Hokuto Shichi Sei

Big Dipper map

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

The Hall of the Big Dipper in a Taoist temple, Wuhan

Throughout eastern Asia, these stars compose the Northern Dipper. They are colloquially named "The Seven Stars of the Northern Dipper":

• Chinese: 北斗七星; <pinyin: běidǒu qīxīng;
• Japanese: 北斗七星, hokutoshichisei;

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.

The Big Dipper's bowl and part of the handle photographed from the International Space Station.
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).

4D proper moving in -/+ 150 000 years. To view this image you need 3D glasses (red-green or red-blue).

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.
• Crossing the top of the bowl from Megrez (δ) to Dubhe (α) takes one in the direction of Capella (α Auriga). A mnemonic for this is "Cap to Capella."

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

• Shichōsei