The Constellation that Is Home to The Big Dipper
The Constellation known as the Big Bear holds double stars, galaxies, and a planetary nebula. You can also use it as a guide to star hop across the sky.
The constellation Ursa Major is known by a couple of names. The entire constellation is Ursa Major, or the Big Bear, also sometimes the Great Bear. The portion of Ursa Major that represents the back half of the bear’s torso and it’s (oddly) long tail is often referred to as the Big Dipper. When part of a constellation has its own unique shape that is easy to identify apart from the entire constellation, that section is called an asterism. Ursa Major’s asterism, the Big Dipper, has also been called The Plough. That’s a lot of names for one grouping of stars.
The familiar dipper shape becomes a bear when you include the stars in front of the bowl of the dipper. These stars make up the bear’s upper torso, head, and front legs. The bear’s rear legs extend out from below the back part of the bear’s body.
Six of the seven stars of the Big Dipper are the brightest stars in Ursa Major. The dimmest of the seven stars is Megrez at magnitude 3.32. This star marks the juncture of the handle and bowl portion of the dipper. The next star up the handle, at magnitude 1.76, is the brightest star in Ursa Major and is called Alioth. The star at the very tip of the handle is magnitude 1.85 Alkaid, the third brightest in the Big Dipper and Ursa Major. In between these handle stars, at the bend of the handle, is a magnitude 2.23 double star, Alcor and Mizar. The ability to split these two stars (see each of them individually without optical aid) was used as an eyesight test before modern equipment was invented. Moving back onto the bowl of the Big Dipper across the top of the bowl to the star farther from the handle is magnitude 1.81 Dubhe. Below this star, diagonal from the handle, is magnitude 2.34 Merak. And finally, in the last corner of the bowl where the rear legs extend downward is magnitude 2.41 Phecda.
These stars can be used to find your way to major stars in other constellations. Most familiarly, take the two stars farthest from the handle in the Big Dipper, Dubhe, and Merak, and extending a line through them up and away from the bear’s back to find the bright star they point to: Polaris, the North Star, in Ursa Minor (the Little Dipper). Sailors, hikers, and countless others have used this to orient themselves with the night sky. Following the curve of the tail stars away from the bear, you can “arc to Arcturus” — the brightest star in the constellation Bootes, and from there you can “spike to Spica” — the brightest star in Virgo. (Spike to Spica is also sometimes said as “speed on down to Spica.”) Take the two bowl stars nearest the handle of the Big Dipper, Megrez, and Phecda, and draw a line below the bear to find the brightest star in Leo, Regulus. Starting at the Big Dipper asterism in Ursa Major, you have now found your way to four other constellations.
Another double star in Ursa Major, slightly less famous than Mizar and Alcor, is given the designation M40. M40 is the only double star in the Messier Catalogue. The 8.4 magnitude pair can be found next to the star Megrez.
One notable planetary nebula, M97 or the Owl Nebula, lies in Ursa Major a little to the east of Merak. The 11.2 magnitude nebula lies beside another Messier object, M108, a 10th magnitude spiral galaxy. Train your telescope in this area to get two for the price of one.
Another “two-for” can be found above the Big Bear’s head. The spiral galaxies M81 and M82 keep close quarters here. M81 is sometimes called Bode’s Galaxy and M82 is sometimes called the Cigar Galaxy. M81 is a fairly bright magnitude 6.9 and M82 is magnitude 8.4. As a truly difficult test of eyesight, try to spot M81 without optical aid. Only a handful of experienced observers have ever been able to do so. The Andromeda Galaxy, M31, is 2.6 million light-years away and is occasionally seen without optical aid at magnitude 3.4. M81 lies more than four times that distance, at 11 million light-years distant.
Two other decent galaxies round out the observing highlights in Ursa Major. First, just off the star Phecda, is M109. M109 is a magnitude 9.8 barred spiral galaxy. The other galaxy, M101, is a gorgeous magnitude 7.7 spiral nicknamed the Pinwheel.