Of the seven stars in the Big Dipper, Dubhe is an outlier in more ways than one.
|Alpha Ursae Majoris (α Uma), HR 4301, HIP 54061
|Orange giant, K0 III
|11h 03m 44s
|+61° 45' 04”
|Yes, spectroscopic binary, plus two additional stellar companions
Almost everyone recognizes the Big Dipper, and you’ve probably seen it countless times yourself. But tonight, go out and take a closer look, paying special attention to the color of the stars. You may notice something that you haven’t before: the stars are all whitish-blue except for one: The lone standout is orange Dubhe.
Even though Dubhe is the Alpha star of Ursa Major, it’s actually slightly dimmer than Alioth, the Epsilon star. Dubhe’s warm color stems from the fact that it is an aging giant. Dubhe has used up the hydrogen in its core and has resorted to fusing helium instead, expanding into a giant 30 times the Sun’s width. It cooled as it grew, and its current temperature gives the star its orange color.
Dubhe differs from other Big Dipper stars in another way. Most of the others are members of the Ursa Major Moving Group, which is exactly what it sounds like: This cohesive group of stars with a similar history moves in the same general direction, even though the stars themselves are more widely separated than, say, a star cluster like the Pleiades. But Dubhe isn’t a member of the Ursa Major Moving Group, so it — along with Alkaid, the star at the end of the Dipper’s tail — travels through space separately.
But Dubhe isn’t alone. First, it’s a spectroscopic binary star; the close companion is a white, but much fainter, F star that’s still burning hydrogen in its core. Additionally, there is another F star and its companion farther out, so Dubhe is actually a four-star system.
One interesting side note: NASA’s Wide Field Infrared Explorer (WIRE) telescope launched in 1999 with the intended goal of observing galaxies. But when a technical issue ended WIRE’s primary mission prematurely, the small tracking telescope was repurposed for asteroseismology, measuring the brightness variations of stars to probe their inner structure. Astronomers thus salvaged some useful science out of an otherwise failed mission. One of the stars the satellite observed was Dubhe, and the findings helped to refine measurements of its mass.
Origin / Mythology
It would be difficult to find a group of stars with more widespread recognition and appeal than the Big Dipper. But folks with even a slight knowledge of astronomy will recall that the Big Dipper — beloved though it may be — is not a true constellation; it’s an asterism, a pattern among a smaller group of stars, within Ursa Major, the Great Bear. The name Dubhe is derived from Arabic for “the back of the bear,” aptly describing the star’s location within its constellation.
For the Greeks, the constellation was the bear Callisto (yes, the same Callisto that is the namesake of the Galilean satellite), of which Homer wrote, “the Bear…turns round and round where it is, facing Orion, and alone never dipping into the stream of Oceanus…” That’s a very poetic way of saying that Ursa Major is circumpolar when viewed from Greece.
The concept of Ursa Major as a bear is common worldwide, but the Big Dipper has had multiple different interpretations throughout time. It’s been a plow, a ladle, a drinking gourd, a coffin, a wagon, oxen, and others. In the Ojibwe culture of the Great Lakes region in the United States, the stars of the Big Dipper were identified as a fisher — a small weasel-like mammal — called ojiig.
How to See Dubhe
As Dubhe forms one of the corners of the Big Dipper’s “bowl,” it couldn’t be easier to find. When looking at the Big Dipper in an upright orientation, Dubhe is the star to the upper-right—one of the two “pointer” stars that lead the way to Polaris. (The other pointer is Merak.)
With a declination of about 61°, Dubhe is circumpolar for much of the U.S, meaning it never sets but rotates continuously in the northern sky all year long. Because of this, the Big Dipper may appear on its side or even upside down at various times throughout the night and the seasons. However, Dubhe, along with Merak, will never be far from the north celestial pole, making it simple to locate.
The color of stars isn’t always easy to detect. Hues are often subtle, and it takes a bit of practice to learn what it is you’re looking for. Dubhe is a good example to try out. A DSLR camera on a tripod will also record Dubhe’s subtle orange color, as well as give you the chance to compare it with the other Big Dipper stars at leisure. Good luck!