Dipper to Polaris, plus Mizar and Alcor
Print Me Email to FriendTonight's Sky for Saturday, Aug 02 2008
The two outer stars in the bowl of the Big Dipper – Duhbe and Merak – always point to Polaris, the North Star. To find this Dipper at this time of year, look toward the northwest in the evening.
And once you’ve found it – after locating Polaris – look more carefully at the second star from the end of the Big Dipper’s handle. If your sky is dark enough, and your eyesight is good, you’ll see that this star, Mizar, has a nearby companion, called Alcor.
Arabian stargazers called Mizor and Alcor the “horse and rider.” These stars are a good test of the night’s viewing conditions: if you can’t see Alcor, there might be thin clouds up there.
These two stars are what’s called “naked-eye double star,” appearing double from our earthly vantage point. But do they orbit each other? Astronomers aren’t sure. The distances to these stars (as to most stars) isn’t precisely known. And if Mizar and Alcor are a true double star, they are a very wide double. If they do lie at the same distance from Earth, their separation is .27 light-years . . . that’s in contrast to eight light-minutes for Earth’s distance from our sun . . . or several light-hours for the distance to our sun of the most distant worlds in our solar system. Still, it’s possible that Mizar and Alcor could be this far apart and still be orbiting one another, with a very long orbital period of three-quarters of a million years.
Does the big dipper contain large bright stars or are they stars that are really close to us because usually the first constellation I can identify when the sun sets is the big dipper.
Bob, essentially every star you see in the night sky without a telescope is bigger and brighter than our Sun. They are easily seen not because they are close, but because they are intrinsically bright. The most common stars are less bright than the Sun, and as such are very difficult to see unless they are quite close. So in answer to your question, the stars in the Big Dipper are generally bigger and brighter than our Sun, and they aren’t prominent because they are particularly close. They certainly aren’t the brightest stars visible, but all the main stars in the Big Dipper part of Ursa Major are probably in the top 100.
LS