Pressemeddelelse

En stjernehob med en hemmelighed

Lithiumproblemet gælder ikke kun stjerner i Mælkevejen, viser undersøgelser med VLT

10. september 2014

Den kugleformede stjernehob M54 ligner til forveksling mange af de andre kuglehobe, som hører til vores galakse, Mælkevejen, men M54 har en hemmelighed. Den hører slet ikke til vores mælkevej, men til en lille satellitgalakse, som kaldes Sagittarius Dværggalaksen. Billedet stammer fra VLT Surveyteleskopet på ESOs Paranal observatorium i det nordlige Chile. Det, at dværggalaksen har en fremmed oprindelse gør, at astronomerne kan undersøge, om der også er uventet lidt af grundstoffet lithium udenfor Mælkevejen. Redskabet til at afgøre problemet er VLT (Very Large Telescope) på Paranal.

Omkring Mælkevejen kredser der mere end 150 kugleformede stjernehobe. De er kugler med hundredetusindvis af gamle stjerner i hver, og stjernerne er lige så gamle som vores galakse. Kometjægeren Charles Messier fandt i slutningen af de attende århundrede en del kuglehobe i omegnen af stjernebilledet Skytten, og en af dem har fået betegnelsen M54 i hans katalog over tåger på himlen.

I mere end to hundrede år efter opdagelsen har vi troet, at Messier 54 var magen til Mælkevejens andre kuglehobe, men i 1994 blev dens afstand målt til omkring 90 000 lysår - og det er mere end tre gange afstanden fra Jorden til Mælkevejens centrum.

Med VLT har astronomerne nu observeret Messier 54 for at løse et af de uafklarede problemer i moderne astronomi - lithium problemet.

Ved Big Bang blev der dannet det meste af grundstoffet lithium, som nu findes i Universet. Samtidig dannedes grundstofferne Helium og Hydrogen (Brint), men i meget større mængder. Det kan beregnes ret nøjagtigt hvor meget lithium vi skulle forvente at finde i det tidlige Univers, og derfra kan man beregne hvor meget, man burde kunne finde i de gamle af stjernerne. Men tallene passer ikke. Der er omkring tre gange mindre lithium i stjernerne end vi skulle forvente. Det er et problem, som har været kendt og forsøgt løst i flere årtier [1].

Indtil nu har det kun været muligt at måle lithiumindholdet i stjerner i Mælkevejen. Med VLT har et forskerhold under ledelse af Alessio Mucciarelli (universitetet i Bologna, Italien) nu målt hvor meget lithium der findes i et udvalg af stjernerne i Messier 54. De har fundet, at mængderne ligger tæt på det, som vi finder i Mælkevejen, så det, der har været årsag til, at der mangler lithium, er altså ikke noget lokalt for Mælkevejen.

Billedet af kuglehoben er nyt, og det blev optaget med VST (VLT Survey Telescope) på Paranal observatoriet. Ud over selve hoben ser man en skov af stjerner i forgrunden. De stammer alle fra Mælkevejen.

Noter

[1] Der er foreslået adskillige løsninger på denne gåde. Den første er, at vore beregninger af lithiummængderne efter Big Bang er forkerte. Men nyere observationer tyder ikke på, at det skulle være løsningen. Den anden løsning kunne være, at lithium på en eller anden måde er blevet ødelagt i de tidligste stjerner i Universet, før Mælkevejen blev dannet. Den tredje er, at nogen processer i stjernerne gradvist har ødelagt lithium i løbet af stjernernes udvikling.

Mere information

Forskningsresultaterne er offentliggjorte i en artikel med titlen “The cosmological Lithium problem outside the Galaxy: the Sagittarius globular cluster M54”, af A. Mucciarelli et al. Artiklen offentliggøres i tidsskriftet Monthly Notices of the Royal Astronomical Society (Oxford University Press).

Forskerholdet består af: A. Mucciarelli (Universitetet i Bologna, Italien), M. Salaris (Liverpool John Moores University, Liverpool, UK), P. Bonifacio (Observatoire de Paris, France), L. Monaco (ESO, Santiago, Chile) and S. Villanova (Universidad de Concepcion, Concepcion, Chile).

ESO er den fremmeste fællesnationale astronomiorganisation i Europa, og verdens langt mest produktive jordbaserede astronomiske observatorium. 15 lande er med i ESO: Belgien, Brazilien, Danmark, Finland, Frankrig, Italien, Nederlandene, Portugal, Spanien, Sverige, Schweiz, Storbritannien, Tjekkiet, Tyskland og Østrig. ESO har et ambitiøst program, som gør det muligt for astronomer at gøre vigtige videnskabelige opdagelser. Programmet har focus på design, konstruktion og drift af stærke jordbaserede observatorier. Desuden har ESO en ledende rolle i formidling og organisering af samarbejde omkring astronomisk forskning. ESO driver tre enestående observatorier i verdensklasse i Chile: La Silla, Paranal og Chajnantor. På Paranal driver ESO VLT, Very Large Telescope, som er verdens mest avancerede observatorium for synligt lys, samt to oversigtsteleskoper. VISTA, som observerer i infrarødt, er verdens største oversigtsteleskop, og VLT Survey Teleskopet er det største teleskop bygget til at overvåge himlen i synligt lys. ESO er Europas partner i ALMA, som er det største eksisterende astronomiprojekt. For tiden planlægges E-ELT, et 39 m optisk og nærinfrarødt teleskop, som bliver "verdens største himmeløje".

Links

Kontakter

Alessio Mucciarelli
University of Bologna
Bologna, Italy
Tel: +39 051 20 95705
E-mail: alessio.mucciarelli2@unibo.it

Lars Lindberg Christensen
Head of ESO ePOD
Garching bei München, Germany
Tel: +49 89 3200 6761
Mobil: +49 173 3872 621
E-mail: lars@eso.org

Ole J. Knudsen (Pressekontakt Danmark)
ESOs formidlingsnetværk og Aarhus Space Centre, Aarhus Universitet
Aarhus, Danmark
Tel: +45 8715 5597
E-mail: eson-denmark@eso.org

Connect with ESO on social media

Dette er en oversættelse af ESO pressemeddelelse eso1428 lavet af ESON - et netværk af personer i ESOs medlemslande, der er kontaktpunkter for medierne i forbindelse med ESO nyheder, pressemeddelelser mm.

Om pressemeddelelsen

Pressemeddelelse nr.:eso1428da
Navn:M 54
Type:Milky Way : Star : Grouping : Cluster : Globular
Facility:VLT Survey Telescope
Instruments:OmegaCAM
Science data:2014MNRAS.444.1812M

Billeder

Den kugleformede stjernehob Messier 54
Den kugleformede stjernehob Messier 54
Den kugleformede stjernehob Messier 54 i stjernebilledet Skytten
Den kugleformede stjernehob Messier 54 i stjernebilledet Skytten
Et stort udsnit af himlen omkring den kugleformede stjernehob Messier 54
Et stort udsnit af himlen omkring den kugleformede stjernehob Messier 54

Videoer

zoom-ind på den kugleformede stjernehob Messier 54
zoom-ind på den kugleformede stjernehob Messier 54
Den kugleformede stjernehob Messier 54 tæt på
Den kugleformede stjernehob Messier 54 tæt på

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