Dubhe (Alpha Ursae Majoris)
Dubhe (Alpha Ursae Majoris) is a star in the constellation Ursa Major, magnitude 1.8.
The Plough’s outer pointer — an orange giant that shows the way to the North Star.
Dubhe is the odd one out in the Plough. Most of the pattern’s stars travel through space together as the Ursa Major moving group; Dubhe does not. It differs in colour, too: an orange giant star of 3.7 solar masses and 280 million years, it has used up its core hydrogen and swollen to 27 times the Sun’s width, shining with 340 times the Sun’s light.
Close beside it, too near to see apart, a hot white star circles it every 44.5 years. Another close pair of stars a few arcminutes away belongs to the same system, making four stars in all.
Its name comes from the Arabic for ‘bear’, from a phrase meaning ‘the back of the Greater Bear’. The ancient Egyptians called it Ak, ‘the Eye’. In India it was Kratu, one of the Seven Rishis, and in China Tiān Shū, the Celestial Pivot.
Dubhe (Alpha Ursae Majoris) is 120 light-years away: the light we see from it set off around 1905, around the time the Wright brothers made the first powered flight.
Did you know?
- Utah’s star. Dubhe is the official state star of Utah.
- A faint flutter. Dubhe varies by about a thousandth of a magnitude — far too little to see. Astronomers have found ten different oscillations in it, with periods from 6.4 hours to 6.4 days.
The orange-yellow star at the outer top corner of the Plough’s bowl. With Merak below it, it forms the Pointers: follow them about five times their separation to reach Polaris. Compare its warm colour with the white stars of the rest of the bowl.
When and where to see it
It is best placed in the evening sky around April, when it crosses the meridian at about 10 pm local time (around midnight in March). It can be seen from everywhere north of 28°S and never sets north of 28°N.
At its highest it stands 78° up from 50°N, 63° up from 35°N, 28° up from the equator, below the horizon from 35°S.
Facts
- Type
- Star
- Constellation
- Ursa Major
- Designations
- Alpha Ursae Majoris, α UMa, 50 UMa, HR 4301, HD 95689, HIP 54061
- Magnitude
- 1.8
- Position
- RA 11h 3.7m, Dec +61° 45′ (J2000)
- Spectral type
- G9III+A7.5
- Colour
- orange (B−V 1.07)
- Distance
- 120 light-years
- Discovered
- Known since antiquity
Open AstroPilot to see when Dubhe (Alpha Ursae Majoris) is highest tonight from your location, how easy it is with your telescope or binoculars, and an eyepiece view.