Kochab (Beta Ursae Minoris)
Kochab (Beta Ursae Minoris) is a star in the constellation Ursa Minor, magnitude 2.1.
A pointer to true north for the ancient Egyptians — and now a Guardian of the Pole.
Long before Polaris took the job, Kochab helped mark north. Around 2500 BC, as Thuban drifted away from the celestial pole, a plumb line hung between Kochab and Mizar in the Big Dipper pointed to true north — something the ancient Egyptians knew well. From about 1700 BC until just after AD 300, Kochab and its neighbour Pherkad circled the pole as twin pole stars.
It is an orange giant some 44 times the Sun’s width and about 540 times as luminous, with a surface near 4,100 kelvin. Astronomers still debate its exact stage of life, and estimates of its mass range from about 1.3 to 2.5 Suns. Its age is put at roughly three billion years.
Kochab also brightens and fades slightly over about 4.6 days, and a giant planet some six times Jupiter’s mass has been reported circling it every 522 days.
Kochab (Beta Ursae Minoris) is 130 light-years away: the light we see from it set off around 1895, 7 years before the Wright brothers made the first powered flight.
Did you know?
- The Indestructibles. Ancient Egyptian astronomers called Kochab and Mizar “the Indestructibles” lighting the north — stars that never set.
- Just “star”?. The name’s meaning is uncertain. It may come from Arabic al-kawkab or Hebrew kōkhāv, both simply “star”, but more likely from Rucaba, an old name for a star in Ursa Major.
- The emperor’s star. In China it is Běi Jí èr, the Second Star of the North Pole asterism, representing the emperor.
An orange 2nd-magnitude star, the brightest in the bowl of the Little Dipper and nearly as bright as Polaris. With fainter Pherkad it forms the Guardians of the Pole; the celestial pole lies almost on the line from Polaris towards Kochab, a handy guide for aligning a mount.
When and where to see it
It is best placed in the evening sky around June, when it crosses the meridian at about 10 pm local time (around midnight in May). It can be seen from everywhere north of 16°S and never sets north of 16°N.
At its highest it stands 66° up from 50°N, 51° up from 35°N, 16° up from the equator, below the horizon from 35°S.
Facts
- Type
- Star
- Constellation
- Ursa Minor
- Designations
- Beta Ursae Minoris, β UMi, 7 UMi, HR 5563, HD 131873, HIP 72607
- Magnitude
- 2.1
- Position
- RA 14h 50.7m, Dec +74° 9′ (J2000)
- Spectral type
- K4-III
- Colour
- orange-red (B−V 1.47)
- Distance
- 130 light-years
- Discovered
- Known since antiquity
Open AstroPilot to see when Kochab (Beta Ursae Minoris) is highest tonight from your location, how easy it is with your telescope or binoculars, and an eyepiece view.