The Sun

A star up close

The Sun is a star like the thousands you see at night; it just happens to be about 93 million miles away instead of light-years. It holds about 99.8% of all the mass in the Solar System, and its gravity keeps every planet, asteroid and comet in orbit.

It is made mostly of hydrogen (about 73% by mass) and helium (about 25%), with small traces of heavier elements. The gas is so hot that it is in the form of plasma, a state of matter where atoms have lost some of their electrons.

Astronomers classify the Sun as a yellow dwarf, a G-type star. That sounds modest, but about 1.3 million Earths could fit inside it.

Layers from core to corona

The Sun has no solid surface. At the center is the core, where temperatures reach about 27 million °F. Energy produced there creeps outward through the radiative zone, a journey that can take more than 100,000 years, then rises through the convection zone in huge boiling cells of plasma.

What we call the surface is the photosphere, the layer that gives off most of the visible light, at about 10,000°F. Above it lie the thin chromosphere and the corona, the outer atmosphere. Strangely, the corona is far hotter than the surface, well over a million degrees, and scientists are still working out exactly why.

Fusion: how the Sun shines

In the core, the pressure and heat are so extreme that hydrogen nuclei collide and fuse into helium. This process is called nuclear fusion. Each time it happens, a tiny amount of mass turns into energy, just as Einstein's equation E = mc² describes.

Every second the Sun converts about 4 million tons of matter into energy. Even so, it has enough hydrogen to keep shining steadily for roughly another 5 billion years. After that it will swell into a red giant and finally settle down as a small, dim white dwarf.

Sunspots and the 11-year cycle

Sunspots are dark patches on the photosphere where strong magnetic fields block some of the heat rising from below. They are still extremely hot, only cooler than their surroundings, so they look dark by contrast. A large one can be several times wider than Earth.

The number of sunspots rises and falls over a cycle of about 11 years, as the Sun's magnetic field winds up and then flips. At solar maximum, sunspots, solar flares (sudden bursts of radiation) and coronal mass ejections (huge clouds of plasma thrown into space) are most common.

The current cycle, Solar Cycle 25, began in late 2019. In October 2024, NASA and NOAA announced that the Sun had entered its maximum phase, and high activity continued through 2025. As of 2026 the Sun is slowly heading toward its next minimum, though big flares are still possible.

The solar wind and space weather

The corona constantly blows charged particles out into space. This flow, the solar wind, travels at roughly 1 million mph and reaches far beyond Pluto, creating a vast bubble called the heliosphere.

When a burst of solar wind or a coronal mass ejection hits Earth, our magnetic field funnels particles toward the poles, where they make the air glow as auroras. Strong storms can also disturb satellites, GPS and power grids, which is why agencies like NOAA issue space weather forecasts.

How to observe the Sun safely

Never look at the Sun directly, and never through binoculars, a telescope or a camera viewfinder without a certified solar filter that meets the ISO 12312-2 standard. Sunglasses, smoked glass and exposed film are not safe. Even a brief glance through optics can cause permanent eye damage in a fraction of a second.

The simplest safe method is projection. Make a pinhole in a piece of card and let sunlight fall through it onto a second card; you will see a small image of the Sun. During a partial eclipse, the gaps between tree leaves do the same thing.

For a closer look, use eclipse glasses or a solar viewer certified to ISO 12312-2, and for binoculars or telescopes, a certified white-light filter fixed securely over the front opening, never at the eyepiece. With that setup you can watch sunspots change from day to day as the Sun rotates about once a month.

Experienced observers also use dedicated hydrogen-alpha telescopes, which reveal prominences, loops of glowing gas on the Sun's edge. Whatever you use, check the filter for scratches or holes before every session, and supervise children closely.