Adam Starr, zeppelin pilot, was a true hero. For several years he'd been little noticed by those around him, and was in the process of going through a painful divorce. But one day the zeppelin he was piloting was hijacked by DRAGON troops on a flight to Washington DC. They planned to detonate a flux-bomb over the city which would kill everyone in it and plunge the country into chaos. Adam was able to hold them off long enough for the Rangers of Freedom to retake the zeppelin, but the bomb was set off. He leaped out of the plane and took the brunt of the bomb himself, disintegrating him into ashes. The young and handsome pilot who sacrificed his life for his country soon became a hero mourned by America and much of the rest of the world.
Two weeks later a strange, luminous figure appeared out of nowhere above Washington, DC. For about a day he simply floated in midair, not reacting to anything. When the light subsided and he descended from the sky the world saw that he was in fact Adam Starr, transformed by the bomb into something more than human. He was giving off powerfully concentrated astro-force and no longer needed to eat or breathe, but psychologically he seemed human enough. His kindness and courage seemed undimmed. It was only natural that he would become a superhero, taking on the name Stellar-Man.
Stellar-Man instantly became an iconic and well-respected figure throughout the world, even more so than his fellow novice hero Astro-Man, a beacon of hope in a world whose superheroes had recently been beset by confusion and scandals. He was accepted by the Legendary Squadron, the privileged elite of the world's heroes. Astro-Man had previously turned them down when they refused to take the somewhat troubled young hero Outworlder, as they feared he would go down the same path as their former member Albedo. Astro-Man went on to form a team of his own instead. The Squadron was more than happy to welcome Stellar-Man as a replacement. His joining the team confirmed his place as a truly great hero and helped restore the team's damaged reputation. Not everyone was celebrating, though. Stellar-Man was staying aloof from those who knew Adam Starr, and some of them suspected that he was not the same person.
They were right. Adam Starr really had been killed by the flux bomb. Stellar-Man was a Viron clone. When Astro-Man refused to join his fellow Virons in their conquest of Earth, Kalthar and Tenebrax decided to make their own champion to replace him. He was created from the DNA and brain patterns of Starr and the Viron General Barbatos, Kalthar's own former mentor. His mission is to lay the groundwork for the Virons to take over the world and to kill Astro-Man in secret.
Of course, there are drawbacks to this plan. Because creating and manipulating the brain circuitry that allows people to wield the Astro-Force is extremely difficult, the process of creating Viron is often difficult. Artificially aging the Stellar Man made him unstable. If he does not drain the life-force of an organically born Viron, they will almost always die within a few years. If he does not he will become an astral vampire, a wraithlike being which drains the lifeforce of others.
Stellar-Man's powers are essentially the same as those of Astro-Man, although as he possesses the brain patterns and memories of a centuries-old Viron general he can manipulate them much more efficiently. He does not suffer from the same weaknesses as Astro-Man. However, he must take great care not to channel too much Astro-Force at once because his biology is inherently unstable. He also has a genius strategic mind.
Stellar-Man is not cruel by temperament, and is able to pass as a hero because he does indeed possess many heroic traits. He is courageous, decent, and a natural leader. But he is unquestioningly loyal to the Viron cause and to his creators. Due to the two sets of memories he possesses he considers himself deeply loyal to both America and Viro, and believes the coutnry would be better off under Viron rule. However, he is at risk of psychologically degenerating as he approaches death.
Note: Another possible origin for Astro-Girl is that she is Stellar-Man's former sidekick, either another clone (who wouldn't be as unstable as Stellar-Man is due to not being aged as much) or an agent from a bloodline based on another humanoid race. Also, the real Adam Starr may still be alive in another world or time, probably heavily mutated, and may return to the Earth to seek help against a new enemy he'd found.