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Ursa Major

From Academic Kids

Ursa Major is a constellation visible throughout the year in the northern hemisphere. Its name means Great Bear in Latin, and is associated with the legend of Callisto.

Its seven brightest stars form a famous asterism known in the United Kingdom as the Plough, and was formerly called by the old name Charles's Wain ("wain" meaning "wagon") as it still is in Scandinavia, Karlavagnen. This common Germanic name originally meant the men's wagon (the churls' wagon) in contrast to the women's wagon (Ursa Minor). There is also a theory that it was named after Charlemagne. In North America it is commonly known as the Big Dipper, because the major stars can be seen to follow the rough outline of a large ladle, or dipper; this is recognized as a grouping of stars in many cultures throughout the eras. In Hindu astronomy, it is referred to as Sapta Rishi meaning "The Seven Sages".


Contents

Notable features

From the bowl to the handle, the stars in the Big Dipper are called Dubhe, Merak, Phecda (or Phad), Megrez, Alioth, Mizar, and Alkaid (or Benetnash), and are given Bayer designations of Alpha to Eta Ursae Majoris, in that order. Mizar has a companion star called Alcor, just visible to the naked eye, that served as a traditional test of sight. Both stars are actually multiple in and of themselves, including the first telescopic and spectroscopic binaries.

The star Polaris, the Pole Star, can be found by measuring a line five times the angular distance between the two pointer stars Dubhe and Merak forming the end of the dipper cup, through those stars and up and away from the dipper. The dipper also points the way to other stars.

Except for Dubhe and Alkaid, the stars of the Big Dipper asterism all have proper motions heading towards a common point in Sagittarius. A few other members have been identified, and it has been given the name Ursa Major Moving Group.

In addition to the Big Dipper, another asterism comes from Arab culture – the "leaps of the gazelle", a series of three pairs of stars:

  • ν and ξ Ursae Majoris, Alula Borealis and Australis, the "first leap";
  • λ and μ Ursae Majoris, Tania Borealis and Australis, the "second leap";
  • ι and κ Ursae Majoris, Talitha Borealis and Australis, the "third leap".

These stars are found along the southwest border of the constellation, the bear's toes.

W Ursae Majoris is the prototype of a class of contact binary variable stars, and ranges in magnitude between 7.75m and 8.48m.

47 Ursae Majoris has a planetary system with three confirmed planets, 2.54 times and 0.76 times the mass of Jupiter. it is also known as the Vrihat Saptarishi.

Notable deep sky objects

Several galaxies are found in Ursa Major, including the pair M81 (one of the brightest galaxies in the sky) and M82 above the bear's head, and M101, a beautiful spiral northwest of η Ursae Majoris. The constellation contains about 50 galaxies, most of which are below 10th magnitude.


It's a not-too-well known fact that the Sun and its planets are located in a somewhat enriched region of the Milky Way Galaxy, from the point of view of stellar populations and especially prevalence of bright stars in the vicinity. At least part of the reason for this is that the Sun happens to be in the midst of a slowly dissolving star cluster, the Ursa Major Stream or Moving Group, of which the major stars of Ursa Major are among the chief members, to which could be added Sirius.

The Ursa Major group a real star cluster in space, which would be noticeable from any given direction for a distance of at least several hundred light years. From the inclusion of Sirius, which is something like 50 degrees of arc across the sky, it is obvious that the Sun is actually inside this loose association of stars, but it is not a member of it. These are relatively young stars, having formed together from a protostellar nebula something like 150 million years ago, since which time they have been gradually drifting apart, and the cluster losing coherence as the stars "evaporate" into the general stream of stars in the galactic plane. The Sun, in its normal streaming orbit around the Galaxy, just drifted on in to their region.

Probably most stars in the galaxy are somewhere near something prominent like a group of bright stars, but this feature (usually called nowadays the "Ursa Major Moving Group" or Ursa Major Stream, and consisting of about 100 stars, most of them dimmer than the Sun), is the most prominent nearby feature for us. 30 or 40 million years ago, the Sun was nowhere near these stars, as it peregrinated on its 250 million year galactic orbit.

History

It was one of the 48 constellations listed by Ptolemy.

This is one of the most widely-known constellations, having been mentioned by such poets as Homer, Spenser, Shakespeare, and Tennyson. The Finnish epic Kalevala mentions them, and Vincent Van Gogh painted them in his painting Starry Night Over the Rhone.

When slavery was still allowed in the southern part of the United States of America, slaves wishing to escape to the Yankee North were advised to "follow the drinking gourd," or the circumpolar constellation of Ursa Major, towards freedom.

Mythology

The constellation of Ursa Major, has been seen by many distinct civilizations have seen this figure as a bear. In consequence, together with the nearby Ursa Minor, it formed the basis of the myth of Callisto.

In earlier times, in Greek mythology, Ursa Major was not considered as a bear, and instead its 3 bright stars (situated in the tail) were considered to be apples growing on a tree (sometimes represented by the fainter stars in the remainder of the constellation). The stars were associated with the Hesperides (considered at the same period to be the stars of Ursa Minor). Together with the other constellations in the zodiac sign of Libra (i.e. [[Bo�tes]], and Draco) these may have formed the origin of the myth of the apples of the Hesperides, which forms part of The Twelve Labours of Hercules.

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