Imagine the Sun going out. It is only a thought experiment, since scientists have no reason to expect the Sun to disappear suddenly, and as of November 2026 it is expected to keep shining for about 5 billion more years. The question is still useful, because a star does not burn like a campfire. Inside the Sun, hydrogen is squeezed together into helium, and this nuclear fusion is what makes the Sun's heat and light. The Sun's middle is so hot and so squeezed by gravity that hydrogen atoms meld into helium. Four hydrogen nuclei (protons) become one helium nucleus. A tiny bit of mass disappears and turns into energy, as Einstein's formula E = mc2 says. Every single second, about 600 million tons of hydrogen in the Sun's core are converted into helium.

The Sun is a giant, glowing ball of hot gas, and its core is its engine room. The core is about 27 million degrees Fahrenheit (15 million degrees Celsius), and it is about 86,000 miles (138,000 kilometers) thick and has a density of about 150 grams per cubic centimeter, roughly 8 times the density of gold. A star stays steady because the push outward from fusion balances gravity's pull inward. Energy made in the core does not leave quickly, since it takes about 170,000 years to travel through the radiative zone, the layer above the core, before it reaches the convection zone. There the temperature drops below 3.5 million degrees Fahrenheit (2 million degrees Celsius) as hot plasma bubbles rise. The Sun has no solid surface, because it is a ball of plasma, super-hot, electrically charged gas.

Light we see from the Sun is made in its photosphere, a word that means light sphere, and it is the layer that emits the most visible light. This part of the Sun is about 10,000 degrees Fahrenheit (5,500 degrees Celsius), much cooler than its core. Above it lies the corona, the Sun's outer atmosphere. It reaches up to 3.5 million degrees Fahrenheit (2 million degrees Celsius), much hotter than the photosphere below it. The solar wind forms a huge magnetic bubble around the Sun called the heliosphere, and it reaches beyond the planets, so Earth is inside the Sun's atmosphere. Tens of trillions of neutrinos from the Sun stream through your body every second, but you cannot feel them because they almost never interact with other matter.

Earth is the third planet from the Sun, and the Sun gives Earth its light and its warmth, and it is much bigger than any planet. At the top of Earth's atmosphere, sunlight delivers about 1,361 watts of power to each square meter facing the Sun, as of November 2026, which scientists call total solar irradiance. Averaged over the whole Earth and the whole year, the Earth system absorbs about 240 watts of solar power per square meter, and Earth's greenhouse effect raises its average surface temperature to about 15 degrees Celsius, more than 30 degrees warmer than it would be with no atmosphere. A star's mass sets how fast it burns fuel. Low-mass stars burn longer, dimmer and cooler, and some will shine for trillions of years, while some massive stars last only a few million years. The Sun is a yellow dwarf star, only an average star in size.

Words to know

• fusion: joining of nuclei
• plasma: super-hot gas
• photosphere: light layer
• corona: outer atmosphere
• neutrinos: tiny particles