The celestial sphere is
the central spherical region of the milky way galaxy, dense with stars.
the orb of the sun; a normal star.
the star-sphere discovered by the greeks and other ancient civilizations which shows the physical location in space of the nearby stars.
a spherical galaxy, centered on the sun. a useful illusion used to map the stars and other objects in the sky.

The star-sphere discovered by the Greeks and other ancient civilizations which shows the physical location in space of the nearby stars.

Explanation:

The celestial sphere is an ideal sphere, without defined radius, concentric with the terrestrial globe, in which the stars apparently move. Some ancient civilizations such as the Greeks assumed that the stars were attached to a celestial sphere, which revolves around the earth, while our planet is always immobile.