Here's why:
* Plasma: Stars are made up of extremely hot, ionized gas called plasma. This means the atoms have been stripped of their electrons, creating a sea of charged particles.
* Luminous: Stars emit light and heat due to nuclear fusion occurring in their core. This fusion process releases immense amounts of energy, making stars incredibly bright.
* Ball: Stars are generally spherical due to the balance between their internal pressure and gravity.
While other things can be considered "luminous balls of plasma" in a more general sense, stars are the most prominent and obvious example.
