Four of Norma's brighter stars—Gamma, Delta, Epsilon and Eta—make up a square in the field of faint stars.Gamma2 Normae is the brightest star with anapparent magnitude of 4.0.Mu Normae is one of the mostluminous stars known, with a luminosity between a quarter million and one million times that of the Sun. Four star systems are known to harbour planets. TheMilky Way, particularly theNorma Arm of the galaxy, passes through Norma, and the constellation contains eightopen clusters visible to observers with binoculars. The constellation also hosts Abell 3627, also called theNorma Cluster, one of the most massivegalaxy clusters known.
Norma was introduced in 1751–52 byNicolas-Louis de Lacaille with the French namel’Équerre et la Règle, "the Square and Rule",[2][3] after he had observed and catalogued 10,000 southern stars during a two-year stay at theCape of Good Hope. He devised 14 new constellations in uncharted regions of theSouthern Celestial Hemisphere not visible from Europe. All but one honoured instruments that symbolised theAge of Enlightenment.[4][a] Lacaille portrayed the constellations of Norma,Circinus andTriangulum Australe, respectively, as a set square and ruler, a compass, and a surveyor's level in a set of draughtsman instruments, in his 1756 map of the southern stars.[5] The level was dangling from the apex of a triangle, leading some astronomers to conclude he was renamingl’Équerre et la Règle to "le Niveau", "the level".[6] In any case, the constellation's name had been shortened and Latinised by Lacaille toNorma by 1763.[2]
Norma is bordered byScorpius to the north,Lupus to the northwest, Circinus to the west, Triangulum Australe to the south and Ara to the east. Covering 165.3 square degrees and 0.401% of the night sky, it ranks 74th of the 88 constellations in size.[7] The three-letter abbreviation for the constellation, as adopted by theInternational Astronomical Union in 1922, is "Nor".[8] The official constellation boundaries, as set by Belgian astronomerEugène Delporte in 1930, are defined by a polygon of ten segments. In theequatorial coordinate system, theright ascension coordinates of these borders lie between15h 12m 13.6119s and16h 36m 08.3235s, while thedeclination coordinates are between −42.27° and −60.44°.[1] The whole constellation is visible to observers south of latitude29°N.[b]
Lacaille charted and designated ten stars with theBayer designations Alpha through to Mu in 1756, however his Alpha Normae was transferred into Scorpius and left unnamed byFrancis Baily, before being namedN Scorpii byBenjamin Apthorp Gould, who felt its brightness warranted recognition. Though Beta Normae was depicted on his star chart, it was inadvertently left out of Lacaille's 1763 catalogue, was likewise transferred to Scorpio by Baily and namedH Scorpii by Gould.[9] Norma's brightest star,Gamma2 Normae, is only ofmagnitude 4.0. Overall, there are 44 stars within the constellation's borders brighter than or equal toapparent magnitude 6.5.[c][7]
The constellation Norma as it can be seen by the naked eye
The four main stars—Gamma, Delta, Epsilon and Eta—make up a square in this region of faint stars.[11]Gamma1 and Gamma2 Normae are anoptical double, and not a truebinary star system. Located 129 ± 1light-years away from Earth,[12] Gamma2 Normae is a yellow giant ofspectral type G8III around 2 to 2.5 times as massive as the Sun. It has swollen to a diameter 10 times that of the Sun and shines with 45 times the Sun's luminosity. It also is half of a close optical double, with a magnitude 10 companion star related by line of sight only.[13] Gamma1 Normae is a yellow-white supergiant, located much further away at around 1500 light-years from Earth.[12]Epsilon Normae is aspectroscopic binary, with twoblue-white main sequence stars of almost equal mass and spectral type (B3V) orbiting each other every 3.26 days. There is a third star separated by 22 arcseconds, which has a magnitude of 7.5 and is likely a smaller B-type main sequence star of spectral type B9V.[14] The system is 530 ± 20 light-years distant from Earth,[12]Eta Normae is a yellow giant of spectral type G8III with an apparent magnitude of 4.65.[15] It shines with aluminosity approximately 66 times that of the Sun.[16]
Iota1 Normae is a multiple star system. The AB (mag 5.2 and 5.76) pair orbit each other with a period of 26.9 years; they are 2.77 and 2.71 times as massive as the Sun respectively.[17] The pair are 128 ± 6 light-years distant from Earth.[12] A third component is a yellow main sequence star of spectral type G8V with an apparent magnitude of 8.02.[17]
Mu Normae is a remote blue supergiant of spectral type O9.7Iab,[18] one of the most luminous stars known, but is partially obscured by distance andcosmic dust. Uncertainties regarding its distance leave open the possibility that Mu Normae could be between 250,000 and one million times as luminous and up to 60 times as massive as the Sun, though it is more likely to have around 500,000 times the Sun's luminosity and 40 times its mass.[19] It is suspected of being anAlpha Cygni variable, with a magnitude range of 4.87–4.98.QU Normae is another hot blue-white star that is a variable, ranging from magnitude 5.27 to 5.41 over 4.8 days.[20] Lying nearEta Normae isR Normae,[21] aMira variable. Its visual magnitude range is 6.5–13.9 and its average period is 507.5 days. Located halfway between Eta Normae andGamma Circini isT Normae, another Mira variable.[21] It ranges from magnitude 6.2 to 13.6, with a period of 244 days.[22]S Normae is a well-knownCepheid variable with a magnitude range of 6.12–6.77 and a period of 9.75411 days.[23] It is located at the centre of theopen clusterNGC 6087. It is a yellow-white supergiant of spectral type F8-G0Ib that is 6.3 times as massive as the Sun. A binary, it has a 2.4 solar mass (M☉) companion that is a blue-white main sequence star of spectral type B9.5V.[24] A binary system composed of two wolf-rayet stars, colloquially calledApep, has been identified as a possible progenitor of a long gamma-ray burst. Located around 8000 light-years distant, it would be the first such in the Milky Way.[25]
IM Normae is one of only tenrecurrent novae known in theMilky Way. It has erupted in 1920 and 2002, reaching magnitude 8.5 from a baseline of 18.3. It was poorly monitored after the first eruption, so it is possible that it erupted in between.[26] Norma hosts two faintR Coronae Borealis variable stars of magnitude 10—RT Normae andRZ Normae—rare degenerate stars thought to have formed from the merger of twowhite dwarfs that fade by several magnitudes periodically as they eject large amounts of carbon dust.[27] A faint object of magnitude 16,QV Normae is a high massX-ray binary star system 15,000–20,000 light-years distant from Earth. It is composed of aneutron star orbiting a blue-white supergiant approximately 20 times as massive as the Sun. Thestellar wind from the more massive star is drawn to the magnetic poles of the neutron star, forming an accretion column and producing X-rays.[28] Located 19,000 light-years away,[29]QX Normae is an activelow mass X ray binary composed of a neutron star and its companion star that is smaller and cooler than the Sun.[30] The neutron star is 1.74 ± 0.14 times as massive as the Sun, yet its radius is a mere 9.3 ± 1.0 km.[29]1E161348-5055 is a neutron star found in the centre ofRCW103supernova remnant. A periodicX-ray source with a period of 6.67 hours, it is approximately 2000 years old and 10,000 light-years away from Earth. It is unusual in that it is spinning much too slowly for its young age, behaving instead like a multi-million-year-old star.[31]SGR J1550-5418 is asoft gamma repeater (SGR)—amagnetar that is emitting gamma ray flares,[32] located some 30,000 light-years distant from Earth. The rotation period, of approximately 2.07 seconds, is the fastest yet observed for a magnetar.[33]XTE J1550-564 is another X-ray binary, this time composed of a large black hole around 10 times as massive as the Sun and a cool orange donor star. The black hole is amicroquasar, firing offjets of material most likely from its accretion disk.[34]
Four star systems are known to harbour planets.HD 330075 is a sunlike star around 164 light-years distant that is orbited by ahot Jupiter every 3.4 days. Announced in 2004, it was the first planet discovered by theHARPSspectrograph.[35]HD 148156 is a star 168 ± 7 light-years distant. Slightly larger and hotter than the Sun, it was found to have a roughly Jupiter-size planet with an orbital period of 2.8 years.[36]HD 143361 is a binary star system composed of a sunlike star and a faint red dwarf separated by 30.9 AU. A planet roughly triple the mass of Jupiter orbits the brighter star every 1057± 20 days.[37]HD 142415 is approximately 113 light-years distant and has a Jupiter-sized planet with an orbital period of around 386 days.[38]
The Ant Nebula, Mz 3, viewed with the Hubble Space Telescope
Due to its location on the Milky Way, this constellation contains manydeep-sky objects such asstar clusters,[21] including eightopen clusters visible through binoculars.[39]NGC 6087 is the brightest of theopen clusters in Norma with a magnitude of 5.4. It lies in the southeastern corner of the constellation betweenAlpha Centauri andZeta Arae. Thought to be around 100 million years old, it is about 3300 light-years away and is around 14 light-years in diameter. Its brightest member is theCepheid variableS Normae.[39] A rich background star field makes it less distinct, though around 36 member stars are visible though a 10 cm telescope at 150x magnification.[40] Located 0.4° north ofKappa Normae isNGC 6067, which has anintegrated magnitude of 5.6 though it is indistinct as it lies in a rich star field.[40] It is thought to be around 102 million years old, and contain 891 solar masses.[41] Two Cepheid variables—QZ Normae andV340 Normae—have been identified as members of the cluster.[42] Fainter open clusters includeNGC 6134 with a combined magnitude of 7.2 and located 4000 light-years away from Earth, the spread-outNGC 6167 of magnitude 6.7,NGC 6115 near Gamma Normae,NGC 6031 andNGC 5999.[11]
Galaxies of theNorma Cluster (yellow) in a 0.5° x 0.5° field
Located around 4900 light-years distant isShapley 1 (or PK 329+02.1), aplanetary nebula better known as theFine-Ring Nebula. Appearing ring-shaped, it is thought that it actually is cylindrical and oriented directly at Earth. Around 8700 years old,[43] it lies about five degrees west-northwest of Gamma1 Normae. Its integrated magnitude is 13.6 and its meansurface brightness is 13.9. The central star is awhite dwarf of magnitude 14.03.Mz 1 is abipolar planetary nebula, thought to be an hourglass shape tilted at an angle to observers on Earth, some 3500 light-years distant.[44]Mz 3—known as the Ant Nebula as it resembles an ant—has a complex appearance, with at least four outflow jets and two large lobes visible.[45]
The relatively weak meteor showerGamma Normids (GNO), which is typically active from March 7 to 23, peaking on March 15, has itsradiant nearGamma2 Normae.[47]
^While parts of the constellation technically rise above the horizon to observers between 29°N and48°N, stars within a few degrees of the horizon are to all intents and purposes unobservable.[7]
^Objects of magnitude 6.5 are among the faintest visible to the unaided eye in suburban-rural transition night skies.[10]
^abStreicher, Magda (2005). "Deepsky Delights: Sparkling clusters in Norma".Monthly Notes of the Astronomical Society of Southern Africa.64 (5–6):107–09.Bibcode:2005MNSSA..64..107S.
^Kaler, James B. (30 July 2010)."Mu Normae".Stars. University of Illinois. Retrieved19 May 2015.
^Otero, Sebastian Alberto (24 May 2012)."QU Normae".The International Variable Star Index. American Association of Variable Star Observers. Retrieved12 April 2015.
^Otero, Sebastian Alberto (19 March 2011)."T Normae".The International Variable Star Index. American Association of Variable Star Observers. Retrieved21 March 2015.
^Watson, Christopher (4 January 2010)."S Normae".The International Variable Star Index. American Association of Variable Star Observers. Retrieved12 March 2014.
^Tisserand; Clayton; Welch; Pilecki; Wyrzykowski; Kilkenny (2012). "The Ongoing Pursuit of R Coronae Borealis Stars: ASAS-3 Survey Strikes Again".arXiv:1211.2475v2 [astro-ph.SR].
^Hemphill, Paul B.; Rothschild, Richard E.; Markowitz, Alex; Fürst, Felix; Pottschmidt, Katja; Wilms, Jörn (2014). "A Clumpy Stellar Wind and Luminosity-dependent Cyclotron Line Revealed by the First Suzaku Observation of the High-mass X-Ray Binary 4U 1538-522".The Astrophysical Journal.792 (1): 15.arXiv:1407.1028.Bibcode:2014ApJ...792...14H.doi:10.1088/0004-637X/792/1/14.S2CID28693293.
^Majaess, D.; Sturch, L.; Moni Bidin, C.; Soto, M.; Gieren, W.; Cohen, R.; Mauro, F.; Geisler, D.; Bonatto, C.; Borissova, J.; Minniti, D.; Turner, D.; Lane, D.; Madore, B.; Carraro, G.; Berdnikov, L. (August 2013). "Anchors for the Cosmic Distance Scale: the Cepheid QZ Normae in the Open Cluster NGC 6067".Astrophysics and Space Science.347 (1):61–70.arXiv:1305.3914.Bibcode:2013Ap&SS.347...61M.doi:10.1007/s10509-013-1495-1.S2CID119207107.
^Monteiro, Hektor; Schwarz, Hugo E.; Gruenwald, Ruth; Guenthner, Katherine; Heathcote, Steve R. (2005). "Three-Dimensional Photoionization Structure and Distances of Planetary Nebulae. II. Menzel 1".The Astrophysical Journal.620 (1):321–29.arXiv:astro-ph/0410496.Bibcode:2005ApJ...620..321M.doi:10.1086/426809.S2CID119442827.
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