G objects
Hesaystherecanbetwomainpossible
explanationsforthis.“Ithinkthey’reeithergas
cloudsorstars,whichref lectstheongoingdebate
betweentheGermangroupattheMaxPlanck
InstituteforExtraterrestrialPhysicsandoursat
UCLA,”hetellsAllAboutSpace.
Morrissaysthathisgrouparestrongproponents
ofthehypothesisthattheGobjectsarestars
surroundedbya cocoonofdustandgas.“Canthey
besomethingelseentirely?”heasks.“Yes,ofcourse,
butI thinktherealmofpossibilitieshasbeenwell
scoured,soalternativehypotheseswouldneedto
bequitesurprisingandunexpected.”
Hecertainlyrejectstherivaltheoryputforward
bytheGermanteamthattheyarecompactclouds
ofgasanddustbecausehesaysthiswouldcause
theobjectstobetornapart.“They’dbetidally
disruptedastheypassthroughthepointofclosest
approachtotheblackhole,andyettwooftheG
objectshavegonethroughtheircloseapproachand
survivedintact,”hesays.
Infact,henotesthattheoutershellofG2
stretchesdramatically,whereasthedustinsidethe
gasdoesn’tstretchmuchwhenitgetsclosetoSgr
A*.Itpointstotherebeingstellarobjectswitha
stronggravitationalpullinsidetheGobjects,and
thiswouldexplainwhytheyarecompactandable
tosurvivetheimmensegravitationalpullofthe
blackhole.Buthowdidtheygettobethatway?
Well,accordingtoUCLAastronomersthesixobjects
weremostlikelybinarystarsthatmergeddueto
thesupermassiveblackhole’sstronggravitational
force.Inotherwords,a systemoftwostarsorbiting
eachotherwouldhavebeenpulledtogetherinto
oneentityoverthecourseofmorethana million
years,Ghezsays.
“Themerged-binaryhypothesisfitsthe
theoreticalexpectationsforwhatshouldhappento binary stars orbiting fairly closely around a
supermassive black hole,” explains Morris. “Binary
stars are commonplace in the galaxy, so one should
expect that to be true in the galactic centre as well,
and their abundance is something that we are
closely looking into.”
He says another hypothesis to consider is one
that suggests stars in the galactic centre are so
close to each other because of the high stellar
density that they occasionally encounter each other
at high speeds in glancing collisions. “This could
levitate much of the stellar atmosphere, causing it
to become distended, cold and dusty, which is what
the G objects are,” Morris says. “It would take a long
time for such bloated stars to settle down.”How are the
G objects behaving?
The G objects are understood
to be in class of their ownContinuedjourney
Oncea Gobjectmovesawayfrom
theblackholeitisslightlymisshapen
followingitscloseencounter,but
itbeginstocompactagainasit
continuesitsorbit.Gasanddust
Underobservation,theGobjects
wouldappeartobea mixofgasand
dust.Astheyapproachtheblackhole
theytidallyinteract,andtheobjectCentralstar
GiventhattheGobjects
arenotdestroyedwhen
theygetclosetothe
blackhole,a recent
studyhassuggesteda
stellar-sizedmassisatthe
centreofeachone.
Distancefrom
theblackhole
EachoftheGobjectsare
within0.04parsecsofthe
blackhole,whichmeans
theyareclose,butstillfar
fromtheeventhorizon.Supermassiveblackhole
TheGobjectsorbitSagittarius
A*,thesupermassiveblackhole
atthecentreoftheMilkyWay.
WhenG2wasobservedin2014,it
wasthoughttheblackholewould
tearittoshreds.Itdidn’t.Differentorbits
TheobjectsG1andG2followa
similarorbitaroundtheblack
hole,butG3toG6takevery
differentpathsthatcantake
anywherebetween 170 and
1,600yearstocomplete.© Tobias Roetsch© JAnna Ciurlo, Tuan Do/UCLA Galactic Center Group