April's Star

This month look for the star Dubhe, the alpha star of the constellation Ursa Major or the Big Dipper. On April evenings, when the Big Dipper is upside-down and pouring water into the Little Dipper, Dubhe is the lower left star in the bowl of the Big Dipper. Dubhe, which is Arabic for “The Bear” is 124 light years away. The light you see tonight left the star in 1890.

Dubhe is a multiple star system consisting of two pairs of stars orbiting each other. Dubhe A and Dubhe B are the star pair we see in the Big Dipper. Dubhe B and A are orbiting each other at a distance a little greater than the distance between the Sun and Uranus. At 124 light years away, Dubhe A and B are too close together to allow most telescopes to see them as separate stars. The second pair of stars is Dubhe C and Dubhe D and they too are also too close together for telescopes to see as separate stars. They orbit the A and B pair at a distance of 1 trillion miles. This means light takes two months to travel the distance between the A and B pair and the C and D pair.

Dubhe A is an orange star, that’s 30 times larger and 300 times brighter than our sun. It’s an old star and its core is fusing helium ash into carbon and oxygen. Outside of its core lies a shell of fusing hydrogen. Unlike Earth, the helium inside Dubhe sinks downwards in the core. That’s because helium is heavier than the hydrogen from where it originated.

Dubhe is one half of the Pointer Stars in the Big Dipper, the other star being the star above it. Follow the Pointers and you’ll run into Polaris, the brightest and closest star to the north celestial pole.