Thursday, June 19, 2008

The Big Dipper Revisited

A couple weeks ago, we located that big drinking vessel in the sky, the Big Dipper. Then we used it to find Polaris, the North Star. This week, let’s take a closer look at the stars of the Big Dipper, both as a group and as individuals.

Most skywatchers know that the famous Big Dipper asterism (recognizable star pattern) is composed of seven stars. What you may not know is that the central five stars are related--what we could call a family group. Collectively they’re known as the Ursa Major Cluster: “Ursa Major” after the constellation that contains the Dipper, Ursa Major the Great Bear, and “Cluster” because they constitute a star cluster, a group of stars that formed around the same time in the same nebula or gas cloud. Astronomers have been able to determine that these five stars are a cluster because of their similar distance from us and the similar direction of their motion through space. All five are around 80 light years from Earth. A light year is the distance light travels in one Earth year, nearly six trillion miles.

Ursa Major Cluster
Chart created with Your Sky

If the Ursa Major Cluster is a family group, then the Big Dipper is an extended family. Let’s get to know each member a little better.

1) If you’re unfamiliar with the location and circular motion of the Big Dipper, read my earlier post.

2) The Summer Solstice on Friday, June 20, is the longest day of the year and, consequently, the shortest night. But even the shortest night gives us plenty of quality time with the Dipper. The bright Moon won’t rise until around two hours after sunset on Friday night and a bit later each subsequent night of our observing week.

Stars of the Big Dipper
Chart created with Your Sky

3) Moving from left to right along the right-side-up Dipper, we’ll start with the three stars of the handle and finish with the four stars of the bowl. The end star of the handle is Alkaid (pronounced AL-kayd). Alkaid, from the Arabic for leader, is a hot, blue-white star six times the size of our Sun.

4) The middle ‘star’ of the handle is probably the best known, because it is a naked-eye double star referred to as the Horse and Rider. The brighter star of the pair is Mizar, and its dimmer companion is Alcor. Mizar and Alcor are reported to have been used by the Romans and the Arabs as a vision test, in the days before they had eye charts with the big “E” on top. Now test your vision: can you see both stars? They are quite close together, and the light from dim Alcor blends together with Mizar’s when you look quickly. If you’re having trouble seeing the double, try using averted vision by looking at a point in space right next to Mizar. Sometimes not looking directly at the main star will help the companion star pop into view. This occurs because our peripheral vision is better than our straight-ahead vision.

Mizar (pronounced MY zahr) is from the Arabic for groin (of the bear). With a telescope, you can split bright Mizar, that is, observe that it is two stars close together. But you won’t have enough magnification to see that each of those two stars is also a double star. That’s right, Mizar is really a system of four stars--two orbiting pairs! Alcor (pronounced AL core) is from the Arabic for black horse. Although Alcor and Mizar both belong to the Ursa Major Cluster and Alcor appears to be incredibly close to Mizar, astronomers believe that Alcor is a bit too distant from Mizar to be in orbit around it.

So, now you can amaze your friends and family when you tell them that what looks like one star in the middle of the Dipper handle is really five stars.

5) The third star in the handle is Alioth (pronounced AL ee ahth), also from the Arabic for black horse. The luminous, white Alioth is the brightest star in the Big Dipper, as well as the entire Ursa Major constellation.

6) Megrez, the top left star of the bowl, is from the Arabic for base of the tail (of the bear). Megrez (pronounced MEG rezz) is the faintest star in the Big Dipper. This white star is about twice the size of our Sun.

7) Phecda (pronounced FEK duh) is the bottom left star of the bowl. Phecda, from the Arabic for thigh (of the bear), is spinning very fast--over 80 times faster than our Sun. It is believed to have a spinning disk of gas surrounding it.

8) Merak, the bottom right star of the bowl, is from the Arabic for flank (of the bear). Merak (pronounced MERR ahk) is one of the two pointer stars that point the way to the North Star. This white star is surrounded by a disk of heated dust, and it may be the most likely candidate of the Dipper stars to have a planetary system.

9) Our final bowl star, Dubhe (pronounced DOO bee), is from the Arabic for--amazingly enough--bear. Dubhe is the second brightest star in the Dipper, just a tad dimmer than Alioth. At around 124 light years from Earth, it’s the most distant Dipper star. In marked contrast to the white spectrum of the other Dipper stars, Dubhe is an orange giant. Can you see the difference in color between the two pointer stars, orange Dubhe and white Merak?

Dubhe is a rebel like Alkaid, moving in a different direction from the five stars of the Ursa Major Cluster. The renegade movements of Dubhe and Alkaid will ultimately distort the Big Dipper’s familiar shape beyond recognition, perhaps in 200,000 years or so. In the way that we now wonder, scratching our heads, how the ancients ever conjured a bear from the stars of Ursa Major, future generations may wonder what possessed us to see a water dipper emerge from a ragtag string of stars.