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The Plough or the Big Dipper is an asterism of seven stars that has been recognized as a distinct grouping in many cultures from time immemorial. The comprising stars are the seven brightest of the formal constellation Ursa Major.
Names and lore
In Hindu astronomy, it is referred to as (Vrihat) Sapta Rishi meaning "The Seven (Great) Sages".
Throughout eastern Asia, these stars compose the Northern Dipper. They are colloquially named "The Seven Stars of the Northern Dipper":
The seven stars are very important in Taoist astrology.
Stars
Within Ursa Major the stars of the Big Dipper have Bayer designations in consecutive Greek alphabetical order from the bowl to the handle.
Mizar and Alcor are at the upper right.
| Proper Name |
Bayer Designation |
Apparent Magnitude |
Distance (L Yrs) |
|---|---|---|---|
| Dubhe | α UMa | 1.8 | 124 |
| Merak | β UMa | 2.4 | 79 |
| Phecda | γ UMa | 2.4 | 84 |
| Megrez | δ UMa | 3.3 | 81 |
| Alioth | ε UMa | 1.8 | 81 |
| Mizar | ζ UMa | 2.1 | 78 |
| Alkaid | η UMa | 1.9 | 101 |
In the same line of sight as Mizar, but three light-years beyond it, is the star Alcor (80 UMa). Together they are known as the Horse and Rider. At fourth magnitude, Alcor would normally be relatively easy to see with the unaided eye, but its proximity to Mizar renders it more difficult to resolve, and it has served as a traditional test of sight. Mizar itself has four components and thus enjoys the distinction of being part of an optical binary as well as being the first-discovered telescopic binary (1617) and the first-discovered spectroscopic binary (1889).
Five of the stars of the Big Dipper are at the core of the Ursa Major Moving Group. The two at the ends, Dubhe and Alkaid, are not part of the swarm, and are moving in the opposite direction. Relative to the central five, they are moving down and to the right in the map. This will slowly change the Dipper's shape, with the bowl opening up and the handle becoming more bent. In 50,000 years the Dipper will no longer exist as we know it, but be re-formed into a new Dipper facing the opposite way. The stars Alkaid to Phecda will then constitute the bowl, while Phecda, Merak, and Dubhe will be the handle.
Guidepost
Not only are the stars in the Big Dipper easily found themselves, they may also be used as guides to yet other stars. Thus it is often the starting point for introducing Northern Hemisphere beginners to the night sky:
- Polaris, the North Star, is found by imagining a line from Merak (β) to Dubhe (α) and then extending it for five times the distance between the two Pointers.
- Extending a line from Megrez (δ) to Phecda (γ), on the inside of the bowl, leads to Regulus (α Leonis) and Alphard (α Hydrae). A mnemonic for this is "A hole in the bowl will leak on Leo."
- Crossing the top of the bowl from Megrez (δ) to Dubhe (α) takes one in the direction of Capella (α Aurigae). A mnemonic for this is "Cap to Capella."
- Castor (α Geminorum) is reached by imagining a diagonal line from Megrez (δ) to Merak (β) and then extending it for approximately five times that distance.
- By following the curve of the handle from Alioth (ε) to Mizar (ζ) to Alkaid (η), one reaches Arcturus (α Boötis) and Spica (α Virginis). A mnemonic for this is "Arc to Arcturus then speed to Spica."
Additionally, the Dipper may be used as a guide to telescopic objects:
- The approximate location of the Hubble Deep Field can be found by following a line from Phecda (γ) to Megrez (δ) and continuing on for the same distance again.
- Crossing the bowl diagonally from Phecda (γ) to Dubhe (α) and proceeding onward for a similar stretch leads to the bright galaxy pair M81 and M82.
- Two spectacular spiral galaxies flank Alkaid (η), the Pinwheel (M101). to the north, and the Whirlpool (M51), to the south.
See also