Getting to Know the Big Dipper
March 2011 :
With the arrival of spring in the northern hemisphere comes the culmination of the northern sky’s most recognized asterism, the Big Dipper. This familiar group of seven stars is notable for having a clear resemblance to its namesake (it is easy to imagine it forming the shape of a large spoon), but how much do you really know about the Big Dipper?
Finding north using the Big Dipper
No matter what the season, the Big Dipper provides us with valuable information to help us orient ourselves here on Earth. First and most significant, the Big Dipper can tell us where north is. Since the Big Dipper is a circumpolar asterism (from our latitude of about 42° north), all of its stars are visible regardless of the time of night or time of year, assuming you have a clear northern horizon. Once you have located the Big Dipper, look at the two outermost stars of the “bowl” of the dipper asterism, opposite of the handle. If you imagine a line connecting these two stars, and extend that line up and beyond the open end of the bowl, the next star you will encounter (just over five times the height of the bowl away) will be Polaris, the North Star. Since Polaris is always within one degree of the north celestial pole (the point in the sky towards which the Earth’s north pole points), finding Polaris is a fairly accurate indicator of true north. Due to the orientation of these two end stars in the bowl of the Big Dipper pointing towards Polaris, they are often referred to as the “pointer” stars. There is a common misconception that Polaris, the North Star is the brightest star in the sky. People often mistake Sirius, or even a planet for the North Star. This is simply not true, and has never been the case. What makes the North Star unique is its seemingly stationary position almost exactly in the north. In fact, Polaris, at magnitude 1.97, is only about the 45th brightest star in the sky, and is in fact outshone by three of the stars the Big Dipper.
What time is it? The Big Dipper can tell
Now that you know where north is, you can use the orientation of the Big Dipper in relation to Polaris to determine the approximate time. Because the Big Dipper is circumpolar, it never rises or sets, but rather rotates around the north celestial pole, marked roughly by the position of Polaris. If you watch the position of the Big Dipper through the course of the night, or observe it at the same time on a number of successive nights, you will notice that it rotates around the pole, counter-clockwise, or bowl-first. The Big Dipper, as with the entire sky, will make one complete revolution about the pole once per sidereal day. The sidereal day is approximately four minutes shorter than the 24-hour solar day we are more familiar with. For simplicity, we can assume a full 24-hour period when observing on a single night, but remember that the Big Dipper will be in the same position in the sky approximately four minutes earlier each successive night.
When using the Big Dipper to tell time, it is easy to imagine the pointer stars as the hour hand of a one-handed clock, but instead of making two revolutions per day as our clocks do, it makes a single revolution. The pointer stars will appear opposite of their observed position 12 hours earlier or 12 hours later; will appear to make one quarter turn every six hours, traverse 45° of arc in three hours, and so on. With this in mind, it is helpful to establish a reference position. Luckily, the timing works out so that the pointer stars are aligned nearly on the meridian at midnight on March 1st. You can imagine that on noon on March 1st, the pointer stars will be aligned directly below Polaris. Since we can approximate one complete revolution in 24 hours, it is easy to visualize the position of the pointer stars at any given hour once you have an initial reference position.
Due to the four minute difference between the sidereal day and the solar day, the pointers will have rotated slightly further to the west at the same hour on each successive night, making a complete revolution over the course of the year. Therefore, each successive month, at the same time, the pointers will be approximately 1/12 of the way around the pole. Since the pointers are oriented close to the meridian at midnight on March 1st, they will be opposite, or directly below Polaris one-half year later, on September 1st. They will be oriented to the left (or at the 9 o’clock position) on June 1st, and to the right (3 o’clock position) on December 1st. Note that 1 hour of time represents the same angular movement of the pointers as one-half month when viewed at the same time.
Now, simply knowing the date and having an unobstructed view of the Big Dipper, you will be able to determine the approximate time. Don’t forget to factor in Daylight Saving Time, which will cause the pointers to be “behind” by an hour, or 15 degrees of angle.
The closest star cluster
Now that the Big Dipper has told us what direction is north and what time it is, let’s learn a little about its stars. While we often refer to the parts of the Big Dipper by their representative names (the handle, the bowl, and the pointers), did you know that all seven stars in the Big Dipper have names? Starting from the end star of the handle and moving along to the back, bottom, and front of the bowl, the stars are named Alkaid, Mizar, Alioth, Megrez, Phecda, Merak, and Dubhe. All of the above stars, with the exception of the two extreme ends (Alkaid and Dubhe) are members of what is known as the Ursa Major Moving Group. These stars were most likely formed in the same nebula and are all traveling through the galaxy together. This makes the Big Dipper the closest star cluster to us.
Taking a tour
When visiting the Big Dipper with a telescope, a great starting point is Mizar, the middle, or “bend” star in the handle. This has always been my favorite place to start because Mizar & Alcor were the first objects after the Moon that I learned to find with my first telescope. That’s right, Mizar is not a lone star, but accompanied by slightly dimmer star to the northeast. Known in ancient times as the horse and rider, Mizar and Alcor are both spectral type A stars, so there will not be any notable color contrast, but the close proximity of the two bright stars in a low power eyepiece makes for a pleasing view. Just a bit to the south of the midpoint of the imaginary line connecting Mizar and Alcor lies a 7th magnitude star makes the view a little more interesting. But it doesn’t end there. Mizar itself is a double star, with a 4th magnitude companion 14 arcseconds away, which is an easy split even in small telescopes.
Once you’ve savored the view of Mizar and Alcor and you’re ready to travel to a distant galaxy, move your scope from Mizar, through Alcor, and continue along that line about eight times the Mizar-Alcor distance until you reach a 6th magnitude star. This is the first star along a shallow zigzag of four similar magnitude, similarly spaced stars that I call the “Mizar-M101 line.” Once you reach this star, move your scope in a line roughly parallel to the inner portion of the handle of the Big Dipper, but moving away from the Dipper’s bowl. Through a low power eyepiece, the shallow zigzag of the Mizar-M101 line will be unmistakable. You shouldn’t get lost here, but if you do, just back-track to Mizar and Alcor and try again. Once you reach the fourth star in the line, stop, and continue in the same direction you moved when you first left Mizar and Alcor, and for approximately the same distance. If you don’t see a large, diffuse glow in your eyepiece, try moving your eye around the field slowly to see if you can pick it up using averted vision. This is the face-on spiral galaxy M101, sometimes called the Pinwheel galaxy. Face-on galaxies are a little tricky to spot because we are looking at them from over one of their poles, so light from the “disk” of the galaxy is spread over the maximum possible area. For best results, use the lowest magnification available. If sky conditions permit, you may even be able to spot it in an 8x50mm finder.
Another spiral galaxy that presents itself as nearly face on to the inhabitants of the Milky Way is somewhat more well known and not too far away (from the perspective of a terrestrial star-gazer). This galaxy lies just across the Dipper’s handle from M101 and is easy to hop to from Alkaid, the very end of the Dipper’s handle. From Alkaid, you’ll want to move the telescope about two degrees to the southwest until you come across a 5th magnitude star known as 24 Canes Venaticorum. From this star, make a 120-degree turn back towards the southeast and travel about the same distance as from Alkaid to 24 CVn. You should find your gaze moving just past a pair of 7th magnitude stars a little less than a degree apart and oriented approximately perpendicular to the vector you arrived from. Nudge the scope just a bit further to the southeast and you should spot not one, but two fuzzy patches of light. This is the double nucleus of the Whirlpool Galaxy, or Messier 51 and NGC 5195. This is one of the most observed and photographed galaxies in the northern sky and once you realize how easy it is to find, you will find yourself taking the brief trip from the end of the Big Dipper’s handle whenever the opportunity presents itself.
Our next star hop brings us to the opposite end of the Big Dipper’s handle, that is the star that joins the handle to the bowl, Megrez. This star hop takes us to what is perhaps the least observed Messier object and certainly the most peculiar, as it is the only double star in Messier’s list of 110 objects. From Megrez, move north-northwest in a line parallel to the back side of the bowl. About 1.5 degrees away you should spot a 6th magnitude star. Keep moving in the same direction about 1/3 of a degree and you will spot a nice, identical pair of 10th magnitude stars. This is Messier 40.
Beginning from the southernmost “pointer” star Merak, or the bottom-front star in the Dipper’s bowl, we can find two Messier objects on a single trip. On this trek we not only encounter another spiral galaxy, but also the second and final Messier in our adventure that lies within our own Milky Way galaxy (the double star M40 being the other one). I tend to start this star hop by moving to the object furthest away from my guide star Merak first, then move inward to spot the other. From Merak, move the scope to the southeast two degrees, about 1/4 of the way from Merak to Phecda along the bottom of the Dipper’s bowl, but deviating from that line by approximately 20 degrees of angle below the bowl. Through low power you should spot a trio of stars that appears to me as a greatly enlarged version of the Trapezium in Orion missing one of its stars. The trick here is to imagine where that fourth star would be, and that is where you should spot M97, the Owl Nebula. You may need to use averted vision on this one since the Owl has a fairly low surface brightness. If you don’t immediately see another fuzzy spot in the eyepiece while viewing M97, move the telescope back towards Merak about 1/3 of the way, and a nudge northward. This is the galaxy M108.
The next star hop is a bit more tricky, but the effort pays off by showcasing what I believe to be one of the finest views in the northern sky. For this star hop, it helps to have a Telrad or red dot finder, because you’re going to aim your scope at an area of sky based on widely-spaced naked-eye guide stars rather than hopping off from one of them. To start, draw an imaginary diagonal line through the bowl of the Big Dipper from the lower back (Phecda) to the upper front (Dubhe). Now extend this line out the same distance as the diagonal through the bowl, but slanted slightly to the north, deviating from the original diagonal line by about 5 degrees of angle. If you point your telescope at this seemingly indistinct patch of sky, you should find the contrasting galaxy pair M81 & M82, once referred to as Bode’s Nebulae. M81 is a bright, textbook-perfect spiral galaxy and M82 is an edge-on irregular galaxy noted for its highly energetic star-forming characteristic, giving it another nickname, the Starburst Galaxy. Using just enough magnification to frame the galaxies with some surrounding sky, the view is unforgettable. I like to view the pair using a 9mm Nagler eyepiece in my Pronto, which shows just over 1.5 degrees of sky at 53x.
Going back to our line diagonally crossing the bowl, move in the opposite direction. Extend this line approximately 80% of the distance through the bowl beyond Phecda, and here you will find galaxy M106. This is one of the brightest galaxies in the region and should be easy to spot.
Our last star hop should be an easy one as it is a relatively short trip. Move back to Phecda and extend the line marking the bottom of the bowl back away from Merak, only about the distance of the width of the Moon. This is the barred spiral galaxy M109.
In all, the Big Dipper has pointed us to nine Messier objects, which should help guide you if you are planning on doing the Messier Marathon this year.
For some observers, Leo is a sign of Spring. For others, it is the rising of Arcturus. For me, the most prominent sign of Spring has always been the Big Dipper shining high overhead.
