We humans really love the Sun. The long history of sun-worship throughout cultures across the world is a testament to that. Today, our rituals typically take the form of sunbathing, or perhaps hunting shade during a record-breaking heatwave, but we probably think of the Sun in much the same way as our distant ancestors – as an overpowering source of light and heat that is felt as much as it is seen. Yet the steady progress of science has provided us with a much deeper view of our neighbouring star, that seldom crosses the minds of most people. One group – the solar physics community – thinks about it a lot, and in the pursuit of understanding, they’ve revealed a rabbit hole of strange and fascinating questions about the Sun’s complex behaviour. Of course, a great deal has been learned, drawing profound connections between the Sun and ourselves. Here are some facts to encourage you to think again about your home star.
The Alpha and Omega
The young Sun surrounded itself with a protoplanetary disk, from which the Earth was born.
The Sun’s behaviour may be steeped in mystery but there are some things we can say with certainty about its impact on us. Perhaps the grandest among them is that, whatever we may individually believe about higher powers, the Sun is both the creator and ultimate destroyer of our world. Our entire Solar System was born cocooned within a collapsing fragment of a large nebula – a cloud of gas and other material spanning many light-years across the Galaxy. At its heart, our young star drew in the surrounding material to form a large, flattened disk, from which many hundreds of ‘planetesimals’ formed. On a scale of hundreds of millions of years, the largest surviving protoplanets became the planets we see today, including our home the Earth. In the distant future, the Sun will meet its end – at least as we know it – and the Earth will go with it.
The Southern Ring Nebula as seen by the James Webb Space Telescope. Our Solar System may look like this billions of years from now.
Not only does life on Earth depend on the Sun’s energy, but our planet itself will probably be consumed as the Sun swells to a tremendous size, swallowing the inner worlds before shrugging off its atmosphere to form a vast and beautiful nebula – the tombstone of a planetary system that once was. Stars create, nurture and eventually sweep away the worlds that orbit them.
A ten-billion-year bomb
The anatomy of the Sun. Energy is generated in the core.
Throughout history scientists have speculated on how the Sun shines, and at the turn of the 20th century, the prevailing hypothesis involved intense heating of the core due to pressure from continuous gravitational collapse. At the extraordinary pressure in the core of the Sun, the temperature soars to millions of degrees. However, the established facts about the size and mass of the Sun presented a problem. Under this gravitational collapse model, it would shine for a few tens of millions of years at most, and geologists already knew that our planet was billions of years old. At almost the same moment in 1919, two physicists – Arthur Eddington and Jean Perrin – arrived at the same idea: that the Sun was powered by a self-sustaining nuclear process. We call it nuclear fusion. The Sun’s core is a colossal fusion bomb, a 15-million-degree maelstrom converting hydrogen into helium at an astonishing rate. With each passing second, the Sun converts approximately 600 million tons of hydrogen into 596 million tons of helium, transforming 4 million tons of matter into energy, which is released as sunshine. One could be forgiven for thinking that, at such at a tremendous rate, the Sun would burn through its available fuel quickly, but this fusion process has been sustained for about five billion years, and will continue for at least five billion more. Thereafter, the Sun will continue to fuse different elements and transform into a different kind of star, which can shine for hundreds of billions of years.
Sunshine from another time
The Sun’s fluid surface, called the photosphere, is the jumping-off point for most of the visible light that reaches the Earth.
You may have heard that it takes about 8 minutes and 20 seconds for the Sun’s light to reach us here on the Earth. Light travels at a finite speed, albeit it the fastest speed in the Universe, and we must always necessarily peer back in time to see objects in the sky. Astronomers call this the ‘look back time’, and it can extend to billions of years when studying distant galaxies. Even the objects close to you have a look back time. For example, light takes approximately one nanosecond (one thousandth of a millionth of a second) to travel from your outstretched hand to your eyes. At a distance of about 92 million miles, the Sun’s look back time is indeed measured in mere minutes, but the light we see and heat we feel from the Sun is actually much more ancient than this. In fact, the Sun has already generated all the energy you will receive from it in your lifetime. In the Sun’s core, radiation is released from nuclear fusion, but in order to reach the surface and leap out into the Solar System, it must make its way through the layered interior. Along the way, it continuously collides with or ‘scatters’ off atoms. The average distance a photon – a packet of radiation – can travel before such a collision is called the mean free path. Calculations of the mean free path inside the Sun vary, but at a lower estimate it takes several thousand years for energy from the Sun’s core to find its way out. It could even take up to half a million years in rare cases! This energy powers our biosphere, grows our food and ultimately fuels us too. It’s a gift from the Sun that was prepared long before any of us were born.
Daylight and nightlight
The Northern Lights (shown here) and Southern Lights illuminate our skies when the Sun is below the horizon, but they occur due to solar radiation.
The Sun’s energy output isn’t just confined to photons. It also radiates a significant quantity of plasma from its own atmosphere into the Solar System. Streams of protons and electrons, riding the Sun’s magnetic field, form the solar wind. Invisible to us, it bathes the entire Solar System, washing over the Earth’s protective magnetic field. Most of the incident solar wind is deflected in the process, but our planet’s magnetic field isn’t completely impervious and as a result, some solar plasma seeps into the so-called ‘magnetosphere’ around us. Focused and accelerated by the magnetic field, these particles strike the Earth’s upper atmosphere around the north and south magnetic poles, energising gas atoms which in turn radiate light in a variety of colours. We call them auroras, and they make the solar wind visible at night like bright foam on a dark wave – as long as you’re at the right latitude to see them! The Aurora Borealis, or Northern Lights, are particularly accessible for travellers, and you can learn all about them (including when, where and how to see them) from my book Northern Lights: The Definitive Guide to Auroras – available now.
Mysteries on our cosmic doorstep
Artist’s impression of Solar Orbiter passing Venus on its way to the Sun.
Our ongoing study of the Sun has revealed much about its origins and inner workings, but arguably it has shown us just how much more we don’t yet understand. What causes sporadic periods of quiescent solar activity? Why is the Sun’s lower corona – its atmosphere – hundreds of times hotter than its surface? How do high-energy events, such as solar flares, play out on very short timescales? These questions and many more are driving a renaissance in solar physics, as a fleet of spacecraft observe the Sun’s every action in unprecedented detail, and supercomputer models compete to unlock the secret mechanisms that escape their scrutiny. NASA’s Parker Solar Probe and ESA’s Solar Orbiter have only recently joined the effort, and a ground-breaking new observatory in Hawaii – the enormous Daniel K. Inouye Solar Telescope (DKIST) – began science operations earlier this year. Our friendly star is poised to become even more familiar to us, as we seek a more complete picture, and there will no doubt be surprising discoveries that delight us along the way.
To find out more about how to safely observe the Sun, pre-order your copy of Observing Our Solar System: A Beginner’s Guide – out 1 September. For a much deeper look at our star, Sungazing will be available 25 May 2023.
Tom Kerss is an astronomer, astrophotographer, writer and speaker, specialising in the rewarding task of connecting people to their shared universe. He is the author of Northern Lights, Moongazing, Stargazing and You Can Explore the Universe, all of which are available now.
His new book, Observing Our Solar System, is out on September 1, 2022.
Image credits:
1. NASA/GSFC/SDO
2. NASA/JPL-Caltech
3. NASA, ESA, CSA, and STScI
4. ESA
5. ESA
6. Tom Kerrs
7. ESA