Final answer:
Movements that occur rapidly and cannot be altered once begun are known as reflex actions, important for swift responses to stimuli. The mention of a star collapsing references a supernova, a massive stellar explosion marking the end of a star's life cycle.
Step-by-step explanation:
Movements that occur very rapidly and generally cannot be altered once begun are known as reflex actions or reflexes. These are involuntary movements that take place in response to a stimulus. A classic example of a reflex action is the knee-jеrk reflex, which occurs when the knee is tapped and the leg kicks out without conscious thought. Reflexes are critical for survival as they allow organisms to respond swiftly to danger or changes in the environment.
The reference to a star exhausting its store of nuclear energy and beginning its final collapse indicates a supernova event. This is a powerful and luminous stellar explosion that occurs at the end of a star's life cycle. The core collapse of massive stars leads to the supernova explosion, releasing vast amounts of energy and often outshining entire galaxies briefly.