Consider a pretend question: what if the Sun went out? It is a thought experiment, and nothing in it is a forecast. Scientists have no reason to expect the Sun to suddenly disappear, and as of November 2026 the Sun has been shining for billions of years and is expected to keep shining for about 5 billion more. The Sun is a star, the one at the center of our solar system, and it is a giant, glowing ball of hot gas. It gives Earth its light and its warmth, and asking what would follow if that stopped is a way to see how much of the world runs on one star, and the answers depend on physics that is worth knowing in its own right. The Sun formed about 4.6 billion years ago when a giant spinning cloud of gas and dust, the solar nebula, collapsed under its own gravity.
Start with what the Sun runs on. The Sun does not shine the way a campfire does. Inside the Sun, hydrogen is squeezed together into helium, which is nuclear fusion, and it makes the Sun's heat and light. Four hydrogen nuclei (protons) become one helium nucleus. Fusion releases energy because the total energy of the helium made is less than the energy of the protons that went into it, a small mass difference that becomes energy, as Einstein's formula E = mc2 says. Every second, about 600 million tons of hydrogen in the Sun's core are converted into helium. There is a puzzle in how this starts. At about 14 million kelvins, the Sun's core is not hot enough for two protons to overcome their electric repulsion, called the Coulomb barrier. Fusion happens because quantum tunneling lets protons pass through it; in the Sun's proton-proton chain, protons fuse into deuterons, then helium-3, then helium-4, and the reactions also release neutrinos, positrons and gamma rays. The proton-proton fusion cycle fuels the Sun and other stars with core temperatures below about 15 million kelvins, and one reaction cycle yields about 25 MeV of energy.
The core is about 27 million degrees Fahrenheit (15 million degrees Celsius), and it is about 86,000 miles (138,000 kilometers) thick, with a density of about 150 grams per cubic centimeter, roughly 8 times the density of gold. The energy then has to get out; energy from fusion tries to leave the Sun at the speed of light, but more than 300 thousand Earth masses of gas lie in the way. Energy made in the core 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 light we see is made in the photosphere, a word that means light sphere, and it is the layer that emits the most visible light. The photosphere is about 10,000 degrees Fahrenheit (5,500 degrees Celsius), much cooler than the core.
A star stays steady because the push outward from fusion balances gravity's pull inward; a star's mass sets how fast it burns fuel. A big star must burn fuel faster to generate the energy that keeps it from collapsing under its own weight, so more massive stars use up their fuel sooner. 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, and astronomers call stars that steadily fuse hydrogen into helium main sequence stars. The Sun is roughly midway through this longest phase of a star's life. The Sun is also moving: it spins once about every 25 Earth days at its equator and once every 36 days at its poles, because it is not solid, and the Sun, with its planets, orbits the center of the Milky Way in a trip that takes about 230 million years.
Only a sliver of the Sun's output reaches us; the Sun's total output is about 3.86 x 10^26 watts, and the whole cross-section of Earth intercepts only about 1.74 x 10^17 watts of it. 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. Spread over the whole globe, the solar input averages about 340 watts per square meter, and it varies by about 0.1% over the 11-year solar cycle. The atmosphere and surface together absorb 71 percent of incoming solar radiation, and they must radiate that much energy back to space for Earth's average temperature to stay stable. Earth's greenhouse effect raises that average surface temperature to about 15 degrees Celsius, more than 30 degrees warmer than it would be with no atmosphere. Every 1.5 millionths of a second, the Sun releases more energy than all humans use in an entire year.
Now picture the Sun vanishing; light from the Sun takes about 8 minutes to reach Earth, so we would not know for about 8 minutes, and then the sky would go dark. The Moon would go dark too, because it only reflects sunlight, but distant stars would still be visible. Gravity does not act instantly across space either: changes in gravity travel at the speed of light. General relativity predicts that gravitational waves travel exactly at the speed of light, and a neutron star merger, seen in both light and gravitational waves arriving at almost the same time after 130 million years of travel, confirmed it. So Earth would keep its path for about 8 minutes, then leave its orbit and move off in a straight line, along with the other planets. Without sunlight, light would come only from other sources: electric lights, fire, living things that glow (bioluminescence) and fluorescence, and people would lose track of day and night.
What would come after the dark? The Sun's energy heats Earth's surface and air, powers ocean currents, drives photosynthesis and evaporation, and melts snow and ice, and Earth's average surface temperature is about 15 degrees Celsius (59 degrees Fahrenheit), as of November 2026. Here the estimates come from a single report, on the science news site Live Science, and they are best read as rough. It says Earth would at first cool by an average of roughly 20 degrees Celsius (36 degrees Fahrenheit) every 24 hours, with the daily drop shrinking as it got colder, and that almost the whole world would pass below the temperature at which water turns to ice within two to three days. Small ponds might turn to ice within a week, and lakes might take weeks or months; plants kept in the dark could not make food by photosynthesis, though some might stay dormant for weeks to months, as they do in winter.
Earth is the only planet in our solar system with liquid water on its surface. The same report says the oceans could stay liquid for many years, maybe decades. In the deepest parts of the oceans, near volcanoes, scientists say water might stay liquid for as long as the volcanoes last, potentially billions of years, because the heat there comes from inside the Earth. As for how cold space can be, even with no star nearby, an object cannot cool all the way to absolute zero (about -273 degrees Celsius), because leftover heat from the Big Bang keeps space at about -270 degrees Celsius, and for comparison, Pluto, about 39 times farther from the Sun than Earth, averages about -232 degrees Celsius. The Sun heats the equator more than the poles, and the atmosphere and oceans work non-stop to even out that heating. Sunlight reaches only the top of the ocean, and below about 1,000 meters the ocean is completely dark.
Not every living thing runs on sunlight. Below about 200 meters there is so little light in the ocean that photosynthesis is no longer possible, yet deep in the dark ocean, hot springs called hydrothermal vents have living things that do not depend on sunlight. The vents are produced by underwater volcanoes or tectonic activity, and they were discovered near the Galapagos Rift in 1977. Some microbes, including bacteria and archaea, use chemosynthesis, getting energy from chemical bonds in rocks and minerals instead of from sunlight. Vent bacteria run on hydrogen sulfide (H2S) chemistry: the same job plants do with sunlight, they do with the energy stored in a chemical that is toxic to most living things. This vent chemosynthesis can be written as 6 CO2 + 6 O2 + 24 H2S = C6H12O6 + 24 S + 18 H2O: carbon dioxide, oxygen and hydrogen sulfide make glucose, sulfur and water. Giant tube worms at vents have symbiotic bacteria that make food through chemosynthesis, and some vent worms grow to almost five feet long in less than two years.
Today the Sun is large: it would take about 1.3 million Earths to fill the Sun's volume, and it holds 99.8% of the solar system's mass, but the real future of the Sun is different from the pretend one: over a billion years from now, the Sun's gradual brightening is expected to vaporize Earth's oceans. Separately, a star's core eventually runs out of hydrogen, and when it does, fusion pressure no longer balances gravity, the core starts to collapse, and squeezing it raises its temperature and pressure and makes the star slowly puff up. In about 5 billion years, as of November 2026, the Sun will swell into a red giant. As a red giant it will become so large that it will engulf Mercury and Venus, and possibly Earth as well. It will then shed its outer layers as a planetary nebula and leave a white dwarf, a roughly Earth-sized stellar cinder that cools over billions of years.
Words to know
- fusion: joining of nuclei
- photosphere: light layer
- chemosynthesis: food from chemicals
- tunneling: slipping through
- neutrinos: tiny particles
