I nyligen publicerade artiklar har forskare uppskattat graden av kollaps av supernovakärna i Vintergatan till 1.63 ± 0.46 händelser per århundrade. Därför, med tanke på den senaste supernovahändelsen, SN 1987A observerades för 35 år sedan 1987, kan nästa supernovahändelse i Vintergatan förväntas när som helst inom en snar framtid.
Life course of a stjärna & supernova
På en tidsskala av miljarder år, stjärnor undergo a life course, they are born, age and finally die with explosion and subsequent dispersal of star materials into interstellar plats as dust or cloud.
Livet för en stjärna begins in a nebula (cloud of dust, hydrogen, helium and other ionized gases) when the gravitational collapse of a giant cloud give rise to a protostar. This continues to grow further with accretion of gas and dust until it reaches its final mass. The final mass of the stjärna determines its lifetime as well as what happens to the star during its life.
Alla stjärnor derive their energy from nuclear fusion. The nuclear fuel burning in the core creates strong outward pressure due to the high core temperature. This balances out the inward gravitational force. The balance is disturbed when the fuel in the core runs out. Temperature drops, outward pressure diminishes. As a result, the gravitational force of the inward squeeze becomes dominant forcing the core to contract and collapse. What a star finally ends up as after collapse depends on the mass of the star. In the case of supermassive stars, When the core collapses in a short span of time, it creates enormous shock waves. The powerful, luminous explosion is called supernova.
This transient astronomical event occurs during the last evolutionary stage of a star and leave behind supernova remnant. Depending on the mass of the star, the remnant could be a neutron star or a svart hål.
SN 1987A, den sista supernovan
The last supernova event was SN 1987A which was seen in southern sky 35 years ago in February 1987. It was the first such supernova event visible to the naked eye since Kepler’s in 1604. Located in the nearby Large Magellanic Cloud (a satellite galax of the Milky Way), it was one of the brightest exploding stars seen in more than 400 years that blazed with the power of 100 million suns for several months and provided unique opportunity to study the phases before, during, and after the death of a star.
Att studera supernova är viktigt
Study of supernova is helpful in several ways such as measuring distances in plats, understanding of expanding universum and the nature of stars as the factories of all the elements that make everything (including us) found in the universum. The heavier elements formed as a result of nuclear fusion (of lighter elements) in the core of stars as well as the newly created elements during core collapse get distributed throughout plats during supernova explosion. The supernovas play a key role in distributing elements throughout the universum.
Unfortunately, there has not been much of opportunity in the past to observe and study supernova explosion closely. Close observation and study of supernova explosion within our home galax Milky Way would be remarkable because the study under those conditions could never be conducted in laboratories on the Earth. Hence the imperative to detect the supernova as soon as it begins. But, how will one know when a supernova explosion is about to begin? Is there any early warning system for impeding supernova explosion?
Neutrino, fyren för supernovaexplosion
Runt slutet av livsförloppet, när en stjärna får slut på lättare element som bränsle för kärnfusionen som driver den, dominerar den inåtgående gravitationen och stjärnans yttre lager börjar falla inåt. Kärnan börjar kollapsa och på några millisekunder blir kärnan så komprimerad att elektroner och protoner kombineras för att bilda neutroner och en neutrino frigörs för varje bildad neutron.
The neutrons thus formed constitute a proto-neutron star inside the core of the star upon which rest of the star fall down under intense gravitational field and bounce back. The shock wave generated disintegrates the star leaving the only core remanent (a neutron star or a svart hål depending on the mass of the star) behind and rest of the mass of the star disperses into interstellar plats.
Den enorma explosionen av neutriner produced as a result of gravitational core-collapse escape into outer plats unimpeded due to its non-interactive nature with matter. About 99% of the gravitational binding energy escape as neutrinos (ahead of photons which are trapped in the field) and acts as beacon of impeding supernova explosion. These neutrinos can be captured on the earth by the neutrino observatories which in turn act as an early warning of a possible optical observation of supernova explosion soon.
De flyende neutrinerna ger också ett unikt fönster till extrema händelser inuti en exploderande stjärna som kan ha implikationer i förståelsen av de grundläggande krafterna och elementarpartiklarna.
Supernova Early Warning System (SNEW)
Vid tidpunkten för den senaste observerade kärnkollapssupernovan (SN1987A) observerades fenomenet med blotta ögat. Neutrinonen upptäcktes av två Cherenkov-vattendetektorer, Kamiokande-II och experimentet Irvine-MichiganBrookhaven (IMB), som hade observerat 19 neutrinointeraktionshändelser. Detektering av neutriner skulle dock kunna fungera som ledstjärna eller larm för att hindra optisk observation av supernovan. Som ett resultat kunde olika observatorier och astronomer inte agera i tid för att studera och samla in data.
Sedan 1987 har neutrino-astronomi gått mycket framåt. Nu är supernovalarmsystemet SNWatch på plats som är programmerat att larma till experter och relevanta organisationer om en eventuell supernovaobservation. Och det finns ett nätverk av neutrinobservatorier över hela världen, kallat Supernova Early Warning System (SNEWS) som kombinerar signaler för att förbättra förtroendet för en upptäckt. Alla vanliga aktiviteter meddelas till en central SNEWS-server av individuella detektorer. Dessutom hade SNEWS nyligen uppgraderats till SNEWS 2.0, vilket också ger varningar med lägre förtroende.
Överhängande supernova i Vintergatan
Neutrino observatories spread across the world are aiming at first detection of neutrinos resulting from gravitational core collapse of the stars in our home galax. Their success therefore, is very much dependent on the rate of supernova core collapse in the Milky Way.
I nyligen publicerade artiklar har forskare uppskattat hastigheten för supernovakärnas kollaps i Vintergatan till 1.63 ± 0.46 händelser per 100 år; ungefär en till två supernovor per århundrade. Vidare tyder uppskattningar på att tidsintervallet mellan supernovans kärnkollaps i Vintergatan kan vara mellan 47 och 85 år.
Therefore, given the last supernova event, SN 1987A was observed 35 years ago, the next supernova event in the Milky Way may be expected any time in the near future. With the neutrino observatories networked to detect the early bursts and the upgraded Supernova Early Warning System (SNEW) in place, the scientists will be in position to have a close look at the next extreme happenings associated with supernova explosion of a dying star. This would a momentous event and an unique opportunity to study the phases before, during, and after the death of a star for a better understanding of the universum.
***
Källor:
- Fyrverkerierna Galaxy, NGC 6946: What Make this Galaxy so Special? Scientific European. Posted 11 January 2021. Available at http://scientificeuropean.co.uk/sciences/space/the-fireworks-galaxy-ngc-6946-what-make-this-galaxy-so-special/
- Scholberg K. 2012. Supernova Neutrino Detektion. Förtryck axRiv. Tillgänglig på https://arxiv.org/pdf/1205.6003.pdf
- Kharusi S Al, et al, 2021. SNEWS 2.0: nästa generations tidig varningssystem för supernova för multibudbärarastronomi. New Journal of Physics, volym 23, mars 2021. 031201. DOI: https://doi.org/10.1088/1367-2630/abde33
- Rozwadowskaab K., Vissaniab F., och Cappellaroc E., 2021. På hastigheten av kärnkollaps supernovor i mjölkvägen. New Astronomy Volume 83, februari 2021, 101498. DOI: https://doi.org/10.1016/j.newast.2020.101498. Preprint axRiv finns på https://arxiv.org/pdf/2009.03438.pdf
- Murphey, CT, et al, 2021. Bevittna historia: himlens utbredning, detekterbarhet och antalet supernovor i Vintergatan med blotta ögat. Månatliga meddelanden från Royal Astronomical Society, volym 507, nummer 1, oktober 2021, sidorna 927–943, DOI: https://doi.org/10.1093/mnras/stab2182. Preprint axRiv Tillgänglig på https://arxiv.org/pdf/2012.06552.pdf
***
