It depends what kind you mean.
First, they need a mature tank with pristine water conditions - esp specific gravity at 1.025-1.026.
They need a long acclimation time, often several hours.
The reef safe seastars can not be spot fed and require very large tanks, typically over 100g, with 125lbs minimum of LR on which they feed.
Non-reef safe stars like chocolate chip stars are easier to keep, but are predatory.
Brittlestars may also be relatively easier to keep as they will also take to spot feeding.
Most reef safe seastars die due to acclimation shock (within a month) or of starvation in about a year.