WhetherGalactusis a Celestial has long been debated within Marvel’s mythos, but the shocking truth is that Marvel may have already answered this question decades ago. Regardless of the answer, though, Galactus has always been portrayed as both a villain and a necessary cosmic force throughout Marvel history.
InFantastic Four Annual #26, written by Tom DeFalco with art by Herb Trimpe, it is theorized that after Galactus has fulfilled his hunger and duty as a Devourer of Worlds,the cosmic being will transcend into a Celestial.While the origins of Galactus and his danger to Marvel’s heroes have been well noted, what happens to him after is complicated.

In the same vein as this theory, it has been confirmed thatGalactus is not currently considered a Celestial. Still, he has the potential to become oneonce he has obtained sufficient power and evolves. Galactus is also not the only World Devourer, with his shared origins shedding some light on the truth behind Galactus’s true purpose.
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Celestials are ancient cosmic beings thatoriginate from the first cosmosand are known for theirsignificant role in shaping the Marvel Universe by creating planets, stars, and experimenting with the lifeformsof these galaxies. While Galactus is not technically a Celestial in his most acknowledged form, the similarities between the two cosmic entities are palpable.
In terms of appearance, both cosmic forces don massive and imposing suits of armor and use planets to sustain themselves, with the Celestials implanting a seed into a planet’s core and Galactus consuming a planet’s life force, consequently resulting in that planet’s destruction.While Galactus is not yet a Celestial, Marvel has established strong parallels and connections.

While a complicated question, Marvel has laid down the foundation and heavily implied that Galactus can evolve into a Celestial.
The enigma of what happens to Galactus after his demise has been touched upon a few times throughout Marvel history, but his death has always remained short-lived, as he has been established as essential to the balance of the Marvel Cosmos. Throughout his many deaths,Galactus has always been reborn to some capacity,sometimes evolving into a new being.
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Galactus Has Died, Been Reborn, and Has Evolved to New Heights
InThor#6 by Donny Cates and Nic Klein, Galactus met his demise at the hands of Thor, but upon being resurrected,he was later resurrected by the Destroyer as a cosmic being who feeds on knowledge, rather than planets.This most recent death hints at an interesting element of evolution in Galactus’s deaths and rebirth.
Similar to Galactus, Mordium was the Sorcerer Supreme of the fifth cosmos, from which magic originates. Upon the cosmos' death, he was reformed into the sixth cosmos and transformed intoOmnimax, the Devourer of Worlds. Both Galactus’s and Onimax’s transformations, as well as Galactus’s recent resurrection,align with the theory of Galactus’s eventual rebirth as a Celestial.
While depicted as separate species, the Celestials and Galactus share a similar role in the cosmic balance of the Marvel Universe. As Galactus evolves, he becomes more akin to the beings of creation from the first cosmos. So, while a complicated question,Marvelhas laid down the foundation and heavily implied thatGalactuscan evolve into a Celestial.