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Supernova explosions are one of the most energetic events in the universe. When a massive star reaches the end of its life, it can explode in a supernova, releasing enormous amounts of energy. This explosion can heat surrounding intercloud gas to extremely high temperatures. The energy released is so vast that it outshines entire galaxies for a short period. Supernovae are essential for dispersing heavy elements throughout the galaxy, contributing to the formation of new stars and planets. They play a key role in shaping the interstellar medium.

The energy from these explosions can heat intercloud gas to millions of degrees, making supernovae a primary source of heating for this hot gas. This energy also generates shock waves that further heat the gas and drive it into motion.

Starlight, or the radiation from stars, plays a significant role in heating intercloud gas, but its effects are generally more limited compared to supernova explosions. Starlight can ionize the gas in star-forming regions, making it slightly warmer. However, it usually doesn't reach the extremely high temperatures necessary to categorize the gas as 'hot'.

While starlight can keep some regions of the interstellar medium warm, its impact on large-scale intercloud gas heating is relatively minor. Stars emit a spectrum of light including ultraviolet (UV) and infrared (IR) radiation, which can heat the dust and gas, but not to the same extent as the cataclysmic energy from supernovae.

Protostars are the early stages of star formation where a dense region of gas and dust collapses under gravity. During this time, the protostar heats up due to gravitational contraction. This heating affects the immediate surrounding area where the protostar is forming, but it doesn’t extend to vast intercloud regions.

Protostars primarily contribute to localized heating in small, dense regions of molecular clouds. The energy output of a protostar is considerably less than that of a mature star or supernova explosion. While they are crucial for heating the immediate environment, they don't significantly impact the temperature of widespread intercloud gas.

Neutrinos are incredibly light, neutral particles that rarely interact with matter. They are produced in vast quantities during nuclear reactions in stars and in supernova explosions. Although they carry away a significant amount of energy, their interaction with interstellar gas is minimal because neutrinos can pass through almost anything without being stopped.

This means that neutrinos do not significantly contribute to the heating of intercloud gas. Their weak interactions make them ineffective in transferring their energy to the gas. Understanding neutrinos, however, is crucial for studying fundamental particle physics and the inner workings of stars.