Ursa Major (also known as the Great Bear) is a constellation in the northern sky, whose associated mythology likely dates back into prehistory. Its Latin name means ‘greater (or larger) she-bear,’ standing as a reference to and in direct contrast with nearby Ursa Minor, the lesser bear.
Appearing in the northern sky, Ursa Major occupies a large area covering 1279.66 square degrees or 3.10% of the total sky, making it the third largest constellations in the night sky. According to the equatorial coordinate system, the constellation stretches between the right ascension coordinates of 08h 08.3m and 14h 29.0m, and the declination coordinates of +28.30° and +73.14°. Ursa Major borders eight other constellations: Draco to the north and northeast, Boötes to the east, Canes Venatici to the east and southeast, Coma Berenices to the southeast, Leo and Leo Minor to the south, Lynx to the southwest and Camelopardalis to the northwest.
Ursa Major is visible throughout the year from most of the northern hemisphere, and appears circumpolar above the mid-northern latitudes. From southern temperate latitudes, the main asterism is invisible, but the southern parts of the constellation can still be viewed. It is primarily known from the asterism of its main seven relatively bright stars comprising the ‘Big Dipper,’ ‘the Wagon,’ or ‘the Plough’ (among others), with its stellar configuration mimicking the shape of the ‘Little Dipper’ (Ursa Minor).
The general constellation outline often significantly features in numerous world cultures, and frequently is used as a symbol of the north (e.g. as the flag of Alaska). Also the asterism’s two brightest stars named Dubhe and Merak (Alpha and Beta Ursae Majoris) can be used as the navigational pointer towards the place of the current northern pole star, Polaris in Ursa Minor.
[https://en.wikipedia.org/wiki/Ursa_Major]
Ursa Major as depicted on Chart VI of the Uranographia of Johann Bode (1801). The familiar shape popularly known as the Plough or Big Dipper is made up of seven stars in the rump and tail of the bear.
Undoubtedly the most familiar star pattern in the entire sky is the seven stars that make up the shape popularly termed the Plough or Big Dipper, part of the third-largest constellation, Ursa Major, the Great Bear. The seven stars form the rump and tail of the bear, while the rest of the animal is comprised of fainter stars. Its Greek name in the Almagest was Ἄρκτος Μεγάλη (Arktos Megale); Ursa Major is the Latin equivalent.
Aratus called the constellation Ἑλίκη (Helike), meaning ‘twister’, apparently from its circling of the pole, and said that the ancient Greeks steered their ships by reference to it. In the Odyssey, for example, we read that Odysseus kept the great bear to his left as he sailed eastwards. The Phoenicians, on the other hand, used the Little Bear (Ursa Minor), which Aratus termed Κυνόσουρα (Kynosoura, or Cynosura in Latin transliteration). Aratus tells us that the bears were also called wagons or wains, and in one place he referred to the figure of Ursa Major as the ‘wagon-bear’ to underline its dual identity.
In mythology (mostly due to Ovid) the Great Bear is identified with Callisto, a paramour of Zeus. Callisto joined the retinue of Artemis, goddess of hunting, and she soon became the favorite hunting partner of Artemis, to whom she swore a vow of chastity. One afternoon, as Callisto laid down her bow and rested in a shady forest grove, Zeus caught sight of her and was entranced. Cunningly assuming the appearance of Artemis, he lay beside her and embraced her. Before the startled girl could react, Zeus revealed his true self and, despite Callisto’s struggles, had his way with her. Zeus returned to Olympus, leaving the shame-filled Callisto scarcely able to face Artemis and the other nymphs.
On a hot afternoon some months later, the hunting party came to a cool river and decided to bathe. Artemis stripped off and led them in, but Callisto hung back. As she reluctantly undressed, her advancing pregnancy was finally revealed. She had broken her vow of chastity! Artemis, scandalized, banished Callisto from her sight. Worse was to come when Callisto gave birth to a son, Arcas. Hera, the wife of Zeus, had not been slow to realize her husband’s infidelity and was now determined to take revenge on her rival. Hurling insults, Hera grabbed Callisto by her hair and pulled her to the ground. As Callisto lay spreadeagled, dark hairs began to sprout from her arms and legs, her hands and feet turned into claws and her beautiful mouth which Zeus had kissed turned into gaping jaws that uttered growls.
For 15 years Callisto roamed the woods in the shape of a bear, but still with a human mind. Once a huntress herself, she was now pursued by hunters. One day she came face to face with her son Arcas. Callisto recognized Arcas and tried to approach him, but he backed off in fear. He would have speared the bear, not knowing it was really his mother, had not Zeus intervened by sending a whirlwind that carried them up into heaven, where Zeus transformed Callisto into the constellation Ursa Major and Arcas into Boötes.
[http://www.ianridpath.com/startales/ursamajor.htm]
Stone-age amulet from northern Europe as representation of stars in Ursa Major and Boötes from Maud Makemson’s ‘Astronomy in Primitive Religion.’ Makemson drew on the work of the pioneer French archaeoastronomer Marcel Baudouin in analyzing a map of the stars in Ursa Major and Boötes incised on a fossilized sea-urchin amulet from stone-age northern Europe. Makemson wrote: “The representation of Ursa Major ... is remarkable for two reasons: first because the relative positions of the stars point to a very great antiquity for the amulet; and second, because the engraver has taken pains to indicate the difference in brightness of the stars, by varying the size of the cavities.”
[http://members.westnet.com.au/gary-david-thompson/page11-2.html]
[http://www.astronomytrek.com/step-6-interesting-facts-about-ursa-major-1/]
[http://jedi-counsel.net/a/astronomy/constellation/Ursa-Major/]
Mapping the night sky using Ursa Major:
Ursa Major is an important reference point for locating several surrounding constellations
Ursa Major: The 7 brightest stars in this constellation form a distinctive shape, known separately as the plough or big dipper, although the entire constellation is spread over a greater area of the sky. It is the most famous of all Northern constellations and is circumpolar in England and the Northern United States.
Ursa Minor: The 7 main stars of this constellation form a shape similar to Ursa Major, but with the tail of the bear pointing in the opposite direction. A line through the last two stars of Ursa Major, also known as ‘the pointers’, leads to the Polaris.
Cassiopeia: A line through the last star at the handle end of the big dipper and through Polaris will lead onto a conspicuous W shaped group of stars called Cassiopeia.
Bootes: Following the tail of the Plough down for 30 degrees leads onto Bootes and the beautiful orange-red giant star Arcturus
Leo: This can be found by following the line on the map through Ursa Major. Leo lies south of the Ursa Major, and its brightest star Regulus is white.
Gemini: Following the line shown on the map we come to Gemini. It is characterized by two nearly parallel rows of stars. The northern row if extended would reach Taurus, the southern one Orion. The two brightest stars in this constellation are Pollux and Castor.
[http://www.astronomytrek.com/step-7-mapping-the-night-sky-using-ursa-major/]
The seven stars forming the Big Dipper asterism are the following:
Alpha Ursae Majoris (Dubhe) is the second brightest star in Ursa Major. It has an apparent magnitude of 1.79, and is 123 light years distant from the solar system. The name Dubhe comes from the Arabic dubb, which means ‘bear,’ from the phrase ‘żahr ad-dubb al-akbar,’ or ‘the back of the Greater Bear.’ Dubhe is a giant star belonging to the spectral class K1 II-III. It is a spectroscopic binary star. The companion is a main sequence star that belongs to the spectral type F0 V. It completes the orbit around the brighter star every 44.4 years from a distance of 23 astronomical units. There is another binary system about 90,000 AU away from the main pair, which makes Alpha Ursae Majoris a four star system.
Beta Ursae Majoris (Merak) is the fifth brightest star in Ursa Major. The name Merak comes from the Arabic ‘al-maraqq,’ which means ‘the loins.’ Beta Ursae Majoris is a main sequence star, approximately 79.7 light years distant, with a visual magnitude of 2.37. It belongs to the spectral class A1 V. It is 2.7 times more massive than the Sun, has 2.84 times the radius, and is about 68 times more luminous. The star has a debris disk of dust orbiting it, one with a mass 0.27 percent that of the Earth.
Gamma Ursae Majoris (Phecda) is the sixth brightest star in the constellation of Ursa Major. It is the lower left star in the bowl of the Big Dipper. The star’s traditional name, Phecda (or Phad), is derived from the Arabic phrase ‘fakhð ad-dubb,’ which means ‘the thigh of the bear.’ It is a main sequence star of the spectral type A0 Ve. It has a visual magnitude of 2.438, and is approximately 83.2 light years distant. The –e in the star’s classification refers to it being an Ae star, one that has an envelope of gas surrounding it and adding emission lines to its spectrum. Phecda’s estimated age is 300 million years. The star is located only 8.55 light years away from the Mizar- Alcor star system.
Delta Ursae Majoris (Megrez) is the faintest of the seven bright stars that form the Big Dipper asterism. The star’s name, Megrez, is derived from the Arabic word ‘al-maghriz,’ which means ‘the base’ (as in, the base of the bear’s tail). It is a main sequence star of the spectral type A3 V. It has a visual magnitude of 3.312, and is approximately 58.4 light years distant from the solar system. It is 14 times more luminous than the Sun and has 63% more mass. The star emits an excess of infrared radiation, which indicates a debris disk in its orbit.
Epsilon Ursae Majoris (Alioth) is the brightest star in Ursa Major, and the 31st brightest star in the night sky. The star’s traditional name comes from the Arabic word ‘alyat,’ which means ‘fat tail of a sheep.’ Alioth is the star in the bear’s tail that is closest to the body of the bear. It belongs to the spectral class A0pCr. The p stands for peculiar because the star’s spectrum of light is similar to that of an Alpha-2 Canum Venaticorum variable. Alioth exhibits fluctuations in its spectral lines with a period of 5.1 days. It has an apparent magnitude of 1.76 and is approximately 81 light years distant. This star belongs to the Ursa Major Moving Group (Collinder 285), a group of stars that includes most of the brightest stars in the constellation Ursa Major. The stars belonging to the group share common velocities in space and are believed to have a common origin.
Zeta Ursae Majoris (Mizar) is the forth brightest star in Ursa Majoris. The name Mizar is derived from the Arabic ‘mīzar,’ which means ‘girdle’ or ‘waistband.’ Zeta Ursae Majoris is a system composed of two binary stars. It can be found in the Big Dipper’s handle- it is the second star from the end. Mizar has an apparent magnitude of 2.23 and is approximately 82.8 light years distant. It was the first double star ever to be photographed.
Eta Ursae Majoris (Alkaid or Benetnash) is the third brightest star in Ursa Majoris, and also the 35th brightest star in the night sky. The star’s traditional names, come from the Arabic phrase ‘qā’id bināt na’sh’ which means ‘the leader of the daughters of the bier.’ The three stars marking the handle of the Big Dipper represent three mourning maidens, while the stars that form the body of the bowl represent the bier. The name Alkaid itself means leader. It is the easternmost star in the Big Dipper asterism. It is a young main sequence star belonging to the spectral class B3 V, approximately 101 light years distant. Alkaid is notable for being one of the hottest stars that can be seen without binoculars. It has a surface temperature of 20,000 kelvins. The star has six solar masses and is about 700 times more luminous than the Sun.
[http://www.constellation-guide.com/constellation-list/ursa-major-constellation/]
M81 and M82 are two bright galaxies in Ursa Major:
M81 versus M82
Here in the Milky Way galaxy we have astronomical front row seats as M81 and M82 face-off, a mere 12 million light-years away. Locked in a gravitational struggle for the past billion years or so, the two bright galaxies are captured in this deep telescopic snapshot, constructed from 25 hours of image data. Their most recent close encounter likely resulted in the enhanced spiral arms of M81 (left) and violent star forming regions in M82 so energetic the galaxy glows in X-rays. After repeated passes, in a few billion years only one galaxy will remain. From our perspective, this cosmic moment is seen through a foreground veil of the Milky Way's stars and clouds of dust. Faintly reflecting the foreground starlight, the pervasive dust clouds are relatively unexplored galactic cirrus, or integrated flux nebulae, only a few hundred light-years above the plane of the Milky Way.
[http://apod.nasa.gov/apod/ap130925.html]
SN 2014J, a bright supernova, was observed in M82, in 2014:
Bright Supernova in M82
Astronomers really don’t find supernovae by looking for the arrows. But in this image taken January 23rd, an arrow does point to an exciting, new supernova, now cataloged as SN 2014J, in nearby bright galaxy M82. Located near the Big Dipper in planet Earth’s sky, M82 is also known as the Cigar Galaxy, a popular target for telescopes in the northern hemisphere. In fact, SN 2014J was first spotted as an unfamiliar source in the otherwise familiar galaxy by teaching fellow Steve Fossey and astronomy workshop students Ben Cooke, Tom Wright, Matthew Wilde and Guy Pollack at the University College London Observatory on the evening of January 21. M82 is a mere 12 million light-years away (so the supernova explosion did happen 12 million years ago, that light just now reaching Earth), making supernova SN 2014J one of the closest to be seen in recent decades. Spectra indicate it is a Type Ia supernova caused by the explosion of a white dwarf accreting matter from a companion star. By some estimates one week away from its maximum brightness, SN 2014J is already the brightest part of M82 and visible in small telescopes in the evening sky.
[http://apod.nasa.gov/apod/ap140124.html]
M101 (Pinwheel Galaxy) is a spectacular spiral galaxy in Ursa Major:
M101: The Pinwheel Galaxy
Why do many galaxies appear as spirals? A striking example is M101, shown above, whose relatively close distance of about 27 million light years allows it to be studied in some detail. Observational evidence indicates that a close gravitational interaction with a neighboring galaxy created waves of high mass and condensed gas which continue to orbit the galaxy center. These waves compress existing gas and cause star formation. One result is that M101, also called the Pinwheel Galaxy, has several extremely bright star-forming regions (called HII regions) spread across its spiral arms. M101 is so large that its immense gravity distorts smaller nearby galaxies.
[http://apod.nasa.gov/apod/ap150614.html]
The Owl Nebula (M97, NGC 3587) is a planetary nebula located approximately 2,030 light years away in the constellation Ursa Major. It was discovered by French astronomer Pierre Méchain on February 16, 1781. When William Parsons, 3rd Earl of Rosse, observed the nebula in 1848, his hand-drawn illustration resembled an owl’s head. It has been known as the Owl Nebula ever since:
Messier 97, Owl Nebula
The nebula is approximately 8,000 years old. It is approximately circular in cross-section with a little visible internal structure. It was formed from the outflow of material from the stellar wind of the central star as it evolved along the asymptotic giant branch. The nebula is arranged in three concentric shells, with the outermost shell being about 20-30% larger than the inner shell. The owl-like appearance of the nebula is the result of an inner shell that is not circularly symmetric, but instead forms a barrel-like structure aligned at an angle of 45° to the line of sight.
The nebula holds about 0.13 solar masses of matter, including hydrogen, helium, nitrogen, oxygen, and sulfur; all with a density of less than 100 particles per cubic centimeter. Its outer radius is around 0.91 ly (0.28 pc) and it is expanding with velocities in the range of 27-39 km/s into the surrounding interstellar medium.
The 14th magnitude central star has since reached the turning point of its evolution where it condenses to form a white dwarf. It has 55–60% of the Sun’s mass, 41-148 times the brightness of the Sun, and an effective temperature of 123,000 K. The star has been successfully resolved by the Spitzer Space Telescope as a point source that does not show the infrared excess characteristic of a circumstellar disk.
[https://en.wikipedia.org/wiki/Owl_Nebula]
NGC 2787 is a barred lenticular galaxy in Ursa Major:
NGC 2787: A barred lenticular galaxy
Lenticular galaxies aren’t supposed to be photogenic. Like spiral galaxies, they contain a disk, but like elliptical galaxies, they are usually short on dust, gas, and pretty spiral arms. Lenticulars are relatively little studied, possibly because of their seemingly benign nature. Famous galaxies historically classified as lenticular include M84, M85, and M86. Recent pictures and evidence, however, indicate that lenticulars can be both photogenic and scientifically interesting. For example, the above image of NGC 2787 taken with the Hubble Space Telescope shows that the center of this lenticular galaxy has interesting structure. The image was taken to help determine how lenticular galaxies formed, and what happens in their centers. The span of NGC 2787 in the above image is about 4500 light years, while the galaxy lies about 25 million light years away toward the constellation of Ursa Major.
[http://apod.nasa.gov/apod/ap020408.html]
NGC 4013 is an edge-on spiral galaxy in Ursa Major, accompanied by a stream of stars:
NGC 4013 and the tidal stream
Nearly 50 million light-years away in the constellation Ursa Major, NGC 4013 was long considered an isolated island universe. Seen edge-on, the gorgeous spiral galaxy was known for its flattened disk and central bulge of stars, cut by silhouetted dust lanes. But this deep color image of the region reveals a previously unknown feature associated with NGC 4013, an enormous, faint looping structure extending (above and toward the left) over 80 thousand light-years from the galaxy’s center. A detailed exploration of the remarkable structure reveals it to be a stream of stars originally belonging to another galaxy, likely a smaller galaxy torn apart by gravitational tides as it merged with the larger spiral. Astronomers argue that the newly discovered tidal stream also explains a warped distribution of neutral hydrogen gas seen in radio images of NGC 4013 and offers parallels to the formation of our own Milky Way galaxy.
[http://apod.nasa.gov/apod/ap080207.html]
I Zwicky 18 is a dwarf galaxy in Ursa Major, once thought to be one of the youngest galaxies:
I Zwicky 18: The Case of the Aging Galaxy
How old is this galaxy? The galaxy on the left, I Zwicky 18, was once thought to be one of the youngest galaxies on record since its bright stars indicated an age of only 500 million years. The galaxy was also intriguing because it resembled galaxies forming in the very early universe, but mysterious since it is so nearby- only 59 million light years away- and surrounded by galaxies that are significantly older. Recent images of I Zwicky 18 by the Hubble Space Telescope have helped resolve this mystery, discovering a population of old faint stars intermixed with the bright star population. Therefore I Zwicky 18 is now thought to be just as old as its neighbors, roughly 10 billion years old, but with an intense episode of relative new star formation. Possibly the trigger for this recent episode of bright star formation is the changing gravitational influence of I Zwicky 18’s smaller companion galaxy, visible at the upper right.
[http://apod.nasa.gov/apod/ap071017.html]
Arp 299 is a system where two galaxies are in the process of merging. Chandra data has revealed 25 bright point-like X-ray sources in Arp 299, 14 of which are categorized as ‘ULXs.’ These ULXs are likely binary systems where a black hole or neutron star is pulling material from a companion star. This is one of the highest numbers of ULXs in a galaxy in the nearby universe, caused by a high rate of star formation triggered by the merger:
Arp 299: Galactic Goulash
What would happen if you took two galaxies and mixed them together over millions of years? A new image including data from NASA’s Chandra X-ray Observatory reveals the cosmic culinary outcome.
Arp 299 is a system located about 140 million light years from Earth. It contains two galaxies that are merging, creating a partially blended mix of stars from each galaxy in the process.
However, this stellar mix is not the only ingredient. New data from Chandra reveals 25 bright X-ray sources sprinkled throughout the Arp 299 concoction. Fourteen of these sources are such strong emitters of X-rays that astronomers categorize them as ‘ultra-luminous X-ray sources,’ or ULXs.
These ULXs are found embedded in regions where stars are currently forming at a rapid rate. Most likely, the ULXs are binary systems where a neutron star or black hole is pulling matter away from a companion star that is much more massive than the Sun. These double star systems are called high-mass X-ray binaries.
This buffet of high-mass X-ray binaries is one of the richest in a galaxy located in the nearby universe, but Arp 299 contains relatively powerful star formation. This is due at least in part to the merger of the two galaxies, which has triggered waves of star formation. The formation of high-mass X-ray binaries is a natural consequence of such blossoming star birth as some of the young massive stars, which often form in pairs, evolve into these systems.
This new composite image of Arp 299 contains X-ray data from Chandra (pink), higher-energy X-ray data from NuSTAR (purple), and optical data from the Hubble Space Telescope (white and faint brown). Arp 299 also emits copious amounts of infrared light that has been detected by observatories such as NASA’s Spitzer Space Telescope, but those data are not included in this composite.
The infrared and X-ray emission of the galaxy is remarkably similar to that of galaxies found in the very distant Universe, offering an opportunity to study a relatively nearby analog of these distant objects. A higher rate of galaxy collisions occurred when the universe was young, but these objects are difficult to study directly because they are located at colossal distances.
The Chandra data also reveal diffuse X-ray emission from hot gas distributed throughout Arp 299. Scientists think the high rate of supernovas, another common trait of star-forming galaxies, has expelled much of this hot gas out of the center of the system.
[http://chandra.harvard.edu/photo/2017/arp299/index.html]
Supermassive black holes are typically stationary objects located at the centers of most galaxies. Under some circumstances, these black holes that contain millions or even billions of times the mass of the Sun can be set in motion. After searching through Chandra’s X-ray and optical data, astronomers found a new candidate of such a ‘recoiling’ black hole. This recoiling black hole candidate is located in an elliptical galaxy about 3.9 billion light years from Earth:
CXO J101527.2+625911: Astronomers Pursue Renegade Supermassive Black Hole
Supermassive black holes are generally stationary objects, sitting at the centers of most galaxies. However, using data from NASA’s Chandra X-ray Observatory and other telescopes, astronomers recently hunted down what could be a supermassive black hole that may be on the move.
This possible renegade black hole, which contains about 160 million times the mass of our Sun, is located in an elliptical galaxy about 3.9 billion light years from Earth. Astronomers are interested in these moving supermassive black holes because they may reveal more about the properties of these enigmatic objects.
This black hole may have ‘recoiled,’ in the terminology used by scientists, when two smaller supermassive black holes collided and merged to form an even larger one. At the same time, this collision would have generated gravitational waves that emitted more strongly in one direction than others. This newly formed black hole could have received a kick in the opposite direction of those stronger gravitational waves. This kick would have pushed the black hole out of the galaxy's center, as depicted in the artist's illustration.
The strength of the kick depends on the rate and direction of spin of the two smaller black holes before they merge. Therefore, information about these important but elusive properties can be obtained by studying the speed of recoiling black holes.
Astronomers found this recoiling black hole candidate by sifting through X-ray and optical data for thousands of galaxies. First, they used Chandra observations to select galaxies that contain a bright X-ray source and were observed as part of the Sloan Digital Sky Survey (SDSS). Bright X-ray emission is a common feature of supermassive black holes that are rapidly growing.
Next, the researchers looked to see if Hubble Space Telescope observations of these X-ray bright galaxies revealed two peaks near their center in the optical image. These two peaks might show that a pair of supermassive black holes is present or that a recoiling black hole has moved away from the cluster of stars in the center of the galaxy.
If those criteria were met, then the astronomers examined the SDSS spectra, which show how the amount of optical light varies with wavelength. If the researchers found telltale signatures in the spectra indicative of the presence of a supermassive black hole, they followed up with an even closer examination of those galaxies.
After all of this searching, a good candidate for a recoiling black hole was discovered. The left image in the inset is from the Hubble data, which shows two bright points near the middle of the galaxy. One of them is located at the center of the galaxy and the other is located about 3,000 light years away from the center. The latter source shows the properties of a growing supermassive black hole and its position matches that of a bright X-ray source detected with Chandra (right image in inset). Using data from the SDSS and the Keck telescope in Hawaii, the team determined that the growing black hole located near, but visibly offset from, the center of the galaxy has a velocity that is different from the galaxy. These properties suggest that this source may be a recoiling supermassive black hole.
The host galaxy of the possible recoiling black hole also shows some evidence of disturbance in its outer regions, which is an indication that a merger between two galaxies occurred in the relatively recent past. Since supermassive black hole mergers are thought to occur when their host galaxies merge, this information supports the idea of a recoiling black hole in the system.
Moreover, stars are forming at a high rate in the galaxy, at several hundred times the mass of the Sun per year. This agrees with computer simulations, which predict that star formation rates may be enhanced for merging galaxies particularly those containing recoiling black holes.
Another possible explanation for the data is that two supermassive black holes are located in the center of the galaxy but one of them is not producing detectable radiation because it is growing too slowly. The researchers favor the recoiling black hole explanation, but more data are needed to strengthen their case.
[http://chandra.harvard.edu/photo/2017/rsmbh/index.html]
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