Here's a breakdown:

* Stars: Stars have incredibly hot and dense atmospheres composed of primarily hydrogen and helium, with smaller amounts of other elements like carbon, oxygen, and nitrogen. These atmospheres are constantly changing due to the intense nuclear reactions happening in the star's core.

* Planets: Planets have atmospheres that vary greatly in composition and density. Some planets have thick, dense atmospheres (like Venus), while others have thin, wispy ones (like Mars). The composition of a planet's atmosphere depends on many factors, including its distance from the sun, its geological activity, and its history.

Examples of atmospheric gases:

* Hydrogen (H2): The most abundant gas in the universe, and often a major component of star and giant planet atmospheres.

* Helium (He): The second most abundant element in the universe, also found in star and giant planet atmospheres.

* Carbon Dioxide (CO2): A major component of the atmospheres of Venus and Mars.

* Nitrogen (N2): The dominant gas in Earth's atmosphere.

* Oxygen (O2): Essential for life on Earth, found in smaller amounts in other planetary atmospheres.

* Water Vapor (H2O): Found in varying amounts in planetary atmospheres, contributing to weather patterns.

Beyond the gases themselves:

* Atmospheric Pressure: The weight of the atmosphere pressing down on a surface.

* Temperature: The average heat of the atmosphere.

* Wind: The movement of air in the atmosphere.

* Clouds: Formed by condensation of water vapor or other substances.

Studying planetary atmospheres provides valuable information about the planet's formation, evolution, and potential for life.