Our Sun is an active yellow dwarf star, frequently bursting out storms of plasma and radiation into the Solar System.
A large-scale eruption from the Sun today would be catastrophic, damaging satellites, taking down Earth’s power grids and communications, endangering astronauts and more.
We study the Sun to learn about its workings, protect our civilization from solar storms, and safely explore the Solar System with our robotic and human endeavors.
Where did the Sun come from?
The Sun formed 4.6 billion years ago from a gigantic collapsing cloud of hydrogen gas and dust called a solar nebula. The leftover material from the Sun’s formation — a mere 0.2% — evolved into the Solar System we know today: planets, moons, asteroids, comets, and all.
The Sun's enormous gravity, about 28 times stronger than Earth’s, produces temperatures of 15 million degrees Celsius (about 27 million degrees Fahrenheit) at its core and enough pressure to fuse hydrogen to helium. This makes the Sun a main sequence star, currently releasing staggering amounts of energy in the form of light and heat.
Why research the Sun?
The Sun’s heat and light make life on Earth possible, which would otherwise be a cold, deserted planet. It drives many of Earth’s phenomena, from providing energy for plants and algae that form the basis of many food chains to influencing ocean currents and weather patterns that change with seasons.
The Sun is the easiest star for us to study, making it very useful to the entire field of astrophysics. It's the only star we can examine up close; Proxima Centauri, the nearest star that you can see in the night sky, is still light-years away. When we study the Sun it can teach us about other stars in the Milky Way galaxy and beyond, and even about the Universe's most unusual phenomena like energetic supernovae and black holes. Learning how the Sun affects the hundreds of billions of kilometers of space surrounding it also teaches us about how its activity affects Earth, and about the possibilities of life on exoplanets around other stars.
What is solar wind?
Space isn’t actually empty. Earth and the planets in our Solar System lie in the extended atmosphere of the Sun. The the nuclear fusion taking place within it, our star doesn’t just emit light and heat but also an incessant stream of charged, energetic particles called the solar wind. Like an inflating balloon, the solar wind carries the Sun’s magnetic field well beyond Pluto. We can see the beginning of the Sun’s extended atmosphere, the corona, during a total solar eclipse.
Solar wind races through the Solar System at no less than hundreds of kilometers per second, slamming everything in its way. It strips away the atmospheres of planets like Venus and Mars and causes comets to develop ion tails that follow its direction.
When the solar wind reaches Earth, our magnetic field largely deflects it, protecting us from its harmful radiation. Sometimes when the solar wind is slightly more intense, some of its particles pierce through and reach Earth’s atmosphere, creating beautiful glowing auroras. But a more vicious solar wind can wreak havoc.
What are coronal mass ejections?
The Sun’s deadliest eruptions, called coronal mass ejections, release billions of tons of material in space, turning solar wind into solar storms. In 1859, 1882, and 1921, particles from the most intense of these reached all the way to Earth’s surface, causing telegraph services to stop working, initiating fires, and even delivering fatal shocks to telegraph operators.
If the Sun emits a coronal mass ejection of such scale today, it could be even more catastrophic. It could damage satellites, which we increasingly depend on, or our robotic explorers across the Solar System. A strong enough coronal mass ejection could even take down Earth’s power grids, disrupt communications, corrode pipelines, endanger astronauts, and more.
What are sunspots?
We know from hundreds of years of telescopic observations that the Sun’s level of activity is linked to sunspots: dark blotches above its surface that appear, grow in number and size, diminish, and go away when the Sun’s magnetic field flips roughly every 11 years.
When the Sun has the most sunspots—which we still track by hand—scientists call it a solar maximum, as that is when solar storms are frequent. During a solar minimum, which last occurred in December 2019, the Sun is quietest.
What were some early missions to the Sun?
Although optical telescopes can see as deep as the Sun's photosphere (the lowest layer of the Sun's atmosphere), other instruments have been used over the decades to peer deeper into the Sun and measure its outer layers and the effect it has on the space surrounding it.
Soon after the Space Age began, we started sending spacecraft to better understand the Sun’s weather. Between 1965 and 1969, NASA launched the Pioneer 6 through 9 spacecraft, which formed a ring of stations spaced approximately along Earth's orbit to provide warnings of incoming solar storms.
NASA also put an 11,000-kilogram (about 24,000-pound) human-operated solar observatory on its first space station, Skylab, in 1973. It observed for the first time coronal mass ejections as well as coronal holes, colder regions of the Sun’s atmosphere that generate part of the solar wind.
Japan’s Yokoh satellite, launched in 1991, studied the Sun for an entire solar cycle in X-rays, where our star’s activity is more prominent compared to visible light. It discovered that a little-understood “magnetic reconnection” process—tangling, breaking, and reconnecting of magnetic lines—are what unleash a flare’s energy output of millions of atomic bombs.
Around the same time, the European Space Agency (ESA) and NASA’s joint mission Ulysses took a gravity slingshot from Jupiter to incline its solar orbit and take the first measurements of the Sun’s poles. Its instruments showed that the poles emit a slower solar wind and larger magnetic waves than was expected, opening a whole new array of questions about the Sun’s magnetic field.
After decades of traveling through the Solar System, NASA’s twin Voyager spacecraft crossed the Sun’s magnetic field bubble in the last decade, measuring its structure and transmitting valuable information about interstellar space. Both Voyagers observed a sharp increase in galactic cosmic rays past the Sun’s magnetic field, which protects us from the majority of this radiation.
Monitoring solar activity
In the last two decades, we have focused extensively on tracking the Sun’s weather. In 2006, NASA launched the twin Solar TErrestrial RElations Observatory (STEREO) spacecraft in the same orbit around the Sun as Earth, with each spacecraft leading and trailing Earth by up to 90 degrees. Their side views, coupled with Earth-based observatories and space telescopes, allowed us to track solar eruptions in 3D. NASA could also detect sunspots and eruptive features while they were developing on the Sun’s far side. We lost contact with STEREO-B in 2014 but STEREO-A continues to provide insights.
Launched in 2010, NASA’s Solar Dynamics Observatory (SDO) monitors the Sun in 13 different wavelengths, each highlighting a particular part from the surface to the corona. Combined with its observations of the Sun’s magnetic field, SDO gives scientists a complete picture of the Sun’s active regions to understand exactly how solar flares develop and erupt. Using this knowledge, scientists predicted seven out of the nine biggest solar flares in the last solar cycle, an increasingly handy ability as we explore more of the Solar System both robotically and with astronauts.
The ESA-NASA Solar and Heliospheric Observatory (SOHO) observatory has been observing the Sun uninterrupted for over 25 years since 1995 from Lagrange point 1, a region of space between Earth and the Sun where their gravities roughly cancel out. SOHO’s coronagraph blocks sunlight to create solar eclipses on-demand to better see the corona and track solar eruptions. The coronagraph has proven to be so valuable that most future missions along the Sun-Earth line will be carrying one, including India’s Aditya-L1 solar observatory and ESA’s Proba-3 spacecraft launching in 2023.
Probing the Sun
In 2018, NASA launched the Parker Solar Probe on arguably the most dangerous robotic mission ever—to study the Sun’s atmosphere by being in it.
The Parker probe is using Venus’ gravity to slow itself down and get close to the Sun. With every Venus flyby, the spacecraft is swinging deeper in the Sun’s corona so it can measure its properties and face the young solar wind, which is turbulent and twisted, unlike the smoother flow it develops near Earth. In 2025, the Parker probe will be just 6 million kilometers (almost 4 million miles) above the Sun’s surface—nine times closer than scorching Mercury.
In its eight passes through the Sun’s outer corona so far, the Parker probe has already revealed intriguing features that the solar wind near Earth, or even Mercury, lacks. A key discovery has been confirming what the Ulysses spacecraft saw hints of—moving zig-zag spikes in the solar wind along which the magnetic field rapidly flips, like a wave traveling through a rope. Scientists think these “switchbacks” are fundamental to understanding how the solar wind originates and accelerates.
The Parker probe’s proximity to the Sun means it couldn’t carry imaging instruments. This is where the ESA-led Solar Orbiter mission, launched in 2020, comes in. The Solar Orbiter will image the Sun and huge swaths of its corona up close for the first time. It'll also measure the properties of solar wind particles.
While its closest point to the Sun lies just within Mercury’s orbit—not as close as the Parker probe—the spacecraft’s complementary set of instruments will allow scientists to uniquely trace the solar wind particles to their root on the Sun’s surface.
One of the biggest fundamental mysteries both spacecraft intend to solve is why the Sun’s corona is much hotter than its surface. Scientists have been debating since the 1940s how the Sun’s atmosphere is heated to 1 million degrees Celsius (almost 2 million degrees Fahrenheit) while the surface remains a comparatively scant 6,000 degrees Celsius (nearly 11,000 degrees Fahrenheit). This is why the Parker probe is studying the corona from within.
Scientists suspect the heating of the corona and the solar wind’s mysterious behavior are both rooted in the Sun’s complex, intertwined magnetic field. To investigate this, the Solar Orbiter will make detailed maps of the Sun’s magnetic field on and below its surface.
The Solar Orbiter’s initial close-up images of the Sun have already turned up a promising clue. It has seen numerous tiny solar eruptions across the Sun’s surface for the first time, each with temperatures around a million degrees Celsius (nearly 2 million degrees Fahrenheit), something which had only been predicted until now. Several scientists think these "nanoflares" play a major role in heating the corona. More images over the next few years should hopefully reveal more clues. The Solar Orbiter will later incline its orbit by 2025 to image the Sun’s poles up-close and measure the drastically different magnetic field and solar wind there.
For the first time, we have a comprehensive view of the Sun’s activity and space weather, from observing the Sun’s full disk to measuring its corona to probing the solar wind near Earth and beyond. By the end of the decade, we hope to unravel quite a few fundamental mysteries about the Sun and get better at dealing with damaging solar storms. In doing so, we’ll be able to better protect our civilization and safely explore the Solar System.
Acknowledgments: this page was initially written by Jatan Mehta in 2021.