Ursa Major - Constellation Guide

Ursa Major

Ursa Majoris

UMa

The Great Bear



Introduction

Ursa Major or the Great Bear is a prominent constellation in the northern celestial hemisphere. It's the third largest constellation in the sky and contains a central feature of seven stars, known as the Plough or Big Dipper, which is one of the most recognisable patterns. It's made up of stars, Dubhe, Merak, Phecda, Megrez, Alioth, Mizar and Alkaid. Ursa Major was listed by Ptolemy as one his 48 constellations in his second century Almagest and remains today as one of the modern 88 constellations.

The constellation spans a vast swath of the northern sky, covering almost 1,280 square degrees in surface area. Of course, the Plough is its standout feature, which has been referenced in drawings and writings since the dawn of civilisation. In Europe, the pattern symbolised a wagon or chariot that was associated with King Arthur. In Roman mythology, Jupiter the King of the God's, lusted after a beautiful woman named Callisto who was transformed into a bear by his jealous wife, Juno. Arcas the son of Callisto almost shoots the bear, but to avoid tragedy, Jupiter turns Arcas also into a bear, placing them both in the sky as Ursa Major and Ursa Minor. The Arabs viewed the grouping as a coffin and Ursa Major, along with Orion and the Pleiades, are mentioned in the Bible.

Most of the stars of the Plough belong to a nearby stellar group, known as the Ursa Major Moving Group or Collinder 285. All member stars are moving in roughly the same direction and at about 80 light-years distant it's the nearest cluster like object to Earth. Of the seven stars, only Dubhe and Alkaid are not actual group members.

The Plough asterism is a very useful guide for locating other stars. Two stars in the bowl Dubhe and Merak act as a pointer to Polaris, the Pole star. An imaginary line extending from Megrez through Dubhe and onwards leads to first magnitude Capella. The handle of the Plough points towards the brightest star in the northern section of the sky Arcturus.

Ursa Major is circumpolar from many Northern Hemisphere locations and is best seen during the months of March, April and May.

Ursa Major Star Chart - pdf format (credit:- freestarcharts)

Interesting Stars

Bright Star, Variable Star

Alioth (epsilon Ursae Majoris - ε UMa) - shines at magnitude +1.76 and is the brightest star in Ursa Major. It's an Alpha2 Canum Venaticorum type star that varies slightly in magnitude over a period of 5.1 days although the change in brightness is too small to be noticed with the naked eye. Alioth is classified as a hot white A1p star. The "p" stands for peculiar due to its overabundance of some metals and slower rotation rate than normal A1 stars. Of all Ap type stars in the sky, it's the brightest.

The star is a member of the Ursa Major moving group and is 81 light-years distant. It's positioned third from the end of the Big Dipper's handle.

Bright Star, Multiple Star

Dubhe (alpha Ursae Majoris - α UMa) - mag. +1.81, is 123 light-years distant and marginally fainter than Alioth. Together with Alkaid (η UMa) they are the only stars of the main seven that are not part of the Ursa Major Moving Group. Dubhe is a further 43 light-years distant from the group centre.

Dubhe is itself a multiple star with a very close magnitude +4.8 companion. The pair orbit each other once every 44.4 years. The main star shines at magnitude +2.0 and is an orange type K0 giant with the secondary a F0 yellow white dwarf star. The current separation is a measly 0.5 arc seconds. Combined with a brightness difference of almost 3 magnitudes, splitting the pair is a great challenge. Under nights of good seeing, a minimum 300mm (12-inch) scope at very high magnifications is required to accomplish the feat.

Visible in binoculars and small scopes is a further fainter wide star. This star, sometimes referred to as alpha UMa C (HD 95638), is a spectroscopic binary positioned 6.5 arc minutes wide of the main pair. It shines at magnitude +7.2.

Dubhe is a beautiful yellow-orange coloured star. It's the northern most star of the Big Dipper and one of the pointers to the Northern Pole Star along with Merak (β UMa). Dubhe also happens to be the official star of the State of Utah.

Bright Stars

Alkaid (eta Ursae Majoris - η UMa) - mag. +1.85, is the easternmost star of the Big Dipper and the constellations third brightest. It's located 104 light-years away and therefore not a member of the Ursa Major Moving group.

Alkaid is a very hot B3 blue-white star and with a surface temperature of 16,800K and therefore much hotter than the Sun (5,778K).

Merak (beta Ursae Majoris - β UMa) - is the fainter of the two pointer stars to Polaris. Merak is a hot blue-white type A1 star. It shines at magnitude +2.37 and is almost three times as large as the Sun and 60 times more luminous.

At 80 light-years distant, Merak is also a member of the Ursa Major moving group.

Phecda (gamma Ursae Majoris - γ UMa) - is the lower left star of the Big Dipper. It's similar in brightness and stellar classification to Merak. Located 83 light-years away, Phecda is one of the more distant members of the Ursa Major Moving group. Tenth magnitude barred spiral galaxy M109 is located 0.75 degrees southeast of the star.

Bright Star, Multiple Star

Mizar (zeta Ursae Majoris - ζ UMa) - is probably the most popular and celebrated double star in the entire sky. At magnitude +2.23, Mizar is the middle star of the Dipper's handle and those with good eyesight will easily notice its mag. +4.0 companion, Alcor. It's some 11.8 arc minutes away or about 1/3 the diameter of the full Moon. For centuries Mizar has served as naked eye vision test. It's often said that those who can split the two stars unaided, without optical aid, can be assured that their eyesight is in good shape. However, the split is actually relatively easy and even those with average eyesight can accomplish it.

A small telescope reveals that Mizar itself is a double consisting of type A2 white stars shining at magnitudes +2.27 and +4.0, separated by 14.4 arc seconds at a PA of 151 degrees. Italian astronomer Giovanni Riccioli first split the star in 1650, making it the first telescopically discovered double star. The brighter component was also the first star to be discovered as a spectroscopic binary by E. C. Pickering in 1889. The fainter component is also a spectroscopic binary, as is Alcor, making this a 6 star system.

For years it was though that the Mizar-Alcor double was purely a line of sight effect with Alcor being about 20 light-years further away. However, recent studies indicate they are actually much closer than previously thought and may be gravitational connected after all, although not yet conclusively proven. The Hipparcos astrometry satellite measured a distance of 86 light-years for Mizar and 82 light-years for Alcor.

The complete star system is approx. 83 light-years away with all members part of the Ursa Major moving group.

Notable Star

Megrez (delta Ursae Majoris - δ UMa) - mag. +3.32, is the faintest star of the Big Dipper asterism and the only one that's not of second magnitude. Megrez is a blue white type A3 star that marks the northeastern point of the bowl of the Big Dipper. The curious double star M40 lies 1.5 degrees northeast of the star.

It's distance is estimate to be between 60 and 80 light-years and it's believed to be a component of the Ursa Major moving group or at least on the outer fringe. Megrez has two faint companions of tenth and eleventh magnitudes.

Multiple Stars

Alula Australis (Xi Ursae Majoris - ξ UMa) - more commonly known as just Xi Ursae Majoris is a close double consisting of magnitude +4.3 and +4.8 components, separated by 1.7 arc seconds. It's a nice small telescope target and a 100mm (4-inch) refractor, on nights of good seeing, at high magnification (above 200x) will split them. The stars are located towards the southern part of Ursa Major.

Xi UMa was also the first double to have its orbital period calculated. French astronomer Felix Savery accomplished the task in 1828 when he correctly calculated a period of just less than 60 years and a separation between 0.9 and 3.1 arc seconds. At closest separation, a 150mm (6-inch) telescope is required to split the pair.

Xi UMa is 29 light-years distant. In addition, both stars are spectroscopic doubles making this a quadruple system.

Talitha (Iota Ursae Majoris - ι UMa) - is a challenging double due to the primary and secondary brightness difference and its close 2 arc seconds separation. A 200mm (8-inch) scope at 250x will split the pair into magnitude +3.1 and +9.2 components. The orbit period is 818 years.

Both stars are also binaries making this a four star system. It lies 47 light-years distant.

Double Stars

Alula Borealis (Nu Ursae Majoris - ν UMa) - is positioned 1.5 degree north of Xi UMa. It's an unequal double star, resolvable in small scopes, that consists of magnitude +3.5 and +9.9 components, separated by 7 arc seconds. The primary star is a type K3 orange giant that's more than 50x the size of the Sun. It's much fainter companion appears greenish-bluish providing a nice telescopic colour contrast.

Alula Borealis is 400 light-years distant.

M40 - is a faint double star consisting of components of magnitudes +9.6 and +10.1, separated by 52 arc seconds. It's faintly visible in 10x50 binoculars and resolvable in small scopes. The brighter star is orange-yellow in colour, the fainter one white.

This is an unremarkable double that's only of significance since Charles Messier catalogued it. Messier was searching for a nebula reported in the area by Johann Hevelius, but despite not seeing any nebula he catalogued this double star instead. Although no nebulosity exists, it has remained on the list.

M40 is also known as Winnecke 4 and is easy to find just 1.5 degrees to the northeast of Megrez.

23 Ursae Majoris - is a yellow-white star of magnitude +3.7 with a wide magnitude +9.2 companion. They are separated by 23 arc seconds and are easily resolvable with small scopes. A fainter much wider magnitude +10.5 star is also visible (sep. 100 arc seconds).

78 Ursae Majoris - another fine double star that's a member of the Ursa Major Moving Group. It's positioned just northeast of Alioth with components of magnitudes +5.0 and +7.7 at a close separation of 1.5 arc seconds. It can be split with a 100mm (4-inch) scope under excellent seeing conditions; high magnifications of between 200x and 225x should do the trick.

Variable Stars

R Ursae Majoris - is a long period Mira type variable that changes between magnitudes +6.6 and +13.5 over a period of 301.7 days. Although never bright enough to be seen with the naked eye, it can be easily seen with binoculars at its peak. When at it's faintest a minimum 150mm (6-inch) scope is required to spot it.

T Ursae Majoris - is another long period Mira type variable star, positioned a few degrees north of the centre of the Plough. It has a brightness range covering magnitudes +6.6 to +13.4 and remarkably similar to R UMa. Its period is 256.5 days.

VY Ursae Majoris - is a semi-regular variable carbon star (C6) with a magnitude range of +5.8 to +6.4 over a period of 120 days. It's located 1.3 degrees south of R UMa.

Near Star

Lalande 21185 - is a red dwarf star (type M2) positioned in southern Ursa Major close to the Leo Minor constellation boundary. At magnitude +7.5, the star is invisible to the naked eye but easy to spot with binoculars. Lalande 21185 is 8.31 light-years from Earth, making it the 6 nearest star / star system to the Solar System.

In approx. 19,900 years time, Lalande 21185 will be at its closest distance to the Sun at 4.65 light-years. It will then shine at mag. +6.2 and at the limit of naked eye brightness.

Deep Sky

Galaxies

M81 (NGC 3031) - at mag. +6.9 is the brightest of the considerable number of Ursa Major galaxies. Also known as Bode's galaxy, it's easily visible with binoculars as a faint patch of light. M81 is a large grand design Sb type spiral galaxy that exhibits near perfect well defined spiral arms and is located 11.8 million light-years distant. It's generally accepted that M33 is the most distant permanent object that can be viewed without a telescope, although a few observers have claimed that they have also spotted M81 under exceptional viewing conditions. At four times more distant than M33 this would be incredible viewing.

M81 is a fine sight in small scopes and a wonderful galaxy for larger instruments. The fainter but equally superb starburst Cigar galaxy, M82, is located in the same binocular and low magnification telescope field of view. Together, the pair form a popular visual and imaging target for amateur astronomers.

M81 has an apparent size of 27 x 14 arc minutes, which corresponds to a spatial diameter of 90,000 light-years. It's estimated to contain more than 250 billion stars and is the largest member of the Ursa Major or M81 group of galaxies.

M82 (NGC 3034) - commonly known as the Cigar galaxy due to its long thin appearance resulting from its edge on orientation. M82 shines at magnitude +8.4 and is separated by only 38 arc minutes from M81. However the two galaxies are very different in appearance. M81 is a grand spiral galaxy, whereas M82 is an irregular galaxy. Although four times fainter than M81, M82 has a high surface brightness and therefore is also visible with binoculars, as a faint thin rod of light. It's best seen with medium or larger scopes, where fine details including dusty patches across the surface are prominent. It forms a wonderful contrast to M81.

M82 is 11.7 million light-years distant and covers 11.2 x 4.3 arc minutes of apparent sky.

M101 (NGC 5457) - also known as the Pinwheel Galaxy, is a large face-on spiral galaxy located 22 million light-years away. With an apparent magnitude of +7.9, it's theoretically within binocular range but due to its low surface brightness in light polluted areas or on nights of bad seeing, the galaxy can be difficult to spot even with a medium size scope.

On good nights, M101 appears as a large nebulous haze through a 80mm (3.1-inch) scope. Large scopes reveal a weak spiral shape and brightness variations. The galaxy covers 29 x 27 arc minutes. Spatially it's extremely large, spanning 180,000 light-years across and is estimated to contain a trillion stars.

M101 is the brightest member of a group of galaxies known as the M101 Group.

M108 (NGC 3556) - is an edge on barred spiral galaxy located just 1.5 degrees southeast of Merak. At magnitude +10.2, it's one of the faintest Messier objects, but due to its edge on orientation exhibits a high surface brightness and can even be spotted with a small 80mm (3.1-inch) telescope on dark nights. Larger scopes, reveal a well-defined thin needle structure with a patchy appearance that looks somewhat like a fainter version of M82. Positioned 50 arc minutes southeast of M108, is the Owl planetary nebula (M97) with both items fitting in the same wide field telescope field of view.

The galaxy spans 8.6 x 2.4 arc minutes of apparent sky and is located 45 million light-years from Earth. It was not included in the final version of the Messier catalogue but added much later in 1953.

M109 (NGC 3992) - is another barred spiral galaxy that was also not included in the original final version of Messier's catalogue. Like M108, it was added in 1953. M109 shines at apparent magnitude +10.3 and covers 7.6 by 4.7 arc minutes of apparent sky. The galaxy appears through a 100mm (4-inch) scope as a faint hazy elongated streak of nebulosity. It helps to switch to higher magnifications and move Phecda outside the field of view. A 150mm (6-inch) telescope reveals a small sharp nucleus, surrounded by a mottled nebulosity with even large amateur scopes hinting at the bar shaped nucleus.

M109 does hold a Messier record. At 83.5 Million light-years from Earth it's the furthest object in the catalogue. It's the main member of the M109 Group that contains 50 mostly smaller galaxies.

NGC 3077 - is a small elliptical galaxy of type E2 that's a member of the M81 Group. At mag. +9.9 and covering 5.0 x 4.2 arc minutes, NGC 3077 can be spotted on dark nights with a 80mm (3.1-inch) telescope appearing as a featureless slightly oval shape. Larger scopes reveal a bright diffuse core that fades gradually outwards.

It's 12.8 million light-years distant.

NGC 2976 - at mag. +10.6, is a small Sd galaxy with an irregular spiral pattern that's also a member of the M81 Group. It lies 85 arc minutes south-southwest of M81, displaying a bright oval without details in a 200mm (8-inch) scope. NGC 2976 spans 5.5 x 2.5 arc minutes of apparent sky and is 11.6 Million light-years distant.

NGC 3953 - is a barred spiral galaxy, one-degree south/southwest of Phecda (γ UMa) and M109. It measures 6.5 x 3.0 arc minutes, shines at magnitude +10.8 and is visible as a small patch of light through a small 80mm (3.1-inch) scope. A 200mm (8-inch) scope reveals subtle variations in brightness, hinting at its spiral nature.

NGC 3953 is a member of the M109 Group of galaxies.

NGC 4051 - is a nice Sb type spiral galaxy located south of the main seven stars, close to the Canes Venatici constellation border. It has an apparent magnitude of +10.9 and measures 4.5 x 3.3 arc minutes in diameter. Under dark skies, a medium size scope will show a compact nucleus surrounded by a faint haze. The galaxy is renowned for its thick spiral arms but they are invisible to backyard scopes.

NGC 2841 - mag. +10.1, is a fine type Sb spiral galaxy located in the western part of the constellation. It has a high surface brightness and therefore quite easy to spot even with a small scopes, when good seeing conditions prevail. NGC 2841 covers 8.0 x 3.0 arc minutes and is a nice object for medium and large size amateur scopes. A 200mm (8-inch) scope reveals an oval haze surrounding a brighter centre, with dust patches also visible on good nights. Its tightly wrapped arms appear mottled in backyard scopes.

NGC 2841 is 46 Million light-years distant.

NGC 3184 - is an Sc type spiral galaxy located 40 Million light-years distant that shines at magnitude +10.0. It's positioned 45 arc minutes west of Tania Australis (μ UMa - mag. +3.1) and therefore in the same low power telescope field of view. The orientation of the galaxy is face-on, so it suffers from low surface brightness.

Despite that it can be spotted as a faint haze in a 150mm (6-inch) scope when μ UMa is moved out of the field of view. The galaxy in total spans 6.9 x 6.8 arc minutes, although of course like all galaxies it appears visually smaller. Larger backyard scopes reveal a hint of spiral structure in moments of good seeing.

NGC 3079 - is a fine example of an edge on barred spiral galaxy. It shines at mag. +10.7 and measures a thin 7.5 x 1.5 arc minutes. Observationally, it appears as a fainter and smaller version of M82. Large scopes may reveal dark dust lanes.

Planetary Nebula

M97 (NGC 3587) - at mag. +9.9 is a famous planetary nebula commonly known as the Owl Nebula. It's easy to find just 2.5 degrees southeast of Merak (β UMa). Barred spiral galaxy M108 is positioned in the same telescope field of view, 50 arc minutes northwest of M97.

The Owl Nebula has a low surface brightness, making it a challenging object for small telescopes. A 100mm (4-inch) scope on good nights reveals a dim circular haze with detail. The standout feature of the Owl Nebula is its famous eyes. They consist of two dark patches superimposed on the face of the nebula and can be seen with 200mm (8-inch) scopes at high powers, but a larger instrument is recommended.

The central star of M97 shines at 14th magnitude making it an elusive target in anything less than a 350mm (14-inch) telescope. M97 spans 3.4 x 3.3 arc minutes and is located 2,600 light-years from Earth.

Ursa Major Data Table

Henry Draper Catalogue (HD) Hipparcos Catalogue (HIP) Bayer Flamsteed Struve Name RA (J2000) DEC (J2000) Visual Mag. Var. Var. Mag. Range Period (days) Double Sep. (arc secs) PA (deg.) Mag. Primary. Sec 112185 62956 Epsilon UMa 77 --- Alioth 12h 54m 02s +55d 57m 35s 1.76 Y 1.74 -> 1.78 5.1 --- --- --- --- 95689 54061 Alpha UMa 50 --- Dubhe 11h 03m 44s +61d 45m 04s 1.81 --- --- --- Y AB 0.5 / AC 380 AB 44 / AC 204 A 2.0 / B 4.8 / C 7.2 120315 67301 Eta UMa 85 --- Alkaid 13h 47m 33s +49d 18m 48s 1.85 --- --- --- --- --- --- --- 116656 65378 Zeta UMa 79 1744 Mizar 13h 23m 55s +54d 55m 32s 2.23 --- --- --- Y AB 14.4 / AC 706 AB 151 / AC 70 A 2.3 / B 4.0 / C 4.0 95418 53910 Beta UMa 48 --- Merak 11h 01m 50s +56d 22m 56s 2.34 --- --- --- --- --- --- --- 103287 58001 Gamma UMa 64 --- Phad 11h 53m 50s +53d 41m 41s 2.41 --- --- --- --- --- --- --- 106591 59774 Delta UMa 69 --- Megrez 12h 15m 26s +57d 01m 57s 3.32 --- --- --- Y AB 182 / AC 175 AB 74 / AC 128 A 3.3 / B 10.2 / C 12.0 98230 55203 Xi UMa 53 1523 Alula Australis 11h 18m 11s +31d 31m 51s 3.79 --- --- --- Y AB 1.7 AB 240 A 4.3 / B 4.8 98262 55219 Nu UMa 54 1524 Alula Borealis 11h 18m 29s +33d 05m 39s 3.49 --- --- --- Y AB 7.4 AB 149 A 3.5 / B 9.9 76644 44127 Iota UMa 9 --- Talitha 08h 59m 13s +48d 02m 33s 3.12 --- --- --- Y AB 2.0 AB 31 A 3.1 / B 9.2 81937 46733 23 UMa 23 1351 23 UMa 09h 31m 32s +63d 03m 43s 3.65 --- --- --- Y AB 23 / AC 105 AB 269 / AC 231 A 3.7 / B 9.2 / C 10.5 113139 63503 78 UMa 78 --- 78 UMa 13h 00m 44s +56d 21m 59s 4.93 --- --- --- Y AB 1.5 AB 97 A 5.0 / B 7.7 92763 52546 R UMa --- --- R UMa 10h 44m 39s +68d 46m 33s 6.6 Y 6.6 -> 13.5 301.7 --- --- --- --- 92839 52577 VY UMa --- --- VY UMa 10h 45m 04s +67d 24m 41s 5.8 Y 5.8 -> 6.4 120 --- --- --- --- 109729 61532 T UMa --- --- T UMa 12h 36m 24s +59d 29m 13s 6.6 Y 6.6 -> 13.4 256.5 --- --- --- --- 238107 --- --- --- --- M40 12h 22m 13s +58d 04m 59s 9.00 --- --- --- Y AB 51.7 AB 75 A 9.6 / B 10.1 95735 54035 --- --- --- Lalande 21185 11h 03m 20s +35d 58m 12s 7.52 --- --- --- --- --- --- ---

Ursa Major Deep Sky Data Table