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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.
Layers of the Sun © NASA
Layers of the Sun © NASA


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.

Radiative and convective zones

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.

Photosphere, chromosphere, and corona

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.

Understanding sunshine

What we see as sunshine is the visible light that reaches Earth and lights our day. But the Sun also gives off energy in invisible wavelengths of light, such as gamma rays, X-rays, ultraviolet, infrared, microwave, and radio.
Spacecraft that orbit Earth and the Sun provide dramatic, close-up images of the Sun in different wavelengths of light. Heliophysicists color code the images to make them easier to interpret: they use artificial color to visualize the Sun in different wavelengths.

The Sun and the electromagnetic spectrum

The electromagnetic spectrum is the entire range of electromagnetic radiation (light). As wavelength increases, frequency and energy decrease. This image of the Sun is actually three images merged into one. Heliophysicists took images of the solar corona at three wavelengths within the invisible UV range. They assigned a color code (red, yellow, blue) to each image, revealing what solar features, like flares, look like at the different wavelengths.
Sun wavelengths
The images in this silent video are observations of the Sun in many wavelengths of light. Seething activity and tremendous outbursts are everyday occurrences.
Khan Academy video wrapper

Solar wind and radiation

The solar wind is a constant flow of hot gas that blasts out from the Sun’s corona at a million miles an hour. Fortunately, Earth’smagnetic field and atmosphere almost always protect us: typically, only a trickle of solar wind gets through, sliding down to the North and South Poles and producing radiant displays of light called auroras. Earth’s magnetic field also protects us from the constant flow of dangerous radiation emitted by the Sun. However, sometimes magnetic explosions on the Sun, called solar flares, create storms in the solar wind. Under rare conditions, they can disrupt radio, cell phones, and GPS, or even cause blackouts on Earth. The Sun, the Sun’s magnetic field, and the solar wind together form a dynamic, interconnected system called the heliosphere, which extends across our Solar System to beyond the Kuiper Belt (a disk of millions of comets that orbit the Sun beyond Neptune).
Satellite images reveal gusts in the solar wind.© AMNH
Solar wind drags the Sun’s magnetic field along with it.
Earth is almost always protected from the solar wind by its own magnetic field and atmosphere. © AMNH

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