Wings of Honor
The Star Saber was one of the many relics contained with the Iacon Hall of Records. Alpha Trion foresaw the conflict of Team Prime with Megatron's forces on Earth and so launched the Star Saber along with many other relics to Earth in hopes that they may one day be found by Optimus Prime and be used against the Decepticons.
The Star Saber would end up lodged in a large boulder. The sword then cast an energy field on the boulder in order to protect and secure it so that only a Prime would be able to remove it.