MYSTE RIE S
UNIVER SE
OF
THE
THE G
OBJECTS
BY THE NUMBERS
G objects that have been
found so far
Size of the G objects
inkilometres
Longestorbitalperiodofa
Gobjectinyears
YearthefirstGobjectwas
firstobserved
Numberoflightyears
betweenEarthandSgrA*
That’s 100 timesthe
distancebetweentheEarth
andtheSun
Number of light years from
the black hole Sgr A*
Shortestorbitalperiod
ofa Gobjectinyears
YearSagittariusA*was
firstdiscovered
Theimmensemassof
theblackhole
6
15
billion
1,600
2005
26,000
0 .1 3
170
1 9 74
4 million
sunS
Atthisstage,G2couldhavebeentreatedasan
outlier– a strangeobjectbehavingoddlyalone.But
someyearsearlierin2005,astronomershadalso
discoveredanotherstrangeobjectclosetoSgrA*.
InlightofG2,thisbecameknownasG1,anditset
GhezwonderingwhetherG1andG2couldbepart
ofa largerclassofobjects.
Assuch,togetherwithhercolleagues,Ghez
subsequentlywentbackover 13 yearsofdata
takenfromobservationsfromtheW.M.Keck
ObservatoryinHawaii,andfoundevidencelinking
fourmore.ThesebecameknownasG3,G4,G5and
G6,collectivelyknownastheGobjects.Certainly,
theywerelookeduponbyastronomerswithgreat
excitement.“Suchobjectshadnotbeenseenbefore
inanyotherenvironment,”saysMarkMorris,a
UCLAprofessorofphysicsandastronomywho
workedwithGhezonthestudy.
“Bystellarstandardstheirdustphotospheresare
verycold,andbecausetheyareina regionwitha
strong background of ultraviolet radiation they are
partially ionised and show up in emissions from
very hot hydrogen atoms, which again hasn’t been
seen elsewhere. They also appear to be present only
inthe entourage of the black hole.”
G3 through to G6 follow a different orbit to G1
and G2 around the supermassive black hole, which
points to them having developed independently of
each other, even though their behaviour in relation
toSgr A* was determined to be the same. “In
every passage the tidal forces of the black hole can
remove some of the material from the outer layers
ofsome of the G objects,” Morris says. “We actually
observed that happening for both G1 and G2.”
“Thetidalforcesof theblack
holecan removesomeofthe
materialfrom theouterlayers
of someoftheg objects” MarkMorris
Above: The
objects may
originate
from binary
stars forced to
merge under
the intense
influence
of Sgr A*
Right: When
observing G2
on its approach
to the cloud
using the ESO’s
Very Large
Telescope,
astronomers
were able
to work out
where on
its orbit the
different parts
of the cloud
were located
© ESO
© ESO