(alphabetical links)
URSA MAJOR
The Big Bear
Ursa Major - Celestial Atlas by Alexander Jamieson - 1822
The most recognizable grouping of stars in the night sky is without a doubt: the Big Dipper. It is also known in different cultures as either a plow, or some sort of wagon, or chariot. But whatever you call it, its shape is unmistakable, and has been used throughout the ages to help find the very important star Polaris. Known as the north star, Polaris is almost directly lined up with Earth's axis, so no matter what time of day or night it is, Polaris is always found in the same place, and all the other stars in the heavens appear to circle around it in a counter clockwise direction. The two stars Dubhe and Merak that form the far edge of the dipper are known as the pointers, because they point almost directly to Polaris.
And if you follow the arc of the dipper's handle, you will arc directly to the bright golden-orange Arcturus, the fourth brightest star in the entire sky, in the constellation Bootes.
But it must be remembered that the Big Dipper is not a constellation. It is merely an asterism (grouping of stars), that is part of the much larger constellation Ursa Major, the big bear. Ursa Major is, in fact, the third largest constellation in the heavens. Almost all cultures all over the world have recognized the constellation as a bear throughout history. Classic writers going all the way back to Homer have identified the stars with a bear. The ancient lore of native North Americans knew the three stars of the dipper's handle as three hunters, tracking the great bear.
In Greek myth, the great bear was originally a beautiful maiden named Callisto, who caught the ever roving eye of the mighty Zeus (Jupiter), king of the gods. In a jealous rage, Zeus's wife, Hera (Juno), turned Callisto into a bear. When Callisto saw her young son, Arcus, she ran to embrace him, but Arcus, seeing only a great bear rushing towards him, and fearing for his life, prepared to shoot it with an arrow.
When Zeus saw this tragedy unfolding he quickly turned the young Arcus into a young bear, and grabbing them both by their tails, he hurled them into the sky, immortalising them in the stars. In the words of the ancient writer Ovid:
"... flung them through the air,
In whirlwinds to the high heavens,
and fixed them there,
Where the new constellations nightly rise,
Lustrous in the northern skies."
The force which Zeus had to use to throw these heavy animals all the way up into the sky is said to have stretched their tails, so that their figures sport much longer tails than their mortal counterparts. But Hera had the final word, (as most wives do), and made sure the bears were placed in the northern sky, close enough to the north star (Polaris), that they would never be able to set, and rest below the horizon, as almost all the other constellations do. Callisto and her son, Arcus, in the form of bears, would be condemned to circle endlessly around Polaris for eternity.
The Planets Of Ursa Major
At last count there were a dozen stars in Ursa Major that support planetary systems, and three of them are visible with the naked eye. Up by the nose of the bear, star 4 Ursa Majoris has one known planet seven times the size of Jupiter in orbit around it, 203 light years from Earth. By the bear's front leg, star HD 81688 also has one known planet. It is 2.7 times the size of Jupiter, and 286 light years away. By the bear's back leg, the star 47 Ursa Majoris is only 43 light years away and has two known planets in its system. Planet 47 Ursa Majoris b is over two times the size of Jupiter, but planet 47 Ursa Majoris c is actually smaller than Jupiter - only 76% as big.
The Stars Of Ursa Major
The brightest stars in a constellation, whether or not they have been assigned proper names, are always designated by letters of the Greek alphabet, such as Alpha Ursae Majoris, Beta Ursae Majoris, and so on. With almost all constellations, the brightest star is designated by the first letter in the Greek Alphabet (Alpha), the second brightest star is given the second letter (Beta), and so on, in descending order. Ursa Major, however, is one of the few exceptions to this rule. In this constellation, the Greek designations for the brightest stars simply begin at one end of the dipper, and follow the stars consecutively to the end of the dipper's handle, regardless of their relative brightness.
Dubhe (Alpha Ursae Majoris)
The name of this star is taken from the Arabic Thahr al Dubb al Akbar - "The Back of the Great Bear". It has a magnitude of 1.81, is 105 light years away, and 145 times brighter than our Sun.
Merak (Beta Ursae Majoris)
From the Arabic Al Marakk - "The Loin of the Bear", Merak is the second of the two pointer stars (along with Dubhe) that point the way to Polaris. It has a magnitude of 2.37, is 80 Light years away, and 65 times brighter than our Sun.
Phecda (Gamma Ursae Majoris)
From the Arabic Al Fahdh - "The Thigh", Phecda has a magnitude of 2.44, is 90 light years away, and 75 times brighter than our Sun.
Megrez (Delta Ursae Majoris)
From the Arabic Al Maghrez - "Root of the Tail", Megrez is the faintest of the stars in the Big Dipper, with a magnitude of 3.30. It is 65 light years away, and 20 times brighter than our Sun.
Alioth (Epsilon Ursae Majoris)
From the Arabic Alyat - "Fat Tail". With a magnitude of 1.79, Alioth is the brightest star in the constellation. It is 70 light years away, and 85 times brighter than our Sun.
Mizar (Zeta Ursae Majoris)
Mizar has a magnitude of 2.40. It is 88 light years away, and 35 times brighter than our Sun.
Alcor
Alcor is so close to Mizar, they are often thought of as one star. Before the days of optometrists and eye charts, a person's eyesight was tested by trying to distinguish the two separate stars, and Alcor was often referred to as Al Sadak - "The Test". At a magnitude of 4.02, Alcor is only half as bright as Mizar, and too dim to warrant a Greek letter designation. The name Alcor means "rider", and along with Mizar, it forms a pair of stars known as "The Horse and Rider".
Alkaid (Eta Ursae Majoris)
At the end of the dipper's handle (or the tail of the bear) we find Alkaid, sometimes also called Benetnasch. Alkaid is a young star, extremely hot and bright, estimated to be 630 times more luminous than our Sun. But at the great distance of 210 light years, we see it with a magnitude of 1.87, just slightly dimmer than Alioth, making it the second brightest star in the constellation.
Muscida (Omicron Ursae Majoris)
Muscida means "muzzle", and marks the nose of the bear. It is 150 light years away and is 85 times brighter than our Sun, giving it a magnitude of 3.37.
The Three Leaps Of The Gazelle
Before we discuss the remaining bright stars in Ursa Major, we must introduce the second animal associated with the constellation: the gazelle. The relatively dark area between Ursa Major and Leo is known as the pond. It is said that a gazelle drinking from the edge of the pond, suddenly noticed the lion on one side of it and the bear on the other, and quickly bounded across the shallow pond to escape them, leaving three sets of tracks in the mud, indicated by three evenly spaced pairs of stars that also represent the feet of the bear. The map below shows Ursa Major as it appears in the evenings during the winter, seemingly standing on its tail, climbing up the sky, and inside the yellow circles, the stars known as the Three Leaps of the Gazelle.
Talitha (Iota and Kappa Ursae Majoris)
Here we have the rare occurrence of two distinct stars (Iota and Kappa) being grouped together under the same proper name. The name Talitha means "The Third Leap", referring to the third leap of the gazelle across "the pond". The two stars also mark the front paw of the bear. Iota, the most forward of the pair, is magnitude 3.12, 50 light years away, and 11 times brighter than our Sun. Kappa is a very much hotter star, estimated to be 250 times brighter than our Sun, but at the great distance of 300 light years, it's apparent magnitude to us on Earth is only 3.68, slightly less than Iota.
Tania Borealis and Australis (Lamda and Mu Ursae Majoris)
Tania is Arabic for - you guessed it - "The Second Leap". The two stars also mark one of the hind paws of the bear. Tania Borealis is 150 light years away, and 75 times brighter than our Sun, giving it an apparent magnitude of 3.45. Tania Australis is a red giant star. It is 105 light years away and 50 times brighter than our sun, giving it a magnitude of 3.05.
Alula Borealis and Australis (Nu and Xi Ursae Majoris)
As you would expect, Alula means "The First Leap". Borealis, of course, means north, and Australis means south.
If you have a telescope, there are several interesting sights in Ursa Major. The first and most obvious is M40, a large, slightly blurred star to the naked eye, that resolves into two separate stars through binoculars, or a telescope. M40 is the only Messier object that is not a nebula or cluster, and is often referred to as Messier's Mistake. The next sight to look for is the pair of bright spiral galaxies M81, magnitude 6.94, and M82, magnitude 8.41. They are both less than 12 million light years away, and appear close enough together to fit into one field of view in a small telescope. M81 is bright enough to almost see with the naked eye, and easy to find with a scope. Through the eyes of the Hubble Space Telescope the true majesty of these galaxies is revealed.
M81 (NGC 3031) - Bode's Galaxy - Hubble Space Telescope - May, 2007
M82 (NGC 3034) - The Starburst Galaxy - Hubble Space Telescope - March, 2006
Close to the Dipper's handle is the spectacular Pinwheel Galaxy, M101. A spiral galaxy similar to our own Milky Way, M101 is 15 million light years away, and contains about 16 million stars. A bright, face-on spiral with a magnitude of 7.86 makes it another good target for a backyard scope.
M101 - The Pinwheel Galaxy - Adam Block/Mount Lemmon SkyCenter/University of Arizona - March, 2009
Under the belly of the bear we find M97, the Owl Nebula. It has a magnitude of 9.9, and is about 3000 light years away. It is an expanding shell of gas emitted by a central star that began to die and shed its outer layers about 6,000 years ago.
M97 - The Owl Nebula - Gary White and Verlenne Monroe/Adam Block/NOAO/AURA/NSF
Right beside the Owl Nebula is the spiral galaxy M108, about 45 million light years away, with a magnitude of 10.7.
M108 (NGC 3556) - NOAO/AURA/NSF - January, 1997
Close to the star Phecda is the barred spiral M109, about 45 million light years away, with a magnitude of 10.7.
M109 (NGC 3992) - NOAO/AURA/NSF - January, 1997
Alphabetical Links to the Constellations
ANDROMEDA
AQUARIUS
AQUILA
ARIES
AURIGA
BOOTES
CAMELOPARDALIS
CANCER
CANES VENATICI
CANIS MAJOR
CANIS MINOR
CAPRICORNUS
CARINA
CASSIOPEIA
CENTAURUS
CEPHEUS
CETUS
COLUMBA
COMA BERENICES
CORONA AUSTRALIS
CORONA BOREALIS
CORVUS
CRATER
CRUX
CYGNUS
DELPHINUS
DORADO
DRACO
EQUULEUS
ERIDANUS
FORNAX
GEMINI
HERCULES
HYDRA
LACERTA
LEO
LEO MINOR
LEPUS
LIBRA
LUPUS
LYNX
LYRA
MONOCEROS
OPHIUCHUS
ORION
PEGASUS
PERSEUS
PISCES
PISCIS AUSTRALIS
PUPPIS
SAGITTA
SAGITTARIUS
SCORPIUS
SCULPTOR
SCUTUM
SERPENS
SEXTANS
TAURUS
TUCANA
URSA MAJOR
URSA MINOR
VELA
VIRGO
VULPECULA
