Spirit stars were points of light said to appear in the sky to represent Toa.
Each Toa had a spirit star. As long as it burned in the night sky, that Toa remained alive. Lhikan's spirit star appeared as a yellow streak of light in the blackness. The direction in which a spirit star moved apparently indicated its Toa's location.
Six stars above Mata Nui were believed to represent the Toa Mata.
Toa stars within the Matoran universe were a kind of electrical impulse from Mata Nui's system. The ones that appeared outside of the Matoran universe probably had some connection to the red star.
A Toa's spirit star didn't go out when they gave up their Toa energy, but instead when they became a Turaga.
Spirit stars couldn't follow their Toa outside of the Matoran universe.