All About Space - UK (2020-02)

The next supernova

Fast forward 500 yearsandstargazersfound
themselvesina goldenera.TychoBrahestudieda
supernovain1572,andJohannesKeplerwitnessed
the lastsupernovaseenwithinourowngalaxy
in 1604.“Twoin 32 yearsandthennoneseenby
humansforthenext 400 years?It’snotfair,”says
SaurabhJha,a supernovaphysicistatRutgers
University,whoalsomissedtheclosestthingwe
know ofsince,backin1987.
Guinanwasmorefortunate.Inthespringofthat
year hewasona cruiseshipjustoffArgentina,
giving astronomytalks.Thelightfromanexploding
star in theLargeMagellanicCloud,whichorbits
the MilkyWay,begantoreachEarth,andhitthe
headlines.Whileitwasa challengetoseeitwith
the nakedeyefrombuilt-upurbanareas,itwas
clearly visibleinthedarkskiesoverthesouthern
Atlantic.Guinanwascompelledtoact.“Ihadto
make uptalksquicklysoI couldtakepeopleouton
deck andlookatit.”
To assessifBetelgeusewilladd 2020 tothe
list of famoussupernovayears,wefirstneedto
understandtheirorigin,andwhichstarsproduce
them. Whiletherearemanytypesandsubtypes,
we can broadlydividetheseexplosiveeventsinto
two groups.Corecollapse,orTypeIIsupernovae,
describethefinalswansongofmassivestarslike
Betelgeuse.Thesesupergiantslivefastanddie
young. Theyspendtheir5 to 10 millionyears
holdingofftheurgetocollapseundertheirown
massivegravitythroughrapidnuclearburningof
their hydrogenfuelreserves.Duringtheirlaterlife,

oncethehydrogenhasbeenusedup,increasingly

largerelementsarefusedinsteadtobalancethe

inwardpressure.Thisactofresistanceholdsuntil

thestar’scoreiscomposedofiron.Theelement’s

refusaltofusemeansgravityfinallywinsout.

Theouterlayersthenrapidlycollapseinwards,

producinga reboundingshockwave.Thisblows

thewholeatmosphericenvelopeoffthestarina

spectacularsupernova.

TheothermaincategoryisTypeIasupernovae,

alsocalledthermonuclearsupernovae.Theseoccur

whena whitedwarfstar,liketheoneourSunwill

leavebehindafteritdies,isfoundina binarypair.

Ifthewhitedwarfisabletoaccretematterfrom

itscompanion,thisextramasswillcompressits

core,triggeringcollapseanda supernova.While

muchrarerthancore-collapseevents,which

gooffroughlyevery 50 yearsinourgalaxy,

thermonuclearsupernovaearebrighterandcan

thereforebeseenfromfurtheraway.

Despiteallwehaveobservedandlearnedabout

thesesupergiantstars,scientistscannotsayforsure

whenBetelgeusewillbecomethenextsupernova

event,onlythatitwill.Anditishighlydebatable

whethertherecentdimmingmakes 2020 any

morelikelythan3030, 9090 ortheyear20020.

Onereasonfordoubt,saysElizabethStanway,an

associateprofessorofastronomyattheUniversity

Above:

Ancient

Chinese

manuscripts

describethe

supernovathat

formedthe

CrabNebulaas

a ‘gueststar’

Right:The

GOTOproject,

runbythe

University

ofWarwick,

looksforthe

gravitational

wavesthat

accompany

supernovae

WWhereeis

Betelggeusee??

Orion

© NASA/ESA

© Antonio González / IAC

© ALMA