Torajii, unlike many other stars, was alive. It was capable of possessing humans, raising their temperature to hundreds of degrees and converting their oxygen into hydrogen. These possessed individuals would gain glowing eyes, which allowed them to incinerate people. The original personality would be subsumed, but Torajii could make use of the person's memories, allowing it to intelligently react to problems.
Torajii could possess Time Lords, but they had the ability to hold back the possession through sheer willpower, but the effect was extremely painful and given time the power of the possession would overwhelm the Time Lord's control, such as it did with the Tenth Doctor. Almost immediately, the Doctor demanded to be flash frozen to remove the solar energy from his body.
Torajii displayed a high level of intelligence instead of just acting like a wounded animal, as it was able to use the infected crew's knowledge to its own advantage and attack the ship in the most effective way. Also, when it sensed the crew were close to escape it spread the infection, allowing it to attack on two fronts. (TV: 42)
Torajii was first encountered by humans as a living being when a cargo ship, the SS Pentallian, hurriedly took some gas from the star as fuel for an illegal fusion scoop. Unaware that they were dealing with a sentient being, the fusion scoop caused Torajii incredible pain and brought it to a maddening rage where it sought to take revenge of the ship and regain what had been stolen from it. Gas from the star travelled to the starship and possessed two members of the crew, Hal Korwin and Dev Ashton, who then went on to sabotage the ship and kill many crew members.
Eventually, the gas possessed the Tenth Doctor, who told the remainder of the crew the source of the problem and asked them to dump the illegal fuel into the star. Torajii then allowed them to leave. The Doctor requested that buoys be placed around Torajii so that no one would attempt to fusion scoop there, stating that as a sentient being Torajii "deserved protection." (TV: 42)