I tried hard to figure out what on earth the "which" suggests.
It keeps confounding me.
Having formed only about 20 million years ago, Beta Pictoris is just about
the age when we would expect it to have a disk, and so far it is the only star around which (= star) one (= disk) has been observed.
If which = the (only) star = it = Beta Pictoris, it seems not to make any sense to me. What does the "one" suggest?
If which = the disk, it makes little sense. But "the disk" is a bit far from "which"...kind of strange.