|Official name (IAU-approved)||Kochab|
|Other Designations||Beta Ursae Minoris, 7 Ursae Minoris, HD 131873, HR 5563, HIP 72607|
|Nicknames||“Guardians of the Pole” (with Pherkad)|
|Distance from Earth||130.9 light-years|
|Type||K4-III B (orange giant)|
|Right Ascension||14h 50m 42s|
|Declination||+74° 04' 19"|
|Probable fate||White dwarf / planetary nebula|
Kochab is also known as Beta Ursae Minoris, a designation that marks it as the second brightest star in Ursa Minor, the Lesser Bear. At magnitude 2, it’s far down the list in terms of overall brightness, but it’s an interesting star with a history of usefulness to humankind.
Kochab is an orange giant star about twice as massive as our Sun. But the hydrogen and helium plasma that make up Kochab is spread out more thinly than in the Sun, spanning a diameter 42 times larger. So, like other giant stars, Kochab’s size is somewhat deceiving because it’s quite diffuse. This effect is caused by the star’s current life stage as it has finished fusing hydrogen in its core and moved off the main sequence that defines stars’ prime. Even though it doesn’t appear as bright as some of the major stars in Earth’s night sky, such as Sirius, Betelgeuse, or Canopus, Kochab does shine with an obvious and pleasing orange color that makes it one of the jewels in Ursa Minor.
Owners of large telescopes may wish to go searching for Kochab’s neighbor (on the sky anyway), a dwarf star that’s woefully dim at magnitude 12.8. However, the distance measurement of this star from Gaia data, which puts it at 613 light-years away, suggests that the two stars are not gravitationally bound.
Telescope or not, you won’t detect Kochab’s other companion with amateur equipment. The star hosts a confirmed exoplanet, Beta Ursae Minoris b — a dense gas giant at least 6 times Jupiter’s mass, although it’s only slightly larger in diameter. Unlike Jupiter’s roughly 5-a.u.-orbit around the Sun, this exoplanet circles Kochab at a distance of 1.4 a.u., just inside Mars’s orbit around the Sun.
Origin / Mythology
The Little Dipper may be smaller than its bigger sibling, but it’s probably the more important for its utility in finding the celestial pole. That said, these stars are so dim that the Greeks initially had them subsumed in Draco, the Dragon, which winds its way around the sky’s north celestial pole. The stars provided the Dragon with wings. The Greek philosopher Thales, however, recognized the importance of Ursa Minor’s prime pole location. He broke these stars off into their own constellation, forming Ursa Minor, and leaving the Dragon with his wings presumably folded away. Thales may have learned the navigational utility of Ursa Minor from Phoenician sailors.
It’s significant that Thales suggested the entire Ursa Minor constellation as a general navigational beacon, rather than singling out Polaris the way we do today. At the time Thales lived — about 600 B.C. — Polaris wasn’t as close to the celestial pole as it is today.
Around 1000 B.C., Kochab was close enough to the pole for Middle Eastern astronomers, who gave it the name Al-Kaukab al-shamaliyy for “star of the north.” This might be the origin of the modern name Kochab.
Even though Kochab was farther from the pole in earlier times, the ancient Egyptians might nevertheless have used Kochab as part of their method for determining north while building their pyramids. When the Great Pyramid of Giza was built some 4,500 years ago, there was no perfect pole star — 3.7-magnitude Thuban was closest, but like Polaris now, it was not directly at the north celestial pole. Egyptologist Kate Spence proposed in the November 16, 2000, Nature, that the pyramid builders could instead have drawn an imaginary line between Kochab and Mizar, in Ursa Major, to identify true celestial north. The two stars almost exactly straddled the pole.
Though later studies contested this theory, it can nevertheless explain why earlier pyramids are now oriented a bit east of true north while later pyramids are oriented a bit west — it’s merely precession at work, unbeknownst to the ancient architects.
How to See Kochab
Ursa Minor is a faint constellation. If it were located anywhere else in the sky, it would probably be among those other dim and little-known groups of stars. As it is, you’ll need a fairly dark sky to pick out the fainter stars of the group. Still, Polaris is an easy target, thanks to its unchanging position, and because Big Dipper stars Dubhe and Merak point the way.
Once you find Polaris, slide your way down the dim stars of the Little Dipper’s handle (the shape isn’t quite the same as the Big Dipper, but the idea is similar). Kochab marks the outer rim of the “bowl,” occupying the equivalent position of Dubhe in the Big Dipper. (Note: Depending on the time and season, Ursa Minor and its dipper may appear “upside-down.”)
Ursa Minor (including Kochab) is circumpolar from all 50 U.S. states, though from Hawai‘i Kochab comes close to “dipping” into the ocean. It can be fun to watch the Little Dipper spin around the celestial pole over a few hours; as it pivots around Polaris, bright Kochab spins in a tight circle. You can try it any clear night of the year — why not tonight?
Daniel Johnson is a Wisconsin-based freelance writer and professional photographer and the co-author of over a dozen books. He’s a longtime amateur astronomer and fortunate enough to live in a rural region with excellent seeing conditions. You can view some of Dan’s photography (he does a lot of animals!) at www.foxhillphoto.com