according to the phase diagram :

 

the melting curve of the helium is parallel to the y-axis

 

thats mean that the melting point of the helium is not affected by the pressure !!!

 

Wy is it so ??

the melting point of helium IS affected by pressure. for instance, it WILL NOT freeze if its at standard pressure. no matter how cold. just because you cannot percieve non parrallel ness does not mean it is parallel.

 

 

EDIT: thought i'd add some linkage

http://www.fluidmech.net/msc/super/super-f.htm < yah looks real parrallel

I'm just curious, chuinhen, did you mean that the curve is (near) parallel to the X-axis? In the link that insane_alien has linked, the curve is definitely not parallel (or near parallel) to the Y-axis

well, the phase diagram is over a ridiculously small pressure/temperature range compared to most substances but thats is only because it is meant to be a useful diagram.