Our Star: The Sun

Our star, the Sun, is a middle-aged yellow star that is more massive than the average star. It is a star that nurtures and supports life on Earth. Its heat and light warm Earth's surface, drive phenomena such as weather and ocean currents, and fuel photosynthesis. We experience the Sun's energy every time we feel its warmth on our skin or see with the aid of its light.


The Sun's energy is generated deep within its core by one of the most powerful processes in the universe: nuclear fusion. Hydrogen nuclei smash together, forming helium and releasing huge amounts of energy. This is why a star shines. It burns its fuel through nuclear fusion (unlike fire, which burns through oxidation). The balance between the outward push of gas heated by fusion and the inward pull of gravity is called hydrostatic equilibrium.

In the radiative zone, closest to the core, the gas is smooth and static, and the energy (light of all wavelengths) diffuses through it as radiation. Above this layer is the convective zone, where swirling currents of gas carry the Sun's energy outward in a process called convection: gas is simultaneously heated from below by fusion, and cooled from above as energy is released into space. Convection causes the gas to churn, like water just before it boils.

The photosphere is the Sun's visible surface, where the atmosphere of the Sun becomes transparent to visible light. Sunspots are cooler regions of the photosphere. The chromosphere and corona are the outermost layers of the Sun. The chromosphere is ten times hotter than the photosphere, but the corona is still hotter—a million degrees—so hot that it escapes the star's gravity and flows out into space as solar wind.

Also at the Museum Beyond Planet Earth


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Journey to the Stars Online Educator's Guide