The Sun: Our Star Explained

Every sunrise is a reminder that we live next to a star.

The Sun lights our days, drives our weather, feeds nearly every food chain on Earth and holds the entire solar system together with its gravity.

Yet most of us rarely stop to think about what it actually is.

This guide explains the Sun in plain terms: what kind of star it is, how it makes its energy,

what is going on inside it and on its restless surface, and how it will change over billions of years.

You will also learn the one rule that matters most when you want to see it for yourself.

What Kind of Star Is the Sun?

The Sun is a fairly ordinary star that astronomers classify as a yellow dwarf.

That name undersells it, because the Sun is brighter and more massive than most stars in our galaxy,

the majority of which are small, dim red dwarfs. It is simply unremarkable next to the true giants.

By human standards, the Sun is enormous.

It is about 1.4 million km (865,000 miles) across, roughly 109 times the width of Earth,

and you could fit more than a million Earths inside it.

It is made mostly of hydrogen, about three quarters by mass,

with nearly all the rest being helium and only a small sprinkling of heavier elements.

The Sun formed about 4.6 billion years ago and is roughly halfway through the stable, hydrogen-burning stage of its life.

That long, steady middle age is a big reason life on Earth had time to develop.

How the Sun Makes Its Energy

The Sun shines because of nuclear fusion in its core.

There, the temperature is about 15 million °C (27 million °F) and the pressure is immense,

enough to force hydrogen nuclei together until they fuse into helium.

Each fusion reaction turns a tiny amount of mass into a large amount of energy.

Added up, the numbers are staggering.

The Sun converts roughly four million tonnes of its own mass into energy every second,

and it has enough hydrogen to keep doing so for billions of years more.

Gravity pulling inward and the heat pushing outward stay in balance, which keeps the Sun remarkably stable.

That energy does not escape quickly.

Light created in the core is absorbed and re-emitted countless times on its way outward,

and by most estimates the journey to the surface takes tens of thousands of years or more.

Once it leaves the surface, the same light crosses the gap to Earth in a little over eight minutes.

Inside the Sun, Layer by Layer

The Sun has no solid surface, but it does have distinct layers, each with its own job.

Moving from the center outward:

1. The core is where fusion happens. It is the hottest and densest part of the Sun.
2. The radiative zone surrounds the core. Energy creeps outward here as light, bouncing from particle to particle in a slow random walk.
3. The convective zone is the outer third or so of the interior. Hot gas rises, cools and sinks again, much like water boiling in a pot.
4. The photosphere is the visible surface, at about 5,500 °C (10,000 °F). Its grainy texture comes from the tops of those rising cells of gas.
5. The chromosphere is a thin, reddish layer just above the photosphere, normally hidden by the glare of the surface.
6. The corona is the Sun's faint outer atmosphere. Strangely, it is heated to more than a million degrees, far hotter than the surface below it, and the exact cause is still an active research question.

The chromosphere and corona are usually invisible to the eye.

They show up in full glory during a total solar eclipse,

when the Moon blocks the bright photosphere and the pearly corona appears around the dark disk.

Sunspots, Flares and the Solar Cycle

The Sun is a ball of electrically charged gas, and it spins faster at its equator than near its poles.

That uneven rotation twists its magnetic field, and where the field becomes especially concentrated it holds back the flow of heat.

The result is a sunspot: a patch that looks dark only because it is cooler than its surroundings.

Sunspots come and go on a rhythm called the solar cycle, which lasts about 11 years.

At solar minimum the disk may show few or no spots for weeks.

At solar maximum there can be many groups at once, and the Sun's magnetic activity is at its most intense.

That activity includes two kinds of eruptions. Solar flares are sudden bursts of radiation from tangled magnetic regions near sunspots.

Coronal mass ejections are huge clouds of charged gas hurled into space,

which, if pointed our way, can reach Earth in as little as 15 to 18 hours, though slower ones take several days.

The Solar Wind and Space Weather

Even on a quiet day, the Sun is always losing material.

A steady stream of charged particles called the solar wind flows outward in every direction,

filling the solar system and carving out a huge bubble around it known as the heliosphere.

When a strong gust from a flare or coronal mass ejection hits Earth, the effects are known as space weather.

Earth's magnetic field shields the surface from the worst of it, but a powerful event can still:

• Light up bright auroras that may be seen much farther from the poles than usual.
• Disrupt radio communication and reduce the accuracy of satellite navigation.
• Damage or disturb satellites in orbit and increase radiation risks for astronauts.
• Induce electric currents in long power lines, which in rare cases has caused blackouts.

Because of these risks, scientists watch the Sun around the clock with ground observatories and spacecraft,

issuing forecasts much as meteorologists do for storms on Earth.

How to Observe the Sun Safely

Never look at the Sun without certified solar filters, not even for a moment and not even when it is partly covered during an eclipse.

Staring at it can permanently damage your eyes, and looking through binoculars or a telescope without the proper filter can cause severe injury almost instantly.

With the right precautions, the Sun is a fascinating target:

• Use certified eclipse glasses or solar viewers from a reputable source, and discard any that are scratched or damaged.
• Fit a certified solar filter over the front of a telescope or binoculars, never over the eyepiece, and keep it firmly attached.
• Try projection. A simple pinhole in a card casts a small image of the Sun onto paper, so you look at the paper, never at the Sun.
• Supervise children closely whenever solar viewing equipment is in use.

Through a properly filtered telescope, you can watch sunspot groups change shape and drift across the disk as the Sun rotates, roughly once a month.

The Sun's Long Future

The Sun will not shine the same way forever.

Over time it is slowly growing brighter, and in about five billion years it will run low on hydrogen in its core.

At that point it will swell into a red giant, becoming so large that it may engulf Mercury, Venus and possibly Earth.

After that, the Sun will shed its outer layers into space, forming a glowing shell of gas,

and what remains will shrink into a white dwarf about the size of Earth.

It is not massive enough to explode as a supernova.

Instead, the white dwarf will simply cool and fade over an immensely long time.

A Star Worth Understanding

The Sun is both ordinary and extraordinary: a common type of star that happens to make our existence possible.

Understanding how it works helps explain everything from auroras to the seasons of solar activity.

Keep that curiosity alive, and whenever you observe it, make certified solar filters the first thing you reach for.