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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 containing 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:

  • 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 and Pollux (α and β Geminorum) can be located by crossing the bottom of the bowl from Phecda (γ) to Merak (β) and continuing onward.
  • 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

References

  • Dickinson, Terence, and Alan Dyer. 2008. The Backyard Astronomer's Guide. Richmond Hill, Ont: Firefly Books. ISBN 978-1554073443.
  • Raymond, John C. 2005. Asterisms for Small Telescopes and Binoculars. Lawrenceville, VA: Brunswick Pub. ISBN 978-1556182051.
  • Seronik, Gary. 2006. Binocular Highlights: 99 Celestial Sights for Binocular Users. Cambridge, MA: New Track Media. ISBN 978-1931559430.
  • Sinnott, Roger W. 2006. Sky & Telescope's Pocket Sky Atlas. Cambridge, MA: Sky Pub. ISBN 978-1931559317.

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