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Small Wonders: Ursa Major
Mar 29 2006 12:36 PM |
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| Small Wonders:
Ursa
Major Tom Trusock 3/06  Ursa
Major Wide Field
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Our constellation for this edition of Small Wonders is familiar to any denizen of the northern hemisphere. Throughout history, Ursa Major has been a recongnizable figure in the night sky. It’s alternately been seen as a bear, three hunters and a bear, a plough, a bear and a wagon. (Did I mention that it's been seen as a bear?) The asterism it contains – the Big Dipper – is probably the most commonly recognized figure in the night sky. The Dipper serves as a guide post to many of the northern hemisphere constellations, and is itself an open cluster. Designated Collinder 285, or the Ursa Major Moving Group, it contains the five central stars of the Dipper and is centered a mere 70 light years from earth. Cr285 best viewed with the naked eye. Many of the targets this month are visible in binoculars. The Dipper is a cornucopia of deep sky delights. Lying well off the Milky Way and covering over 1280 degrees of sky, this is a very large patch of space looking far out into intergalactic reaches. Ursa Major is therefore, unsurprisingly, rich in galaxies and galactic clusters. But it’s home to a number of other interesting targets as well: thousands of galaxies brighter than mag 20 (on a more realistic limit, there are 812 at mag 15 or brighter, and 56 brighter than mag 12), 7 Hickson groups, 327 Abell Galaxy Clusters, 641 quasars (MKN 421 @ 11h05, +38 deg 11 min and mag 13.5 is the brightest), two planetary nebula, 9 diffuse nebula, and one globular cluster (Palomar 4) – to list a few. There are several stars of note in Ursa Major – outside of the stars of the dipper, UMa also contains Lalande 21185 – a mag 7.49 red dwarf – which is the 4th closest star to the solar system and located a mere 8.1 light years away. Lalande 21185 is the brightest red dwarf visible from the northern hemisphere. UMa is also home to Groombridge 1830 – at mag 6.45, it lies 28 light years away and has the third fastest proper motion of any other known stars. Groombridge 1830 is a population II object and as old as the galaxy’s globulars. Another star of note in Ursa Major is 47 Ursae Majoris - which is one of the (many) extrasolar systems to have a (suspected) planetary inhabitant. Ursa Major houses a total of 7 Messier Objects – 6 that are of interest visually. (We’ll toss out M40, although double star observers might want to take a gander at it.) The Great Bear is also home to the first Hubble Deep Field at 12h 36m 49.4000s +62d 12' 58.000", this tiny window (about the size of a grain of rice held at arms length) let the HST look outside our galaxy and capture at least 1500 galaxies in a 10 day long exposure. Nearly everything you can see in the image below is a galaxy. (If you have a high bandwidth connection, be sure to see "Zooming In to the Hubble deep field" in the links at the end of this article.)   Star names in the Dipper Before we go further, lets be on a first name basis with the stars that make up the Dipper proper. Starting at the handle, we have Alkaid, then the naked eye double Alcor and Mizar is found at the handles bend. Moving on down the ladle we come to Alioth, and a little further leads us to the first of the stars that mark the bowl of the Dipper – Megrez. Down the first side we hit Phecda, then Merak and Dubhe. One of the first things any novice learns is to line up Merak and Dubhe and use them as pointers to find Polaris, the North Star in Ursa Minor.  Various sources I’ve
read have said that many civilizations and
cultures used Alcor and Mizar as tests of visual
acuity, but this has often puzzled me a bit, as I’ve never found them
all that difficult to separate.Frankly, Ursa Major is a daunting consellation to write a sky tour for: it’s absolutely massive, and home to dozens of targets for an observer with even the most modest of telescopes. Because of this, I’ve concentrated on what I consider to be the brightest and most spectacular objects, but one area I’ve neglected is what Walter Scott Houston called “The Bowl Of Night” – the bowl of the Big Dipper itself. Once you’ve finished this month’s tour, I’d encourage you to spend some time scanning in the bowl itself – you’ll find several targets worthy of a moderate telescope, and I provide a map and listing of some of the brighter galaxies in and around the bowl at the end of this article.  Bottom of the Bowl Finder Chart Discovered by Merchain, M109 was known by Messier but didn’t appear on “his” list until the middle of the 20th century. Messiers original list consisted of 103 targets with some of them questionable (M40 – a double star, and the “missing” messier – M102).  M109 Image courtesy Jason Blaschka  M109
Sketch Courtesy Jay Michaels
Jay Michaels excellent drawing is a good example of what can be seen in an 8-10 inch scope on a good night. While you are in the area, take a moment to look for NGC 3953 about a degree south of M109. Afterwards move up towards the middle of the bottom then drop a bit south to find a nice cluster of objects – NGC3718, NGC 3729 and one of our challenge objects for the month – Hickson 56.   Visually you’ll
find both 3718 and 3729 lie within the same moderate
power field of view – I find 3718 to be about three times larger than
3729, but to my eye both galaxies are fairly similar. In larger
telescopes, I find that both have visible (if faint) cores, and a
dispersed outer halo. Just a bit to the south, you’ll find
Hickson 56
– but we’ll come back to that one in a bit.Move back up towards the bottom pointer star (Merak) with a low power wide field eyepiece, and you’ll run across a celestial odd couple. First into view should be M97 – the Owl Nebula. A planetary nebula discovered by Merchain in 1781, I’ve found that this is one of those few objects which actually resembles it’s nickname. Even in small scopes (under good conditions) I can catch a glimpse of the dark spots which form the owls eyes. It’s fairly large, and as such the surface brightness is quite low. Some observers have claimed to see blue or green across the face of the disk – on excellent nights, I’ve caught hints of green in large telescopes, but it usually appears simply grey.  Image Courtesy Rick Krejecki Rick Krejecki’s shot of M97 is amazing. Take a gander at the full resolution version on his web site (http://www.ricksastro.com/DSOs/owl_XT_xscope.shtml) – you can spend a fair amount of time just counting the tiny background galaxies. I wonder if any of these have been spotted visually by observers with giant telescopes? If we want to look at extragalactic targets we don’t have to look far away – just a little closer to Merak you’ll find the edge on barred spiral M108. Spend some time experimenting with different magnifications on this target – see what, if any mottling you can make out, and if you can detect any presence of an outer halo.  M108
and M97 Image Courtesy Tom Nicolades
Tom Nicolades excellent shot shows a bruised and battered M108 and an electric blue M97 in the same frame, and in low power wide field views (anything providing more than 1 degree TFOV), both can easily be caught in the same field of view.  NGC
2841 Area
 While we’re here, let’s hop down the front legs of the bowl and take a quick gander at 2841. This mag 9.2 galaxy will present a nice sliver of light for moderately sized telescopes. A bright core region is surrounded by a slightly dimmer halo. If you have a larger telescope, look for a dust lane marked by a sharp fading of the halo on one side of the galaxy.  M81
/ M82 Image Courtesy John Moody
When you’ve finished with 2841, let’s move on to two of the real
showpieces in the Great Bear; M81 and M82. M81 / M82 Area
M81 and 82 are a spectacular pairing of galaxies visible even through small binoculars. Separated by a mere 3/4 degree both are visible in wide field telescopes and present a wonderful pairing. Discovered by Bode in 1774, they are examples of galactic morphology which couldn’t appear to be more different from one another other. Both are members of a small cluster of galaxies called the M81 group (which lies a neighborly 10 million light years away), so I suppose it would be appropriate to discuss M81 first. In smaller telescopes, M81 is a bright oval, but large scopes begin to show it’s spiral structure. M81 is by far the larger and brighter of the two, and in long exposure photographs is a classic spiral. M82, by contrast, is a tortured irregular that looks like it’s been the looser in some vast celestial conflict. In my 18”, I see it bent at one end while the mottling is clearly apparent, along with a nearly clean division around 1/3 of the way from one edge. While it’s slightly fainter than M81, visually I find it far more impressive. It’s worth noting that this is one of the few DSO’s visual observers have reported color in, but don’t grab those 80mm scopes in anticipation yet. While a friend of mine in Arizona with access to a 30 inch telescope describes seeing red or pink, I’ve yet to see it even tho I’ve gazed at this object with telescopes up to 25” in size. I suspect it takes an excellent night, good optics, and as much aperture as you can throw at it. Don’t despair however – in my opinion, M82 is one of the finest targets in the night sky – color or not. Even in small scopes, these two are stunning, and can be picked out from dark skies with the slightest optical aid.  Image Courtesy Carol Lakomiak Carol Lakomiak’s sketch of the area provides an excellent example of what can been seen in large binoculars or a small telescope. As you’ll note in the area chart, there are a number of other targets in this region as well – take the time to scout around a little bit - keep an eye out for NGC 3077, 2976 and IC 2574. In large scopes, I find that NGC 3077 and 2976 bear a resemblance to glimpses I’ve had of M81 in smaller apertures. When I’m star hopping and stumble on one of these one first, it can get quite confusing trying to find M81. Always keep your expectations in line with your aperture. We haven’t even really begun to cover the possibilities in Ursa Major, but we’ll make one more basic pit stop for the night and then introduce two challenge objects.  M101 Area M101 is a large, loose face on object that can be decptively difficult in small telescopes. Keep in mind when searching you are looking for a large object - around 2/3 the size of the full moon, but the surface brightness is very faint so keep your eyes peeled for a gradual lightning of the sky background. The galaxy is huge – sources put it from 170,000 to 190,000 light years across, some 25 million light years away and home to some of the most spectacular and immense star forming areas known. Many of these giant starbirth areas are bright enough to merit their own NGC numbers - NGC 5441, 5447, 5450, 5449, 5451, 5453, 5458, 5461, 5462, and 5471. 5471 is the largest and brightest HII region in M101 – far larger than anything comparable in the Milky Way (and has been proposed to contain a hypernova in 5471B). These are visible in larger telescopes, and while often I’d recommend a power to view the galaxy at (my favorite galaxy buster is my 13t6 nagler in my 18” Obsession, it provides a magnification of around 180x, and a nice wide field), because of its detailed structure, I’d rather recommend that you spend a while examining it at high and low powers to find what works best for you. Be sure to take a gander for some of the brighter HII regions. Note that 5450 and 5447 are overlapped in the image below - 5447 is the region directly to the south of 5450.  HII Regions in M101 Like M81, M101 is the dominant member of it’s own cluster of galaxies, so while you are in the area keep an eye out for other interlopers. Brighter neighbors include NGC 5474 and NGC 5473 but numerous other galaxies reside in the area.  M101 image courtesy James Jacobson Challenge Objects There are several objects worthy of the challenge tag in Ursa Major – 6 Hickson groups, Palomar 4 (a globular cluster), and a fairly bright quasar were the first that came to mind. Quasars are interesting for what they are – not what you see, and Palomar 4 is certainly doable with a large scope from a dark site, but on the whole I tend to prefer galaxy groupings. With that in mind, I present as challenge objects two of the “brightest” Hickson’s in Ursa Major – Hickson 56 and Hickson 41. Hickson 56 Finder Chart Hickson 56 lies just to the south of a galaxy pair we visited earlier – NGC 3729 and 3718.  Hickson 56 finder chart Hickson 56 has 5 components (although not all may be visible) that range from magnitude 16.2 to 15.8, and all the components are tiny (the largest is 1.3x.2 arc seconds) – so make sure you try for this one under good skies with plenty of aperture. Iiro Sairanen from Härskiänsaari, Ruokolahti, Finland observed Hickson 56 with his 16 inch Newtonian at 292x and provided the following sketch:  Sketch of Hickson 56 Courtesy Iiro Sairanen. The other challenge object this month is Hickson 41.  Wide field Chart  Area Finder Chart  Hickson 41 is a
little more difficult to get to, but it’s a little
brighter. Again, note that it does not perfectly match up with the charts provided. Use the DSS images for reference. There are 4 listed components ranging in magnitude from
14.6
to 18.1, with the largest being a mere 1.5x.2 arcseconds in size.
Alvin Huey, in his excellent Hickson Group Observer’s Guide observing
at 377 and 528x, was unable to catch the 4th member of the group with
his 22” f4.1 dobsonian. I was able to catch three of the four galaxies in my 18” f4.5 from my driveway, but it necessitated some extreme measures; I used a towel to cover my head and shield all extraneous light, I needed a good evening (for my driveway), and I used extremely high powers (600x) in order to darken the sky background sufficiently. Finally, I had to resort to rapping the side of the telescope in order to ensure that I had spotted all three of the members. The Hicksons, for the most part, are not casual observation or quick look targets. Use every trick in the book including high magnification and perserverence to glimpse these tiny groups of interacting galaxies. Additional Targets As I alluded above, Walter Scott Houston called this area the "Bowl of Night - with that in mind, here's a map to some other suggestions for the Dipper's bowl.  More
targets in the "Bowl of Night"
And here's the necessary info on the additional targets:
Till next time - Tom T. Additional Links The Hubble Deep Field http://www.stsci.edu/ftp/science/hdf/hdf.html Zooming in to the Hubble Deep Field http://hubblesite.org/newscenter/newsdesk/archive/releases/1996/01/video/a A CRITICAL EXAMINATION OF HYPERNOVA REMNANT CANDIDATES IN M101. II. NGC 5471B http://www.journals.uchicago.edu I'd
love to hear of your experiences under the night sky - please feel
free to
e-mail me or send any observing reports to: tomt@cloudynights.com Please indicate if I can cite your observations in future columns. Photographic Images Courtesy DSS: copyright notice http://archive.stsci.edu/dss/acknowledging.html Star Charts Courtesy Chris Marriott, SkyMap Pro 10 Printed with Permission http://www.skymap.com Special Thanks to Collin Smith for his editorial assistance, Olivier Biot for assistance with the PDF’s and all those who take the time to read and contribute to this series. |
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