What happens if someone takes off his space helmet in space?

Take your pick... Suffocation (there's no air in space) Severe heamorraging as the cavities and blood vessels expand and rupture. (Space is a vacumn) Freezing. (It's very cold in space) Assuming you could survive all of that, the radiation from the sun would eventually kill you. Quite quickly if there happen's to be a star (like our sun) nearby that's going through a period instability; coronal mass ejections, solar flares etc..