In the end, Darth Vader left his mask and walked away... in the shadows of the dark side ... in search of their own essence!
In the Expanded Universe novels it is shown that most Jedi do NOT disappear upon death. Those who do have 'become One with the Force'.
It is implied that only those who disappear can re-appear as Force Ghosts, but Vader seems to invalidate this - Vader's organic body DID disappear, what was burned was just his suit. A better theory is that learning how to become a Force Ghost - a technique Qui-Gonn is said to have rediscovered enables a Jedi who is prepared for death to disappear upon their physical form dying.
To disappear upon death requires great strength in the Force (equivalent to a Jedi Master), great wisdom and understanding of its ways, and for the person who dies to be at peace.
"Eternal life…"
"The ultimate goal of the Sith, yet they can never achieve it; it comes only through the release of self, not the exaltation of self. It comes through compassion, not greed. Love is the answer to the darkness."
"The ultimate goal of the Sith, yet they can never achieve it; it comes only through the release of self, not the exaltation of self. It comes through compassion, not greed. Love is the answer to the darkness."
