Sentinel Major
From Halopedia, the Halo wiki
Template:SentinelSentinel Majors are artificial flying constructs designed by the Forerunners and serve on the Halo Installations. They are similar to a regular sentinel, but are gold in color, have stronger beams, are protected by a light energy shield, and fire more accurately. They were only observed on Installation 05.
In combat, their shields are less powerful than that of an Elite, and the beam fired is blue instead of orange. It is also more powerful, but its battery drains faster as a result. You can pick up this beam and the normal beam as your primary and secondary weapons, but there is no difference in it's design so if you hold both you won't know which is which without firing it.
Sentinels have been known to have been created long before Installation 04 was ever conceived. They operated automated facilities such as Gas Mines like the one found on Threshold for "untold centuries" before the facility was redesigned to research the Flood. Therefore it is possible that they were only outfitted for combat after the Flood was discovered.
The Sentinels are the "police" of the Halo installations, and are controlled by the Monitor of the Installation. Their primary purpose is to contain small outbreaks of Flood, however they can only manage flood for a short time. Should an outbreak grow beyond the sentinels' ability to control, Enforcers and Sentinel Majors are activated by the Monitor of the Installation to assist the Sentinels.
Trivia
Although they only appear in Halo 2, in the Halo levels, Two Betrayals and The Maw, a few Sentinel Minors have shields.
