Boötes

The Herdsman behind the bears

Boötes, the Herdsman, is a large constellation of spring and summer evenings. Its name comes from a Greek word for a ploughman or ox driver, and the figure seems to follow the Great Bear around the pole, as if driving it on.

Most of its stars are of second and third magnitude, so the outline is easy only from a dark site. But one star makes Boötes impossible to miss: Arcturus, a brilliant orange point that dominates the eastern sky on spring evenings.

How to find it

Start with the Big Dipper. Follow the gentle curve of its handle away from the bowl, about 30 degrees, roughly three fists at arm's length, and you reach Arcturus. The old phrase 'arc to Arcturus' makes it easy to remember.

From Arcturus the rest of the constellation rises toward the north in a narrow kite shape, about 20 degrees long. Its northern tip, Nekkar, marks the Herdsman's head. Just east of the kite, a small half circle of stars is Corona Borealis, the Northern Crown.

From 40°N in June, around 10 p.m., Arcturus stands high in the south-west, about 60 degrees above the horizon.

The main stars and their names

Arcturus (Alpha Boötis) shines at magnitude about −0.05 and lies about 37 light-years away. Its name is Greek for 'guardian of the bear'. It is an orange giant about 25 times the Sun's diameter, an older star that has used up the hydrogen in its core.

Arcturus also moves quickly across our sky compared with most bright stars, about the width of the full Moon in roughly 800 years. In 1933 its light, collected by a telescope and turned into an electric signal, switched on the lights of the Chicago World's Fair.

Izar (Epsilon Boötis), magnitude about 2.4, has a name from the Arabic for 'loincloth'. Muphrid (Eta Boötis), magnitude about 2.7, sits only about 3.3 light-years from Arcturus, so the two are true neighbors in space. Nekkar (Beta Boötis), at magnitude about 3.5, is a yellow giant at the top of the kite.

Double stars and deep-sky objects

Izar is the showpiece for a small telescope. A bright golden giant has a much fainter blue-green companion very close to it, and the color contrast is striking on a steady night. The 19th-century astronomer Friedrich Georg Wilhelm Struve called it Pulcherrima, 'the most beautiful'.

Kappa Boötis is an easier pair, white and bluish, but it is an optical double: the two stars only line up by chance, at different distances. Boötes lies far from the Milky Way, so its galaxies and clusters are faint. The globular cluster NGC 5466 is a loose, dim glow that needs a fair-sized telescope.

Boötes also contains the Boötes Void, a huge region with very few galaxies, a few hundred million light-years across. It cannot be seen through a telescope, but it shows that galaxies are arranged in walls and threads around large empty spaces.

The Quadrantid meteors

The Quadrantids are named after Quadrans Muralis, the Mural Quadrant, an old constellation that is no longer recognized; its stars now belong to Boötes. The shower peaks around January 3–4 and can produce more than 100 meteors an hour under ideal skies.

The catch is that the peak is short, only a few hours, and it often falls in daylight for one region or another. Check the predicted time for your year, and watch after midnight, when the radiant in northern Boötes climbs in the north-east. Their parent is thought to be asteroid 2003 EH1, possibly a dead comet.

Mythology

In Greek sky lore, Boötes is the man who drives the oxen of the plough, the seven stars we call the Big Dipper, or who guards the bears circling the pole. In one story he is Arcas, son of Callisto; in another he is Icarius, who learned to make wine from the god Dionysus and shared it with his neighbors, with tragic results.

Arcturus itself is one of the stars named in ancient Greek poetry; the poet Hesiod used its rising to mark the season for pruning vines and for the grape harvest. In Turkish the constellation is usually called Çoban, the Shepherd, and the star keeps its old name, Arktürüs.

Practical observing tips

Arcturus is bright enough for any city, and its orange color is clear to the eye. Compare it with blue-white Spica to the south and Vega rising in the north-east on early summer evenings.

The faint kite needs a darker site, away from lights, on a moonless night. For Izar, use a telescope at high power, 100 times or more, and wait for steady air; when the image stops shimmering, the companion appears. For the Quadrantids, dress very warmly, lie back and watch a wide area of sky.