Mass of this invisible object is determined by observing the orbit of the visible star around it.
(Mass of this invisible object; is determined; by observing the orbit of the visible star around it) (Mass of this invisible object; by observing; the orbit of the visible star around it)
Mass of this invisible object Mass of this invisible object
is determined by observing
by observing the orbit of the visible star around it
the orbit of the visible star around it