Dubhe

Dubhe, also known as Alpha Ursae Majoris, second brightest star in the constellation Ursa Major, the Great Bear. Dubhe’s name comes from the Arabic phrase Thahr al Dubb al Akbar, which means “The Back of the Bear,” a reference to its position in Ursa Major. Modern observers find Dubhe more easily as one of the two stars at the outer edge of the bowl of the Big Dipper. Dubhe and the other star on this outer edge, Merak, are frequently referred to as the Pointers, since a line drawn from Merak through Dubhe points almost directly at Polaris, the North Star. Dubhe is less than 30° from the north celestial pole, a point in the sky about which the stars in the northern hemisphere appear to rotate. This appearance is actually due to the rotation of Earth. Because Dubhe is so close to the pole, it is visible to observers in most areas of the northern hemisphere each night of the year, circling the north celestial pole. For this reason, Dubhe is known as a north circumpolar star. Because of Dubhe’s far northerly position, it is not visible in southern South America or southern Australia. In other parts of the southern hemisphere and the tropics, Dubhe is visible in the night sky from mid-January through mid-April.

Stars that are visible to the unaided eye, such as Dubhe, belong to Earth’s home galaxy, the Milky Way, and tend to be very bright or relatively close. Dubhe is fairly close to our solar system and shines in the night sky as one of the 50 brightest stars as seen from Earth.

Dubhe’s estimated surface temperature is 4200°C (7600°F), which is about 20 percent cooler than the surface temperature of the Sun and gives the star an orange color. Its diameter is about 25 million km (about 15 million mi), which is about 18 times the diameter of the Sun. From its size, temperature, and composition, astronomers classify Dubhe as an orange giant star—a star that has used up most of the hydrogen in its core and has begun burning the hydrogen in its outer layers.

Dubhe is orbited by a much fainter companion star known as Dubhe B. For this reason, Dubhe is known as a binary star system. Dubhe B is a white dwarf star but is too close to Dubhe A to be readily detected with most amateur telescopes.