Is the sun a sunlike star?

If you’re not an astronomer, the obvious answer is yes. That’s tautological! Sunlike stars are stars like the sun. The sun is the sun, so it’s the most sunlike of them all!

Astronomers, though, can be persnickety. When asked that question, they immediately shoot another right back: What do you mean by “sunlike”?

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That’s a fair response. If the definition is too broad, the sun starts to look a little funny, appearing quite unlike other stars. For a much tighter definition—well, it’s still a little off but not nearly as much.

So why ask this question in the first place?

Astronomers like to look at large populations of objects because bigger sample sizes should better reveal trends in their properties. This can lead to greater insights on their structure, their characteristics and the physics that underlie it all.

We already know a lot about the sun, given that it’s the nearest star in the universe to us. If we then compare it with other, similar stars, we can extrapolate the properties we know to those we may not, thus learning more about them, too.

For example, when compared with stars of similar mass, the sun sits right at the average in terms of its temperature and the amount of energy that it emits. In those cases, it’s quite normal.

But stars have other properties! An important one for a star is its amount of magnetic activity, which depends on the strength and variability of its magnetic field and the effect that field has on the star and its immediate surroundings. Research published in the journal Science in 2020 suggested that the sun was not normal when compared with the magnetic activity of other, similar stars. Our very own star was instead something of an underachiever.

Magnetic fields are extremely complex phenomena. They can, however, be explored in stars using a surprisingly simple method: looking at star spots—sunspots on other stars.

At the risk of oversimplifying, magnetism is created when charged particles (such as electrons, protons or ionized atoms) move around. In general, the more coherently these particles move as a group, the stronger the magnetic field is. All of them ping-ponging around higgledy-piggledy won’t generate a magnetic field, but a cloud of them moving along together will.

The sun and stars like it aren’t just miasmas of incandescent plasma. They have internal layers, such as each’s core, where energy is generated by thermonuclear fusion power. At some distance above that, material heated by the core rises up, just like warm air rises in Earth’s atmosphere, flowing upward until it reaches the star’s surface. It then radiates its heat into space, cools and sinks back down in a process called convection. It’s like a conveyor belt of heat, and there are many thousands of such towers of upward-and-downward-flowing cells in the sun.

But that gas is hot enough to have electrons stripped from its atoms, making it ionized. Each vast tower of flowing material therefore creates its own magnetic field, which it drags along as it flows vertically. Sometimes, the magnetic fields from neighboring convection cells get tangled together, and when these tangles reach the surface, they prevent material from easily flowing back down. So that stuff spends more time at the surface, where it cools a bit more than usual. Cooler gas doesn’t glow as brightly, so these patches are a little bit darker than the surrounding material. These are the aforementioned star spots. And besides looking menacingly dark, they are sites of enhanced magnetic activity.

It’s easy to see them on the sun, but detecting them on distant stars is much harder. It is possible, however, because stars rotate. As one does, star spots spinning into view can slightly dim the star, as seen from afar, with bigger spots or groups of spots causing greater dimming. The star brightens again when the spots rotate back out of view. By carefully measuring the star’s fluctuating brightness, we can get a measure of its star spot activity and use that as a proxy for its magnetic field.

This is more important than you might think because a star’s magnetic field isn’t constant. The sun, for example, has a roughly 11-year cycle in which the magnetic field waxes and wanes in strength. When it’s strong, the number of sunspots increases. At the peak of the cycle, the sun blasts out more solar flares, as well as coronal mass ejections, immense explosions of material that scream outward into the solar system. These can profoundly affect our orbiting satellites and power grid, so understanding the sun’s magnetic activity actually very much impacts us here on Earth.

And this is where the sun appeared anomalous in the 2020 research: it didn’t show as much magnetic activity as similar stars. So maybe the sun is a magnetic weakling.

A team of astronomers wanted to take a closer look at this. The scientists used data from NASA’s exoplanet-hunting satellite Kepler, which searched for exoplanets via very precise, high-cadence brightness measurements of hundreds of thousands of stars. Those measurements, it turns out, can be used to look for star spots as well.

Their results, published in 2023 in the journal Astronomy and Astrophysics, is that the sun is indeed less active compared with other stars in a broad range of characteristics such as temperature and mass. But by narrowing the range of these parameters—for instance, by only looking at stars whose temperature was within 100 degrees Celsius of that of the sun—they found our home star is only a tad bit less active, well within the range of normal.

So the sun is a sunlike star.

Still, why are the other stars stronger at all? If they have the same properties as the sun, what makes them more magnetically excited?

The answer, the team found, is that the stars observed by Kepler are composed of somewhat higher levels of heavy elements. Stars are mostly hydrogen and helium, with much smaller amounts of elements like iron, carbon, and so on. But these trace elements can have an outsize effect on overall stellar characteristics.

For example, heavy elements are better at absorbing heat from the core, which affects the size of a star’s convective layer. More heavy elements make the convective layer deeper, allowing it to generate a more powerful magnetic field.

Surprisingly, this also affects a star’s rotation. Stars like the sun blow a wind of subatomic particles; in the case of the sun, we call it the solar wind. As the star spins, the magnetic field coming out of the surface catches that wind and sweeps it up. This imparts a drag on the star—imagine spinning around on your heels while holding an open trash bag; the bag will act like a parachute, slowing you. Over millions and billions of years, the drag from a star’s magnetic field slows its rotation, and in fact, the astronomers found that stars with stronger magnetism tended to rotate more slowly.

So that dash of heavier elements in other stars has wide-ranging effects and shows that, in the end, it’s not so much that the sun isn’t sunlike but rather that astronomers need to do a little extra bookkeeping when they compare it with other stars.

It’s reassuring to know the sun is sunlike after all. And while that might seem like a silly question, it actually reveals a lot of fascinating science behind stars and their behavior. It also forces us to question our assumptions, something every scientist (and everybody) should do.