shower. It was discovered in January 1835 and named in 1864 by Alexander Hershell. The radiant is located in northern Boötes near Kappa Boötis, in its namesake former constellation of Quadrans Muralis. Quadrantid meteors are dim, but have a peak visible hourly rate of approximately 100 per hour on January 3–4. On April 28, 1984, a remarkable outburst of the normally placid Alpha Bootids was observed by visual observer Frank Witte from 00:00 to 2:30 UTC. The Alpha Bootids normally begin on April 14, peaking on April 27 and 28, and finishing on May 12. Its meteors are slow-moving, with a velocity of 20.9 kilometers per second. The June Bootids, also known as the Iota Draconids, is a meteor shower associated with the comet 7P/Pons-Winnecke, first recognized on May 27, 1916, by William F. Denning. The average June Bootid has a magnitude of 5.0. It is related to the Alpha Draconids and the Bootids-Draconids. The shower lasts from June 27 to July 5, with a peak on the night of June 28. Several other weak meteor showers exist in the Bootes Constellation.

Credit: Constellation List, IAU, Wikipedia.