What is Star mean by the tower on the airport diagram?

That normally means that there is a beacon on the top of the tower - These beacons can be red to warn of the towers height - most tall obstructions on the perimeter and approach to an airport will also have a red light for pilots to see. Beacons can be used on the tower to show a pilot the type of airport they are approaching Some of the most common used are;

White and Green - Lighted land airport

Green alone - Lighted land airport

White and Yellow - Lighted water airport

Yellow alone* - Lighted water airport

Green, Yellow, and White - Lighted heliport

White, White, Green - Military Airport

White, Green, Amber - Hospital and/or Emergency Services Heliport