杏吧原创

YOU should never stare directly at the sun. This isn鈥檛 a euphemism of some kind 鈥� you really shouldn鈥檛 because it will damage your eyes.

Of course, some among us may have stared at it anyway, perhaps in what we irresponsibly perceived to be a show of daring. But maybe you have been good and have never stared at it or only used special glasses to look, if you have ever witnessed a solar eclipse.

Either way, you have probably never noticed that the sun seemed different. Sure, clouds, fog or even the moon might get in the way, but, in general, it is just this extremely bright yellowish circle in the sky that we are either longing for, annoyed with or contented by. It is, by all other measures, timeless.

But the sun is more fantastical than it appears to be to us in everyday life. It is easy to forget that, like many of the pinpoints we see in the sky at night, it is just an average so-called main sequence star 鈥� and that means it is changing all the time.

To start with, the sun hasn鈥檛 always existed. In fact, it isn鈥檛 a first-generation star. If it were, we wouldn鈥檛 be here on Earth. The first generation of stars, counter-intuitively called population III stars, were made entirely of primordial hydrogen and helium, the by-products of big bang processes. These stars formed when clumps of gas collapsed under their own gravitational pull, leading to the ignition of nuclear fusion.

Fusion in stars is the universe鈥檚 primary source of elements that are more massive than hydrogen and lighter than iron. The process changes with time. At the start of its life, a star has a lot of hydrogen. By the end, there will be less because some will have fused into helium and other more massive elements like oxygen and carbon. One quirk of the astronomy community is that everything heavier than hydrogen or helium is called a 鈥渕etal鈥�. So, in our lingua franca, you are mainly metal.

At the end of their lives, population III stars either underwent supernova explosions or slowly blew off their outer layers. Both scenarios left behind a lot of hydrogen, helium and some metal-rich gases. In the case of supernovae, elements heavier than iron might have fused during the explosion.

鈥淲e are carbon-based life forms, requiring heavy elements that didn鈥檛 exist in the era of the first stars鈥�

The evaporated remains, mixed with other hydrogen and helium gas, eventually repeated the gravitational collapse and nuclear ignition process, birthing second generation population II stars. These stars have a different composition to their ancestors because metals were available in their nursery.

It is from the mix of population III and population II remains that stars like our sun were born. These generational stages are vital for our existence: those metals are what make up the planets orbiting the sun, including our own, and we are carbon-based life forms, requiring heavy elements that didn鈥檛 exist in the population III era.

Our sun, like its forebears, has a finite lifetime. Our calculations, and dating of elements on Earth, suggest it is about 4.5 billion years old, halfway through its life. With each passing day, the sun has less hydrogen and more helium.

It also has an outer magnetic field whose behaviour and effects on Earth remain somewhat unpredictable. Several of my colleagues at the University of New Hampshire鈥檚 Space Science Center work in the area of heliophysics, trying to figure out the exact dynamics of the magnetosphere and other parts of the sun that are constantly in flux. This work, sometimes known as space weather, points to how dynamical and constantly evolving our sun actually is.

It might look the same to you from the park, but the sun is changing every day in ways that are noticeable with missions like NASA鈥檚 Parker Solar Probe. In about 4.5 billion years, when it begins its transition into a planetary nebula, the sun will look different by anyone鈥檚 standards.