What is the difference in taste between star of anise and fennel
ChefOno is correct. Often recipes will advise you to break off a single point from star anise, as it is that intense. It's one of the flavors is Chinese Five Spice.
Star Anise has a much stronger taste, it's a component of many Asian style broths--like "pho" or red-cooking meat in Asian style. (braised in soy sauce)..and as mentioned in the 'five spice powder'.
It also has undertones of cinnamon to me as a back note to the flavor; a more sharp aggressive taste than fennel seeds.
Fennel seed has a more pronounced licorice flavor, for taste; you're probably more familiar with it as the primary flavoring in Italian sausage meat on pizzas. In fact crushed fennel seeds on a pizza (just a pinch) will give it a boost of that flavoring.
They're cousins in flavor, but like a garlic, or shallot or onion, they aren't oh so quite equal in flavor to sub one for the other.