Boötes the Herdsman

Boötes the Herdsman
Boötes (pronounced boh-oh-teez) is one of the oldest constellations in the sky, though today it's not generally well known.

Homer mentioned Boötes in The Odyssey three thousand years ago. It was already old then. The name comes from Sumeria, a Middle Eastern civilization with a well-developed astronomical tradition, which flourished some five thousand years ago.

You can find Boötes by looking first for the Big Dipper (known as the Plough in the UK). Follow the curved handle around to the next bright star, Arcturus. A skywatcher's mnemonic says to "arc to Arcturus."

Arcturus (alpha Boötis) is one of the brightest stars in the sky, a red giant with a diameter 25 times that of the Sun. It has a yellow color like spring flowers and when Arcturus appears, spring won't be far behind. At 37 light years distance, it's a near neighbor in galactic terms.

Once you've found Arcturus, in a fairly dark sky you should be able to see the rest of Boötes. In the past Boötes may have been envisaged as a person, but it looks very much like a kite to modern eyes. Arcturus is right where the kite is attached to the tail – it's labeled with a Greek alpha.

There is also a horseshoe of stars alongside Boötes, called Corona Borealis (Northern Crown), an attractive little constellation.

The older the constellation, the more stories are attached to it and Boötes is no exception, so I'll just mention one. Boötes is a herdsman who is driving a bear around the sky. The bear, of course, is Ursa Major (Great Bear) which the constellation Boötes follows around the North Pole. The name Arcturus means "bearkeeper" and seventeenth century astronomer Hevelius gave the herdsman two dogs on a leash, represented by the constellation Canes Venatici, as shown in his star atlas.

Soon after New Year's Day, the section of the sky where we see Boötes is the radiant (the apparent source) of the annual Quadrantid meteor shower. But since meteor showers are named for the location of the radiant of the shower, this one should be called the Boötids. What's up?

The shower was first observed early in the nineteenth century. The radiant was in the constellation of the Mural Quadrant, called after an an old astronomical instrument. The radiant hasn't moved, but the Mural Quadrant wasn't included in the 88 constellations officially agreed by the International Astronomical Union (IAU).

Tau Boötis is a star which isn't particularly bright seen from Earth, even though it's somewhat larger and hotter than the Sun. Its main claim to fame is that it's orbited by one of the first exoplanets to be discovered.

There are three other stars in Boötes also known to have exoplanets, the most interesting being a gas giant in the habitable zone of its star. The habitable zone is the area around a star in which liquid water could exist and therefore there could be life. Although we wouldn't expect the planet itself to be inhabited, it could have a suitable rocky moon.

Boötes contains a large region full of galaxies so far away that light takes from 6-11 billion years to get here. This means we're looking back in time when we see these thousand galaxies with their supermassive black holes, some having the mass of billions of Suns.

In 2007 the Chandra X-Ray Observatory, the Spitzer Space Telescope and some ground-based telescopes observed this region. Since we have evidence that most galaxies have a supermassive black hole at the center – including ours – this observation can help us to understand how galaxies evolve.

Nonetheless perhaps the most surprising thing found in Boötes is nothing. The Boötes Void was discovered in 1981 – an area 350 million light years across, apparently containing no galaxies. By now, some have been discovered, yet it's very underpopulated compared to other regions of space. There are theories, but no one is really sure why it's so empty.

(1) Ridpath, I. "Boötes the herdsman" [accessed 08.09.10]
(2) "New Panorama Reveals More Than a Thousand Black Holes"

You Should Also Read:
What Are Constellations
Absolute Beginners - Spring Skies
The Starry Crowns - Corona Borealis

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