Featured Deep Sky Objects Stars in Ursa Major

Deep Sky Object Chart | Stars in Ursa Major | M40 | M81/M82 | M97 | M101 | M109 | NGC2841	Alpha Ursae Majoris (Duhbe)    owes it's "alpha" designation not to preeminent brightness (as is usually the case in constellations), but to it's position as the first of the seven stars of the "big dipper". The name "Duhbe" is derived from the Arabic for "back of the great bear", which the star marks in the larger Ursa Major proper. The second brightest of Ursa Major's stars (magnitude 1.81), Duhbe can be distinguished from the other "dipper" stars by it's color - a tawny yellow, very unlike it's much hotter blue-white dipper mates. A giant star, Duhbe puts out about 145 times the light of our sun, and is at 105 light years more distant than the other dipper stars. Duhbe is a member of this group only visually; it is an unrelated star, passing fairly near the dipper cluster at this time. Due to it's very different space motion, Duhbe will drift away from the dipper, heading roughly southwest - in 100,000 years, the pattern will thus be significantly disturbed.
For binoculars, Duhbe is an example of an extremely wide separation "common proper motion" pair - so named because the stars apparently lie near another and share a course through the galaxy. Such wide pairs are too separated, however, to show evidence of orbital motion over the period since the invention of the telescope, and no actual orbital motion can be seen or predicted. Duhbe's far flung friend is a 7th magnitude white star, 6.3 arc minutes away (a fourth the apparent diameter of the moon). This separation works out to 12,000 AU, or 200 times the width of our solar system - an orbit of that girth would take many thousands of years to complete. The system holds more of interest. If you have an incredible telescope, here's a test: Duhbe's primary star is a close binary as well, with a 5th magnitude companion just - get this - one THIRD of a one arc second away! Good luck, you'll need it. By the way, spectroscopes reveal the distant companion to be double as well, raising the stellar population to four.
Beta Ursae Majoris (Merak)    is located 5.4 degrees south of Duhbe, and is noticeably fainter at magnitude 2.37. The name is derived from the Arabic for "the loin of the bear". Unlike Duhbe, and like the other dipper stars, Merak is a hot bluish star, of spectral type A1V. The actual luminosity of Merak is about 65 times the light of our sun, about three times that of the much closer Sirius in Canis Major. If Merak were to replace Sirius, it would be magnitude -2.5! Merak is a solitary star, about 80 light years away, and is a proper member of the Ursa Major moving group.
Gamma Ursae Majoris (Phecda)    forms, with Beta, the bottom of the dipper's handle, and is a blue-white star very similar to Beta: 90 light years away, 75 times as bright as our sun, and also a true member of the dipper cluster. Phecda is not much of a telescopic target, being a solitary star, but serves as a useful starting point for galaxy hunters headed for NGC 3992 (sometimes referred to as M109), which lies three-fourths of a degree to the southeast.
Delta Ursae Majoris (Megrez)    is the faintest of the dipper stars, at magnitude 3.3. Of spectral type A3V. Megrez is bluish, and is located some 65 light years away. Interestingly, this star was recorded by Tycho Brahe, certainly a careful observer, as being a full magnitude brighter 400 years ago than it appears today. Astronomers have not detected any change in the star in recent history, so perhaps this was a simple mistake.
Epsilon Ursae Majoris (Alioth)    is the brightest star in the constellation by a small measure, tipping the scales at magnitude 1.79 - just 0.11 magnitudes shy of the first magnitude. Designated "Epsilon" only in order of it's dipper position, this star would be an "Alpha" in many a constellation, if brightness were the criterion - it outshines the alpha of Aries, for example. Alioth gets far less attention than it's fainter dipper neighbor Mizar, as Alioth is not a binary or multiple star. Like most of the dipper's stars, solitary Alioth is a hot bluish sun rather like Vega, but twice as bright: 85 times the light of our sun. Alioth is just barely a variable star in light output (0.02 magnitude variation), but during its extremely regular 5.089-day cycle the spectroscope reveals strange alterations in the chemical signatures of that light. These odd "spectrum variables" are not much of a visual spectacle, but are a fascinating puzzle for astrophysicists, who believe the changes relate to a strong magnetic field in the star.
Zeta Ursae Majoris (Mizar)    is one of the most famous and remarkable multiple stars in the sky, easily located as the middle star in the handle of the big dipper. To the unaided eye, Mizar appears as a bluish star of magnitude 2.4, with a fourth magnitude star just to it's east. The separation of the stars is 11 arc minutes, a little less than a third the apparent width of the moon. Named Alcor, this easily seen companion does indeed accompany Mizar, although the actual separation of the stars is a fourth of a light year and any orbital motion might take hundreds of thousands of years. Owners of telescopes taking a closer look at Mizar itself will find it is itself a double star, of magnitudes 2.4 and 4, with an easily detected separation of 14.4 arc seconds. At the calculated distance of 88 light years, this closer pair turns out to be anything but close: they are separated by 5 times the width of our solar system.
	The true majesty of the Mizar system is revealed only through the spectroscope, which finds that Mizar's two stars are actually three and two stars - a quintuple. Almost unbelievably, Alcor is also found to be a binary - thus, the Mizar/Alcor system is a septuple star system, with seven suns performing a complex gravitational dance. The imaginary scene here shows what it might be like on a planet orbiting the Mizar A pair; Mizar B is the dazzling triple sun at left, and the other dipper stars are scattered across the sky (Alcor is out of view to the upper right). There would be no sense of being inside a cluster. In fact, the densest clump of stars above the horizon at center is a highly distorted Orion (look for red Betelgeuse there). We can be grateful we do not live in such a star system; the seven stars would be lovely, surely, but night would come rarely for stargazers living in a seven-sun environment. The Mizar visual binary was the first known, and also the first photographed, in 1857.
Eta Ursae Majoris (Alkaid)    is the final star in the handle of the big dipper, and is sometimes referred to as "Benetnasch". The name "Alkaid" is also taken from the same Arabic, a phrase meaning "the leader of the daughters of the mourners". At magnitude 1.87, Alkaid is one of the brightest stars of the constellation, barely losing out to Epsilon and Alpha. Hotter than the other dipper stars, Alkaid is spectral type B3V, making it's blue color more pronounced. The star is also not a member of the dipper group - like Alpha, it merely happens to be in the area, and is headed a different way. Alkaid is located somewhat beyond the true dipper group (110 light years) and is brighter than them as well: about 630 times brighter than our sun. Alkaid is a road sign to a wonder of an adjoining constellation: Canes Venatici's lovely "whirlpool" galaxy M51, located some 3.5 degrees to the southwest.
Iota Ursae Majoris (Talitha)   is the brighter of two stars marking the front foot of the great bear, in the southwestern portion of Ursa Major where it borders Leo Minor. Talitha, of magnitude 3.12, is separated from magnitude 3.68 by just one degree; there is no real color contrast, as both stars are bluish. Iota is a very unequal double star for moderate telescopes, the companion being tenth magnitude and a fairly tight 4 arc seconds away. Very large instruments might be able to split the reddish companion itself - this is a pair with just 0.7 arc seconds between them, and an orbital period of 39 years. Talitha is located about 50 light years from us, closer than the dipper cluster, and is not a member of the group.
Kappa Ursae Majoris (Al Kaprah)    is located alongside Talitha, and is somewhat hotter and bluer than it's seeming neighbor (the spectral type is B9). In truth, Al Kaprah is 300 light years away, much farther than Talitha, and the alignment is by chance only. Al Kaprah is a very close double star, suitable for large telescopes only - the separation is a tiny 0.3 arc seconds, and the magnitudes are an even pair of 4.3 and 4.5. The actual separation of these stars must be similar to Uranus and the sun, and the orbital period is just under 58 years.
Lambda Ursae Majoris (Tania Borealis)   marks the hind foot of the bear, and shares this job with the adjoining red star Mu, just 1.5 degrees to the southeast. The stars, of magnitudes 3.45 and 3.05 respectively, form a lovely color contrast pair, as Lambda is a pale blue star of spectral type A2. The two stars are unrelated, with Lamba at 150 light years being beyond the red star. Although located thirty degrees from the center of the dipper pattern, Lambda is a member of the Ursa Major moving group.
Mu Ursae Majoris (Tania Australis),   with its distinctive reddish color, serves as a fine signpost for a face-on spiral galaxy, NGC 3184. This 10th magnitude target is suitable for moderate amateur instruments only if your sky is very dark. Look just three-quarters of a degree to the west of Mu - this will be within the field of view of many telescopes at lower magnifications, so you may be able to see both the (relatively, at 105 light years) nearby star and the much more distant galaxy together.
Xi Ursae Majoris (Alula Australis)    derives the "Alula" portion of it's name from the Arabic for "the first spring"; it shares this name with the star Nu Ursae Majoris, just 1.6 degrees to the north, which is known as Alula Borealis. Nu is of the third magnitude; the more southerly (and thus "australis") Xi is of the fourth, and these stars lie in the far southern reaches of the great bear. In fact, these stars are far enough off the beaten track that they are off the bottom of our star chart! They can perhaps most easily be located by extending a line straight north through the two stars marking the hindquarters of Leo (see our wide angle sky chart this month). Xi is an interesting but close binary star, of about 60 years orbital period. The magnitudes of Xi's stars are 4.3 and 4.8, a fairly equal pair, separated by about one arc second - either your telescope or imagination will need to be properly tuned! This star system, located just 26 light years from us, is in actuality a quadruple, as both stars we can see are known to be double themselves.
Omicron Ursae Majoris (Muscida)    is a third magnitude yellow star marking the nose of the bear, in the far western part of the constellation. Much brighter than our sun (85 times so), Muscida is located about 150 light years. This star is not a member of the dipper cluster. Muscida is apparently attended by a very faint companion star (15th magnitude) 7 arc seconds away, but the faintness of the second star will prove too much for anything but the largest amateur instruments.
	47 Ursae Majoris is a star with no proper name, but one that perhaps should have one, now that we know that this visually undistinguished star (magnitude 5.05) is definitely attended by a giant planet not unlike our own Jupiter. While the planet is of course utterly invisible in any present telescope, the star itself can be located in the southern portion of Ursa Major, about 4 degrees south-southwest of the "knee" of the bear's hind leg. 47 Ursae Majoris is similar in color and brightness to our own sun. The planet was detected by looking for subtle variations in the star's light that result from gravitational interaction between the star and the unseen planet. Astronomers expect that 47 Ursae Majoris's planet is a cold gas giant world, and is itself unlikely to harbor life - but perhaps there are other planets here, closer to their sun's warmth?
Lalande 21185    is a reminder that nearby stars rarely appear bright in our sky. Just 8.3 light years away, little red Lalande 21185 is a small cool sun, typical of the majority of stars in the galaxy. It puts out a pathetic 0.0048 the light of our hotter, larger star, and thus manages to shine at magnitude 7.6 - hardly a startling scene in your telescope. However, Lalande 21185 holds our interest for other reasons. First, it is the fourth nearest of all the stars, with only Alpha Centauri, Barnard's Star, and Wolf 359 being closer. Secondly, Lalande has a high apparent motion among the stars - in this case, the star is not speeding, though, but is merely so close to us that it's movement is fairly obvious: about 4.78 arc seconds per year. Rumors of an unseen, tiny companion to the star - perhaps a planet - have been around for years; as of this writing, the evidence is still being debated. If you would like to hunt down this galactic neighbor, you'll need a star chart; your hunting area will be in the southern part of Ursa Major, not far from 47 Ursae Majoris.
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