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The nucleus of a comet is the core of this celestial traveler. It is composed mainly of ice, dust, and rocky particles.
The nucleus is typically the only part of a comet that remains solid, lying dormant when far from the Sun.
It is often described as looking like a small, icy asteroid or large snowball floating through space. Despite its hard and frozen state, the nucleus holds volatile materials, mainly frozen water and gases, that can turn into gas or vapor through sublimation when heated. When a comet is distant from the Sun, no sublimation occurs, and therefore little to no visible activity surrounds the nucleus.
This is why comets appearing in these remote orbits lack the brilliant display often associated with them.

As a comet approaches the Sun, the heat starts a fascinating transformation.
The increase in temperature causes the comet's ice to vaporize in a process called sublimation, releasing gas and dust.
This forms a luminous cloud around the nucleus known as the coma. The coma can grow large, often thousands of kilometers across, while the nucleus itself remains relatively small. Around this area, the force of the solar wind causes dust and gas to stretch outward, forming the comet's impressive tail.
The tail is really two separate tails: a bluish ion tail, made of charged particles, and a dust tail that appears more white or yellow.
The ion tail points directly away from the Sun due to the solar wind, while the dust tail curves off along the comet's path.

Sublimation plays a key role in the evolution of a comet's visual characteristics.
This process is the change of solid ice directly into vapor, bypassing the liquid phase.
In the chill of space, sublimation occurs as comets approach the Sun and receive its warmth. Frozen gases like carbon dioxide and methane within the nucleus begin to sublimate, releasing these gases along with embedded dust particles into space.
The result is the creation of the coma and the characteristic tails of the comet.
Continuous sublimation is why comets become such stunning spectacles, with their glowing comas and long tails visible from Earth.

The solar wind is a flow of charged particles released by the Sun that has a significant effect on comets.
As these particles encounter a comet, they exert pressure and blow the ion tail away from the Sun. The ion tail will thus always point directly away from the Sun, while the slower-moving dust particles form the broader, curved dust tail.
The solar wind not only shapes these tails but also contributes to the loss of material from the comet. Repeated passes past the Sun will see the comet shed more mass, as solar wind effects, coupled with sublimation, wear down its surface.
Over many orbits, this can lead to significant erosion of the nucleus, ultimately causing the comet to shrink and sometimes disintegrate entirely.