Roiurile globulare. Eterne diamante ceresti

Un roi globular este o colectie sferica de stele care orbiteaza in jurul centrului galactic, ca un satelit natural. Roiurile globulare nu au o preferinta pentru planul galactic ci sunt raspandite uniform in exterior, ca un halou inconjurator. Stelele din componenta roiurilor globulare, sunt foarte strans legate intre ele prin gravitatie, densitatea stelara fiind foarte [...]

Un roi globular este o colectie sferica de stele care orbiteaza in jurul centrului galactic, ca un satelit natural.

Roiurile globulare nu au o preferinta pentru planul galactic ci sunt raspandite uniform in exterior, ca un halou inconjurator.

Stelele din componenta roiurilor globulare, sunt foarte strans legate intre ele prin gravitatie, densitatea stelara fiind foarte mare pe unitatea de spatiu si ea crescand de la periferie catre centru. Aceste proprietati  confera roiului forma de glob sau sfera, de unde deriva si numele de “roiuri globulare”.

Aceste roiuri orbiteaza in zona de halo a galaxiei, deci nu in planul galactic.

Pana in prezent se cunosc numai in Galaxia noastra, 159 de roiuri globulare. Numarul lor se estimeaza a fi mai mare insa posibilitatile tehnice de observare de azi sunt limitate. Unele roiuri globulare pot fi ascunse in spatele unor nori de materie absorbanta (praf si gaz din galaxie) si astfel sunt invizibile de pe Pamant.

Formarea roiurilor globulare ramane un fenomen foarte putin inteles la ora actuala. In interiorul roiurilor globulare, cele mai multe stele se afla in aproximativ acelasi stadiu de evolutie, sugerand ca ele au cam aceeasi varsta si s-au format toate in acelasi timp.

Nu se cunosc roiuri globulare in care sa existe activitate de formare stelara. Stelele din r.g. sunt stele batrane iar acestea sunt formatiuni foarte vechi si stabile in timp.

De asemenea, in interiorul roiurilor nu sunt prezente gaze, praf si materie nebulara. Se pare ca acestea au fost consumate la formarea stelelor roiului cu multe milioane de ani in urma.

Roiurile globulare contin sute de mii de stele iar unele mai mari, ajung pana la milioane de stele.

Stelele componente, prezinta metalicitate redusa, predominand populatia de tip II.

Tipul spectral al stelelor din r.g. este asemanator cu cel din bulbul galactic, numai ca sunt concentrate intr-un spatiu foarte redus, densitatea de stele pe unitatea de volum fiind foarte mare si crescand si mai mult spre centrul roiului.

Distanta tipica dintre stelele componente ale unui roi globular, este de aproximativ 1 an-lumina, dar spre centru se poate ajunge si la distante comparabile cu dimensiunea sistemului nostru solar.

Unele roiuri globulare (ex. M15), au centre extrem de masive, care au condus la ipoteza ca in interiorul lor s-ar gasi o gaura neagra sau ar exista aglomerari de stele neutronice ori pitice albe de mare densitate care ar putea explica aceasta imensa masa centrala.

Metalicitatea roiurilor globulare

Roiurile globulare sunt compuse in mod normal din populatii stelare de tip 2 ( II – metal poor), sarace in elemente mai grele, care contin preponderent hidrogen si heliu si foarte putin elemente mai grele, denumite impropriu  “metalice” (carbon, azot, oxigen, neon…etc), spre deosebire de populatiile stelare de tip I (unu) ( metal rich), care sunt mai bogate in elemente “metalice”, cum ar fi Soarele, care contine si unele elemente mai grele in afara de hidrogen si heliu.

Aceste elemente mai grele (C, N,O,Ne…) determina “metalicitatea” stelelor si sunt produse prin nucleosinteza stelara, care apoi sunt expulzate in spatiu prin distrugerea stelelor care le-au produs si “reciclarea” lor in mediul interstelar, unde vor fi utilizate in nasterea urmatoarelor stele mai bogate in elemente metalice si eventual a sistemelor lor planetare (unde va fi cazul si daca acestea se vor forma).

S-a calculat ca stelele din interiorul roiurilor globulare nu pot detine sisteme planetare, deoarece acestea ar fi dinamic instabile, din cauza prea marii apropieri intre stelele vecine.

Roiurile globulare au densitati stelare foarte mari si de aceea au loc interactiuni si coliziuni frecvente intre stelele componente, mai ales spre centrul acestor roiuri. Din cauza acestor interactiuni, s-au observat cateva categorii “exotice” de stele, si anume “blue stragglers” adica “hoinarii albastri”, “pulsarii de milisecunde” sau miliseconds pulsars” si binarele cu masa redusa, emitatoare de raze X.

Un “hoinar albastru” (blue straggler) se formeaza din unirea sau fuzionarea a doua stele, posibil chiar din intalnirea unei singure stele cu un sistem binar. Steaua care rezulta din aceasta contopire, are o temperatura mai mare decat celelalte stele de aceeasi luminozitate din roi si difera ca tip spectral fata de ele.

Spre exemplu in roiul Messier 53 astronomii au observat cu surprindere mai multi astfel de “hoinari albastri”, atribuiti unor de coliziuni stelare in interiorul clusterului.

Cel mai cunoscut roi globular in tara noastra si vizibil prin instrumente mici, este Messier 13 (sau M13) care se proiecteaza pe bolta cereasca in constelatia Hercule. El este situat la o distanta de 25.100 ani lumina iar in partea lui rezolvabila prin telescop, au putut fi numarate peste 40 mii de stele.

Desigur ca sunt si multe alte roiuri vizibile de la noi iar acestea pot fi identificate cu ajutorul atlaselor ceresti.

Roiurile globulare nu sunt specifice doar galaxiei noastre, Calea Lactee, ci ele au fost observate si in alte galaxii. Astfel in galaxia M 31 din constelatia Andromeda, au fost observate peste 70 de roiuri globulare.

Se pare ca numarul total de roiuri globulare al acestei galaxii este chiar mai mare decat cel al galaxiei noastre.

De asemenea, s-au emis ipoteze ca unele roiuri globulare au fost captate prin acretie de la alte galaxii.

 

Roiurile globulare nu pot fi rezolvate complet in stele. Numarul total de stele dintr-un astfel de roi, se determina cu ajutorul observatiilor fotografice si prin aplicarea unor metode de calcul statistic.

Roiurile globulare contin si stele variabile. Peste 90% dintre stelele variabile ale unui r.g. sunt de tipul RR-Lyrae, iar restul sunt stele diferite, din care mare parte o formeaza cefeidele.

Cu ajutorul stelelor RR-Lyrae, se pot determina distantele pana la roiurile globulare.

Astfel s-a determinat distanta la roiul M13 din Hercule. Roiul din constelatia Canes Venatici se gaseste la o distanta de 13000 parseci iar roiul Omega Centauri, cel mai mare si spectaculos din emisfera sudica (invizibil din Romania) se gaseste la o distanta de 4850 parseci (adica12800 ani lumina) de Pamant.

Privite prin telescop, pe un cer negru si fara poluare luminoasa, roiurile globulare ofera un spectacol celest minunat si imagini foarte frumoase care raman mult timp in amintirea observatorului.

 

Autor articol :  Irimia Teodorian  (Teo)                                                 22 iunie 2016

 

76 Responses

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It was calculated that the stars inside the globular clusters cannot have planetary systems, because they would be dynamically unstable, due to the close proximity between the neighboring stars.

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The best-known globular cluster in our country and visible through small instruments.

Globular clusters do not have a preference for the galactic plane, but are spread evenly outward, like a surrounding halo.

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There are many other swarms visible from us and they can be identified with the help of celestial atlases.

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There aren’t any known globular clusters where star formation is taking place. The stars in the rg are very old stars, and this formation has remained stable over time.

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