Master Chief does not die. After completing Halo on any difficulty, if you wait for the credits to end (might want to get something to eat and drink while waiting, they take ages), and if you do not skip it afterwards you will see the final cutscene. It depicts Master Chief floating inside the Remaining half of the frigate Forward Unto Dawn. The replacement ring collapsed upon firing and it's blast radius was only big enough to wipe out the Gravemind and damage the Ark such that all flood parasite forms were destroyed by the collapse or by the sentinels, so he is still alive but is drifting in space. He enters a cryogenic state, similar to the state he left at the beginning of Combat Evolved. The ship is then caught in the gravitational pull of another planet. It is most likely a shield world similar to Onyx, or it may be a totally new planet which will be a possible feature of Halo Chronicles or another possible sequel from 343 industries. HE IS ALIVE!
EDIT: to make it even cooler, the planet he is on is his training planet, if you read the first halo book. This planet is Sigma Octunas IV. In the books, this happens, and it continues by him exiting cyro-sleep, walking on the planet and finding his training officer's corpse, Chief Mendez.
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