Others have suggested that the objects are a
protostar surrounded by an accretion disc. “But
that would necessarily be very young, and there
are reasons to question whether star formation has
taken place sufficiently recently for that hypothesis
to be viable,” Morris counters.
As for the German team, Morris says: “They are
not willing to admit that we can rule out the gas
cloudhypothesis.Theyhavearguedthatsucha
cloudcanstaytogetherduringitsclosepassage
becauseofthehighexternalpressureofthegas
there.I don’tthinkthatpreventsthecloudfrom
beingtidallydisrupted.”
Theevidenceforthembeingcocoonedstarsis
backedupbytheangularmomentumoftheobjects
intheirorbitsaroundtheblackhole.“Becausethey
haveangularmomentumintheirorbitsaroundthe
blackhole,theywillkeepgoinginthesameway
thattheEarthkeepsgoinginitsorbitaroundthe
Sunwithoutgettingpulledin,especiallyiftheyare
stars,becausethatangularmomentumcannotbe
removed,”hesays.
IftheGobjectsareindeedmergedbinarystars,
thenithaswideramificationsforastronomyand
scientificunderstanding.Ina statementannouncing
thediscoveryofG3throughtoG6andthestudy
intosuchobjects,itwaspointedoutthattheblack
holemaybedrivingbinarystarstomerge.
Ghezsaidinthestatement:“It’spossiblethat
manyofthestarswe’vebeenwatchingandnotunderstandingmaybetheendproductofmergers
thatarecalmnow.Wearelearninghowgalaxies
andblackholesevolve.Thewaybinarystars
interactwitheachotherandwiththeblackholeis
verydifferentfromhowsinglestarsinteractwith
othersinglestarsandwiththeblackhole.”
MorrisclaimsGobjectsarelikelyrelevantto
thegrowthofsupermassiveblackholesingalactic
nuclei.“Thereisa long-standingproblemof
howsuchblackholesgetfedandhowtheycan
growtohavethemassesthattheyhave,”he
tellsus.
“Bothstarsandinterstellargascontributetotheir
growth,butinspiteoftheirstronggravityblack
holesaresosmallthatitisveryhardtogetmatter
toreachthemandtogoallthewaydowntotheir
eventhorizontobecomepartoftheblackhole.In the chair with G2’s discoverer
AstronomerStefanGillessenoftheMaxPlanckInstituteforExtraterrestrialPhysicsinGermany discovered the
objectG2in2011.Hebelievesit ismostlikelya compactgascloudWhenG2wasseenheading
towardsSgrA*in2014,how
surprisedwereyoutoseeit
emergerelativelyunscathed?
I wasn'treallythatsurprised– the
simulationswecarriedoutwere
notsuggestingtherewouldbea
collision:theyshowedthatthe
gascouldrunaroundSgrA*.Whatpreventedtheobjectfrom
beingtornapartinitsentirety?
There’sabalancebetween
centrifugalforcesand
gravitationalforcesasanobject
approachesablackhole,butit’s
extremelydifficulttofallin.Fora
circularobjectthere'saverynice
balance,butforaneccentricorbitthereisswingingbackandforth.
SincetheorbitofG2ishighly
eccentric,yousimplyexpectthat
itcomesclosetoablackholeonly
fortidalforcestostepin.The
facttheobjectwastidallytorn
apartmeansitwasnotboundto
anythingelse.Wouldthatdispelthetheory
there’sastellarmassinside?
It'shardtoprove.Theproblemis
thatyoucanmakesomethinghot
enoughorsmallenoughforitnot
tobeseenintheory,butitisvery
hardtotest.I tooktheposition
thattherecouldbealumpofgas
becausethereisgaseverywhere
inthegalaxy.Arethere strong arguments
against a hidden stellar mass?
Ithasbeen shown that G2 slowed
down,and this points to the gas
notbeing gravitationally bound.
Youwould expect the stellar
sourceto continue on the original
orbitat the same time as the gas
isdecelerating. But we haven't
seenanything moving out. There
isalsoa theory that this is a
merged object – former binary
starscolliding and shielding
themselves in a cocoon – but six
Gobjects would be too many
merged objects out of the 40
starswe see in the galactic centre.
Mergers are really rare and they'd
needto live for millions of years.Above:
When G2 was
observed to
be getting
close to Sgr A*,
astronomers
believed it
would be
ripped apartRight: When
using Chandra
to observe Sgr
A*’s reaction
to G2 in 2014,
a large X-ray
flare was
detected
from the
supermassive
black hole© ESO© ESOMYSTE RIE S
UNIVER SE
OF
THE