The galactic center is part of the chair being held by the figure, which we interpret to mean that it symbolizes the "throne" of the center of the Milky Way Galaxy whose galactic center is located there.
Avebury Stone #42 Left Side Photograph
Avebury Stone #42 Left Side Photo Tracing
Avebury Stone #42 Left Side and its Corresponding Stars in Ophiuchus, Scorpio & Ara to the left of the stars of Sagittarius marked on the henge-outward facing front face
The head at the bottom would seem to signify the underworld of the deceased, who, as we know from Avebury Stone #10 and from our previous decipherment of the Stonehenge sarsens and stones, marked the destination (presumably presumed "origin") to which the souls of the deceased returned.