The Big Dipper is one of the most easily recognizable asterisms in the night sky, found in the constellation Ursa Major, the Great Bear. The star pattern, formed by the seven brightest stars of Ursa Major, is well-known in many cultures and goes by many other names, among them the Plough, the Great Wagon, Saptarishi, and the Saucepan. The Big Dipper is particularly prominent in the northern sky in the summer, and is one of the first star patterns we learn to identify.
The Big Dipper is often confused for the constellation Ursa Major itself and its name used synonymously with the Great Bear. However, the Big Dipper itself is not a constellation, but only the most visible part of Ursa Major, the third largest of all 88 constellations. Ursa Major constellation covers a much larger area of the sky, but the stars marking the bear’s head, torso, legs and feet are not as bright or as easy to see as the seven stars marking its tail and hindquarters.
The seven stars of the Big Dipper are Alkaid (Eta Ursae Majoris), Mizar (Zeta Ursae Majoris), Alioth (Epsilon Ursae Majoris), Megrez (Delta Ursae Majoris), Phecda (Gamma Ursae Majoris), Dubhe (Alpha Ursae Majoris) and Merak (Beta Ursae Majoris). Alkaid, Mizar and Alioth mark the Big Dipper’s handle or the Great Bear’s tail, while Megrez, Phecda, Dubhe and Merak outline the Dipper’s bowl or the Bear’s hindquarters.
The star names in Big Dipper mostly refer to the stars’ positions in Ursa Major. The name Alioth refers to a tail (of a sheep), Megrez to the base of the tail, Phecda to the bear’s thigh, and Merak to the loins.
The brightest star in the Big Dipper asterism is Alioth, Epsilon Ursae Majoris. Alioth is also the brightest star in the constellation Ursa Major and the 32nd brightest star in the sky.
Five of the seven Dipper stars belong to the Ursa Major Moving Group, also known as Collinder 285. The Ursa Major Moving Group is a group of stars that share a common origin, proper motion, and common velocities in space. The white (class A) stars Mizar, Alioth, Megrez, Phecda and Merak are members of the group. The blue main sequence star Alkaid and orange giant Dubhe are not.
Alkaid, or Benetnash, (from the Arabic qā’id bināt na’sh, meaning “the leader of the daughters of the bier”) is one of the hottest stars visible to the naked eye. It is the star marking the tip of the handle of the Big Dipper, or alternatively the tip of the Great Bear’s tail. The name Alkaid means “the leader.”
Alkaid is a young blue main sequence star of the spectral type B3V. It has an apparent magnitude of 1.86 and is about 103.9 light years distant from Earth.
Alkaid is the third brightest star in Ursa Major and the 38th brightest star in the sky. It is 3.4 times larger, 6.1 times more massive and, with a surface temperature of 15,540 K, 594 times more luminous than the Sun.
Mizar (from the Arabic mīzar, meaning “girdle”) is the primary component of a multiple star system that consists of two spectroscopic binary stars. It was the first double star to be photographed, in 1857. It has an apparent magnitude of 2.23 and is 82.9 light years distant.
Mizar is the middle star in the Big Dipper’s handle. It forms a naked-eye double with the fainter Alcor, with which it may be physically associated. Alcor itself has a fainter companion, so if it is indeed gravitationally bound to Mizar, this would make Zeta Ursae Majoris a sextuple star system. Both Mizar and Alcor are members of the Ursa Major Moving Group.
Mizar, the primary component in the Zeta UMa system, is a white main sequence star of the spectral type A2Vp. It has a mass 2.2224 times that of the Sun and a radius 2.4 times solar. With a surface temperature of 9,000 K, it shines with 33.3 solar luminosities. The star is believed to be about 370 million years old.
Alioth (from the Arabic alyat, meaning “fat tail of a sheep”) is the star in Ursa Major’s tail which is the closest to the bear’s body. It has a visual magnitude of 1.77 and is about 82.6 light years distant. It has the stellar classification of A1III-IVp kB9, indicating a white star that is coming to the end of its main sequence lifetime.
Alioth has a mass of 2.91 solar masses and is 4.14 times larger than the Sun. It shines with 102 solar luminosities with an effective temperature of about 9,020 K. The star’s estimated age is 300 million years.
Alioth is a peculiar star, one that shows variations in its spectral lines over a period of 5.1 days. It is classified as an Alpha2 Canum Venaticorum variable. It is the brightest of the seven stars in the Big Dipper asterism.
Megrez (from the Arabic al-maghriz, “the base,” referring to the base of the Big Bear’s tail), is the dimmest of the seven stars. It has an apparent magnitude of 3.312 and lies at a distance of 80.5 light years.
Megrez is a white main sequence star of the spectral type A3 V. It has a mass of 1.63 solar masses and a radius of 1.4 solar radii. With a surface temperature of about 9,480 K, it is 14 times more luminous than the Sun. The star is a fast rotator, with a projected rotational velocity of 233 km/s. Like its Big Dipper neighbours, it is believed to be about 300 million years old.
Phecda, or Phad (from the Arabic fakhð ad-dubb, “the thigh of the bear”), has the stellar classification A0Ve, indicating another white main sequence dwarf. The star has a mass 2.94 times that of the Sun and a radius 3.04 times solar. It is 65.255 times more luminous than the Sun with an effective temperature of 9,355 K.
Phecda has an astrometric binary companion, an orange dwarf of the spectral type K2 V that perturbs it and causes it to wobble around the centre of mass. The two stars have an orbital period of 20.5 years. The companion has a mass of 0.79 solar masses and is considerably cooler than the primary, with a surface temperature of 4,780 K. It shines with only 0.397 solar luminosities.
Phecda has an apparent magnitude of 2.438 and lies at a distance of 83.2 light years from Earth.
Dubhe (from the Arabic dubb, meaning “bear,” abbreviated from the phrase żahr ad-dubb al-akbar, meaning “the back of the Greater Bear”) has a visual magnitude of 1.79 and is about 123 light years distant from Earth. It is the second brightest star in Ursa Major.
Dubhe is an orange giant with the stellar classification of K0III. It is a spectroscopic binary star, with a white main sequence companion of the spectral type F0V. More recent sources classify Dubhe as a yellow giant of the spectral class G9III and the companion as an A7.5 class star. The two stars are 23 astronomical units apart and have an orbital period of 44.4 years.
Dubhe is 4.25 times more massive than the Sun and 316 times more luminous. It is a slow spinner, with a projected rotational velocity of 2.6 km/s. The companion is less massive, with about 1.6 solar masses. It has a visual magnitude of 4.86.
Merak (from the Arabic al-maraqq, meaning “the loins”) is a white subgiant star of the spectral type A1IVps. It has an apparent magnitude of 2.37 and is 79.7 light years distant. It is classified as a suspected variable. The star has a mass of 2.7 solar masses and a radius 3.021 times that of the Sun. With a surface temperature of 9,377 K, it is 63.015 times more luminous than the Sun. The star’s estimated age is about 500 million years.
BIG DIPPER FACTS AND LOCATION
Ursa Major lies in the second quadrant of the northern hemisphere (NQ2), which makes it visible at latitudes between +90° and -30°. It is best seen in the evenings in April.
The Big Dipper is circumpolar in most of the northern hemisphere, which means that it does not sink below the horizon at night. As a result of the Earth’s rotation, Ursa Major appears to rotate slowly counterclockwise at night around the north celestial pole.
The Big Dipper can be found in different parts of the sky at different times of year. In spring and summer, the Big and Little Dippers are higher overhead, and in autumn and winter, they are closer to the horizon. The rule is, spring up and fall down.
The appearance of the Big Dipper changes from season to season. In autumn, it rests on the horizon in the evening. In winter evenings, the handle appears to be dangling from the bowl. In spring, it is upside down in the evening hours, and in summer the bowl leans toward the ground.
The Big Dipper is located in the region of the sky that contains several famous deep sky objects, including the Whirlpool Galaxy (Messier 51), located under the Big Dipper’s handle in Canes Venatici constellation, and the Pinwheel Galaxy (Messier 101) in Ursa Major, which can be found with binoculars or small telescopes. Other notable deep sky objects in the area include the double star Messier 40 (Winnecke 4), the spiral galaxy Messier 81 (Bode’s Galaxy), the irregular galaxy Messier 82 (Cigar Galaxy), the planetary nebula Messier 97 (Owl Nebula), and spiral galaxies Messier 108 and Messier 109.
The asterism serves as a guide to a number of bright stars, too. The arc of the Big Dipper’s handle leads to Arcturus, the bear keeper, the brightest star in the constellation Boötes, the Herdsman. Following the line further leads to Spica, also one of the brightest stars in the sky, located in the constellation Virgo.
Two of the stars marking the cup of the Big Dipper lead the way to Polaris, the North Star, and another pair of stars, Megrez and Phecda, point the way to Regulus, the brightest star in Leo and also one of the brightest stars in the night sky, and also to Alphard, the brightest star in Hydra constellation.
The line from Megrez to Dubhe points the way to Capella in Auriga constellation, and one drawn from Megrez to Merak leads to Castor in Gemini when extended by about five times the distance between the two stars.
In about 50,000 years, the stars of the Big Dipper will be at different locations, which will result in the asterism changing shape and facing the opposite way. Still, as most of the stars that form the asterism (all except Alkaid and Dubhe) are members of the Ursa Major Moving Group, which means that they share common motion through space, the asterism will not look significantly different. The pattern will be present even 100,000 years from now, but the shape of the handle, with Alkaid marking the tip, and the end of the bowl marked by Dubhe, will appear slightly different.
NORTH STAR AND THE LITTLE DIPPER
Finding the Big Dipper in the night sky is the easiest way to find Polaris, the North Star, located in the constellation Ursa Minor, the Little Bear. The Big Dipper rotates around the north celestial pole, and always points the way to the North Star.
The Little Dipper, formed by the seven brightest stars in Ursa Minor constellation, lies in the vicinity of the Big Dipper, but as the stars of the Little Dipper aren’t quite as bright, especially the four located between Polaris on one end and Kochab and Pherkad on the other, the Little Dipper is not as easy to find in the sky, especially in areas polluted by light.
Since the Little Dipper is not quite as prominent in the sky as its larger neighbour, it is easier to use the stars of the Big Dipper to find both the North Star and true north. To find Polaris, follow the line from the Pointer Stars, Merak and Dubhe, to the first bright star along the same line. That is the North Star. Merak and Dubhe are the stars that mark the end of the bowl of the Big Dipper. They are called the Pointer Stars because they point the way to Polaris and true north.
The Big Dipper is associated with a number of different myths and folk tales in cultures across the world. In Hindu astronomy, the asterism is called Sapta Rishi, or The Seven Great Sages. In eastern Asia, it is known as the Northern Dipper. The Chinese know the seven stars as the Government, or Tseih Sing. In Malaysia, the asterism is called Buruj Biduk, or The Ladle, and in Mongolia, it is known as the Seven Gods. In an Arabian story, the stars that form the bowl represent a coffin and the three stars marking the handle are mourners following it. The name of the star Alkaid (or Benetnash), located at the tip of the handle, refers to that story.
In the UK and Ireland, the asterism is known as the Plough, and sometimes as the Butcher’s Cleaver in northern parts of England. The old English name for the asterism is Charles’ Wain (wagon), which is derived from the Scandinavian Karlavagnen, Karlsvognen, or Karlsvogna. Charles or Karl was a common name in Germanic languages and the name of the asterism meant “the men’s wagon,” as opposed to the Little Dipper, which was “the women’s wagon.” An even older name for the stars of the Big Dipper was Odin’s Wain, or Odin’s Wagon, referring to Scandinavian mythology.
In Slavic languages and in Romanian, the Big and Little Dipper are known as the Great and Small Wagon, and Germans know the Big Dipper as Großer Wagen, or the Great Cart. The Romans knew the seven stars as the “seven plough oxen,” or Septentrio, with only two of the seven stars representing oxen and the others forming a wagon pulled by the oxen.
Some Native American groups saw the bowl as a bear and the three stars of the handle either as three cubs or three hunters following the bear. The second interpretation is linked to a folk tale explaining why the leaves turn red in autumn: the hunters are chasing a wounded bear and, since the asterism is low in the sky that time of year, the bear’s blood is falling on the leaves, making them turn red.
In more recent history, black slaves in the United States knew the constellation as the Drinking Gourd and used it to find their way north, to freedom. The folk song, “Follow the Drinking Gourd,” gave runaway slaves directions to follow the Big Dipper to get to north. In Africa, the seven stars were sometimes seen as a drinking gourd, which is believed to be the origin of the name the Big Dipper, most commonly used for the figuration in the U.S. and Canada.