Galactic Nova, often simply Nova, is a giant comet made out of random pieces of metal, machinery and debris. He originally appeared in the Milky Way Wishes sub-game in Kirby Super Star. He has the ability to grant one wish when summoned, but that can only happen when the Planets' fountains surrounding Planet Popstar are harnessed. Kirby summoned him to stop a war between the Sun and Moon of Popstar.
As Kirby was about to make his wish, Marx knocked him off screen and wished for world domination on Popstar. Nova begins a collision course towards Popstar to fulfill Marx's wish, ignoring the fact that Kirby was the one to collect the power required to summon him. Fortunately, the Sun and Moon delay Nova's journey for just enough time so Kirby could fly into the comet and disable his heart. Nova's heart serves as a boss battle which involves shooting at the pillars surrounding Nova's actual heart, which are guarded by rocket-like auto-turrets. When all of the pillars are destroyed, the Nucleus shoots out and part of Nova's left side explodes, paralyzing him. Upon Marx's defeat, he gets launched into the destroyed part of Nova, thus causing a chain reaction that creates a massive explosion. Nova was annihilated, as his body was shown to have blown to pieces during the sequence. Both Marx and the Heart of Nova return as bosses in The Arena.
In the Nintendo DS remake, Kirby Super Star Ultra, Nova has an expanded role as shown in the Meta Knightmare Ultra and The True Arena sub-games. In the former sub-game, it's shown that Meta Knight summoned Nova and wished to fight "the greatest warrior in the galaxy". Nova, in turn, fulfilled his wish by summoning Galacta Knight, a warrior sealed away in fear of his power. In the latter sub-game, it's revealed that Marx was near death since the explosion after his battle, but he merged Nova's pieces with himself, becoming the powerful Marx Soul.
Spoiler alert: Plot-specific details are within this box
Although Nova does not directly feature within the story, the Mother Computer Star Dream is heavily implied to be the direct result of Haltmann's attempts at reverse engineering it. This is reflected in the fact that its third form makes it resemble Nova in terms of appearance, even more so as Star Dream Soul OS where the equivalent form is golden in color. It even attacks using the objects surrounding original Nova (such as piano keys, light bulbs and compasses). The beginning of Star Dream Soul OS's final phase plays like an on-foot version of the original Galactic Nova Nucleus battle, and later on during the same fight, it will use the enemies found within the short level before the nucleus as projectiles.
End of spoilers
Nova is a large, mechanical comet made in the shape of a pocket watch. Nova's center is designed to look like a sun and he has a shooting star above his right eye. All around the edges of Nova are seemingly random items. Going clockwise the items are: a weather vane, a telescope, a wind-up key, a drafting compass, a globe (strangely absent in both Kirby Super Star and Kirby Super Star Ultra's official artwork), two atomic tubes, a pendulum, a numberless pocket watch with the hands bent, a triangular ruler, a chain, several piano keys, a compass, and a light bulb next to the ring that lies on top of his head. Nova has two purple eyes and a cat-like mouth. Nova has a small hole on his upper left corner of his face. The back of him shows four nozzles that form his main engine cluster.
After the destruction of the Heart of Nova, the small hole on Nova's face becomes bigger and his left eye closes.
You will now fight a warrior who has been sealed away for fear. His power was too great. I will now summon this warrior once more!”
— Nova to Meta Knight • Kirby Super Star Ultra
Nova is missing his globe in his official artwork for Kirby Super Star Ultra because he had been destroyed and reassembled.
The chain that emerges from the piano keys and end at the pocket watch is gray and is also on top of the triangular ruler in Kirby Super Star. However, in Kirby Super Star Ultra, the chain turns dark blue and it appears under the triangular ruler.
In Kirby Super Star Ultra, it is implied in the pause screen description of Marx Soul that Nova is not the only being of his kind. It states that Marx "absorbed a Nova's power" instead of simply saying "he absorbed Nova's power". If this is not a translation error, then it would seem to indicate the existence of more than one Nova, although only one has been confirmed. Despite a hint at there being more than one Nova, the one summoned by Meta Knight is obviously the same one since he was reassembled from various pieces that seemingly come from distant space rather than simply appearing, and this Nova displayed clear signs of damage from the fight with Kirby, such as the closed eye. Though, in the European version of the game, this hint about there being multiple Novas is not present, so it was likely a translation error.
Star Dream from Kirby: Planet Robobot could possibly be one of the other Novas created on Halcandra. The pause menu description of Star Dream Soul OS's second and third phases, as well as the first half of the fourth phase hints that Haltmann simply reactivated a dormant machine. This machine could have just been the Mother Computer, or an entire Nova.
Haltmann also claims that he reactivated Star Dream with the knowledge gained from analyzing "the most advanced civilizations in the known universe," one of which was likely The Ancients of Halcandra.
After the scene where Marx makes his wish and transforms, Kirby is taken back to the game map. As the Sun and Moon hold Nova back from reaching Pop Star, it is seen that Nova is the same size as the Sun and Moon. Therefore the size relationship between Kirby and the Sun and Moon is equivalent to the size relationship between Kirby and Nova.
An unused splash screen within Kirby Mass Attack's code depicts an image of Nova, presumably used as a placeholder or error handling message. The word written on the splash screen says, "Image does not exist" (画像がありませぬ).