Thom Kallor was born among the stars, in a research outpost orbiting the planet Xanthu. His parents were astronomers, and it is thought that the unique location of his birth allowed for him to develop super powers. Early on in his childhood, he discovered that he could temporarily increase the mass of an object, up to the mass of a star. He briefly developed a second set of powers very similar to those of a Kryptonian when he was caught in the tail of a comet, but in time those faded, and he was left with only his density abilities. As a teenager, he applied and was accepted into the Legion of Super-Heroes.
Star Boy begins a long relationship with Nura Nal, the precog Legionnaire know as Dream Girl. However, when he kills Dream Girl's ex-boyfriend in self-defense, he is expelled from the Legion for violating their rules against killing. Dream Girl leaves the Legion as well, and the pair join the Legion of Substitute Heroes. However, some time later, the two disguise themselves as "Sir Prize" and "Miss Terious," and reapply for Legion membership, and are accepted back on the team. Not long after, Star Boy changes his costume from a purple, caped uniform resembling Superboy's, into a suit with that has a full body starfield, which he would wear for many years.
After the events of ZERO HOUR, we are introduced to a new version of Star Boy, who replaces the deceased Kid Quantum as Xanthu's representative to the Legion. As with the previous Star Boy, this version of Thom Kallor also temporarily acquires several Kryptonian-like super-powers, like the power to breathe fire, but he finds these new abilities difficult to control. The post-ZERO HOUR version of Star Boy and all his teammates were eventually revealed to be living on the parallel world of Earth-247.
A third version of Star Boy later emerged, this time on the parallel world of Earth-Prime. Although the previous versions of Star Boy were presented as white, this Thom Kallor was black. During his tenure with the Legion, he saw himself as founding member Cosmic Boy's right hand man.
The original Star Boy appears once again, this time in the 21st Century. It was revealed that he traveled from his future to the KINGDOM COME universe, known as Earth-22, and then to the 21st Century. Because of the limits of 21st Century medical knowledge, Kallor began to hear voices in his head, and he is then diagnosed as borderline schizophrenic, a condition that was held at bay due to future medicine. He joins the Justice Society of America as Starman, but when he was not adventuring with them, he was a voluntary patient at the Sunshine Sanitarium.
While in the sanitarium he assumes the identity of Danny Blaine, a name that Kallor took from his favorite boyhood fictional hero growing up Xanthu. Eventually, the supremely powerful Gog uses his powers to restore Thom's sanity. Starman leaves the sanitarium and works as a gravedigger, which he believes will help him carry out his mission in the present: procure the body of Superboy Conner Kent in order for him to be resurrected in the future. He later learns that his starfield suit was designed by three different Brainiac 5s from three parallel universes, and the costume itself is a map of the recreated multiverse.
Starman later became a part of a secret Legion team sent by the late R.J. Brande to the 21st century to ensure their future comes to pass. In the post-FLASHPOINT reality, Kallor is once again seen in the future as a member of the Legion, although he is now a paraplegic. Star Boy dies when the Legion headquarters collapses during an attack from the Fatal Five's onslaught, but later is resurrected and rejoins the Legion.