16. SupernovaD 89-12
Astronomersobserveasupernovathatisdimmerthanexpectedbecauseofdustscreening，Ifasupernovatriggeredstarformationfromdensecloudsofgasanddust, andifthemostmassivestartobeformedfromthecloudevolvedintoasupernovaandtriggeredanewroundofstarformation, andsoon, thenachainofstar-formingregionswouldresult. Ifmanysuchchainswerecreatedinadifferentiallyrotatinggalaxy, thedistributionofstarswouldresembletheobserveddistributeinaspiralgalaxy.
Thislineofreasoningunderliesanexcitingnewtheoryofspiral-galaxystructure. Acomputersimulationbasedonthistheoryhasreproducedtheappearanceofmanyspiralgalaxieswithoutassuminganunderlyingdensitywave, thehallmarkofthemostwidelyacceptedtheoryofthelarge-scalestructureofspiralgalaxies. Thattheorymaintainsthatadensitywaveofspiralformsweepsthroughthecentralplaneofagalaxy, compressingcloudsofgasanddust, whichcollapseintostarsthatformaspiralpattern.
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Ifasupernovatriggeredstarformationfromdensecloudsofgasanddust, andifthemostmassivestartobeformedfromthecloudevolvedintoasupernovaandtriggeredanewroundofstarformation, andsoon, thenachainofstar-formingregionswouldresult. Ifmanysuchchainswerecreatedinadifferentiallyrotatinggalaxy, thedistributionofstarswouldresembletheobserveddistributioninaspiralgalaxy 螺旋星云,旋涡星云).
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