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Sunday, June 11, 2017

Mensa


Mensa is a small constellation in the Southern Celestial Hemisphere between Scorpius and Centaurus, one of twelve drawn up in the 18th century by French astronomer Nicolas Louis de Lacaille. Its name is Latin for table, though it originally depicted Table Mountain and was known as Mons Mensae. It covers a keystone-shaped wedge of sky stretching from approximately 4h to 7.5h of right ascension, and −71 to −85.5 degrees of declination. Other than the south polar constellation of Octans, it is the most southerly of constellations. As a result, it is essentially unobservable from the Northern Hemisphere.

Mensa is bordered by Dorado to the north, Hydrus to the northwest and west, Octans to the south, Chamaeleon to the east and Volans to the northeast. Covering 153.5 square degrees and 0.372% of the night sky, it ranks 75th of the 88 constellations in size. In the equatorial coordinate system, the right ascension coordinates of the constellation lie between 03h 12m 55.9008s and 07h 36m 51.5289s, while the declination coordinates are between −69.75° and −85.26°. The whole constellation is visible to observers south of latitude 5°N.

Mensa, introduced by Lacaille under the name Mons Mensae, as illustrated on Chart XX of the Uranographia of Johann Bode (1801). Nubecula Major is the Large Magellanic Cloud, representing the cloud that caps the real Table Mountain.
[http://www.ianridpath.com/startales/mensa.htm]

Initially known as Mons Mensae, Mensa was created by Nicolas Louis de Lacaille out of dim Southern Hemisphere stars in honor of Table Mountain, a South African mountain overlooking Cape Town. He recalled that the Magellanic Clouds were sometimes known as Cape Clouds, and that Table Mountain was often covered in cloud when a southeasterly stormy wind blew. Hence he made a ‘table’ in the sky under the clouds. Lacaille had observed and catalogued 10,000 southern stars during a two-year stay at the Cape of Good Hope. He devised 14 new constellations in uncharted regions of the Southern Celestial Hemisphere not visible from Europe. Mensa was the only constellation that did not honor an instrument that symbolised the Age of Enlightenment. John Herschel proposed shrinking the name to one word in 1844, noting that Lacaille himself had abbreviated his constellations thus on occasion.

Although the stars of Mensa do not feature in any ancient mythology, the mountain it is named after has a rich mythology. Called ‘Tafelberg’ in Dutch and German, the mesa (Portuguese and Spanish for table) has two neighboring mountains called ‘Devil’s Peak’ and ‘Lion’s Head.’ Table Mountain features in the mythology of the Cape of Good Hope, notorious for its storms- the explorer Bartolomeu Dias saw the mesa as a mythical anvil for storms. Another myth relating to its dangers comes from Sinbad the Sailor, an Arabic folk hero who saw the mountain as a magnet pulling his ships to the bottom of the sea.

[http://www.dibonsmith.com/men_con.htm]

[http://www.southernskyphoto.com/constellations/south_celestial_pole.htm]

Lacaille labelled eleven stars with Bayer designations Alpha through to Lambda (excluding Kappa). Gould later added Kappa, Mu, Nu, Xi and Pi Mensae. Stars as dim as these were not generally given designations; however, Gould felt their closeness to the South Celestial Pole warranted naming. Overall, there are 22 stars within the constellation’s borders brighter than or equal to apparent magnitude 6.5

Alpha Mensae is the brightest star in the constellation. It has an apparent visual magnitude of 5.09 and is 33.1 light years distant from Earth. It is the dimmest of all lucidae (constellations’ brightest stars). The star is a yellow main sequence dwarf with the stellar classification of G5 V. It has roughly the same mass and radius as the Sun, and 83 percent of the Sun’s luminosity. It has a red dwarf companion at a separation of 3.05 arcseconds.

Gamma Mensae is the second brightest star in Mensa. It has an apparent visual magnitude of 5.18 and is approximately 101 light years distant from the Sun. Gamma Mensae is a double star with the stellar classifcation K4III, which means that the primary component in the system is an orange giant.

Beta Mensae is the third brightest star in the constellation. It has an apparent magnitude of 5.302 and is approximately 640 light years distant. It is a yellow giant star belonging to the stellar class G8III.

W Mensae is a yellow-white supergiant star with the stellar classification of F8Iabp. It is classified as an R Coronae Borealis type variable star. The star is located in the Large Magellanic Cloud galaxy, approximately 168,000 light years from earth. It has a maximum apparent magnitude of 13.8 and can only be seen in a larger telescope. R Coronae Borealis variables are a rare class of stars, often called ‘inverse novae’ because they exhibit sudden and dramatic drops in brightness. When at minumum, W Mensae has a visual magnitude of only 18.3. The star’s variability was discovered by the Dutch-American astronomer W. J. Luyten in 1927.

Pi Mensae is a yellow subgiant belonging to the stellar class G1IV. It has a visual magnitude of 5.67 and is 59.7 light years distant from Earth. The star has a high proper motion. The star has 1.1 times the Sun’s mass, 2.1 times the radius, and is 4.73 times more luminous than the Sun. It is one of the top 100 target stars for the planned Terrestrial Planet Finder mission.

An extrasolar planet was discovered orbiting the star in October 2001. It is a massive superjovian planet, with a mass at least 10.27 times that of Jupiter, which makes it one of the most massive planets known. It orbits the star every 2,151 days (5.89 years). It has an eccentric orbit and passes through the habitable zone over the course of its orbit, so if there were any Earth-like planets there, the planet would likely have disrupted their orbits and maybe even thrown them into the star or out of the system, into the interstellar medium.
[http://www.constellation-guide.com/constellation-list/mensa-constellation/]

The first images taken by the Chandra X-Ray Observatory were of PKS 0637-752, a quasar in Mensa with a large gas jet visible in both optical and x-ray wavelengths:

[http://chandra.harvard.edu/photo/1999/0064/]

PKS 0637-72 is so distant that we see it as it was 6 billion years ago. It is a luminous quasar that radiates with the power of 10 trillion suns from a region smaller than our solar system. The source of this prodigious energy is believed to be a supermassive black hole.

Radio observations of PKS 0637-752 show that it has an extended radio jet that stretches across several hundred thousand light years. Chandra’s x-ray image reveals a powerful x-ray jet of similar size that is probably due to a beam of extremely high-energy particles.

The x-ray jet observed for the first time by Chandra in PKS 0637-752, is a dramatic example of a cosmic jet. It has blasted outward from the quasar into intergalactic space for a distance of at least 200,000 light years! The jet’s presence means that electromagnetic forces are continually accelerating electrons to extremely high energies over enormous distances. Chandra observations, combined with radio observations, should provide insight into this important cosmic energy conversion process.
[http://www.nasa.gov/centers/marshall/news/background/facts/PKS_0637-752_Fact_Sheet.html]

DEM L50 is a super-bubble found in the constellation Mensa:

DEM L50: Stellar effervescence on display

This composite image shows the superbubble DEM L50 (a.k.a. N186) located in the Large Magellanic Cloud about 160,000 light years from Earth. Superbubbles are found in regions where massive stars have formed in the last few million years. The massive stars produce intense radiation, expel matter at high speeds, and race through their evolution to explode as supernovas. The winds and supernova shock waves carve out huge cavities called superbubbles in the surrounding gas.

X-rays from NASA’s Chandra X-ray Observatory are shown in pink and optical data from the Magellanic Cloud Emission Line Survey (MCELS) are colored in red, green and blue. The MCELS data were obtained with the University of Michigan’s 0.9-meter Curtis Schmidt telescope at Cerro Tololo Inter-American Observatory (CTIO). The shape of DEM L50 is approximately an ellipse, with a supernova remnant named SNR N186 D located on its northern edge.

Like another superbubble in the LMC, N44 , DEM L50 gives off about 20 times more X-rays than expected from standard models for the evolution of superbubbles. A Chandra study published in 2011 showed that there are two extra sources of the bright X-ray emission: supernova shock waves striking the walls of the cavities, and hot material evaporating from the cavity walls.
[http://chandra.harvard.edu/photo/2013/deml50/index.html]

Mensa also contains part of the Large Magellanic Cloud (the rest being in Dorado).

[https://en.wikipedia.org/wiki/Mensa_%28constellation%29]






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