Introducing Our Sun
The key facts about our closest star and what we still don't know.
The Sun is an average yellow dwarf star that lies at the heart of our Solar System. It is the engine that drives the orbits and dynamics of the planets, and gives warmth and life to planet Earth.
It is an enormous ball of hot plasma gas that is powered by nuclear fusion in its core. For 4.6 billion years, the Sun has been fusing hydrogen into helium, releasing huge amounts of energy in the process that causes the Sun to shine.
Leftover debris from the Sun’s formation collapsed under gravity to form the 8 planets as well as the countless dwarf planets, asteroids, and comets that continue to orbit the Sun.
The Sun is about halfway through its 10 billion year lifetime. In 5 billion years, the Sun will run out of hydrogen fuel and will begin to die, eventually puffing off its outer layers of gas into a planetary nebula while the core collapses into a white dwarf.
The Sun is a typical star, similar to hundreds of billions of stars in our Milky Way galaxy, and among the trillions of other stars throughout the universe. Stars are the building blocks of all galaxies, and the powerhouses of countless other solar systems.
Size: The Sun is a ball of burning plasma 1.4 million kilometres in diameter. Compared to the Earth, 1.3 million Earths could fit inside the Sun.
Gravity: The Sun has a mass of 1.9 × 1030 kilograms or 333,000 times heavier than Earth. It contains 99.8% of all the mass in the Solar System. If you could stand on the surface of the Sun (not recommended!), gravity would be 28 times stronger than on Earth.
Age: The Sun is 4.6 billion years old, and is about halfway through its 10 billion year lifetime.
Distance from Earth: The Sun is 150 million kilometres from Earth. It takes light 8 minutes to travel from the Sun to Earth.
Composition: The Sun is a ball of hot plasma gas that burns hydrogen into helium in a process called nuclear fusion. The Sun is made of 73% hydrogen, 25% helium, and trace amounts of heavier elements such as oxygen, carbon, neon, and iron.
Star type: The Sun is an average yellow dwarf star (or in astronomer-speak: a G-type main-sequence star).
Temperature: The Sun’s surface (or photosphere) is about 5,500 degrees Celsius. For reasons we don’t yet understand, the atmosphere above the surface (the corona) is much hotter, around 1 million degrees Celsius.
Rotation: The Sun’s surface is a fluid plasma and rotates at different speeds. On the equator, the surface rotates once every 25 days while near the poles, the surface moves slower and takes 35 dates to rotate.
Speed: The Sun moves at 220 kilometres per second around our Milky Way galaxy.
Moons: The Sun has 0 moons, but it has many other satellites including 8 planets, at least 5 dwarf planets, and countless asteroids and comets that orbit the Sun.
Discovered by: Ancient humans! The Sun has been observed, worshipped, and studied for all of human history, but we are still unravelling its mysteries.
Timeline of Solar Science - before space missions
Sunspots sketched by Carrington in 1859.
Long before telescopes, humans have kept an eye on the Sun. They studied its movements, predicted its eclipses, and speculated on its place in the Solar System.
Here’s a brief history of key solar discoveries, pre-space race.
1375 BCE: Babylonians used stone tablets to record solar eclipses.
800 BCE: Astronomers in ancient China recorded the first observations of sunspots in I Ching, the Book of Changes.
1600s: Galileo used a telescope to observe the cosmos. Around 1610, he began to track mysterious dark spots on the Sun.
1842: During an eclipse over Italy, English astronomer Francis Baily suggested that the mysterious haze encircling the Sun — known as the corona — is the Sun’s atmosphere.
1859: English astronomer Richard Carrington observed a sudden brightening on the Sun’s surface through his telescope. Seventeen hours later, strong aurora are seen as far south as Cuba and telegraph systems across the western world fail and catch fire. This Carrington Event was the first link between a coronal mass ejection on the Sun and a geomagnetic storm on Earth.
1908: American astronomer George Ellery Hale and colleagues discovered the solar cycle, whereby the number of sunspots varies over an 11 year cycle.
1942: Discovery that the corona is millions of degrees Celsius, due to observations of iron spectra with 13 missing electrons. This coronal heating problem, where the solar atmosphere is so much hotter than its surface, is still an open question.
1946: High energy particles detected on Earth were first linked with the Sun (and a recent flare), confirming the concept of a solar wind.
Sun missions - past and present
Since the dawn of the space race, many missions have focused on the Sun. Here is a sampling of some of the key solar missions, past and present.
Skylark – 1961 – This British sounding rocket carried up an X-ray instrument from the University of Leicester, and captured the first X-ray image of the Sun’s corona. Skylark rockets continued to carry instruments to study the Sun for decades to come.
Ariel 1 – 1962 – The first British satellite was primarily focused on studying the Sun and its interaction with our atmosphere.
OSO – 1962-1975 – NASA launched 8 identical OSO missions over 12 years in order to fully study the 11-year cycle of the Sun. The spacecraft measured the Sun in UV, X-rays, and gamma rays. X-ray instruments built by the University of Leicester flew on two of these missions, OSO-4 and OSO-5. A flight spare of OSO can be seen at the National Space Centre.
Ulysses – 1990 – This ESA-NASA mission studied the solar wind and the heliosphere that encompasses our Solar System. It was the first mission to fly over the poles of the Sun. Although it did not carry cameras to image the poles, its instruments sampled the plasma and magnetic field streaming from the Sun for 18 years.
SOHO – 1995 – Another ESA-NASA collaboration, SOHO has studied the Sun from the L1 point between the Earth and the Sun for more than 20 years, and for two full cycles of solar activity.
STEREO – 2006 – Two twin spacecraft, A and B, that orbited the Sun just in front and behind the Earth, in order to capture a 3D picture of the Sun’s activity.
SDO – 2010 – An ongoing solar workhorse that captures images of solar flares and the sources of these eruptions.
Parker Solar Probe – 2018 – This NASA mission is “touching” the Sun by flying closer than we’ve ever been to the Sun and sampling the million degree temperatures of the corona in order to understand how the solar wind is created.
Solar Orbiter – 2020 – This ESA-NASA mission will arrive at the Sun in 2022 and capture the first ever images of the Sun’s poles. It will be able to orbit in sync with sunspots and evolving flares, helping us to better understand and predict solar storms that could damage the Earth.
What don't we know?
Despite studying the Sun for thousands of years, we still don’t fully understand the complicated physics that drives our closest star. We want to solve these mysteries so that we can better predict solar flares and solar storms that might have harmful consequences to technology here on Earth, but also so that we can better understand the fundamental science of all stars, near or far.
Here are four key things we still don’t know:
1. Magnetic field: From a distance, the Sun has a magnetic field shaped similar to Earth’s magnetic field – a donut-shaped dipole with a north and south pole. Every 11 years, at the peak of a cycle of sunspots and flares, this magnetic field flips upside down, and north becomes south. Close up, the Sun’s magnetic field is far more complicated than Earth’s. If we could see it, it would look like a tangled mass of lines and loops, constantly moving and flowing with the gas plasma. If these loops of magnetic field lines are stretched or twisted too much, they can snap and then instantly recombine in a different way, throwing clouds of plasma out into space.
Currently, we struggle to model the magnetic field of the Sun, which is why we’re still sending solar missions out to make more measurements and study it up close. We want to understand how the magnetic field is formed and how it weaves up from the surface and out into space.
2. One million degree corona: The surface of the Sun (the photosphere) is about 5,500 degrees Celsius. But in 1942, we detected an unusual spectra of iron in the atmosphere (corona) above the surface, with 13 electrons missing. Iron is an incredibly stable element, and this stripping of electrons could only happen if the temperature of the corona is more than 1 million degrees Celsius. This doesn’t make any sense – why is the corona hotter than the Sun’s surface? If a pot of water boils on the hob, the surface of the water is hotter than the air above it. There must be some extra energy that is added to the corona in order to make it so much hotter than the surface, and we want to know what that is. NASA’s Parker Solar Probe is currently flying through the Sun’s corona to sample it close up and understand more.
3. Launching the Solar Wind: The Solar Wind is a constant stream of high-energy particles, radiation, and magnetic fields that launches from the Sun and blows out beyond the orbits of Neptune and and Pluto. It carves out a bubble in space called the heliosphere, and it is this bubble that defines the edge of the Solar System. This far-reaching wind governs the aurora and radiation across the whole Solar System, but we still don’t know how it is launched and accelerated away from the Sun’s surface. NASA and ESA missions are currently flying around the Sun to study this more.
4. Solar Flares and Storms: A solar storm (or coronal mass ejection) that is launched from the Sun travels at hundreds of thousands of kilometres per hour through space, and can reach Earth in just a few days. In 1859, a solar storm hit Earth that was the biggest ever observed. Known as the Carrington Event, it created aurora that were seen from the equator and knocked out telegraph systems across the world.
Smaller solar storms have hit more recently, creating local power black outs, but we’ve been lucky to avoid a similar Carrington-scale event in modern times. We’re far more dependent on our energy grids, our communication networks, and our satellites for navigation. A severe solar storm could cause huge disruptions and costly damage, potentially putting lives at risk (especially the astronauts that work on the International Space Station).
We can’t yet fully predict solar storms because our models of the Sun are not accurate enough. In order to have a few days warning of a massive solar storm, we need satellites in orbit around the Sun that can witness storms developing and give us warning of these complicated but hazardous events.
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About the author: Dr Tamela Maciel is the Space Communications Manager at the National Space Centre.