since the diodes, transistor and resistor effectivly form a constant current source one of two conditions will occur
1 the bulb will turn on and settle at some voltage set by the current flow. bulb voltage will be approximatly (.2*bulb resistance at that current) that resistance is fillament temperature dependant and must be determined from the dreaded data sheet for the bulb selected.
In that case the transistor will see the remaining voltage across its c-E junction thus the potential dissapation issue.

Or the second case the current set by the constant currant source will require more voltage across the bulb than is available. The current that actualy flows will then be less than the designed current and the transistor will have minimum saturation voltage across it so the bulb voltage will see 12v-(vce sat transistor + Ic*re) again the dreaded data sheet must be consulted for probable numbers for actual Ic for the bulb selected. remember Ic = I bulb.

It seems that you are a little light in the fundamentals of electronics and could benifit from some extra curricular work. Can anybody recomend a suitable basic text easily available in India