You're right, comets are primarily composed of ice and dust. So, how do they shine so brightly in the night sky? It's not because they're inherently glowing. Instead, it's due to reflection and interaction with the Sun's energy:

* Sunlight Reflection: As a comet approaches the Sun, its icy and dusty surface heats up. This causes some of the ice to sublimate (transform directly from solid to gas), releasing dust and gas into a vast cloud around the comet called a coma. These dust particles reflect sunlight, making the comet appear bright.

* Solar Wind Interaction: The solar wind (a stream of charged particles from the Sun) interacts with the coma's gas, causing it to fluoresce, emitting its own light. This process is called ionisation, and it contributes to the comet's visible brightness.

Essentially, comets are like giant mirrors in space, reflecting sunlight and glowing from the interaction of solar wind with their gases. The brightness of a comet depends on its size, composition, and how close it is to the Sun.

Here's a simplified analogy: Imagine a snowball thrown into a hot oven. The snowball would melt and release water vapor, creating a cloud. This cloud would both reflect the oven's light and glow faintly due to the heat. A comet is similar, reflecting the Sun's light and glowing from the interaction with the solar wind.