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The Sun, explained one question at a time

The Sun is an ordinary star that happens to be 270,000 times closer than the next one, which makes it the only star we can study in detail. Everything below is an episode of Watt & Why written out in full, with its sources and a ten-question quiz, arranged so that each answer sets up the next.

17 written answers
17 quizzes
Free, read or watch

Take it as a course Jump to the quizzes

Start with the basics

The Sun is a ball of plasma about 1.4 million kilometres across, roughly 109 Earths side by side, made of about three quarters hydrogen and one quarter helium by mass. In its core, at around 15 million degrees, hydrogen nuclei fuse into helium and release the energy that, after a very long climb to the surface, leaves as the light that reaches Earth eight minutes later. It is about 4.6 billion years old and roughly halfway through a stable life. Every episode below pulls on one thread of that picture.

What the Sun actually is

A star, close enough to study. What it is made of, how it is layered, and how it turns hydrogen into light.

1
What is the Sun, really?
The Sun is a star, the closest one to Earth. It is a vast ball of glowing gas, mostly hydrogen with some helium, held together by its own gravity. It...
5:06 episode · quiz
2
What are the layers of the Sun, from core to corona?
The Sun has six layers. Working outward: the core, where fusion happens; the radiative zone, where light bounces slowly outward; the convective zone...
4:25 episode · quiz

Light, heat and time

The journey of a photon from the core to your skin, and why the Sun sets the clock and the calendar for everything alive.

1
Why does sunlight take thousands of years to escape the Sun?
Light made in the Sun's core cannot travel in a straight line. The gas is so densely packed that a photon is absorbed and re-emitted in a random...
4:00 episode · quiz
2
How long does sunlight take to reach Earth?
Sunlight takes 8 minutes and 20 seconds to travel from the Sun to Earth, covering about 150 million kilometres at 300,000 kilometres per second...
4:07 episode · quiz
3
Why do we have day, night and seasons?
Day and night happen because Earth spins once every 24 hours, not because the Sun moves. Seasons happen because Earth is tilted about 23.5 degrees on...
4:38 episode · quiz

The active Sun

Dark spots, snapping magnetic fields and the storms that light up the poles and worry the power companies.

1
Why do sunspots look black?
Sunspots are not dark. They are patches of the Sun's surface that are cooler than their surroundings, roughly 3,500C against about 5,500C. That...
4:50 episode · quiz

Eclipses

Why the Moon fits the Sun so exactly, why it does not happen every month, and what to expect on 2 August 2027.

1
What causes a total solar eclipse?
A total solar eclipse happens when the Moon passes exactly between the Sun and Earth, blocking the Sun's light and casting a shadow onto the planet...
7:03 episode · quiz
2
How do you watch a solar eclipse safely?
Use eclipse glasses certified to the ISO 12312-2 standard, or skip glasses entirely and project the Sun's image onto paper through a pinhole...
5:52 episode · quiz
3
What does a total solar eclipse actually look like?
For about an hour the Sun is slowly eaten away with almost no visible change. In the final half hour the light turns silvery and strange, shadows...
5:59 episode · quiz
4
How did a solar eclipse prove Einstein right?
In 1919, expeditions led by Arthur Eddington photographed stars near the Sun during a total solar eclipse. The stars appeared shifted from their...
5:05 episode · quiz
5
Why is the path of totality so narrow?
The Moon's shadow narrows to a cone, and only its tip reaches Earth. That tip paints a strip typically around a hundred to a few hundred kilometres...
5:38 episode · quiz
6
Why isn't there a solar eclipse every month?
The Moon's orbit is tilted about 5 degrees compared with Earth's orbit around the Sun. Most months the new moon passes above or below the Sun from...
6:52 episode · quiz
7
Why do the Moon and Sun look the same size in the sky?
The Sun is about 400 times wider than the Moon, and it also sits about 400 times further away. The two factors cancel out, so both discs appear...
7:04 episode · quiz
8
Why does the Moon turn red during a lunar eclipse?
During a lunar eclipse the Moon passes into Earth's shadow, but it does not go dark. Earth's atmosphere bends sunlight around the planet's edge and...
7:39 episode · quiz

Birth, middle age and death

From a collapsing gas cloud to a slowly cooling ember: the four-and-a-half-billion-year story so far and the five billion still to come.

1
How was the Sun born, and how will it end?
The Sun formed about 4.6 billion years ago when a vast cold cloud of gas and dust collapsed under its own gravity, heated up, and ignited fusion. It...
5:03 episode · quiz