The Sun, a typical star

Foundations

Ordinary, and ours

The Sun is not unique. It is one star among a few hundred billion in one galaxy among hundreds of billions, and its physics is the physics of lesson 2.2: hydrogen fusing in a core held in balance by gravity. It is not even average in the sense people usually mean. Most stars are small red dwarfs, and the Sun is more massive than about nine in every ten stars. But nothing about it is special. What is special is that it is eight light-minutes away instead of light-years, so it is the one star we can study in detail, and everything we know about other stars starts here.

It is not unchanging either. It has weather, and seasons of a sort, and occasional violence that reaches the Earth.

By the numbers

The Sun holds 99.86 percent of all the mass in the solar system; every planet, moon, asteroid and comet together is the remaining seventh of a percent. It is 1.39 million kilometres across, a hundred and nine Earths side by side, and weighs 333,000 Earths. By mass it is about three quarters hydrogen and one quarter helium, with two percent of everything else, the same recipe the first three minutes fixed, lightly seasoned by generations of earlier stars. It is 4.57 billion years old, dated from the oldest solids that condensed around it, which survive in meteorites. At Earth’s distance it delivers 1,361 watts to every square metre facing it, a figure that varies by only a tenth of a percent as the Sun’s mood changes.

The layers

The core, the inner quarter, is where fusion happens, at fifteen million kelvin. Around it the radiative zone, where light staggers outward for a hundred thousand years. Then the convective zone, the outer third, where the gas boils; the granular pattern of the boiling is visible on the surface as cells the size of India rising and sinking every few minutes.

The surface itself, the photosphere, is not a surface at all but the layer where the gas becomes transparent and light finally escapes, at about 5,800 kelvin. Above it lies the chromosphere, a few thousand kilometres of thin, reddish gas seen at the edge of the Sun during eclipses. And above that the corona, the pearly crown that surrounds the eclipsed Sun, stretching millions of kilometres into space.

The corona is a puzzle a century old. It is at one to three million kelvin, hundreds of times hotter than the photosphere beneath it, though heat should flow the other way. The energy must be carried up by the Sun’s magnetic field, in waves or in countless tiny flares, and which of these dominates is still argued over. From the corona blows the solar wind, a stream of charged particles predicted by Eugene Parker in 1958 and confirmed by spacecraft within a few years, that fills the solar system to well beyond Pluto.

The active Sun

Sunspots are dark patches on the photosphere where strong magnetic fields choke off the flow of heat from below; they are cooler than their surroundings, though still brighter than an arc lamp. Galileo drew them in 1610. In 1843, after seventeen years of drawing them every clear day, an amateur astronomer in Dessau named Heinrich Schwabe announced that their number rises and falls in a cycle of about eleven years. The cycle is magnetic: the Sun’s field winds up, erupts and reverses, and the spots, flares and eruptions follow.

The eruptions matter. A flare is a magnetic explosion that releases in minutes the energy of billions of nuclear bombs; a coronal mass ejection hurls a billion tonnes of gas into space. On 1 September 1859 Richard Carrington, sketching sunspots in his observatory, saw two brilliant white patches flare up, the first flare ever recorded. Within a day the Earth’s magnetic field was convulsing, telegraph operators were receiving shocks and the aurora was seen near the tropics. A storm of that size today would threaten power grids and satellites, which is the practical reason nations send spacecraft to watch the Sun.

Watching it

The Sun rings like a bell. Millions of sound waves trapped inside it make the surface rise and fall by a few metres every five minutes, and reading those oscillations reveals the interior the way earthquakes reveal the Earth’s: the depth of the convective zone, the run of temperature, the fact that the surface rotates faster at the equator than at the poles while the deep interior turns as one body. The interior matches the model of lesson 2.2 to better than a percent.

Spacecraft watch from closer. The Parker Solar Probe flew into the corona itself in 2021. And since January 2024 India has had its own eye on the Sun: Aditya-L1, launched from Sriharikota on 2 September 2023, hovers at the L1 point one and a half million kilometres sunward of Earth, where the pulls of Sun and Earth balance and the view is never interrupted. Its seven instruments image the corona and the ultraviolet Sun, record X-rays from flares, and sample the solar wind and its magnetic field as they stream past on their way to us. Its coronagraph can watch eruptions from the moment they leave the Sun, which is what space-weather forecasting needs.

A warning belongs here. Never look at the Sun without a proper filter, and never through binoculars or a telescope without one designed for it. The instruments on Aditya-L1 exist because the Sun is too bright for eyes.

Halfway

The Sun is about halfway through its ten billion years on the main sequence. It is brightening slowly, by about ten percent every billion years, and in five billion it will swell into the red giant of lesson 2.3. On the cosmic calendar it was born on the first of September; its eleven-year cycle is a quarter of a second; and it has, in the year to come, until about July.

Deeper

Composition and the abundance problem

Spectroscopic analysis of the photosphere, with three-dimensional models of the convecting surface, gives mass fractions of about 0.74 hydrogen, 0.25 helium and 0.013 heavier elements, the last dominated by oxygen, carbon, neon, nitrogen and iron. The 2009 revision lowered the heavy-element fraction by about a third from earlier values, and solar models built with the new abundances agree less well with helioseismology than the old ones did. The disagreement, a few percent in the sound speed just below the convective zone, is unresolved.

The energy budget

The solar constant of 1,361 watts per square metre, multiplied by the area of a sphere at Earth’s distance, gives a luminosity of 3.83 × 10²⁶ watts. Over the solar cycle the total varies by about 0.1 percent, brighter at maximum because the bright faculae around sunspots more than compensate for the dark spots. Over the Sun’s life the luminosity has risen by about thirty percent as the core’s helium has accumulated, which raises a puzzle for the early Earth taken up in chapter four.

The dynamo and the cycle

The Sun’s differential rotation, faster at the equator, stretches the north–south magnetic field into an east–west field wound around the star; convection twists that field back into a north–south field of opposite sign. The result is a cycle of about twenty-two years in the magnetic polarity and eleven in the sunspot count. Spots appear first at mid-latitudes and migrate toward the equator as the cycle progresses. The cycle’s length and strength vary; between about 1645 and 1715 spots almost vanished for seventy years, a period that coincided with cold winters in Europe.

Coronal heating

The photosphere radiates as a blackbody at 5,800 kelvin; the corona emits X-rays and lines of iron ionised a dozen times, requiring millions of kelvin. The energy flux needed to sustain it is small, of order a few hundred watts per square metre against the photosphere’s sixty million, but delivering it across a temperature inversion requires a non-thermal channel. Two families of models compete: dissipation of magnetohydrodynamic waves launched from the convection below, and the release of magnetic energy in vast numbers of small reconnection events, nanoflares. Parker Solar Probe’s measurements from inside the corona, including magnetic switchbacks in the young solar wind, are constraining both.

Aditya-L1’s instruments

The Visible Emission Line Coronagraph images the corona from close to the solar limb outward and tracks coronal mass ejections; the Solar Ultraviolet Imaging Telescope images the photosphere and chromosphere in near-ultraviolet bands; two X-ray spectrometers record the energy distribution of flares; two particle analysers sample the solar wind’s protons, alpha particles and heavier ions; and a magnetometer measures the interplanetary field. Together they connect what happens on the Sun to what arrives at Earth, from a vantage point 1.5 million kilometres upstream that gives about an hour’s warning of an incoming storm.

Frontier

Forecasting the storm

The next Carrington-class event is a matter of when, not whether; estimates of the chance per decade run from a few percent to over ten. Predicting whether a given eruption will hit Earth, and how hard, depends on knowing the magnetic orientation of the cloud before it arrives, which is measured only when it passes L1. Aditya-L1, alongside NASA’s and ESA’s spacecraft, is part of the effort to push the warning time out from an hour to a day.

The abundance puzzle and the corona

Both of the Sun’s open problems, the heavy-element abundances and the heating of the corona, are being attacked with new instruments: Solar Orbiter’s views of the poles, Parker’s plunges through the corona, and the Daniel K. Inouye Solar Telescope’s images of magnetic structures a few tens of kilometres across. The best-known star is still yielding surprises.

Transcript and shot list

Target runtime about 6 minutes. Built as one star examined from the inside out, with the unsolved thing placed in the middle rather than saved for the end. Shot types: ANIM is rendered from code with Manim, AI is a generated shot from Higgsfield with the shared style reference, REAL is agency imagery with credit, CAM is the narrator.

[00:00] S01 · REAL · The Sun in extreme ultraviolet from the Solar Dynamics Observatory, with loops and an eruption (NASA SDO; public domain; confirm credit line).

This is the Sun, in ultraviolet, on an ordinary day. Not a smooth yellow ball. A boiling, magnetised, erupting star with weather of its own. And a few minutes from now I am going to show you something about it that nobody has explained in a hundred years of trying. It’s not unique, and it’s not unchanging. It’s one star among hundreds of billions, and it happens to be eight light-minutes away instead of light-years, which makes it the one star we can really study.

[00:39] S02 · ANIM · Cosmic calendar; the Sun’s birth at 1 September; a tiny tick labelled “one eleven-year cycle: a quarter of a second”.

Born on the first of September on the cosmic calendar. About halfway through its life. Its eleven-year activity cycle is a quarter of a second.

[00:51] S03 · ANIM · A row of 109 Earths across the Sun’s disc; a balance with 333,000 Earths; a pie with a sliver labelled “everything else in the solar system”; a recipe bar of hydrogen, helium, two percent other.

By the numbers. Ninety-nine point eight six percent of all the mass in the solar system. Every planet, moon, asteroid and comet together is the other seventh of a percent. A hundred and nine Earths across. Three hundred and thirty-three thousand Earths in mass. Three quarters hydrogen, a quarter helium, two percent everything else: the recipe from the first three minutes, lightly seasoned by earlier stars. Four and a half billion years old.

[01:24] S04 · ANIM · A cutaway: core, radiative zone, convective zone with granules the size of India, photosphere, chromosphere, corona; a temperature readout at each layer, with the corona’s jumping to millions.

The layers. A core at fifteen million degrees, where fusion happens. A radiative zone where light staggers outward for a hundred thousand years. A convective zone where the gas boils, in cells the size of India that rise and sink every few minutes. The photosphere, the visible surface, which isn’t a surface at all, just the layer where the gas turns transparent, at five thousand eight hundred degrees. Then the chromosphere. Then the corona, the pearly crown you see during an eclipse. And here’s a century-old puzzle: the corona is at millions of degrees, hundreds of times hotter than the surface beneath it, though heat should flow the other way. The magnetic field must carry the energy up. How, exactly, is still argued over.

[02:22] S05 · ANIM · The solar wind streaming from the corona through the planets to beyond Pluto; Parker, 1958.

From the corona blows the solar wind, a stream of charged particles predicted by Eugene Parker in 1958, confirmed by spacecraft within a few years, filling the solar system to well beyond Pluto.

[02:37] S06 · ANIM · Sunspots on the disc; Galileo’s drawings; Schwabe’s seventeen years of daily counts rising into a curve; the eleven-year cycle; the magnetic field winding up and flipping.

Now the weather. Sunspots: dark patches where strong magnetic fields choke off the heat from below. Galileo drew them in 1610. In 1843, after seventeen years of drawing them every clear day, an amateur astronomer in Dessau named Heinrich Schwabe announced that their number rises and falls in a cycle of about eleven years. The cycle is magnetic. The Sun’s field winds up, erupts, and reverses, and the spots and flares follow.

[03:10] S07 · ANIM · 1 September 1859: Carrington at his telescope; two white flashes on a sunspot group; a coronal mass ejection crosses to Earth; telegraph sparks; aurora at low latitudes.

The eruptions matter. On the first of September 1859, Richard Carrington, sketching sunspots, saw two brilliant white patches flare up: the first flare ever recorded. Within a day the Earth’s magnetic field was convulsing, telegraph operators were getting shocks, and the aurora was seen near the tropics. A storm that size today would threaten power grids and satellites. That is the practical reason nations send spacecraft to watch the Sun.

[03:42] S08 · ANIM · The Sun’s surface rippling with sound waves; the interior revealed layer by layer; the rotation profile: fast equator, slow poles, solid interior.

The Sun rings like a bell. Millions of sound waves trapped inside make its surface rise and fall by a few metres every five minutes, and reading them reveals the interior the way earthquakes reveal the Earth. The depth of the boiling layer. The run of temperature. The surface turning faster at the equator than at the poles while the deep interior turns as one. It matches the model from lesson 2.2 to better than a percent.

[04:18] S09 · REAL · Aditya-L1 launch from Sriharikota, 2 September 2023 (ISRO; confirm licence and credit). S10 · ANIM · The L1 point 1.5 million kilometres sunward; the spacecraft’s halo orbit; its seven instruments labelled.

And since January 2024, India has had its own eye on the Sun. Aditya-L1, launched from Sriharikota on the second of September 2023, hovers at the L1 point, one and a half million kilometres sunward of Earth, where the pulls of Sun and Earth balance and the view is never interrupted. Seven instruments image the corona and the ultraviolet Sun, record the X-rays from flares, and sample the solar wind and its magnetic field as they stream past on the way to us. Its coronagraph can watch an eruption from the moment it leaves the Sun, which is exactly what forecasting a storm needs.

[05:06] S11 · ANIM · A pair of eyes, a pair of binoculars, a telescope, each crossed with a red bar; then a proper solar filter.

One warning, and it isn’t optional. Never look at the Sun without a proper filter, and never through binoculars or a telescope without one built for it. There’s a reason we send instruments.

[05:21] S12 · ANIM · A brightness curve rising ten percent per billion years; the Sun swelling at five billion years; the calendar page of next year with July marked.

The Sun is halfway through its ten billion years. It’s brightening slowly, ten percent every billion years, and in five billion it will swell into the red giant of lesson 2.3. On the calendar, it has until about July of next year.

[05:41] S13 · ANIM · Labels: ESTABLISHED for the numbers, the layers, fusion and the cycle; ACTIVE RESEARCH for the corona, the abundances and storm forecasting.

The scorecard. The Sun’s mass, size, age, recipe, structure and cycle: established, better than for any other star. Why its corona is so hot, exactly how much oxygen and carbon it holds, and how to forecast its storms a day ahead instead of an hour: active research, and some of it is happening at L1 under an Indian flag.

[06:08] S14 · AI into ANIM · An eclipse: the corona blooms around the black disc; the calendar returns.

So, not unique, and not unchanging. An ordinary star with weather, seen close up. Everything we know about the other stars starts here. And there’s one chart that puts the Sun and every other star in its place. Next lesson.

Shot list

One AI shot, eleven code-rendered shots, two agency images. Estimated Higgsfield use: 2 to 4 generations including retries.