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| Brightest Stars on Jun 10, 2014

Kochab and Pherkad guard the pole star

Two noticeable stars in the Little Dipper are said to guard the north celestial pole because they circle so close to Polaris.

Kochab and Pherkad are part of the famous Little Dipper asterism in the constellation Ursa Minor the Lesser Bear. They mark the outer part of the cup of the Little Dipper, the part farthest from Polaris, the North Star. Kochab is designated Beta Ursa Minoris, and Pherkad is Gamma Ursa Minoris. Their proximity to the famous Polaris might cause you to overlook them, but Kochab and Pherkad have their own claims to fame. Follow the links below to learn more.

How to see Kochab and Pherkad

Science of Kochab and Pherkad

History and mythology of Kochab and Pherkad

Use the two outer stars in the bowl of the Big Dipper to find Polaris, which marks the end of the handle of the Little Dipper.  Kochab and Pherkad are in the bowl of the Little Dipper.

Use the two outer stars in the bowl of the Big Dipper to find Polaris, which marks the end of the handle of the Little Dipper. Kochab and Pherkad are in the bowl of the Little Dipper.

Chart via IAU, skyandtelescope.com, Wikimedia Commons.

Chart via IAU, skyandtelescope.com, Wikimedia Commons.

How to see Kochab and Pherkad Because they are so close to Polaris and the north celestial pole, Kochab and Pherkad 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, if you look first for the Big Dipper and then Polaris. Use the two outer stars in the bowl of the Big Dipper to find Polaris, which marks the end of the handle of the Little Dipper. Kochab and Pherkad are in the bowl of the Little Dipper. Along with Polaris, Kochab and Pherkad are the only stars of Ursa Minor easily visible from most urban locations.

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. Make your own star clock.

A view of the giant orange star Kochab, from the point of view of two hypothetical bodies in close orbit.  Caption and artist's illustration via ESky

Artist’s illustration showing the giant orange star Kochab, from the point of view of two hypothetical bodies in close orbit, via ESky In May 2014, astronomers confirmed at least one planet orbiting Kochab.

Science of Kochab and Pherkad. Although they appear close together in our sky, Kochab and Pherkad are not related in any way, in space. Kochab lies about 130 light-years distant, while 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. David Darling says that Kochab has exhausted its core supply of hydrogen and is an evolving orange giant star probably at the stage of helium-burning.

Pherkad is not as large (a mere 15 times larger than our sun), but because it is much more massive and hotter than our star, it pumps out energy at 1,100 times the rate. Actually, we should refer to it as Pherkad A to distinguish it from a nearby faint star, Pherkad B. These two are not related gravitationally, either, by the way. Pherkad B is barely visible to the unaided eye under very good dark sky conditions.

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.

Ursa Minor, the Lesser Bear, via ianridpath.com

Ursa Minor the Little Bear, via ianridpath.com. This constellation contains the Little Dipper asterism, of which Kochab and Pherkad are a part.

History and mythology of Kochab and Pherkad. Continually circling around Polaris, near the north celestial pole, Kochab and Pherkad are often called Guardians of the Pole.

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.

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

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