The Big Dipper - Tour Guide to the Stars
The Big Dipper has a whole bunch of names: The Drinking Gourd, The Plow (or The Starry Plough), The Butcher's Cleaver, The Great Bear, The Great Wagon, Odin's Wain, Charlie's (Charlemagne's) Wagon, Göncöl's Cart, The Coffin and Mourners, The Ladle, The Fisher Cat, Saptarishi (seven sacred stars of asterism), The Seven Gods and Samantha (from rural India so probably not a reference to Elizabeth Montgomery, the star of "Bewitched"). The constellation is one of the most prominent in the northern night sky and since 90% of all humans live north of the equator, it's collected a lot of names in a lot of cultures. It's official astronomical name, Ursa Major, from the Latin means "Great Bear".
The Dipper has several cool features you'll want to know about. When you show this constellation to the kids, you can point these out and in the process check off some more items on the star honor. The Dipper is part of the North Sky Circle and as such, is visible year round in most of the civilized parts of the Northern Hemisphere. It's long been used as a "pointer" constellation to help people find other objects like prominent stars and the Pole Star which marks true north. The two stars located at the outside edge of the dipper's bowl (Dubhe and Merak) are the North Star "pointers". As you can see from the picture below, if you draw an imaginary line through the two stars going "up" from the bowl, the first bright star you come to is Polaris, the north star.
There are four stars in the Big Dipper you should know my name: Dubhe and Merak (the pointer stars we discussed above) and Mizar and Alcor. They are labeled on the picture below.
You will notice that the bright star in the center of the dipper's handle is labeled twice. That's because it is a naked-eye double star. Alcor and Mizar are part of the same system. They used to be considered something of an eye test as you needed nearly 20/20 vision to distinguish them as two stars. You can easily see that the star is a double with a pair of binoculars. As it turns out, however, Mizar and Alcor are not a binary system.
There are, in fact, six stars in the system. Alcor itself is a binary star and Mizar is a quadruple star made up of two binary stars rotating around each other. In addition, the Alcor system and the Mizar system rotate around each other and travel with the rest of the Ursa Major "moving group" group of dispersed stars that share a common birth point. The other bright stars of of the Big Dipper, except Dubhe and Alkaid belong to the group. A telescope shows Mizar and Alcor quite clearly.
There are also a couple of interesting "Messier" objects - nebula and galaxies as identified by Charles Messier in his original 1781 astronomical catalog. The really spectacular one can be seen through a small telescope as a dim smudge. With a big scope you get a top down view of the Whirlpool Galaxy.
The smaller galaxy is thought to have passed M51 (The Whirlpool Galaxy) roughly along our line of sight and is now behind its large neighbor.
The lower portion of the Big Dipper handle between Mizar/Alcor and the bucket acts as a pointer. If you draw a line away from the bucket toward Mizar it points roughly at the second prominent Messier object near the Big Dipper - M101 or the Pinwheel Galaxy. Another face-on galaxy this one is a so-called "grand-design" spiral galaxy located about 27 million light-years from earth. It's twice as big as the Milky way. Astronomers think a recent encounter with another galaxy threw it out of shape and triggered strong star formation in the areas that look like red spots in this picture.
As an extra activity, have your kids search the Internet for the Hubble images of these two galaxies. They are quite spectacular and in August 2011, astronomers discovered a new supernova in M101.
In March 1956, a supernova was observed in the galaxy that reached a magnitude of 12.8. You can find it by drawing an arrow from Megrez to Mizar and searching among the stars just past Mizar.
The Big Dipper is a handy astronomical guide for viewing the sky with a telescope. If you want to spend an enjoyable evening picking out constellations, learning stars by name and finding nebula and galaxies, you need only find The Big Dipper to get started. This diagram shows some of the main pointers in the Dipper. Others are described below.
Lets find some things:
- Polaris - We already discussed finding the North Star above, but if you follow the line from Merak to Dubhe to Polaris, you will have arrived at the tip of the handle to the Little Dipper or Ursa Minor.
- M51 and M101 - The two galaxies we discussed can be spotted above and below Alkaid at the tip of the Dipper's handle
- M81 and M82 - Cross the bowl diagonally from Phecda to Dubhe and proceed about the same distance as to the North Star and there is another bright galaxy pair - M81 and M82
- Capella - Crossing the top of the bowl from Megrez to Dubhe points in the direction of the star Capella, the sixth brightest star in the night sky. Capella is found in the constellation Auriga. A way to remember this pointer is with the mnemonic "Cap to Capella."
- Castor and Pollux - Draw a a diagonal line from Megrez to Merak and then go straight for approximately five times that distance and you'll arive at Castor in the constellation Gemini, the Twins. The bright star nearby is Pollux, the other "twin". Castor is a visual binary star, but each of the binaries is a binary making it a quadruple star system. Castor also has a companion star that is an eclipsing binary with a period of one day so technically Castor is also a sextuple star system. Pollux is an evolved giant star and, in 2006, was found to have at least one planet circling it.
- Arcturus, Bootes and Spica - If you draw an ark along the curve of the Big Dipper's handle from Alioth to Mizar to Alkaid and travel on in that same arc, the first bright star you reach is Arcturus in the constellation Bootes. Arcturus is an orange giant star and the fourth brightest star in the heavens. Astronomers believe Arcturus has either a giant planet or a substellar companion circling it. Keep traveling on in that arc from Ursa Major and the next bright star you hit will be Spica in the constellation Virgo. Spica is interesting because it is a blue giant star. It is the 15th brightest star in the sky.
- Regulus - If you extend a line from Megrez to Pheceda on the inside of the bowl, the first bright star you come to will be Regulus in the constellation Leo. Regulus is another multiple star system with four stars organized into two pairs. The mnemonic to help you remember this pointer is "A hole in the bowl will leak on Leo."
- Cassiopeia and the Pleiades - If you project a line from Alkaid through Polaris, the North Star, you will come first to the constellation Cassiopeia and continuing on will come to the Seven Sisters or the Pleiades.
- The Hubble Deep Field - If you draw a line from Phecda to Megrez and continuing on for the same distance again you will come to a small spot recently observed by the Hubble Space Telescope. The pictures showed an incredible almost 3000 galaxies in an area of a mere 2.5 seconds of arc.
|The Hubble Deep Field|
You can learn to identify all the stars by name that you need to learn for your Star Honor patch just there in the neighborhood of the Big Dipper. The rest of the constellations the Big Dipper points to will give you all the constellations you need as well, but don't quit there. There are more cool objects to look at in the night sky coming, including my personal favorite. Tune in next week to see actual light from heaven through your telescope.
(c) Tom King - April 2013