First: how to observe the Sun safely

This is the one part of astronomy that can blind you in a second, and Edexcel examines it.

• NEVER look at the Sun directly — with the naked eye, and absolutely never through binoculars, a telescope or a finderscope. A telescope concentrates the Sun's heat and light enough to destroy the retina instantly and painlessly (the retina has no pain receptors).
• Do not use sunglasses, smoked glass, exposed film or a "solar filter" that screws into an eyepiece — these are not safe.
• ✅ The safe school method is projection: point the telescope at the Sun using its shadow (never by looking through it), cap the finderscope, and project the image onto a white card held behind the eyepiece. Everyone can look at once — and you can draw and track sunspots day by day.
• Professional-grade full-aperture filters (fitted over the front) and dedicated H-alpha telescopes are also safe, but they must be properly made and undamaged.

The exam answer: "project the image onto a white card" plus "never look through the telescope or finder". That earns the marks — and keeps your eyes.

Observing safely

Inside the Sun

The Sun is a ball of plasma — roughly 73% hydrogen and 25% helium — about 1.4 million km across. From the middle outwards:

• Core — about 15 million °C. Here nuclear fusion joins hydrogen nuclei into helium, converting a tiny fraction of the mass into an enormous amount of energy.
• Radiative zone — energy crawls outwards as photons, absorbed and re-emitted over and over. A photon can take thousands of years to cross it.
• Convective zone — hot gas physically rises, cools, and sinks again. It gives the photosphere its mottled granulation.
• Photosphere — the visible "surface", about 5500 °C. This is what you see in a projected image.
• Chromosphere — a thin reddish layer above it, seen during a total eclipse.
• Corona — the pearly outer atmosphere, visible during totality, and — bizarrely — millions of degrees, far hotter than the surface below it.

Fusion, not burning: the Sun is not on fire. Chemical burning would exhaust it in a few thousand years. Fusion of hydrogen to helium keeps it shining for about 10 billion years.

What powers the Sun?

Sunspots and the solar cycle

Sunspots are dark patches on the photosphere. They look dark only by contrast: they are around 4000 °C against a 5500 °C background, so they are still dazzlingly bright. Each has a dark umbra and a lighter penumbra.

• They are cooler because intense magnetic fields suppress the convection that carries heat up to the surface.
• They usually come in pairs or groups with opposite magnetic polarity.
• Their number rises and falls on an 11-year solar cycle. Spots appear at high latitudes early in a cycle and closer to the equator later on.
• Watching a sunspot cross the disc reveals the Sun's rotation — and shows it rotates differentially: about 25 days at the equator but over 30 days near the poles. Only a fluid can do that.

Other magnetic activity: prominences (huge loops of glowing gas arching above the surface), solar flares (sudden violent releases of energy) and coronal mass ejections (billions of tonnes of plasma flung into space).

Space weather: the solar wind — charged particles streaming from the corona — is deflected by the Earth's magnetic field and funnelled towards the poles, where it excites atmospheric gases and produces the aurorae. Big flares and CMEs can also disrupt satellites, radio and power grids.

Your turn — how fast does the Sun spin?

Your turn — sunlight in transit

Your turn — the size of the Sun

Why are sunspots dark?

Which part of the Sun?

Tap a feature, then tap where it belongs.

Match the description to the term

Tap a description on the left, then its matching term on the right.

Northern lights

Reading the Sun's spectrum

Split sunlight with a prism or diffraction grating and the continuous rainbow is crossed by hundreds of dark lines — the Fraunhofer lines. They are absorption lines: cooler gas in the Sun's outer photosphere absorbs specific wavelengths on the way out.

• Every element absorbs a unique fingerprint of wavelengths, so the lines tell us what the Sun is made of — mostly hydrogen and helium. (Helium was in fact discovered in the Sun's spectrum in 1868, before it was found on Earth — hence its name, from helios.)
• The same trick works on any star, which is how we know stellar compositions and temperatures.
• Special H-alpha telescopes isolate one deep-red hydrogen line and reveal prominences and flares on the chromosphere.

Careful with the words: a hot dense object gives a continuous spectrum; a hot thin gas gives bright emission lines; a cool gas in front of a hot source gives dark absorption lines. The Sun shows the third case.

Dark lines in the rainbow

The big ideas to know

That is Solar Astronomy covered for Edexcel GCSE Astronomy. Press Finish to see your score.