STAR Forms are a new type of transformation introduced in Tech 10: Star Spirit.
When the StarTrix Requiem begins to overheat, it will present the user with two options: the first is to shut down the StarTrix and let it cool; the second is to redirect the excess energy flow into the user's body. When the second option is chosen, the transformation in question assumes their STAR form.
STAR forms are possible due to a unique type of nanomachine that inhabits the StarTrix Requiem. When a STAR transformation is activated, these nanobots spread throughout the user's body, carrying the excess energy from the StarTrix's Star Connection Drive and infusing it into the user. This quite literally infuses the transformation in question with the power of a star, giving their abilities a massive boost.
It's worth noting that activating the STAR Form function on All☆Star or any of its fusions will not change their physiology, as Heliosinge are already designed to absorb and utilise large amounts of solar power.
When aliens assume their STAR form, their overall appearance doesn't change too much, aside from possibly changing color and gaining some bulk. The StarTrix automatically changes their uniform to accommodate the STAR forms, changing its color scheme and adding a golden five-pointed star symbol somewhere. The StarTrix itself changes its dial color from Blue to Yellow.