Conversation so far:
What year did Bobby Orr retire?
Bobby Orr retired in 1978 at age 30.
What was Bobby Orr doing for work at time of retirement?
At the time of retirement, Bobby Orr was a player for the Chicago Black Hawks.
What was Bobby Orr doing with Chicago?
Bobby Orr was Defenseman for the Black Hawks.
Why could Bobby Orr no longer play?
Repeated injuries had effectively destroyed Bobby Orr's left knee.
How did they respond to Bobby's Orr's retirement announcement?
What did he do after retiring?
After retiring as a player, Bobby Orr started a new role as an assistant to Chicago general manager Bob Pulford.
What else can you tell me about Bobby Orr's retirement?
Bobby Orr's number 4 jersey was retired by the Bruins on January 9, 1979.

Question: Why did they do that?
Self-contained rewrite: Why did the Bruins retire Bobby Orr's number?