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| Brightest Stars on Dec 01, 2010

Kochab and Pherkad guard the North Celestial Pole

Two stars of Ursa Minor constantly circle the north celestial pole (and Polaris) as if to guard it. In addition, they served as a star clock for those who knew how to read it.

How to see Kochab and Pherkad
Kochab and Pherkad are stars in Ursa Minor, the Little Bear or Little Dipper. They mark the part of the cup of the dipper farthest from the north celestial pole. Because they are so close to Polaris and the north celestial pole, they can be observed every night of the year from the heavily populated areas of the northern hemisphere, and at some times from as far south as Brazil, much of Africa and far northern Australia. For northern observers, these stars can be seen at any hour of the night, any night of the year, but are best high overhead on midsummer evenings.

Finding this pair is easy. A line drawn from Polaris to Eta Ursa Majoris (Alkaid, the star at the end of the handle of the Big Dipper) passes very near Kochab. Along with Polaris, Kochab and Pherkad are the only stars of Ursa Minor easily visible from most urban locations.

Science
Although they appear close together in the sky, Kochab and Pherkad are not related in any way. Kochab is about 126 light-years away, whereas Pherkad is nearly four times farther, at about 480 light-years. Both are giant stars, larger and brighter than our sun.

Although not as hot as the sun, Kochab is roughly 40 times larger in diameter, and 500 times more energetic. In fact, if placed where our sun is now, Kochab would extend halfway to the planet Mercury.

Pherkad is not as large (a mere 15 times larger than the sun), but because it is much more massive and hotter than our star, it pumps out energy at 1100 times the rate. Actually, we should refer to it as Pherkad A to distinguish it from a nearby faint star, Pherkad B. The two are not related, but Pherkad B, barely visible to the unaided eye under very good conditions, is interesting in itself – it has at least one planet orbiting it.

Visually, Kochab is magnitude 2.07 and Pherkad is more than twice as dim at magnitude 3. Both are easily visible from dark locations with low light pollution. Spectroscopically, Kochab is a K4IIIvar – an orange giant with slight variability. Pherkad is an A3II-III – a hot white giant.

History and Mythology
Continually circling around Polaris, near the north celestial pole, Kochab and Pherkad are sometimes called “Guardians of the Pole.” Scientifically, they are designated Beta Ursa Minoris and Gamma Ursa Minoris, respectively.

The name Kochab is derived from an Arabic title that apparently refers to its nearness to the north celestial pole, and in fact about 3000 years ago it was closer to the pole than Polaris.

At times Kochab and Pherkad were seen as two calves, because they keep as close to the pole as calves to their mother. The name Pherkad appears to be derived from an Arabic term for calf.

Interestingly, these two stars were used by some as a timepiece, circling steadily as they do around Polaris like the hands of a celestial clock.

Kochab’s position is: RA: 14h 50m 42s, dec: +74° 09′ 20″
Pherkad’s position is: RA: 15h 20m 44s, dec: +71° 50′ 02″