The synonym for 'handy' in this context is 'convenient'. Some reasons:
Grouping stars by their constellations would be a convenient way of mapping the sky. It would make the sky easier to navigate and understand.
'Handful' does not fit well, as a handful implies a small or limited number, while constellations contain many stars.
'Complicated' is the opposite meaning of 'handy'. Something that is handy is easy and simple, not complicated.
'Unique' is also not a close match. While constellations are unique formations of stars, 'unique' does not convey the sense of being practical and useful, like 'convenient' does for 'handy'.
So in summary, the most suitable synonym for 'handy' in this context is 'convenient'. Grouping stars by constellations would provide a convenient method to map and study the night sky. This is because constellations serve as recognizable patterns that simplify comprehending the vast number of stars.
The key clue is that grouping the stars by constellations would make mapping the sky "a HANDY way". This suggests it would be an easy and practical method, for which 'convenient' is the closest synonym.
Thanks.