There are two main variations on the Purple Coin Star: the first is the collection of 100 coins of which there are only 100, without a time limit. The second is timed, but there are many more than 100 of the coins, generally around 150.
Purple Coins reappear in Super Mario Galaxy 2 when a Purple Comet orbits a galaxy. Their missions are identical to those from Super Mario Galaxy; 100 Purple Coins must be collected to get a Power Star. Purple Coins are worth 5 gold coins.