The vast majority of Pokémon fans will be wondering why the Pokémon Sun Legendary, the Solgaleo be type Psychic / Steel and not Psychic / Fire, after all, as the game has the biggest star name in the solar system, which is usually represented by fire.
As everyone knows, the sun is a star. Despite he seemed to be burning, this star is a lot of dust and gas, and hydrogen most of this gas. In their cores, they undergo a process called "nuclear fusion" where hydrogen atoms are crushed by gravity, thus forming helium. This fusion is so powerful that produces energy, radiation. The radiation is pushed by gravity and maintains the star safely without it explodes. You know the sun's rays? So they are this radiation that is pushed by gravity.
While this merger continue producing this radiation, the star will continue burning, and thus will remain stable. However, in very heavy stars, the merger does not stop on helium. When hydrogen is exhausted, it begins to fuse helium to create radiation. And the process continues, passing through various elements such as carbon, neon, oxygen and even silicon.
But to some extent, we know that the stars fuels will run out, and thus the merger cease. And when a star reaches that point, the fused atoms begin to become a specific element, which is iron.
This element, which would be the representation of the type of Solgaleo, is the "final stage" of the merger, and it starts to accumulate in the nucleus of the star. At that point, the stars lose the ability to create radiation entirely, and thus begin to collapse until they die in a supernova. For this reason, Solgaleo has the steel type, likely to be related to a star about to "die." What makes sense also because as it is known, "the beast that devours the sun," the element that destroys a star.