The central frozen body of a comet plunging inward toward the Sun is called the nucleus. This small, solid chunk consists of volatiles like water ice and is responsible for the creation of the comet's atmosphere and tail.
Step-by-step explanation:
For a comet that is plunging inward toward the Sun, its central frozen body is called the nucleus of the comet. The nucleus is considered the real comet, containing the fragment of ancient icy material responsible for creating the comet's atmosphere, known as the coma, and its tail when it gets close enough to the Sun. These cometary nuclei are typically only a few kilometers across and are composed of volatile substances like water ice mixed with dust and other solid materials, earning them the nickname "dirty snowballs" from their description first suggested by Fred Whipple in 1950.
Comets generally spend most of their time in the cold regions of space, such as the Oort cloud or the Kuiper belt, and only start losing mass and forming a visible atmosphere and tail when they approach the inner solar system and the Sun's warmth causes their volatiles to evaporate.