Experimental osmotic second virial coefficients are reported for polystyrene in toluene (good solvent), cyclohexane (Theta solvent) and methylcyclohexane (poor solvent) in the temperature range 10-60 degreesC. The Theta temperature for eight arm star polystyrene in methylcyclohexane is 29 +/- 3 degreesC. Intrinsic viscosity for polystyrene in cyclohexane and methylcyclohexane has been measured over a wide temperature range. A coil-globule transition has been observed for eight-arm star polystyrene in methylcyclohexane at temperatures close to the Theta temperature. The Theta point for star polymers with three, four, five, and six arms have been determined by standard Monte Carlo simulation calculations. For six-arm star polymers at the Theta point, defined as the well depth at which the radius of gyration squared scales linearly with the number of segments, the osmotic second virial coefficient is zero. As shown by others, for a branched polymer, the osmotic second virial coefficient at good solvent conditions and the Theta temperature are lower than those for a corresponding linear homologue in the same solvent.