Sterrekunde

Hoeveel sterre is daar in 'n bolvormige groep van 10 ^ 5 sonmassas?

Hoeveel sterre is daar in 'n bolvormige groep van 10 ^ 5 sonmassas?


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Ek het my afgevra of daar 'n maklike manier is om die aantal sterre in 'n bolvormige groep (GC) met 10 ^ 5 sonmassas te benader.

Kan 'n mens byvoorbeeld net aanneem dat die GC uit sonagtige sterre bestaan ​​en dus 10 ^ 5 sterre het? Of is dit te eenvoudig?

Dankie vir u antwoorde!


Die gemiddelde massa van die aanvanklike massafunksie is ~ 0,3 sonmassas (Maschberger 2013). 'N GC met 10 ^ 5 sonmassas sal dus ~ 3 x 10 ^ 5 sterre bevat. Aangesien die massiefste sterre reeds in 'n GC gesterf het, sal die gemiddelde massa van die hedendaagse massaverspreiding van die GC effens laer wees, wat 'n effens groter aantal sterre impliseer, maar dit sal 'n relatiewe klein regstelling wees (beslis nie meer as 'n faktor van 3).


Prikkelende getuienis: word die naaste sterretros aan die son vernietig?

Die Hyades-sterreswerm smelt geleidelik saam met die agtergrond van sterre in die Melkweg. Die tros is 153 ligjare weg geleë en is sigbaar vir die blote oog omdat die helderste lede 'n 'V'-vorm van sterre in die sterrebeeld Taurus, die Bul, vorm. Hierdie beeld wys die lede van die Hyades soos geïdentifiseer in die Gaia-gegewens. Die sterre is pienk gemerk en die vorms van die verskillende konstellasies word in groen opgespoor. Sterre van die Hyades kan gesien word wat uit die sentrale groep strek en twee 'sterte' vorm. Hierdie sterte staan ​​bekend as getysterte en dit is hierdeur dat sterre die groep verlaat. Die prent is met behulp van Gaia Sky geskep. Krediet: ESA / Gaia / DPAC, CC BY-SA 3.0 IGO-erkenning: S. Jordan / T. Sagrista.

Gegewens van die ESA se Gaia-ster-kaartsatelliet het ontstellende bewyse getoon dat die naaste sterreswerm aan die son onderbreek word deur die swaartekrag-invloed van 'n massiewe, maar onsigbare struktuur in ons sterrestelsel.

As dit waar is, kan dit bewyse lewer vir 'n vermeende bevolking van 'donker materie-subhalo's'. Hierdie onsigbare wolke van deeltjies is vermoedelik oorblyfsels van die vorming van die Melkweg en is nou versprei oor die sterrestelsel en vorm 'n onsigbare onderbou wat 'n merkbare swaartekrag-invloed uitoefen op enigiets wat te naby dryf.

ESA-navorsingsgenoot Tereza Jerabkova en kollegas van ESA en die Europese Suider-sterrewag het die ontdekking gedoen terwyl hulle bestudeer hoe 'n nabygeleë sterretros saamsmelt in die algemene agtergrond van sterre in ons sterrestelsel. Hierdie ontdekking was gebaseer op Gaia se vroeë derde vrystelling (EDR3) en data van die tweede vrystelling.

Die ware omvang van die Hyades-getysterte is vir die eerste keer onthul deur data van die ESA se Gaia-missie. Met die Gaia-data kan die voormalige lede van die sterretros (in pienk vertoon) deur die hele lug opgespoor word. Daardie sterre is pienk gemerk en die vorms van die verskillende konstellasies word in groen opgespoor. Die prent is met behulp van Gaia Sky geskep. Krediet: ESA / Gaia / DPAC, CC BY-SA 3.0 IGO-erkenning: S. Jordan / T. Sagrista

Die span het die Hyades as teiken gekies omdat dit die naaste sterreswerm aan die Son is. Dit is net meer as 153 ligjaar verder geleë, en is maklik sigbaar vir hemelkykers in die noordelike en suidelike halfrond as 'n opvallende 'V'-vorm van helder sterre wat die kop van die bul in die sterrebeeld Taurus aandui. Behalwe die helder sigbare helder sterre, tel teleskope 'n honderd flouer sterre in 'n sferiese gebied van die ruimte, ongeveer 60 ligjaar.

'N Sterkluster sal natuurlik sterre verloor, want as die sterre binne die groep beweeg, trek hulle swaartekrag aan mekaar. Hierdie konstante ruk verander die sterre se snelhede effens en beweeg dit na die rande van die tros. Van daar af kan die sterre deur die swaartekrag van die sterrestelsel uitgevee word, wat twee lang sterte vorm.

Die een stert volg die sterretros, die ander trek voor dit uit. Hulle staan ​​bekend as getysterte en is wyd bestudeer in botsende sterrestelsels, maar niemand het hulle nog tot onlangs gesien uit 'n nabygeleë oop sterrehoop nie.

Die sleutel tot die opsporing van getysterte is om te sien watter sterre in die lug op 'n soortgelyke manier as die sterretros beweeg. Gaia maak dit maklik omdat dit die afstand en beweging van meer as 'n miljard sterre in ons sterrestelsel meet. "Dit is die twee belangrikste hoeveelhede wat ons nodig het om getysterte van sterretrosse in die Melkweg te soek," sê Tereza.

Vorige pogings van ander spanne het slegs beperkte sukses behaal omdat die navorsers slegs na sterre gesoek het wat ooreenstem met die beweging van die sterregroep. Dit het lede uitgesluit wat vroeër in die 600-700 miljoen jaar geskiedenis vertrek het en dus nou op verskillende bane reis.

Om die omvang van die wentelbane te begryp, het Tereza 'n rekenaarmodel gebou wat die verskillende versteurings sal simuleer wat ontsnapende sterre in die groep kan voel gedurende hul honderde miljoene jare in die ruimte. Dit was nadat die kode uitgevoer is en die simulasies met die regte data vergelyk is, dat die ware omvang van die Hyades-getysterte aan die lig gekom het. Tereza en kollegas het duisende voormalige lede in die Gaia-data gevind. Hierdie sterre strek nou duisende ligjare oor die sterrestelsel in twee enorme getysterte.

Maar die ware verrassing was dat dit lyk of die sleepstert ontbreek. Dit dui aan dat iets baie brutaler plaasvind as die sterretros wat sagkens 'oplos'.

Tereza het die simulasies weer uitgevoer en getoon dat die data weergegee kan word as die stert met 'n wolk stof bots wat ongeveer 10 miljoen sonmassas bevat. "Daar moes 'n noue interaksie met hierdie baie massiewe klomp gewees het, en die Hyades is net verpletter," sê sy.

Maar wat kan daardie klomp wees? Daar is geen waarnemings van 'n gaswolk of sterretros wat in die omgewing groot is nie. As geen sigbare struktuur opgespoor word nie, selfs in toekomstige, gerigte soektogte, stel Tereza voor dat die voorwerp 'n subhalo van die donker stof kan wees. Dit is natuurlik klompe donker materie wat voorkom dat dit die sterrestelsel help vorm tydens die vorming daarvan. Hierdie nuwe werk wys hoe Gaia sterrekundiges help om hierdie onsigbare raamwerk van die donker materie in die sterrestelsel te karteer.

'Met Gaia het die manier waarop ons die Melkweg sien heeltemal verander. En met hierdie ontdekkings sal ons die Melkweg se substrukture baie beter as ooit tevore kan karteer, ”sê Tereza. En nadat hy die tegniek met die Hyades bewys het, brei Tereza en kollegas nou die werk uit deur op soek na getysterte van ander sterre trosse.

Verwysing: & # 8220Die 800 stuks lang getysterte van die Hyades-sterreswerm: Moontlike ontdekking van kandidaat-episikliese oormatighede uit 'n oop sterrehoop & # 8221 deur Tereza Jerabkova, Henri MJ Boffin, Giacomo Beccari, Guido de Marchi, Jos HJ de Bruijne en Timo Prusti, 24 Maart 2021, Sterrekunde en astrofisika.
DOI: 10.1051 / 0004-6361 / 202039949


Kapteyn's Star

Kapteyn's Star is 'n baie dowwe rooi onderdwerg, ietwat blouer en
dowwer as Gliese 623 A (M2.5V) en B (M5.8Ve) regs onder.
(Sien 'n 2MASS-opname van Kapteyn's van die NASA Star
en Exoplanet-databasis.)

Op 3 Junie 2014 meld 'n span sterrekundiges die ontdekking van twee super-Aarde wat om hierdie antieke ster wentel. Kapteyn b kan vloeibare water op die oppervlak dra, alhoewel dit minstens 4,8 aardmassas het en sy baan binne slegs 48,6 dae voltooi op 'n gemiddelde baanafstand van 0,17, met 'n orbitale eksentrisiteit van 0,21. Kapteyn c is selfs meer massief teen 'n minimum van 7,0 Aardmassas en sy jaar duur 121,5 dae op 'n gemiddelde baanafstand van 0,31, met 'n orbitale eksentrisiteit van 0,23, en moet te koud wees om vloeibare water te dra. (GOS-nuusverklaring en Anglada-Escud et al, 2014).

Victor Robles, James Bullock, Miguel Rocha,
Joel Primack, UC Irvine, UC Santa Cruz

Twee super-Aarde is rondom opgespoor
Kapteyn's Star ('n weesster wat geskeur word uit 'n
antieke dwerg-satellietstelsel van die Milky
Way), een binne sy bewoonbare sone (meer).

Hierdie ster is slegs ongeveer 12,8 ligjare (ly) van ons Son, Sol, in die deel (05: 11: 40.58-45: 01: 06.27, ICRS 2000.0) van Constellation Pictor, die skilderskild - noordwes van Beta, geleë Pictoris. Omdat hy baie kleiner en koeler is as Sol, is Kapteyn's Star egter nie met die blote oog sigbaar nie. Dit het die tweede hoogste bekende bewegingsbeweging na Barnard's Star en beweeg na die suidooste teen 8,7 "per jaar. Op grond van sy eksentrieke, retrograde galaktiese baan, kan die ster gebore word as 'n bolvormige tros soortgelyk aan Omega Centauri (Wylie-de Boer) et al, 2009 Ken Croswell, New Scientist, 12 November 2009 Ken Croswell, Astronomy, 2005).

Skynbare mosie van Kapteyn's Star verby
drie jaar.

Volgens Ken Croswell is die uiters hoë beweging van hierdie ster in 1897 ontdek deur Jacobus Cornelius Kapteyn (1851-1922) van die Universiteit van Groningen en deur Robert Thorburn Ayton Innes (1861-1933) van Edinburgh, Skotland. Innes het vanaf die Kaap die Goeie Hoop in Suid-Afrika waargeneem, waarskynlik met die 7-duim-vuurvastes by die Royal Observatory. Kapteyn, wat nie 'n teleskoop gehad het nie, was vrywillig om fotografiese plate te meet wat deur David Gill (1843-1914) geneem is, ook aan die Kaapse sterrewag. Gevolglik het Kapteyn die Cape Photographic Durchmusterung of CP (D) ontwikkel, 'n katalogus van 454.875 suidelike sterre en waardes vir die digtheid van sterre in die ruimte as 'n funksie van afstand, helderheid en spektrale klas. Daarbenewens het sy ontdekking van 'ster-streaming' gelei tot die konsep van galaktiese rotasie. Dit lyk asof die ster in die twintigerjare na sy dood hernoem is tot Kapteyn, moontlik omdat hy die eerste was wat opgemerk het dat die dowwe ster in 'n ou katalogus as 'Cordoba Zone 5 uur 243' gelys het en op een van Gill se fotografiese plate ontbreek. totdat Innes 'n ster gevind het wat ooreenstem met die beskrywing daarvan oos van sy oorspronklike posisie. (Meer oor Kapteyn).

Sterrekundiges het Kapteyn's Star geïdentifiseer
as 'n "Tier 1" teiken vir NASA's optiese
SIM-sending, nou onbepaald vertraag.

As gevolg van Kapteyn se nabyheid aan Sol en sy afgeleide antieke oorsprong van buite die galaktiese skyf, is die stelsel 'n voorwerp van groot belang by sterrekundiges. Die ster is gekies as 'Tier 1' teikensterre vir NASA se optiese Space Interferometry Mission (SIM). Die missie sal probeer om planete so klein as drie aardmassas binne twee AU's van elke ster op te spoor. Alhoewel sommige opsommingstelselinligting en beelde van Kapteyn's Star moontlik nog by die SIM-spanne beskikbaar is, het die SIM-projekbestuurder op 8 November 2010 aangekondig dat die missie onbepaald uitgestel is weens die onttrekking van NASA-finansiering.

Orbitaal
Afstand
(a = AU's)
Orbitaal
Tydperk
(P = dae)
Orbitaal
Eksentrisiteit
(e)
Orbitaal
Neiging
(i = grade)

Massa
(Aarde)

Deursnee
(Aarde)

Digtheid
(Aarde)
Oppervlak
Swaartekrag
(Aarde)

Metallisiteit
(Sonkrag)
Kapteyn's Star0.0. . . 96,600-129,90031.7-34.9. . 0.074
Innerlike H.Z. Rand?0.12629.80.0?. . . . .
Planeet "b"0.16848.60.21?=>4.8. . . .
Buitenste H.Z. Rand?0.23676.50.0?. . . . .
Planeet "c"0.311121.50.23?=>7.0. . . .

Kapteyn's Star is 'n dowwe rooi subdwerg of hoofreeks (sdM0-1.5 of V), halo-ster (John E. Gizis, 1997, bladsy 809 en NASA Star and Exoplanet Database), wat vermoedelik oorspronklik 'n lid van die Melkweg is. sterrestelsel se stralende stralekrans. Die ster het ongeveer 7,4 persent van Sol se ystervloed en 'n geweegde gemiddelde metaalvloed van elemente wat swaarder is as waterstof en helium van byna 10,5 persent van Sol's (Woolf en Wallerstein, 2004). As gevolg van die skaarste aan swaarder elemente, lyk die ster 'n bietjie meer blouerig as 'n hoofreeks rooi dwerg van klas M. Dit kan ongeveer 29 tot 39 persent van Sol se massa hê (Ken Croswell, 2005 en RECONS), ongeveer 'n derde (29 tot 32 persent) van sy deursnee, en minder as 4/1 000ste van sy helderheid. Dit is 'n veranderlike ster wat VZ Pictoris genoem word. Sommige nuttige sterrekatalogusnommers en -benamings vir hierdie ster is: VZ Pic, Gl 191, Hip 24186, HD 33793, CD-45 1841, CP (D) -44 612, SAO 217223, LHS 29, LTT 2200, LFT 395, GC 6369, U 628 en Cordova Zone 5 uur 243.


Kapteyn's Star is groter as
Alpha Centauri C (Proxima)
maar aansienlik kleiner
as Sol (meer van ESO).

Volgens 'n 2014-analise is die binnekant van Kapteyn se bewoonbare sone relatief naby die ster geleë op ongeveer 0,126 AE van die ster, terwyl die buitenste rand verder uit lê teen ongeveer 0,236 AE (Anglada-Escud et al, 2014, tabel) 2 op bladsy 4). Rekening hou met die relatief groter infrarooi-uitset van M-sterre soos Kapteyn's Star, die afstand vanaf Kapteyn's waar 'n aardagtige planeet vloeibare water op sy oppervlak sou hê, sentreer ongeveer 0.158 AU - ver binne die baanafstand van Mercurius in die son Stelsel. As ons aanneem dat Kapteyn s'n 39 persent van Sol se massa het, sal so 'n planeet sy baan om die ster binne minder as 37 dae voltooi teen 0.158 AE - minder as ses weke.

'N Super-Aarde, Kapteyn b, kan vloeibare water op die oppervlak dra. Dit het minstens 4,8 aardmassas en voltooi sy baan binne slegs 48,6 dae op 'n gemiddelde baanafstand van 0,17, met 'n wentel-eksentrisiteit van 0,21. (GOS-nuusverklaring en Anglada-Escud et al, 2014).

'N Nog groter super-Aarde, Kapteyn c, is massiewer met 'n minimum van 7,0 aardmassas. Sy jaar duur 121,5 dae op 'n gemiddelde baanafstand van 0,31, met 'n orbitale eksentrisiteit van 0,23. Die planeet is egter ver genoeg geleë vanaf die begin van sy dowwe gasheer dat dit te koud moet wees om vloeibare water te kan dra. (GOS-nuusverklaring en Anglada-Escud et al, 2014).

'N Dwerg en halo ster? - Subdwerge, soos die nabygeleë Kapteyn's Star, is dowwer en blouer as jonger hoofreeksdwergsterre (Wing et al, 1976). Hulle het 'n laer "metaal" -inhoud van elemente wat swaarder is as helium. Hierdie lae metallisiteit word vermoedelik te wyte aan hul geboorte in 'n vroeëre ouderdom (of streek) van die sterrestelsel toe relatief min supernovas hul metale nog in die omliggende stofwolke gespuie het (John E. Gizis, 1997). Sterrekundiges het teen 1998 15 subdwerge in die woonbuurt Solar gevind (Fuchs en Jahrei , 1998).

Die meeste sterre in die sentrale bult en in die bolvormige trosse van die galaktiese stralekrans is ou, lae metale sterre, en halo sterre is slegs 0,1 tot 0,2 persent van die sterre naby Sol (Ken Croswell, 1995, pp. 62-63). ). Kapteyn's Star is die naaste bekende halo-ster, 'n lid van 'n plaaslike groep sterretjies genoem die "Kapteyn's star group" of sterre "bewegende groep" wat deel kan uitmaak van 'n plaaslike superkluster van halo-sterre versprei oor 2000 tot 3000 parsec (6.500 tot 9 800 ligjaar) in die Melkweg se galaktiese stralekrans (Olin Jeuck Eggen, 1996).

Halo-sterre is ietwat sferies rondom die galaktiese kern versprei, maar die meeste lede van die stralekrans lê ver bo of ver onder die galaktiese vlak. Met uiters elliptiese galaktiese wentelbane kan hulle so ver as 100 000 ligjaar van die galaktiese middelpunt af beweeg en so 'n paar duisend ly. Met inbegrip van die sterre van die verre bolvormige trosse, tel die sterretjies van die sterrestelsel een van die oudste sterrestelsels, waarvan hulle vermoedelik meestal 10 miljard jaar en ouer is. Terwyl halo-sterre slegs baie swak gekonsentreer is na die galaktiese vlak, vertoon hulle 'n sterk konsentrasie na en insluitend die galaktiese kern, maar met baie eksentrieke wentelbane. As 'n groep vertoon hierdie sterre as 'n groep min of geen netto rotasie rondom die sterrestelsel, en dus het 'n halo-lid 'n baie negatiewe V-snelheid ('retrograde rigting'), aangesien die beweging van die son rondom die galaktiese middelpunt in die positiewe V is rigting.

Hierdie sterre bevat 'n baie lae metale-oorvloed van een tot tien persent van Sol's (met 'n gemiddelde van ongeveer twee persent), maar die tekort aan Kapteyn-metale as 'n M-tipe dwerg is verrassend sag in vergelyking met tipe K- en G-subdwerge (JR Mold, 1976) . Terwyl halo-sterre so min as 0,1 persent van die sterre in die sonbuurt kan beloop, bevat dit plaaslike subdwerge, Kapteyn's Star en Groombridge 1830 ('n G8VIp met 'superflare' wat nou glo 'n enkele ster is - sonder 'n M- tipe flare ster metgesel). Vanweë hul latere ontdekking, ook bekend as Population II-sterre, bevat hierdie groep ook RR Lyrae-veranderlikes met periodes langer as 12 uur, subdwerge en ander uiters metaalarm sterre, en 'n paar rooi reuse.


Kapteyn's Star mag
gebore is as
'n lid van die
Omega Centauri
Globular Cluster
voordat hy versteur word
in 'n wentelbaan
die Melkweg (meer).

In 'n voordruk van 20 Oktober 2009 het 'n groep sterrekundiges wat na 16 sterre in die Kapetyn-sterre bewegende groep gekyk het, vermoed dat Kapteyn's Star een van die 14 stralekranssterre in die groep is met dieselfde elementêre oorvloed as wat baie lede van die Omega Centauri Globular Cluster, wat ongeveer 17 000 ligjare vanaf Sol lê. Die 12 miljard-jarige Omega Centauri, die helderste bolvormige groep wat van die Aarde waargeneem is, het sterre van verskillende ouderdomsgroepe en spoorelemente, wat daarop dui dat die tros die kern van 'n dwergstelsel kan wees wat saamgesmelt het met die Melkweg (Bekki en Freeman, 2005 en 2003). Tydens die antieke samesmelting is die meeste van die afgeleë sterre van die dwergstelsel in nuwe retrograde wentelbane om die kern van die Melkweg versteur, wat blykbaar Kapteyn's Star insluit, gebaseer op kinematiese en chemiese ontleding (Wylie-de Boer et al., 2009 en Ken Croswell. , Nuwe Wetenskaplike, 12 November 2009).


Hoeveel sterre is daar in 'n bolvormige groep van 10 ^ 5 sonmassas? - Sterrekunde




'N Gids vir bolvormige trosse
DEUR MARK ARMSTRONG
ASTRONOMIE NOU
Geplaas: 14 Mei 2013

Met die waarneming van geleenthede wat begin afneem namate die nagte korter en ligter word, sorg bolvormige trosse vir goeie teikens in die minder as ideale waarnemingsomstandighede, met hul helderheid oor die algemeen.


Van die beste bolvormige trosse word hierdie maand vertoon. Gebruik hierdie soekgrafiek om hulle op te spoor. Beeld gemaak met die Sky weergawe 5. Sien groter weergawe.

Laat lente en vroeë somer lug bevat 'n oorvloed groot bolvormige trosse vir waarnemers op die Noordelike Halfrond, wat hieronder getoon word.

Globulêre trosse is skouspelagtige, dig verpak naby sferiese versamelings van antieke sterre wat hoofsaaklik die uitgebreide buitenste stralekrans van ons sterrestelsel bevolk. Daar word vermoed dat hulle al meer as 11 miljard jaar gelede in die vroeë lewe van ons sterrestelsel gevorm het, wat hulle baie ouer as oop trosse maak. Ons Melkweg het 150 tot 200 lede, 'n druppel in die see in vergelyking met getalle wat aan ander sterrestelsels gekoppel is. Die reuse elliptiese M87 in Maagd het 'n ongelooflike 16.000 globulars en M31 het meer as 300 bevestigde globulars, terwyl ander kandidate die totaal meer as 1000 stoot. Galaktiese bolletjies kom in 'n reeks diameters en massas, die kleinste kan net net massiewer wees as die grootste oop trosse, maar die grootste soos M19 en M54, met 'n gewig van 1,5 miljoen sonmassa's en bevat etlike miljoene sterre, en is 'n paar dwergstelsels. Dit is inderdaad moontlik dat die helderste en skouspelagtigste van almal, die magtige Omega Centauri, die gestreepte kern is van 'n klein sterrestelsel wat gely het as gevolg van 'n noue ontmoeting met ons sterrestelsel eeue gelede.

Hoe maklik 'n bolvormig is om te sien, is nie net die helderheid nie, maar ook die mate van kondensasie (hoe dig verpak dit is) is 'n kritieke faktor. 'N Digte, kompakte, sterryke bolvormige toon 'n groter kontras met die agtergrondhemel en is dus makliker om te sien as diffuus, wat op die agtergrond makliker verdwaal. Ervare amateurs gebruik 'n skaal van 12 punte wat die mate van kondensasie aandui, wat wissel van I (baie dig en kompak) tot XII (uiters diffuus sonder sentrale konsentrasie). M13 is V, net soos M5, alhoewel baie mense reken dat dit minder dig is as M13, dus miskien VI. M3 is V1, met Omega Centauri diffuser by VII. M92 word beoordeel as IV, dus is dit kompakter as die meeste, dus dit moet makliker wees om op te tel, dit is 'n maklike voorwerp in 'n verkyker, duidelik nie-sterre. Kyk na M2 (klas II) in die Waterman, hierdie herfs vir 'n blik op 'n baie digte bolvormige.

Een van die beste in die hele lug, Messier 3 (NGC 5272), wat slegs deur die M13 in die noordelike hemel verduister is, is nog steeds goed te sien in die groot konstellasie Canes Venatici. M3 was Messier se eerste ware ontdekking in Mei 1764 en is groot, helder en maklik om in 'n verkyker raak te sien, en is ook met die blote oog sigbaar in donker, deursigtige lug. M3 is een van die grootste en mees massiewe trosse, met 'n gewig van 800.000 sonmassas en bevat vermoedelik ongeveer 500,000 sterre in 'n sfeer van ongeveer 190 ligjaar. Dit het 'n baie elliptiese galaktiese baan en word tans vermoedelik 34 000 ligjare weg gelê.


Die Messier 3 bolvormige groep. Krediet: Jeremy Perez

Dit benodig nie veel diafragma om M3 se ware aard te herken nie, want selfs 'n 80 mm-omvang by lae krag toon dit korrelig en diafragma's van 100 mm en hoër sal die cluster begin oplos, wat dit 'n moeiliker voorstel maak hierin. respek as die belangrikste teenstanders in die noordelike lug, M13 en M5. M3 word in die 12-puntskaal van kondensasie as klas V1 aangewys (met ek baie dig en kompak teenoor XII wat uiters diffus is), wat dit effens diffuser maak as die bogenoemde mededingers, albei beoordeel V. Die skynbare grootte deur die oculair hang af van die diafragma en verskyn ongeveer sewe boogminute in 'n 100 mm en verhoog tot 15 boogminute in groot amateurbereik. Spektakulêre beelde is moontlik deur selfs beskeie omvang met diep LRGB CCD-beelde wat M3 se asemrowende mooi vorm onthul wat tot byna 20 boogminute strek.

Ondanks die feit dat M3 in 'n relatief dorre deel van die lug is, is dit baie maklik om M3 te sien, kyk net halfpad tussen die briljante Arcturus (alfa Bo & oumltis) en Cor Caroli (alfa CVn), 'n bietjie nader aan eersgenoemde. Op die oomblik kulmineer M3 suid net na 23:00 v.G.J. teen 'n baie gesonde 65 grade of so, en daar is 'n vyf-uur lange waarnemingsvenster totdat die oggendskemer tussen 04:00 ingryp.

As u die lug met 'n verkyker in die rigting van Serpens, naby die grens met Maagd, skandeer, kan u 'n wasige plek raakloop soos 'n ster wat nie fokus nie. Dit is die groot, helder en pragtige bolvormige tros Messier 5 (NGC 5904), so goed in werklikheid dat die groot waarnemer, Edward Emerson Barnard, dit 'veel mooier as M13' gedink het.


Die Messier 5 bolvormige groep. Krediet: Jim Misti

Gottfried Kirch in Berlyn het die eerste keer in Mei 1702 die M5 opgeneem, Charles Messier het dit in 1764 opgemerk, maar William Herschel was dit die eerste wat dit in 1791 opgelos het. Dit is in baie kategorieë soortgelyk aan M3 en M13, dit is gelyk aan M13 in helderheid en skyn op mag. +5,7 en het net 'n effens minderwaardige skynbare grootte van 20 boogminute in teenstelling met M13 se 21 boogminute. Dit gee hom 'n werklike grootte van 150 ligjare op 'n afstand van 26 620 ligjare. M13 is effens nader en groter met M3 wat albei 190 ligjare groot is op 'n meer afgeleë afstand van 34 170 ligjare. M5 kan soveel as 'n halfmiljoen sterre bevat en weeg 800 000 sonmassas. Die ouderdom van M5 is 'n twispunt wat voorheen as een van die oudste genoem is. Studie in 1997 deur Raul Jimenez en Paolo Padoan het 'n jeugdige tien biljoen jaar gerapporteer, wat dit as een van die jongste een sou maak.

M5 kan met die blote oog gevind word deur waarnemers van arendsoë op donker plekke ongeveer 25 grade suid-oos van Arcturus en agt grade wes van alfa Serpentis. Die impak daarvan word ietwat verminder deur die lae noordelike afname, en dit is waar M3 en M13 swaar is. Aan die ander kant beteken dit wel dat M5 vanaf beide hemisfere sigbaar is. Klein omvang toon 'n duidelike elliptiese vorm met 'n helder kern en resolusie van die afgeleë sterre. Om tot 'scopes' te beweeg in die 150-200 mm-klas, bied 'n pragtige uitsig, met die resolusie min of meer tot by die kern met matige vergrotings. M5 kan waargeneem word sodra dit donker word en is op sy beste om 12.45 vm BST wanneer dit 40 grade hoër is. Neem 'n eksemplaar van die Mei-uitgawe van Astronomy Now op vir 'n diepgaande, omvattende waarnemings-, skets- en beeldvormingsgids vir M5.

Ongetwyfeld die beste bolvormige groep in die Noordelike Halfrond, M13 (NGC 6205), is baie waarnemers se eerste ervaring van 'n bolvormige groep en is van toe af verslaaf. M13 word slegs verduister deur die groot suidelike globulars Omega Centauri en 47 Tucanae, met M22 in Boogskutter wat baie strawwe kompetisie bied. M13 is baie maklik om aan die westekant van die Keystone-sterretjie van Hercules te vind. Dit lê 'n derde van die eta tot by zeta Herculis en dit is net binne die blote oogafstand vanaf die donkerste plekke, met 'n geïntegreerde grootte + 5.7.


Die Messier 13 bolvormige groep. Krediet: Nik Szymanek

'N Verkyker sal M13 maklik vang, geflankeer deur twee sterre van die sewende grootte, maar toon net 'n lang, vaag kol met 'n helderder kern. Klein teleskope van goeie gehalte in die 80-100 mm-klas moet 'n skynbare deursnee van agt tot tien boogminute vertoon en sommige van die afgeleë sterre in hierdie reuse-balletjie begin oplos. Die opgradering na 'n 150 mm en die vergroting van 200x gebruik eenvoudig 'n pragtige uitsig.

M13 is in 1714 deur Edmond Halley ontdek en Messier het dit in 1764 by sy lys van komeetagtige newelagtige voorwerpe gevoeg. Die onvermoeide en briljante William Herschel was 20 jaar later die eerste om die ware aard daarvan te herken. M13 het 'n eksentrieke baan van 500 miljoen jaar rondom die galaktiese middelpunt en dit kan so ver soos 80 000 ligjare van ons af wees, maar op die oomblik lê dit baie nader op 26 000 ligjare. M13 is een van die groter trosse met 'n fisiese deursnee van 160 ligjaar, wat gelyk is aan 'n skynbare deursnee op die hemelsfeer van 21 boogminute. Sterrekundiges meen M13 bevat nie meer as een miljoen sterre met 'n totale massa van 600 000 sonmassas nie.

M92 (NGC 6341) is 'n baie goeie bolvormige tros wat in Hercules woon, maar word dikwels oorskadu deur die groot bolvormige groep M13, slegs tien grade suidwes. M92 is kleiner en dowwer (mag. +6.5 en 14 ') as M13 (+5.7 en 21') omdat dit fisies kleiner en verder weg is. M92 weeg by 400 000 sonmassas, ingeprop in 'n 110 ligjaar deursnee wat 27 000 ligjare weg is.


Die Messier 92 bolvormige groep. Krediet: Nik Szymanek

Dit is dus maklik om M92 in die noorde van Hercules op te spoor, en sy hoë noordelike afwyking is nog 'n pluspunt in die opsporing daarvan. Maar hoe lyk die uitsig deur die okularis? Amateurs hou daarvan om individuele sterre op te los in globusse en probeer sterre oplos tot in die kern van die groep. Dit is moeiliker om die bolle met 'n hoër mate van kondensasie op te los en die hoë gradering van M92, wat 'n voordeel is om dit te vind, word 'n hindernis wanneer u dit probeer oplos. Diafragma's in die omgewing van 75-100 mm sal die buitenste streke van M92 begin oplos, met die kompakte, newelagtige kern wat daarop dui. Maar dit sal waarskynlik 'n diafragma van 250-300 mm benodig om die M92 ten volle tot in die kern op te los.

Probeer om die aansig tussen M13 en M92 te verander en kyk hoe u dit beoordeel? Wissel die vergroting uit en neem baie tyd om elke groep te waarneem. M92 lyk asimmetries, selfs in omvang van so klein as 80 mm, met die sentrale kondensasie in die noord-ooste en daar is geen duidelike sterre kettings nie, anders as in M5 en M13.

Beide M92 en M13 is uitstekend geplaas op Mei-aande, waarna dit die hele nag waarneembaar is, wat op die vroeë oggendure uitloop. M92 is sirkumpolêr van die Verenigde Koninkryk (stel nooit af nie).

Ophiuchus is die tuiste van nie minder nie as sewe Messier-bolvormige trosse, met die paar M10 (NGC6254) en M12 (NGC6218) (slegs effens meer as drie grade uitmekaar) wat die mees toeganklike is vir die Noordelike Halfrond waarnemer en toevallig twee van die mooiste ter wêreld is. hemele. M10 is effens beter as sy buurman in alle kategorieë, en skyn op mag. +6,6 en strek tot bykans 20 boogminute in skynbare deursnee.


Die Messier 12 bolvormige groep. Krediet: Jim Misti

Dit is sigbaar in 'n verkyker, maar die aansig sal soveel beter wees in 'n klein teleskoop en selfs 'n 80 mm-omvang by vergroting x100 sal individuele sterre begin oplos. M10 is van medium kompaktheid en konsentrasie, geklassifiseer as VII, en dit verg slegs 150 mm om sterre ten volle tot in die kern op te los. M10 is redelik groot bolvormig met 'n fisiese deursnee van 140 ligjare, maar gemeen met M107 (ook in Ophiuchus) is dit ietwat gemiddeld en bestaan ​​uit ongeveer 250.000 sonmassas.

M12 is 'n merkbaar losser globulêr, selfs deur klein openings (IX-klassifikasie), en as u 'n 100-150 mm-omvang het, moet u hierdie groep tot in sy kern kan oplos as die siening genoeg krag toelaat. M12 is kleiner en dowwer as M10, alhoewel daar in helderheidstaal nie veel is nie, net twee tiendes op +6,8, in grootte verloor dit met vyf boogminute, ondanks dat hy 4000 ligjare nader as sy buurman op 20.760 ligjare lê. Dit is die ware fisiese grootte van ongeveer 85 ligjare, baie kleiner as M10. Aangesien dit suid van die hemelse ewenaar is (al is dit maar effens), trek beide M10 en M12 eers middernag goed van die suidoostelike horison af en bereik dit omstreeks 02:00, albei is gemaklik meer as 30 grade bo die suidelike horison.


FASE OORGANGS IN DENSE ASTROFISIESE PLASMAS

3 VRYSTELLING VAN H / A WIT DORWE EN DIE OUDERDOM VAN DIE GALAXIE

Tot onlangs was die algemeen aanvaarde siening dat ons Melkweg in 'n vinnige ineenstorting 19 gevorm is. In hierdie prentjie moet die ouderdomme van al die komponente van die Melkweg dieselfde wees as die ouderdomme van die oudste bolvormige trosse (15 ± 3) × 10 9 jaar ≡ 15 ± 3 Gyr 20. Onlangs is daar egter metodes ingestel wat gebaseer is op die nukleokosmochronologie en die verkoelingsouderdom van die wit dwerge, wat albei ouderdomme oplewer vir die galaktiese skyf wat baie jonger as hierdie is. Malaney en Fowler 21 het byvoorbeeld 'n galaktiese ouderdom verkry tG ≲ 12 Gyr van beide Th / Nd en Eu / Ba verhoudings.

Die ander nuwe metode om die ouderdom van die galaktiese skyf te verkry, wat deur Winget bekendgestel is et al. 22, kombineer die waargenome tekort aan wit dwerge met helderheid L & lt 10 −4.5 L 23 met wit dwergverkoelingsteorie. Die resultaat gee tG ≈ 9 Gyr, in ooreenstemming met die resultate van die nukleokosmochronologie, maar strydig met die ouderdomme van die bolvormige trosse. 'N Meer onlangse en volledig onafhanklike berekening deur Iben en Laughlin 24 verkry 'n skyfouderdom ∼ 9 Gyr, in ooreenstemming met die resultate van Winget et al. 22 .

Kan die skeiding tussen koolstof en suurstoffase hierdie afwyking verklaar? Daar is tot onlangs gedink dat die skeiding van die C / O-plasma in 'n wit dwerg in C-ryke en O-ryke fases na bevriesing, soos die eerste voorgestelde Stevenson 25, die verskil tussen die wit-dwerg-verkoeling en die ouderdomme van die bolvormige trosse. Soos getoon deur Mochkovitch 26, laat die digter, O-ryke vaste stof verswak aan ernstige gravitasie-energie, wat die afkoeling van 'n wit dwerg vertraag en die ouderdom daarvan verleng. As dit sou gebeur, kan die Galaxy se skyf baie ouer wees as die huidige skatting wat deur die 'wit dwergchronometer' gegee word.

Lengthening the age of a cool white dwarf also increases the luminosity function at these faint magnitudes, however. Garcia-Berro et al. 27 have recently computed the effect of complete phase separation upon the luminosity function, and they find a large and potentially observable effect. Thus, if phase separation were to occur, it could eventually be subjected to observational test.

It now seems very unlikely that C/O phase separation will take place, however. Barrat et al. 28 have recomputed the phase diagram for a C/O mixture and have found it to be of the spindle type rather than the eutectic type suggested by Stevenson. They conclude that significant phase separation does not occur, and that the maximum increase in the white dwarf ages associated with the freezing of this binary plasma is about 0.5 Gyr. Still more recently, Ichimaru, lyetomi, and Ogata 29 have also recomputed the C/O phase diagram and found it to be of an “azeotropic” form. Like Barrat et al., they have concluded that significant phase separation does not occur and that the effect on white dwarf ages is minimal. These two studies essentially close the book on this effect.

The most attractive possibility for resolving the difference between the ages obtained from white dwarf cooling and nucleocosmochronology, on the one hand, and from cluster ages, on the other, seems to me to be to abandon the hypothesis that the disk of the Galaxy is the same age as the halo. Indeed, according to Norris and Green 30 , “There seems no compelling reason to believe that the Galactic disk in the solar neighborhood has any major stellar component as old as the disk globular clusters.” They favor a more gradual formation process and point out that the pressure-supported collapse models of Larson 31 are in best accord with observations. In Larson's model 6, after ∼ 2 Gyr, the disk is confined to within ∼ 5 kpc of center. Only after a further several Gyr does the disk form at the solar distance from the center. They regard this as “the most natural explanation of the apparent relative youth of the disk in the solar neighborhood.” Larson 32 , too, advocates this solution.

In the end, the definitive determination of the age of the galactic disk will almost certainly require the Hubble Space Telescope. In preparation for the launch of this instrument, Tamanaha et al. 33 have recently undertaken a calculation of the contribution to the white dwarf luminosity function of the stars which completed their evolution during the formation of the galactic halo. Such a possibility was first discussed by Larson 34 who postulated “bimodal star formation,” with a “high-mass” star formation mode occurring preferentially in the early history of the galactic disk.

Tamanaha et al. 33 point out that after ∼ 15 Gyr, a 0.8 M white dwarf will have cooled to L ∼ 10 −6 L, implying MV ∼ +20, just below the current limit of detection. Though there exist increasing uncertainties in the microphysics at lower luminosities, these authors have made a first effort to explore this domain. Their most interesting finding is that the most extreme models do allow the entire halo dark matter to consist of white dwarfs, with ages thalo = 12 to 13 Gyr. Younger halos cannot supply all the dark matter. This is a testable result, and when the HST is launched, it will surely be one of the priorities for observational investigation.


10 Beautiful Star Clusters for Stargazers

Credit & Copyright: Jewel Box (NGC 4755) by Dieter Willasch (Astro-Cabinet)

Star clusters represent distinct steps in the evolution of the Universe. On the one hand, globular clusters can be thought of as “fossils”, in the sense that in many cases, they are all that remain of smaller galaxies that were devoured when large galaxies formed. Open clusters on the other hand, are evidence of the evolution of the Universe, in that in almost all cases, the constituent stars in these groupings all formed at roughly the same time from the remnants of their predecessors, to shine until eventually they too will die, with their remains in turn supplying the material for new stars to form.

By keeping the above in mind when looking at the star clusters on this list, the items presented here may become more than just pretty pictures instead, they may come to be seen as the building blocks of the Universe, without which our Sun and solar system may never have existed.

Omega Centauri (NGC 5139)

– Constellation: Centaurus
– Class: VIII
– Coordinates: RA 13h 26m 47.28s | Dec. -47° 28′ 46.1?
– Distance: About 15,800 light years
– Mass: 4 million solar masses
– Radius: 86 light years
– Magnitude: 3.9
– Estimated age: 12 billion years
– Other designations: NGC 5139, GCl 24, Caldwell 80

Image Credit: ESO

Under really dark skies, Omega Centauri appears almost as big as the full Moon, which is not surprising given that this, the biggest cluster orbiting the Milky Way (and the second biggest cluster in the Local Group) spans a full 150 light years, and contains roughly 10 million stars that are on average separated by only about one tenth of a light year. In fact, the cluster is so distinctively different from other similar globular clusters that some investigators believe that it is not the remaining core of a dwarf galaxy that had been assimilated into the Milky Way.

The most distinguishing feature of this cluster is the large number of old, evolved, red giant (Population II) stars that account for a significant percentage of its total stellar population. This and other factors explain the wide distribution of metallicity and stellar ages in the cluster, which suggests that unlike other globular clusters that formed from the same material at roughly the same time, Omega Centauri may be the product of an accretion process in which smaller clusters may have been cannibalized to produce the mixed stellar population. Investigations are continuing.

47 Tucanae (NGC 104)

Image Credit: ESA/Hubble

– Constellation: Tucana
– Class: III
– Coordinates: RA 00h 24m 05.22s | Dec. -72° 04′ 57.9″
– Distance: 16,700 light years
– Mass: 1 million solar masses
– Radius: 60 light years (120 ly diameter)
– Magnitude: 4.95
– Estimated age: 13.06 billion years
– Other designations: NGC 104, GCl 1, Caldwell 106, 1RXS J002404.6-720456

Apart from the fact that 47 Tucanae is the second biggest and brightest globular cluster around the Milky Way, and also contains several million stars, it contains many stars of scientific interest. For instance, the cluster contains at least 21 blue stragglers (near its nucleus), which are stars that have merged with other stars through one of several mechanisms. 47 Tucanae is also contains several hundred X-ray sources.

Furthermore, the cluster houses at least 25 millisecond pulsars, which is the largest population of such objects in any known globular cluster. Moreover, based on recently obtained data, the cluster is very likely to contain a 2000+ solar mass black hole, and given this mix of extreme gravity, exotic radiation, and a low overall metal content, it is perhaps not surprising that despite a diligent search, no planets have been discovered in, or around the cluster.

Messier 2 (NGC 7089)

Image Credit: NASA/STScI/WikiSky

– Constellation: Aquarius
– Class: II
– Coordinates: RA 21h 33m 27.02s | Dec. –00° 49′ 23.7″
– Distance: 37,500 light-years
– Mass: 900,000 solar masses
– Radius: 87.3 light years (174.6ly diameter)
– Magnitude: +6.3
– Estimated age: 13 billion years
– Other designations: NGC 7089

Discovered in 1746 by Jean-Dominique Maraldi and Jacques Cassini, this large cluster was at first thought to be nothing more than a cloud of nebulosity, until William Herschel was able to resolve individual stars in 1783. Spanning about 175 light years, M2 is just visible to the naked eye under dark skies, but a small telescope will easily resolve some of the brighter stars in its outer fringes. In total, M2 contains only about 150,000 stars, which number includes at least 21 variable stars. The brightest members of the cluster are either red or yellow giants, which no doubt helps to make the cluster as conspicuous as it is.

NGC 1049 (in Fornax Dwarf Galaxy)

Image Credit: NASA / ESA / S. Larsen

– Constellation: Fornax
– Class: V
– Coordinates: RA 02h 39m 52.5s | Dec. -34° 16′ 08″
– Distance: 630,000 light years
– Radius: 12 light years (24ly diameter)
– Magnitude: +12.9
– Other designations: Hodge 3

Located a whopping 630,000 light years away, orbiting one of the Milky Way’s satellite galaxies, this pretty cluster is well worth observing in telescopes of moderate aperture. However, its parent galaxy, the Fornax Dwarf Galaxy, is nearly invisible, which makes finding, and observing the cluster a whole lot easier

Mayall II (Andromeda’s Cluster)

Image Credit: Michael Rich, Kenneth Mighell, and James D. Neill (Columbia University), and Wendy Freedman (Carnegie Observatories) and NASA

– Constellation: Andromeda
– Coordinates: RA 00h 32m 46.51s | Dec. +39° 34′ 39.7″
– Distance: 2.9 million light years
– Mass: 1×107 (Solar masses)
– Radius: 21.2 (42.4ly diameter)
– Magnitude: +13.8
– Estimated age: About 12 billion years
– Other designations: G1, NGC-224-G1, SKHB 1, GSC 2788:2139, HBK 0-1, Andromeda’s Cluster

Mayall II is the biggest and brightest globular cluster in the entire Local Group of Galaxies, consisting of around 300,000 old stars which orbits the Andromeda Galaxy at a distance of about 130,000 light years from that galaxy’s centre. While it has been difficult to calculate the exact mass of this cluster, most estimates put the value at about twice that of Omega Centauri. Mayall II also almost certainly contains an intermediate mass black hole in its core.

Like Omega Centauri, Mayall II is characterized by a wide distribution of metallicity and stellar ages, which suggests that it is not a true globular cluster, but the remains of a dwarf galaxy that had been tidally stripped by the Andromeda galaxy. Note that the two bright objects in the frame are unrelated, over-exposed foreground stars.

Southern Pleiades (IC 2602)

Image Credit: Marcin Paciorek

– Constellation: Carina
– Coordinates: RA 10h 42m 57.5s | Dec. -64° 23′ 39″
– Distance: 479 light years
– Dimensions: 50’ × 50’
– Magnitude: 1.9
– Estimated age: 50 million years
– Other designations: Theta Carinae Cluster, Melotte 102, Collinder 229, VDBH 103

Like its namesake in the Northern Hemisphere, the Southern Pleiades is a conspicuous naked-eye object, even though it is 70% fainter than the Taurean Pleiades. It has fewer members, though, consisting of only about 60 or so stars, the brightest of which is Theta Carinae, which shines at magnitude +2.74. The age of the Southern Pleiades is still uncertain, but it is thought to be at least as old as the cluster IC 2391, which is estimated to be about 50 million years old, based on its lithium depletion boundary.

Jewel Box (NGC 4755)

Image Credit: ESO

– Constellation: Crux
– Coordinates: RA 12h 53m 42s | Dec. -60° 22.0′
– Distance: 6,440 light years
– Magnitude: 4.2 (Total, integrated magnitude)
– Estimated age: 16 million years
– Other designations: NGC 4755, Herschel’s Jewel Box, Kappa Crucis Cluster, Caldwell 94

Forming part of the Southern Cross, the aptly Jewel Box [cluster] is among the youngest known open star clusters, being only about 14 million years or so old. Clearly visible without optical aid, it contains about 100 brightly colored stars, which John Herschel described as follows: “..this cluster, though neither a large nor a rich one, is yet an extremely brilliant and beautiful object when viewed through an instrument of sufficient aperture to show distinctly the very different colour of its constituent stars, which give it the effect of a superb piece of fancy jewellery”.

The most luminous stars in this little cluster are all supergiants, and count among the most luminous stars in the entire Milky Way galaxy, which gives some credence to the statement often made by northern observers that “..the South has all the best stuff”. It does, indeed.

Pearl Cluster (NGC 3766)

Image Credit: ESO

– Constellation: Centaurus
– Coordinates: RA 11h 36.1m | Dec. -61° 37′
– Distance: 5,500 light-years
– Diameter: 12.0 (arc mins)
– Magnitude: 5.3
– Estimated age: 14.4 million years
– Other designations: NGC 3766, Lacaille III.7, Dunlop 289, Melotte 107, Collinder 248, C1133-613, Caldwell 97

The most attractive aspect of this little open cluster that’s bearing down on us at the rate of 14.8 km per second is the two red giant stars that bracket the core of younger, blue stars. Although there appears to be about 137 stars in this cluster, accurate photometric data exists for only 36, meaning that many stars in the cluster may not be members of the group.

However, the uniform color, and hence the temperature, of the stars in this cluster seems to suggest that they all formed at the same time, from a common origin, which is supported by their largely common proper motion. Investigations into the origin of the cluster are ongoing, but regardless of the outcome, the cluster will remain an easy binocular or small scope target for at least the next million years or so.

Hodge 301 (in Large Magellanic Cloud)

Image Credit: ESO

– Constellation: Dorado
– Coordinates: RA 05h 38m 27s | Dec. -69° 04′ 26?
– Distance: 168,000 light years
– Magnitude: 11
– Estimated age: 20 million years

Unlike the next item on this list, Hodge 301 is a fairly sedate open cluster the is also located in Tarantula Nebula within the Large Magallanic Cloud galaxy. Removed from the far more energetic R136 by about 150 light years, many stars this cluster have had time to evolve into giants, and it is thought that as many as 40 supernova events have occurred in this part of nebula, which have collectively contributed to the violent currents and shock waves within the Tarantula Nebula that have in turn, sparked periods of intense star formation.

The presence of two major star clusters within the nebula, one of which is at least ten times older than the other, has enabled investigators to observe relatively close-up the effects of supernova events and fast solar winds on dense concentrations of gas and dust.

R136 (in Large Magellanic Cloud)

Image Credit: NASA, ESA

– Constellation: Dorado
– Coordinates: RA 05h 38m 42.396s | Dec. -69° 06′ 03.36?
– Distance: 157,000 light years
– Mass: 450,000 solar masses
– Magnitude: 9.5
– Estimated age: About 1.5 million years
– Other designations: UCAC2 1803442, SAO 249329, HD 38268, TYC 9163-1014-1, CD-69 324, GC 7114

While open clusters-within-open-clusters are not exactly rare, it is not often that an inner cluster such as this one is bright enough to illuminate an entire nebula, which in this case, also happens to be the Tarantula Nebula in the Large Magellanic Cloud.

Previously known as RMC 136, this tightly packed knot of stars constitutes the inner regions of the larger open cluster NGC 2070, and spans about 6.5 light years. It similarly contains some of the most massive and luminous stars known, among which are 32 very hot O-type blue stars, at least 40 other, slightly cooler O-type stars, and at least 12 massive Wolf-Rayet stars, most of which are of the extremely energetic WNh type- all packed into the innermost 5 parsecs, or 16.3 light years, of the inner cluster.

Other stars within about 150 parsecs (490ly) of the core include an additional 325 O-type stars, and a further 19 Wolf-Rayet stars. However, this cluster-within-a-cluster is only about 1.5 to 2 million years old, which means that there are no, old stars among its stellar population.


How many stars are there in a Globular Cluster of 10^5 solar masses? - Sterrekunde



Above, a wide-field view of the cluster (Image Credit N.A.Sharp, REU program/AURA/NSF/NOAO)
Below, an HST image of its core (Image Credit ESA/Hubble/NASA)


NGC 104 (= PGC 2802612 = 47 Tucanae)
Discovered (1751) by Nicolas Lacaille
A 4th-magnitude globular cluster in Tucana (RA 00 24 05.2, Dec - 72 04 49)
Above, a 7 arcmin wide image of the cluster core (North is about 30 to the right of up in this image Credit ESO)
Below, the region studied in detail with the HST to measure stellar motions in the cluster's core
(North is about 30 to the right of up in this image Credits Ground-based image at top, VLT, R. Kotak & H. Boffin, ESO
2/3 arcmin wide HST closeup at bottom, ESA, G. Meylan (Ecole Polytechnique Federale de Lausanne), NASA)


Below, a 25 by 37 arcmin wide view of nearly the entire cluster (North is nearly at the top in this image)
(Credit & © Daniel Verschatse, Observatorio Antilhue, Chile used by permission)



NGC 5139 = ω (Omega) Centauri
Discovered (1677) by Edmond Halley
A 4th-magnitude globular cluster in Centaurus (RA 13 26 47.0, Dec -47 28 51)
Above, a half degree wide view of the core of NGC 5139 (Image Credit ESO)
Below, a one degree wide DSS image centered on the cluster

Below, the center of Omega Centauri, where stars are packed ten thousand times more densely than in the Solar neighborhood. (Adrienne Cool (SFSU) et al., Hubble Heritage Team (STScI/AURA), NASA, apod011010)


NGC 2419 (= PGC 2802643): "The Intergalactic Wanderer"
Discovered (Dec 31, 1788) by William Herschel
A 10th-magnitude globular cluster in Lynx (RA 07 38 08.5, Dec +38 52 57)

Open Clusters

An open cluster (also sometimes referred to as a galactic cluster) can be thought of as a loosely gravitationally bound collection of tens or hundreds of stars, many of which are young, bright, blue stars. These stars were formed at the same time (give or take a few thousands of years!) from the same initial cloud of gas (mostly hydrogen) and dust. There are approximately 1500 or so of these open clusters in our Galaxy and we know that they also exist in nearby galaxies such as the Large and Small Magellanic Clouds (LMC and SMC). The stars in an open cluster are therefore relatively close to each other which makes them different to constellations (such as Orion and Ursa Major) since constellations are group of stars that only appear to be close to each other but which are in fact at different distances from us.

When they are young (a few million or tens of millions of years old), these clusters can contain some very massive and bright stars (perhaps as massive as 200 times the mass of our Sun) with spectral types O or B. The youngest open clusters (less than 10 million years old) often contain the remnants of the gas cloud from which they were formed. This gas is now visible as 'cloudiness' within many astronomical images and is known as nebulosity.

Stars in open clusters have proved very useful to astronomers since they were all formed from the same giant cloud (so they have the same chemical composition) and are all at approximately the same distance from us. It is therefore safe to assume that any differences between the stars in an open cluster are really caused by their different masses. These differences manifest themselves in terms of brightness, surface temperature and the star's life-time and make them perfect targets for studies such as those that produce colour magnitude diagrams from photometry.

Stars obey Wien's Law - the more massive stars are usually very blue (and therefore hot - perhaps around 30,000 K), intermediate mass stars (like the Sun) are yellow (cooler - approximately 6,000 K), and the very lowest mass stars are red (cool - around 3,000 K).

Many of these open clusters are included in the Messier catalogue. This is a list of around deep sky objects, mostly visible from the northern hemisphere. Among the most striking objects in this list are M25, M44 (the Beehive), M45 (the Pleiades), M67 and NGC290 (see Figure 1).

Figure 1: A Hubble Space Telescope image of the open cluster, NGC290.
Credit: ESA, NASA, E.Olszewski

Further information on open clusters can be found at the WEBDA and Dias databases.

While we can describe open clusters subjectively, science teaches us that more thorough analysis can allow us to understand more about these objects. With this in mind, a classification system for open clusters was designed in the 1930s by the Swiss astronomer, Robert Trumpler. The Trumpler system classifies a cluster based on three properties

  • a Roman numeral from I-IV denoting concentration (I = strongly concentrated, IV = loosely concentrated)
  • a number from 1 to 3 indicating the range in stellar brightness (1 = small, 3 = large)
  • the letter p, m or r to indicate whether the cluster is poor, medium or rich in stars

An additional 'n' is given if the cluster shows signs of nebulosity.

Examples using this system include the Pleiades which are I3rn (strongly concentrated with a large range in brightness, richly populated and containing nebulosity), while the nearby Hyades are classified as II3m (more dispersed with fewer stars and no nebulosity).

Read more about spectral types.

Which of the following best describes an open cluster?

A collection of tens to hundreds of old stars No, it is generally younger stars that are found in open clusters A collection of hundreds to thousands of young stars No, open clusters do not normally have this many stars A collection of tens to hundreds of young stars Yes, this is correct A collection of hundreds to thousands of old stars No, open clusters generally have less stars and the stars are younger

How many clusters are there in our Galaxy (open and globular)?

Around 150 of each type No, there are more open clusters than this Around 1500 of each type No, there are fewer open clusters than this 150 open and 1500 globular No, these values are the wrong way around 150 globular and 1500 open Yes, this is correct


Hidden Black Hole in Globular Cluster May Be a Cosmic Middle Child

For decades, astronomers have tracked black holes with masses millions of times that of the sun, as well as those with tens of solar masses. But black holes between those two extremes have proved elusive. Now, astronomers studying a globular cluster have found just such a black hole at its center, showing that intermediate-mass black holes could be hiding out in these compact agglomerations of stars.

Lead study author Bülent Kiziltan, an astronomer at the Harvard-Smithsonian Center for Astrophysics (CfA), and his co-authors Holger Baumgardt (of Australia's University of Queensland) and Abraham Loeb (also of CfA) found a black hole between 1,400 and 3,700 solar masses at the center of 47 Tucanae, a globular cluster in the southern sky some 16,700 light-years from Earth.

Black holes are usually found because they emit massive amounts of X-rays as matter falls in. Midsize black-hole candidates have been found in galaxies a group from the University of Maryland and NASA's Goddard Space Flight Center found one in another galaxy in 2015, and there are about a dozen objects in total. [Die vreemdste swart gate in die heelal]

Kiziltan and his team found this one by measuring motions of pulsars within the cluster. They found the telltale signs of a compact, massive object in the cluster's heart. The likeliest explanation for the motions was a black hole.

"Intermediate-mass black holes have been expected [in globular clusters] for many decades," Kiziltan told Space.com. "But we've not been able to find one conclusively."

Theorists think stellar-mass black holes form from stars that are at least a few dozen times the mass of the sun. When they run out of nuclear fuel, there is no longer enough energy from radiation to hold the star's outer layers against its immense gravity. The star collapses, and then explodes as a supernova. (Supernovas can outshine the galaxies in which they reside.) What's left of the star then shrinks into a tiny volume. A 100-solar-mass star, as a black hole, would have a radius of about 180 miles (290 kilometers). The former star's escape velocity exceeds that of light, resulting in a black hole, from which nothing can escape.

A big question for astronomers is what the population of black holes looks like. Given that there are supermassive black holes, and stellar-mass ones, there should be a population of black holes with masses between those two. But there don't seem to be as many as expected. The centers of globular clusters, which are agglomerations of old stars, seemed a good place to look, as earlier studies indicated they might be there, according to the new study. [No Escape: Black Holes Explained (Infographic)]

The problem is, black holes are visible only when stuff falls in them. As such, the researchers needed another method that didn't depend on picking up radio emissions.

That's why Kiziltan and his colleagues decided to look at the pulsars that inhabit a globular cluster. Pulsars form from stars less massive than those that make black holes. After those stars go supernova, they collapse into neutron stars.

Some neutron stars spin rapidly and emit radio waves along a line offset from their rotational axes. These are called pulsars. Earthbound observers see them if Earth is in the radio beam as it sweeps across the sky. Pulsars' rotation rates change so little that they are precise timekeepers. They are precise enough that by timing the signal and looking for any Doppler shifts, it's possible to measure a pulsar's movement along one's line of sight.

Kiziltan's group tracked the movement of some two dozen pulsars and used computer simulations to model the cluster to track down their black-hole candidate.

"We're proposing a brand-new approach to the study of globular clusters," Kiziltan said. "It's not only that we see the dynamical signature of a black hole, but how to probe the region near it without going too close to it." Probing the centers of globular clusters is usually difficult, because the density of stars makes it hard to see what's going on.

Finding the intermediate-mass black hole raises more questions about how these black holes form, said Cole Miller, a professor of astronomy at the University of Maryland who studies black-hole formation. "Let's say it's an intermediate-mass black hole," he said. "How did it get there?"

"Globular clusters have small escape speeds," he said. "So the stars should blow away all the gas." There will be some as stars age, such as a red giant's stellar winds. "But that amount of gas is nowhere close enough to make an intermediate-mass black hole."

This differs from the supermassive black holes at galactic centers, he added, because one would expect lots of matter to accumulate there, feeding a black hole and allowing it to grow very fast.

Both Kiziltan and Cole said there are several ways to grow black holes early in a cluster's history. "One of my favorites is runaway collisions of stars or stellar- mass black holes," Miller said. "An interesting effect is, if you have a bunch of stars in a dense stellar region, the heaviest will start runaway collisions." Once a black hole forms &mdash perhaps when a star that's absorbed a few neighbors dies ― all the matter that isn't in a stable orbit around the black hole will fall in or get ejected from the cluster, he said. That puts an automatic stop on the black hole's growth.

For scientists to get a better handle on how such black holes might form in clusters, more of them need to be found &mdash but that won't be easy, Kiziltan said. The only reason it worked for 47 Tucanae was that there were enough pulsars in it to begin with, and they were close enough to see. Not every globular cluster has the right combination of distance and bright pulsars.


Researchers explore the surroundings of globular cluster NGC 6809

NGC 6809. Credit: Hewholooks/Wikimedia Commons

Using the 4-meter Blanco telescope at the Cerro Tololo InterAmerican Observatory (CTIO), astronomers have mapped the outermost regions of a galactic globular cluster known as NGC 6809. Results of the study, published May 24 on the arXiv pre-print server, could improve our understanding of this cluster and its surroundings.

Globular clusters (GCs) are collections of tightly bound stars orbiting galaxies. Astronomers perceive them as natural laboratories enabling studies on the evolution of stars and galaxies. In particular, globular clusters could help researchers to better understand the formation history and evolution of early-type galaxies as the origin of GCs seems to be closely linked to periods of intense star formation.

NGC 6809 (also known as Messier 55 or M55) is a galactic GC in the constellation Sagittarius, located some 17,600 light years away. It has a radius of about 48 light years, mass of approximately 269,000 solar masses and is estimated to be 12.3 billion years old. Although many studies of NGC 6809 have been conducted, still little is known about its outermost regions.

In order to change this, a team of researchers led by Andres E. Piatti of the National University of Cuyo in Mendoza, Argentina, decided to investigate this cluster and its surroundings with the Dark Energy Camera (DECam) of the CTIO's 4-m Blanco telescope.

"Here, we explored the outermost regions of NGC 6809. We built its CMD [color-magnitude diagram] from DECam images centered on the cluster, which reached nearly 6 mag below the cluster MS [main sequence] turnoff," the astronomers wrote in the paper.

Piatti's team constructed stellar density maps for stars distributed in five different magnitude intervals along the cluster main sequence. Such maps are useful tools when it comes to identifying extra-tidal features distributed around the cluster's main body.

By analyzing the stellar density maps, the astronomers found that only stars—with NGC 6809 membership probability over 70 percent and more than 4 mag fainter than those at the MS turnoff—exhibit some excesses of stars at opposite sides from the cluster center. This suggests that less massive stars are prone to leave the cluster more easily.

The research detected no signs of tidal tails in the studied inner globular cluster sample. This is in agreement with recent results from numerical simulations and suggests that it could be due to a comparative shorter diffusion time. The researchers explained that the diffusion time of streams (tidal tails in GCs) is reduced by gravitational potentials that sustain chaotic orbits.

"The lack of detection of tidal tails in the studied inner globular cluster sample could be due to the reduced diffusion time of tidal tails by the kinematically chaotic nature of the orbits of these globular clusters, thus shortening the time interval during which the tidal tails can be detected," the authors of the paper concluded.


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