The Celestials are a race of ancient god-like beings of immense power and strength who appear to be humanoid in appearance, their bodies contain similar skeletal and neurological structures but on a much larger scale. They posses the ability to manipulate matter and energy and have an exceptionally strong constitution, allowing them the ability to wield the Infinity Stones. These beings were present in the universe long before the Dark Elves or Asgardians and while they can be killed, appear to be imperious to the effects of time.
There are currently three “known” Celestials, Eson the Searcher, Ego and the Celestial whose head became Knowhere. Peter Quill is a half Terran, half Celestial hybrid.
Eson the Searcher is the Celestial depicted in the hologram the Collector showed the Guardians of the Galaxy. He is a previous owner of the Power Stone which he used to destroy an alien race on an unknown planet.
Ego appears to be a unique Celestial as he does not possess a humanoid form and instead is essentially a large brain. He possesses incredible cosmic power and the ability to manipulate matter and over the millennia manipulated the molecules around him to form a protective planet around his vulnerable physical form. Ego was able to mimic and create the humanoid forms of the races he encountered. Ego is the biological father of Peter Quill/Star-Lord
Peter Quill is unique in the fact he is the only known living offspring of a Celestial. As the biological son of the Celestial, Ego, Quill appeared to posses the immortality and the same matter manipulation and cosmic powers of his father, although it is believed that he lost these powers when Ego was killed.
Knowhere is the severed head of an ancient Celestial and home to the mining colony, Exitar and previous home of The Collector’s museum,. Hundreds of years ago, the Tivan Group set up a mining colony inside of this severed Celestial’s head to harvest the valuable bone, brain tissue and spinal fluid. The Guardians of the Galaxy traveled to Knowhere to sell the Orb to The Collector but were eventually ambushed by both Ronan and Yondu.