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The most recognizable shape 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, immortalizing them in the stars. In the words of the ancient writer Ovid:
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 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.
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.
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.
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.
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.
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 has a magnitude of 2.40. It is 88 light years away, and 35 times brighter than our Sun.
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".
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 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.
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 and the bear, 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. 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.
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 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.
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 galaxies M81 and M82. They are close enough together to fit into one field of view at low power, and a wondrous sight in a small telescope. They are both approximately 7 million light years from Earth. The time exposure below was taken by Ray Gralak. Click to enlarge.
In May, 2007, the Hubble Space Telescope took a two and a half day time exposure of M81, resulting in a photo that shows details in the galaxy never before seen. Aside from the wealth of scientific data the photo provides, it shows the incredible grandeur and beauty of a galaxy that is a mirror image of our own Milky way galaxy, except much larger. It gives us an idea of what our galaxy would look like from a few million light years away. Like our own galaxy, M81 contains a massive black hole at its centre, only the black hole at the centre of M81 is 15 times larger. Click on the small photo below, for the full high resolution image, which will take your breath away.
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. That we can see it at all at that great distance is testament to the combined brilliance of the 16 billion stars it contains. Although not much more than a blur in a small telescope, the NASA time exposure below brings out its true splendour. Click to enlarge.
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