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. Earth is the third planet from the Sun. The Sun is a star, the one at the center of our solar system, and the real story of the Sun is a long one. It formed about 4.6 billion years ago when a giant spinning cloud of gas and dust, the solar nebula, collapsed under its own gravity. Astronomers call stars that steadily fuse hydrogen into helium main sequence stars, and the Sun is roughly midway through this longest phase of a star's life, a phase in which it stays steady because the push outward from fusion balances gravity's pull inward.
The Sun is one star among 100 billion or more in our galaxy, the Milky Way. Inside the Sun, the fusion that holds it steady follows a set path. 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. Neutrinos are made whenever atomic nuclei come together, as in the Sun, or break apart, and they are tiny, neutral and so light that no one has been able to measure their mass. Every second, about 600 million tons of hydrogen in the Sun's core are converted into helium, and a tiny bit of mass disappears and turns into energy, as Einstein's formula E = mc2 says. 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.
The Sun gives Earth its light and its warmth. Spread over the whole globe, the solar input to Earth 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. The Sun heats the equator more than the poles, and the atmosphere and oceans work non-stop to even out that heating. The Sun is also in motion: because it is not solid, it spins once about every 25 Earth days at its equator and once every 36 days at its poles, and the Sun, with its planets, orbits the center of the Milky Way, which takes about 230 million years for one trip.
Much later, the Sun's story will change. A star's mass sets how fast it burns fuel, and a star's core eventually runs out of hydrogen. 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, and it will become so large that it will engulf Mercury and Venus, and possibly Earth as well. A low-mass star such as the Sun 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. Long before that, over a billion years from now, the Sun's gradual brightening is expected to vaporize Earth's oceans.
All of this lies far ahead, and light from the Sun takes about 8 minutes to reach Earth. For our pretend question, the useful fact is about speed: gravity does not act instantly across space, and 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. Gravitational waves are ripples in space-time made by accelerating massive objects. The Sun's gravity pulls on Earth and the other planets and keeps them circling around it. That is why, if the Sun vanished, 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.
Words to know
- nebula: cloud of gas
- neutrinos: tiny particles
- fusion: joining of nuclei
- dwarf: smaller star type
- orbits: circles around
