Cryptocurrency exchange market structure is fragmented, since cryptocurrencies are fungible, but customers cannot move freely across cryptocurrency exchanges. We build a model where exchanges with captive customers are linked by arbitrageurs, showing that ``star-shaped'' equilibria can exist, in which arbitrageurs endogenously coordinate on one exchange as a liquidity hub. The model predicts that large exchanges' listing decisions should influence price dispersion, arbitrage flows, trade volumes, and listing decisions on small exchanges. We provide evidence for the model's predictions using data on exchange prices and trade volumes, as well as blockchain token flows across exchanges.