Known as the plough or the wagon in Western mythology, the Big Dipper is an asterism that is part of the larger constellation Ursa Major, the great bear. The seven stars (well, eight if you include both Mizar and Alcor) comprise the hindquarters and long tail of the big bear. The nose and legs are much fainter and tough to pick out (here is a good star chart).
In Greek mythology (from Ovid's book Metamorphoses), the bear is the Princess Callisto, who was a follower of Artemis, the goddess of hunting. Zeus trapped Callisto in the woods one day and had his way with her, and when she became pregnant, Artemis assumed Callisto was someone of poor character and kicked her out of the group. Callisto had a son, Arcas, and after this, Hera piled on by transforming Callisto into a bear, as if the whole thing were Callisto's fault.
For years, Callisto roamed the woods as a bear with a human mind, always on the run from hunters. One day, she encountered her son, Arcas, who didn't recognize her, but before Arcas could shoot her, Zeus transformed her into a heavenly creature. For Northern observers, the bear never sets, though she is low on the horizon in the evening during winter, presumably hibernating. In Spring, you can find Ursa Major in the evenings climbing up from the Northeastern horizon, presumably coming out of hibernation, and then during Fall evenings, the bear is headed downward toward the Northwestern horizon, going into hibernation.
The Big Dipper was also important during U. S. History in the 19th century. As slaves were instructed to runaway toward the North along such routes as the Underground Railroad, the knew which way to go by finding the "drinking gourd" in the sky, whose pointer stars (the two stars at the end of the bowl, Dubhe and Merak) lead the eye to Polaris, which marks true north. The Big Dipper's bowl can also be used to find several other stars in the sky, including Regulus, Castor, Betelgeuse and Capella following lines from Megrez, which connects the bowl and handle). If you instead follow the arc of the handle, that curve leads to Arcturus and further on to Spica.
Now for the stars in the Big Dipper portion of Ursa Major, starting with Alpha UMa, or Dubhe, which comes from the Arabic for bear. This isn't the brightest star in the constellation by just a little bit, but it definitely stands out from the other bright stars of the Dipper. The middle five stars of the Dipper are all similar stars at about the same distance from Earth (80 light years) and are thought to be from the same loose association of stars known as the Ursa Major moving group.
A "moving group" is basically a very loosely associated cluster. The stars aren't close enough together to be called a cluster, but they all move in the same general direction in space and seem to have other similarities (such as age and/or composition), and the Ursa Major moving group is the closest such association to the Earth. This is likely the remnant of an open cluster that formed about 300-500 million years ago and was subsequently pulled part by tidal forces within the disk of our galaxy.
Beta Aurigae (Menkalinan) is also a part of this group. Our Sun is on the outskirts of the group, but it is moving in a different direction and is also more than 10 times older than the average age of moving group stars. There is some debate over whether Sirius should be considered a member. It is about the right age, composition and is similar to the Dipper stars, and it is moving in the right direction on the sky, in parallel with the stars in the moving group, but it is pretty far from the center of the group, so it is hard to say for sure.
Anyway, Dubhe is a bit further than this moving group (120 light years approximately) and is a K-class giant star rather than a blue main sequence star. It is moving roughly in the opposite direction on the sky is the moving group, so Ursa Major will look very different in a few million years as a result of these divergent proper motions. Here is a nice animated gif showing the changes over the millennia.
The other of the two North Celestial Pole pointer stars is Merak, a star very similar to Sirius but much further away (80 light years instead of 8.7 ... recall that Sirius is in the nearest ten stars to the Earth). Rounding out the bowl, the other star at the base of the bowl of the dipper is Phad (or Phecda), from the Arabic for thigh. Similar to the other moving group stars, it is a hot, blue main sequence star roughly 80 light years distant. Unlike the others, it is a rapid rotator and has thrown off a bright disk of hot gas.
Moving to the junction of bowl and handle, we come to Delta UMa (Megrez), the faintest star in the Big Dipper asterism. Its name appropriately comes from the Arabic for root of the tail. Kaler mentions an age for Megrez of about 50 million years old, which would make it far younger than the other moving group members, so I wonder if that's a typo as I can't find any other reference at hand that estimates its age.
Moving up the handle, the next star is the brightest star in Ursa Major, Epsilon UMa (Alioth). As with many other constellations, the greek letter designations don't indicate brightness, but instead here just are in order from West to East for the brightest stars. This star ranks about 32nd in brightness in the entire sky. Like the others in the moving group, it is a hot blue A star, and a basic rule of Astronomers who study individual stars, there is no such thing as a "normal" A star. Alioth is no exception. It rotates about every 5 days, and is it does, its composition appears to change (we are just looking at different parts on the star's surface) due to the strong effect its magnetic field has on which elements are pushed by radiation from the inner parts of the envelope to the surface.
The 2nd star in the handle is the famous pair Mizar and Alcor, or "horse and rider". The proximity of these two stars in the sky is close enough (14 arcseconds) to be picked out if you have very good eyesight at a dark site, and it is quite nice to look at through a small telescope. There you can see that this is actually a double-double system, with each of the two bright visible stars having faint nearby orbiting companions. Because they rotate very slowly, it is easier for heavy elements to be pushed to the surface by radiation pressure (without being plunged back into the star's interior by turbulence or magnetic effects), so these stars have a lot of rarely seen heavy elements in their spectra. Burnham also notes that Mizar and Alcor is a perfect starting point for a star-hop to the famous spiral galaxy M101 which I will talk about a little later.
Finally, at the tip of the handle, we come to Eta UMa (Alkaid) whose name comes from the leader as this star leads the rest of the Dipper stars across the sky. Like Dubhe, this one doesn't belong to the moving group. It is much hotter and younger than the moving group stars, and it is moving in a different direction on the sky, helping to change the shape of the Dipper over time.
Three pairs of stars on the sky form three of the feet of Ursa Major. In Arabic culture, these three pairs south of the Dipper and right on the border with Leo Minor are known as the three leaps of the gazelle. The westernmost pair, marking the front foot, is Talitha Borealis (Iota UMa) and Talitha Australis (Kappa UMa). The middle pair is Tania Borealis and Tania Australis, representing the northern and southern stars, and the easternmost pair is Alula Borealis and Alula Australis. The star names actually go from west to east, with Alula meaning "first" in Arabic, Tania meaning "second" and Talitha meaning "third".
Talitha is an A-class star about 50 light years away, 1.7 solar masses and most of the way through the main sequence part of its lifetime. It has a close, faint companion about 5 AU away (roughly Jupiter's orbital distance from the Sun), and It is orbited very remotely by a close pair of M-dwarfs, taking about 800 years to go all the way around. The other of the pair is Kappa UMa, a close binary system consisting of two hot stars, both of which seem to be spinning rapidly and throwing off a mutual circumstellar disk that creates emission lines in the spectrum.
The next pair starts with Lambda UMa (Tania Borealis), a slow rotating hot blue A star. The other is Mu UMa (Tania Austalis), a red giant on the verge of igniting Helium fusion in its interior. The westernmost pair starts with Alula Borealis (Nu UMa), which is technically the first leap since the gazelles leap from west to east. Alula Borealis is a bit further along in its evolution compared to Tania Austalis, being a hotter K giant probably fusing heavier elements like Carbon and Oxygen in its core instead of Hydrogen or Helium. And Alula Australis (Xi UMa) is perhaps the most historically significant of the six, according to Kaler.
As William Herschel discovered, Alula is not only a visual double in the sky, it is the first such double that was discovered to be an orbiting binary system, and it was the first such system to have its orbit successfully predicted, which allowed the first successful determination of a stellar mass other than our own Sun. This system is two stars nearly identical to the Sun with an orbital period of about 60 years, and each of the two stars is itself orbited by a very low mass, closeby companion, making it at least a quadruple star system, and further observations reveal that there may be as many as five or six stars in this system.
There are also three rather dim stars in Ursa Major around which planets have been discovered. Kaler has more on each of the three systems, which are HR 4067, Pi-2 Ursa Majoris and the famous 47 Ursa Majoris, one of the first systems discovered with multiple planets. The two planets here are spaced from their star in a way somewhat similar to that of Jupiter and Saturn, though they are (like all planets discovered to date) much closer to their parent star, being only 2.1 and 3.7 AU away on average. This system is 46 light years away and has a central star very similar to our own Sun in mass, luminosity, size and composition. Could there be terrestrial planets here as well? Probably not because the big planets are so close in.
For the deep sky objects in Ursa Major, there are many, so I will start with some of the brightest and best known Messier Objects within Ursa Major. If you follow a line from Phad to Dubhe and again as far, you will run into a small group of bright galaxies including M81 and M82. M81 is a spectacular spiral galaxy about 12 million light years away from us. Since it is nearby galaxy so similar to our own, it has been closely studied.
Here is an image of the same galaxy in Ultraviolet light, which reveals more sharply the presence of hot, young stars populating the spiral arms of the galaxy. Notice in the image how sharp is the boundary between the core of the galaxy and the disk filled with young stars. This next image is an even deeper visible light image of M81 shows the extent of the disk a little more clearly and also reveals a lot of nebulosity along our line of sight to this galaxy from within our own disk. My favorite image of M81 is this incredibly deep and detailed image from Hubble. Click on the link and then click again on the image and scroll around for a truly breathtaking tour of a grand design spiral galaxy.
Very close by in the sky, a fraction of the degree to the North, is M82, also known as the Cigar Galaxy. As the image shows, M82 has been disrupted tidally by nearby M81. This tidal disruption trigged not only changes in the structure of M82 but likely also sparked a lot of star formation in the galaxy. Perhaps this is responsible for the enormous combined stellar wind of thousands of hot, massive stars that is blowing filaments of Hydrogen gas out of the galaxy now.
A few degrees East of this pair is IC 2574, also known as Coddington's Nebula. It isn't a nebula but instead a galaxy and a member of the M81 group. Close examination of this galaxy's neutral Hydrogen reveals that it is sprinkled with enormous cavities, regions where the gas has apparently been cleared out. Since these regions are associated with active star formation (which also contributes to the very blue, irregular color distribution of the galaxy), we think this is due to many recent supernova explosions or perhaps the combined stellar wind of several young OB associations of stars.
Next up, a few degrees North of Alkaid at the tip of the handle of the Dipper is Messier 101, also known as the Pinwheel galaxy (often confused with the Triangulum galaxy). This enormous spiral is about 50 percent larger than our own (pretty big) Milky Way galaxy and be seen even more clearly in this higher resolution image from the Hubble Space Telescope. This was one of the Key Project galaxies in which we were looking for Cepheid variables since we had already seen supernovae go off in this galaxy within the past few decades. Here is another image with a wider, deeper field of view showing better the extent and asymmetry of the disk of M101, almost certainly due to a recent gravitational close encounter with one of its many small satellite galaxies. About 3000 individual H II regions have been detected on such photographs.
Next up is a pair of Messier objects together in the sky: M97 and M108, a couple of degrees southeast of Merak. M 97 is a planetary nebula known as the Owl Nebula as it resembles the round face of an owl with two circular faint spots that stand out like eyes against the bright background. This nebula is about 1500 light years away. We believe the appearance is that of a circular torus seen edge-on so that the two "holes" are either side of the torus.
The green color of the nebula in this image is common to many such planetary nebulae, and that's partly due to the filter and photographic mixing technique used. However, it is true that most of the light from planetaries comes in the form of emission lines, and by far the brightest emission line in the visible part of the spectrum is an emission line from doubly-ionized Oxygen. The history of this emission line is rather interesting, actually. When it was originally discovered, we didn't know what element corresponded to this line, so it was dubbed "nebulium" as though it were a new element only existing in nebulae.
Turns out that the reason we hadn't seen this line before is that it represents an atomic transition that isn't possible in the lab. You can put doubly-ionized Oxygen in a tube and measure its emission lines all day, and you'll never see the nebulium line. That's because the density of gas in our labs is much too high, but let me back up a minute.
Atoms can emit light in the form of spectral emission lines in two main ways. First, if the electron is excited somehow and moves to a high energy level in the atom, it will normally very quickly jump back down to a lower energy level, giving up energy in the form of an emission line photon. Sometimes, though, the atom is bumped by another particle, and the electron gains or loses energy that way, sometimes moving out of its energy level without absorbing or emitting a photon.
For this particular energy level of doubly-ionized Oxygen in question, it is known as a meta-stable state. That is, when an electron gets excited to this energy level somehow, it tends to stay there for a long time (many seconds, minutes, hours or even years instead of the more usual microseconds). If the atom is left alone long enough, then the electron will eventually drop down, emitting the "forbidden" line, but if the atom is in a high density gas, it will get bumped in a collision long before it has a chance to emit a photon on its own. Hence "forbidden lines" only occur in the low density environment of space, which is a vacuum literally billions of times better than anything we get in our labs on Earth.
So, back to the Messier objects: next to M97 in the sky (about one and a half full moon diameters away) but about 30,000 times further away is the spiral galaxy M 108, seen here in a pretty good close up image. This is a very dusty spiral with no clear features, and it is a member of a loose group of galaxies known as the Ursa Major group.
The final Messier object in Ursa Major is M 109, a very pretty barred spiral galaxy seen in better detail here right next to the bright star Phad, a part of the Big Dipper I mentioned earlier. Though a few degrees away from M108, this galaxy is also considered to be a member of the Ursa Major group, and there are several other smaller galaxies around it on the sky, though much fainter. Among them are the spiral galaxy NGC 3893 (with its nearby small companion NGC 3896), NGC 3953, a lovely tilted spiral with clearly defined arms, and the more face-on NGC 3982.
Now for the other galaxies in Ursa Major, starting at the Western end (near the nose) and working our way East. About 2-3 degrees southeast of Muscida, the star marking the nose of bear is the Helix Galaxy, NGC 2685. This odd spiral galaxy has clearly defined polar-oriented rings around its center, perpendicular to its disk, and this ring is likely the result of a galaxy merger in its recent past. In many of these galaxies, the stars and gas in the polar ring orbit the center of the galaxy faster than the material in the disk, which indicates that the dark matter halo of the galaxy is flattened in the polar ring plane.
Further southeast near Talitha is a tightly wound spiral, NGC 2841. Recently, the Chandra X-Ray observatory took a photo of this galaxy in X-rays revealing a surrounding galactic corona. Like the Sun's corona, this is hot gas streaming away as a galactic wind thanks to the outward-pushing pressure of all of the hot stars (and perhaps supernova explosions) in the disk. We think most galaxies have a similar corona. This galaxy is the largest of a small cluster of galaxies about 20 million light years away, ten times more distant than the nearest large spiral to our own, the Andromeda galaxy.
A few degrees West of the M81/M82 pair I talked about previously is NGC 2787, a barred lenticular galaxy. Lenticular galaxies are basically spiral galaxies but without any obvious arms due to a lack of gas (and thus, a lack of a notable pattern of star formation to emphasize regions of different density as in true spirals). Galaxies like these consist of an older population of stars, more like ellipticals than spirals, but there haven't been any waves of star formation recently, and that's in part due to the relatively isolated nature of this galaxy. There are no nearby galaxies to spark star formation. Though this is close in the sky to M81 and M82, it isn't part of their group as it is twice as far away from us.
Moving closer to the Big Dipper, we find NGC 3359 about a degree West of Dubhe, the northernmost of the two pointer stars that leads the eye to Polaris. This is a barred spiral galaxy about 49 million light years away. Deep radio images of neutral Hydrogen shows that this galaxy has a small companion that has recently disrupted the disk and may have been the spur that formed the central bar in this galaxy (which we think is very young due to the stellar population there).
In the other direction, a couple of degrees south of Merak is a pair of galaxies that are pretty faint but nice to look at in deep images. NGC 3310 is a face-on spiral galaxy about the same distance as NGC 3359 (50 million light years) and part of a loose cluster at that distance. It is undergoing a massive burst of star formation thanks to a recent galaxy merger. Just a bit to the Northeast of this, also at the same distance, is NGC 3709, an edge-on spiral galaxy also shown in Hubble images to be giving off large arcs and bubbles of gas from its central regions, likely the result of either stellar winds or stellar explosions.
While talking about all of these galaxies in Ursa Major, I should at least mention my selection criteria: I'm trying to limit the ones I'm talking about to the ones that are fairly bright and can be seen with a typical amateur telescope from a dark site. So, the next galaxy I want to talk about is NGC 3184, located less than half a degree West of Tania Australis (the southernmost of the middle pair of stars marking the leaps of the gazelle). This galaxy is also very near HR 4067, one of the stars recently discovered in Ursa Major that has a planetary companion.
NGC 3184 is a face-on spiral that recently drew the attention of the Hubble Space Telescope thanks to a Type II supernova that occured there in 1999. In fact, the first spectrum of this supernova was taken by a very talented guy I worked with in graduate school by the name of Peter Garnavich, who is now at the Harvard Smithsonian Center for Astrophysics. This galaxy is somewhere between 25 and 40 million light years away, and sources I've seen quote different distances. How can they be off by so much?
Well, one major source of uncertainty is the interstellar material that is along our line of sight to the starlight in the galaxy, which is mainly in our own galaxy but also inside the target galaxy (especially if mergers are creating a lot of infall of gas and dust). It is very difficult to isolate this absorbing material and account for it perfectly, though we do have some tricks. For example, with certain kinds of supernovae, we know what the spectra should look like at maximum light. That is, we know how much light should appear at each wavelength, but interstellar gas and dust absorbs differently at different wavelengths, so by comparing a "standard" supernova spectrum with what we actually see, we can reverse engineer the properties of the material along our line of sight that are obscuring the galaxy.
Next, we move back closer to the pair of Messier objects we studied earlier just East of Merak, M97 and M108. Moving a couple of degrees further Southeast from these two objects, there is NGC 3631, a dim face-on spiral very similar to NGC 3184. This is an isolated spiral like NGC 2787 (the lenticular spiral) I mentioned earlier, so the spiral structure is very uniform and not disturbed much due to a lack of nearby galaxies to gravitationally interact with. I found a very nice analysis of the properties of this galaxy published on the web as the result of an undergraduate research project, and this is an impressive effort.
This effort is part of a nationwide program sponsored by the National Science Foundation (NSF) called Research Experiences for Undergraduates (REU). Astronomy departments (and other science departments, of course) submit a proposal to the NSF, explaining what faculty are available during the summer along with a list of resources available and suggested projects for undergraduates. If the proposal is accepted, then the NSF will financially support a summer research program that usually lasts around 6-8 weeks.
Undergraduates will then apply to various REU programs around the country, usually at schools they may be considering for graduate school, and once everyone finds a slot, the NSF financially supports the research program, and the students get a great experience working closely with faculty from another department during the summer. It is great for everyone involved, and I've seen a lot of really good research come out of these focused efforts, including peer-reviewed publications. It is a very cheap program from the point of view of the NSF, and the return on investment is significant.
A couple of degrees to the East from NGC 3631, the region just south of the bowl of the Big Dipper, we find the very interesting warped spiral galaxy known as NGC 3718. The odd extended shape of this galaxy is due to an interaction with a fainter nearby spiral galaxy, itself showing a strong ring structure. In the same image behind NGC 3718 is a much more distant group of five interacting galaxies, seen more closely here.
The final galaxies I want to look at are all on the Eastern edge of Ursa Major, near the border it shares with the small constellation Canes Venatici, about 7-8 degrees South from Phad in the Dipper. The galaxy trio is a set of spirals known as NGC 3938, NGC 4013 and NGC 4051. NGC 3938 is a face-on grand design spiral galaxy perhaps best known recently for hosting supernova 2005ay, discovered first by a dedicated amateur with a very nice home observatory and 16" telescope.
NGC 4013 is an edge-on spiral that drew the attention of the Hubble Space Telescope in 2001, perhaps because of the huge tidal stream passing through the galaxy, likely a remnant of a recent merger. About 55 million light years away, this is nearly a perfectly edge-on spiral similar to our Milky Way. The foreground star in the middle has led some observers to refer to this as the Diamond Ring Galaxy, as though we are seeing a diamond ring edge on with the foreground star in the center as the diamond. Though it looks pretty quiet in the optical image, looking at this galaxy's neutral Hydrogen radio emissions reveal a very warped disk and lots of activity. This is a perfect example of why Astronomers are so keen to look at objects in many different wavelengths, not just what we can see in visible light.
Finally, NGC 4051 is a tilted spiral with very well-defined arms and lots of bright star forming regions shining with the reddish light characteristic of H-II regions (ionized Hydrogen surrounding clusters of hot, young stars). This is a Seyfert galaxy, which means its nucleus is very bright in radio wavelengths and varies on rapid timescales. This galaxy is close enough to NGC 4013 that it may be the culprit responsible for the tidal stream passing through that galaxy. Astronomers have been closely monitoring the variations of the nucleus of NGC 4051 for several years as it has gone from a bright stage to a very quiescent mode, perhaps because the supermassive black hole at the center doesn't currently have anything in its vicinity to tear apart.
In closing, I should also mention another interesting field of view in Ursa Major: the Hubble Deep Field, one of the deepest observations ever made and with amazing resolution, the deep field is a panorama of about 1500 very distant galaxies, some of which are being seen from a distance of several billion light years, meaning the light left these objects when the Universe was young. Images like this help us understand how the Universe has evolved over time.Posted by Observer at March 11, 2008 04:01 PM
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