Question

Is a comet hot, dense, and fiery? If so, what is the energy source? If not, why do comets look like burning masses of gas?

Short answer

Comets are not hot, dense, or fiery; their glow is due to solar radiation sublimating their icy components.

Step-by-step solution

Understanding what comets are

Comets are icy celestial bodies orbiting the Sun, mainly made up of ice, dust, and small rocky particles. They are not typically dense or fiery like a star.

Exploring the appearance of comets

When comets come close to the Sun, the heat causes the ice to vaporize, releasing gas and dust. This process creates a glowing coma around the comet and a visible tail, making it appear bright and fiery from Earth.

Explaining why comets are not hot or dense

Comets themselves are not hot, dense, or fiery because they are primarily composed of cold materials like ice and dust. Their fiery appearance is due to solar radiation and the effects of sublimation.

Identifying the energy source of comets

The apparent energy or brightness of a comet is due to solar heating, not an internal energy source. The Sun's radiation causes the comet's ice to sublimate, creating the visible glow.

Key concepts

Solar Heating

Solar heating is crucial for understanding the apparent behavior of comets. When a comet travels through the solar system, it draws close to the Sun where solar heating becomes significant. This process refers to the absorption of the Sun's radiant energy by the comet.
As the Sun's energy hits the comet's icy surface, it transfers heat to the particles. This heat causes changes in the physical state of components on the comet's surface.
- The result is the transformation of solid ice to gas, a process known as sublimation. - This process is responsible for the comet's bright appearance rather than any fiery properties. Solar heating, therefore, acts as the external trigger that leads to the subsequent creation of other cometary features.

Coma

The coma is one of the most distinctive features of a comet. As solar heating continues, sublimation intensifies, forming a large cloud of gas and dust around the nucleus of the comet. This cloud is referred to as the coma.
The coma forms whenever a comet approaches the Sun, typically beyond the orbit of Mars. - It can grow to thousands of kilometers in diameter. - Composed mainly of water vapor, carbon dioxide, and small dust particles, it reflects sunlight and appears as a glowing aura.
The expansion of the coma is a visible marker of a comet heating up and beginning its transition through the inner solar system.

Tail

The tail of a comet is a fascinating feature developed from the effects of solar heating and sublimation. As a comet nears the Sun, solar winds and radiation pressure act on the material in the coma, pushing it away to form a tail.
This tail always points away from the Sun due to solar wind effects.
- There are usually two types of comet tails: the dust tail and the ion tail. - The dust tail is composed of small particles and reflects sunlight, giving it a bright appearance. - The ion tail is formed from gases that emit light when ionized, often appearing bluish. The tail can stretch millions of kilometers, creating the comet's iconic streak across the sky.

Sublimation

Sublimation is a key process governing the activity of a comet. It happens when solid ice changes directly into gas, bypassing the liquid state, when heated. - This is driven primarily by solar radiation as the comet approaches closer to the Sun. - Sublimation results in the formation of the coma, releasing gas and dust from the comet's nucleus.
Understanding sublimation helps demystify why comets appear like fiery objects. The glow and tail formation are all consequences of the high-energy sublimated material reflecting or emitting sunlight. It is this sublimated material that, under the influence of solar forces, forms the majestic vision we see when observing comets.