The Sun is a nearly perfect sphere of hot plasma, about 1.39 million km wide, more than 100 Earths placed side-by-side. It contains 99.86% of the Solar System’s mass, and its gravity holds everything in orbit: planets, moons, asteroids, comets.

About 4.6 billion years ago, it formed from a collapsing cloud of gas and dust. Nuclear fusion ignited at the center, creating the Sun, while the leftover material flattened into a disk that became the planets.

Though it appears steady from 150 million kilometers away, the Sun is a roaring nuclear engine. Every second, it fuses around 600 million tons of hydrogen into helium, releasing energy equivalent to a trillion megaton bombs.

Why the Sun Matters

Without the Sun, nothing familiar would exist:

> No life
> No atmosphere
> No weather
> No oceans
> No blue sky

Its light fuels photosynthesis, drives Earth’s climate, and powers nearly every biological process on our planet. And the atoms in your body (carbon, nitrogen, oxygen) were created in previous generations of stars. The Sun is part of that long cosmic lineage. It’s also a unique laboratory. Out of the 200+ billion stars in the Milky Way, it’s the only one close enough to study in real time and in extraordinary detail. By understanding the Sun, we understand stars everywhere.

Where It Sits in the Cosmos

The Sun orbits the center of the Milky Way once every 230 million years, traveling at about 220 km/s. Its journey carries the entire solar system along with it.
It’s classified as a Population I star, meaning it’s relatively rich in heavy elements formed by earlier generations of stars. That chemistry is what allowed planets, life, and you to exist.

The Sun–Earth Connection

Light from the Sun travels the 150 million km to Earth in just 8 minutes, warming oceans, driving weather, powering climate, and supporting life. But the Sun also blows a constant stream of charged particles, the solar wind, which creates a vast magnetic bubble called the heliosphere. Earth lives inside this bubble, sheltered from interstellar radiation. Yet when the Sun becomes active, its influence can disrupt:

> Satellites
> GPS
> Radio signals
> Power grids

How we study the Sun

Modern missions including SOHO, SDO, Parker Solar Probe, Solar Orbiter, Hinode, IRIS observe the Sun from every angle and wavelength. We study: X-rays, UV light, Radio waves, magnetic fields, plasma flows, sunspots, flares, the solar wind

We monitor the Sun continuously because understanding its behavior helps us understand space weather and protect modern technology.

Quick Facts

Stellar type	G2V (main-sequence)
Diameter	1.39 million km (~109 Earths)
Mass	1.989 × 1030 kg (~333,000 Earths)
Age	~4.6 billion years
Surface temperature	~5,500 °C (9,932 °F)
Core temperature	~15 million °C
Composition	~74% Hydrogen, ~24% Helium, ~2% heavier elements
Distance to Earth	1 AU (149,597,870 km)
Light travel time	~8 min 20 s
Luminosity	3.8 × 1026 W

References

References & Credits

• NASA/GSFC – Solar Dynamics Observatory
• Johns Hopkins APL – Parker Solar Probe
• ESA – Solar Orbiter
• NOAA SWPC – Space Weather Prediction Center

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The inner layers: Core, Radiative Zone, Convective Zone

At the center of the Sun, temperatures reach about 15 million degrees Celsius, and pressures are immense. Under these conditions, hydrogen nuclei move fast enough to collide and combine, forming helium. Even though each individual reaction releases only a small amount of energy, the sheer number of reactions, trillions upon trillions every second, powers the entire Sun. The core is the only place in the Sun where fusion occurs. Everywhere else is simply the pathway for this energy to escape.The core is about 25% of the Sun’s radius.

After energy is produced in the core, it begins a long journey outward. In the radiative zone, energy moves as photons, which are repeatedly absorbed and re-emitted by nearby particles. This process, called radiative diffusion, is incredibly slow; a photon may take hundreds of thousands to millions of years to make it through this region. The radiative zone is dense enough that light cannot travel freely and is constantly scattered.

When energy reaches the convective zone, conditions change. The plasma here is cooler and more opaque, making it harder for radiation to pass through. Instead, hot plasma rises toward the surface while cooler plasma sinks, transporting energy by convection.
This motion creates the granulated appearance of the Sun’s visible surface and helps shape the Sun’s magnetic field. Convection is efficient, and energy moves through this layer much faster than through the radiative zone.

The outer layers: The Photosphere, Chromosphere & Corona

The Sun’s visible surface, the photosphere, is where most sunlight escapes. It’s what we actually see. The temperature drops to ~5,500 °C and this is where we can see things like sunspots.

Above it lies the chromosphere, where the temperature begins to rise to thousands of degrees. We can observe intense magnetic activity, waves, and jets in this region.

Beyond that stretches the corona, the Sun’s outer atmosphere, extending millions of kilometers into space, surprisingly hotter than the surface. Temperatures in the corona exceed a million degrees.

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The Sun shines because of nuclear fusion, a process that takes place only in its core. Fusion is the fundamental engine that drives everything the Sun does.

What Fusion Is

In the Sun’s core, temperatures reach about 15 million°C and pressures are enormous. Under these extreme conditions, hydrogen nuclei move fast enough to overcome their natural repulsion and combine to form helium.
This reaction releases energy because a tiny amount of mass is converted into energy, following E = mc². Even though each reaction releases only a small amount of energy, the number of reactions occurring every second is so large that it powers the entire Sun.

Fusion only happens in the core; no other layer reaches the temperatures and pressures required.

Why Fusion Makes the Sun Shine

The energy produced in the core starts as high-energy photons. These photons then take a long, indirect journey outward through the Sun’s interior before they finally escape from the surface as sunlight.
Once radiation reaches space, it travels to Earth in just 8 minutes, after spending a vast amount of time moving through the Sun itself.

Although most of the Sun’s energy escapes as light, some of it also leaves as the solar wind, a stream of charged particles flowing outward from the corona.

Balancing Gravity and Pressure

Fusion is not just a power source, it also keeps the Sun structurally stable. The outward pressure created by fusion-produced energy balances the inward pull of gravity. This balance, called hydrostatic equilibrium, is why the Sun doesn’t collapse under its own weight or blow itself apart.

As long as fusion continues at a steady rate, the Sun remains stable.

How Long the Sun Will Shine

The Sun has enough hydrogen fuel to sustain fusion for about 10 billion years in total. It is currently around 4.6 billion years old, meaning it is roughly halfway through its stable, energy-producing lifetime.
Eventually, the Sun will expand into a red giant and change dramatically, but this will not happen for billions of years.

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Sunspots are the most visible signs that the Sun’s magnetic field is constantly changing. They mark areas where magnetic forces are strong enough to alter the flow of energy inside the Sun.

What Are Sunspots?

Sunspots are darker, cooler regions on the Sun’s photosphere.

> They appear dark only by contrast; they are still extremely hot.
> Typical temperature: ~3,500–4,000°C (vs. ~5,500°C for the surrounding surface).
> Many are large enough to fit Earth inside.

Sunspots form when strong magnetic fields rise up from within the Sun and suppress convection, the upward movement of hot plasma. With less heat reaching the surface, the area becomes cooler and appears darker.

Sunspots usually occur in pairs or clusters, representing opposite magnetic polarities (a north and south pole emerging through the surface).

Where the Magnetism Comes From

The Sun’s magnetic field is generated by motion within its interior. Hot, ionized plasma circulates in the convective zone while the Sun rotates at different speeds at different latitudes. These combined motions create electric currents and, ultimately, the Sun’s global magnetic field, a process known as the solar dynamo.
Because plasma is constantly moving and flowing, the magnetic field is never static. It stretches, twists, and folds over time, contributing to the formation of active regions and sunspots.

Magnetic Fields and Active Regions

Sunspots typically form in larger magnetic structures called active regions. In these areas, magnetic field lines extend above the surface, guiding the movement of plasma and shaping features such as bright loops seen in ultraviolet and X-ray images.
Active regions can vary from simple and well-organized to highly complex. The more tangled the field becomes, the more energy is stored within it, increasing the likelihood of eruptions.

Reconnection and Solar Eruptions

When magnetic fields become too stretched or stressed, they can suddenly change shape. This process is called magnetic reconnection and releases stored energy rapidly. Reconnection is responsible for producing solar flares, which are bursts of intense radiation, and coronal mass ejections, which are large eruptions of plasma and magnetic field into space.
Most major space-weather events originate from active regions associated with sunspots.

Why Sunspots Matter

Because sunspots form where magnetic fields are strongest, they serve as useful indicators of solar activity. Tracking how many sunspots appear, how large they are, and how their magnetic fields are arranged helps scientists monitor the Sun’s activity level and assess the potential for flares or eruptions.
Sunspots therefore play an essential role in understanding the Sun’s magnetic behavior and in forecasting events that can affect Earth and space-based technology.

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The Sun’s magnetic field does not stay the same over time. Instead, it follows a repeating pattern of activity that rises and falls over roughly 11 years. This pattern is known as the solar cycle, and it governs how many sunspots appear on the surface, how active the Sun becomes, and how often solar storms occur.

How the Cycle Works

The solar cycle is driven by the Sun’s internal magnetic field. Because the Sun is made of plasma and not a solid surface, different parts of it rotate at different speeds. The equator spins faster than the poles, and the convective zone constantly stirs and stretches magnetic fields. Over time, these motions cause the magnetic field to become twisted and tangled.

As the field becomes more distorted, sunspots and active regions become more numerous. Eventually, the field grows so stressed that it reorganizes itself, and the Sun’s north and south magnetic poles switch places. This magnetic “reset” marks the transition between cycles. After the flip, the field gradually straightens out again and the Sun’s surface grows quieter.

Solar Minimum

Solar minimum is the quiet phase of the cycle. During this time, the Sun has few or no visible sunspots, and its magnetic field is relatively simple and stable.
Although the Sun is not entirely inactive, small flares and eruptions still occur, the overall level of solar disturbance is low. Space weather effects on Earth are generally mild, and the solar wind is slower and more uniform.

Solar Maximum

As the cycle progresses toward solar maximum, sunspots become far more common. The Sun’s magnetic field becomes increasingly complex, and active regions can produce frequent solar flares and coronal mass ejections.
This is the phase when the Sun is most likely to generate strong space weather events, including radiation storms, fast CMEs, and radio blackouts. The corona becomes more dynamic, and the heliospheric current sheet becomes highly warped as the magnetic field twists.

The Magnetic Pole Flip

Near the peak of each cycle, the Sun’s global magnetic field reverses directions. The north pole becomes the south, and the south becomes the north.
This flip doesn’t happen instantaneously; instead, the magnetic fields gradually weaken, break apart, and reform with opposite polarity. Once the reversal is complete, a new solar cycle has begun.

Why the Solar Cycle Matters

The solar cycle shapes the rhythm of space weather throughout the Solar System. During active years, Earth is more likely to experience:

> Intense auroras
> Strong geomagnetic storms
> Satellite disruptions
> Increased radiation exposure for astronauts
> Changes in the upper atmosphere that affect satellite drag

Even during quiet phases, the Sun remains a dynamic star, but the level of activity is much lower.

Tracking the solar cycle helps scientists anticipate periods of heightened activity, plan spacecraft operations, and better understand how the Sun’s magnetic field evolves over time.

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The Sun produces light across the entire electromagnetic spectrum, not just the visible colours we see with our eyes. Each type of light reveals different information about the Sun’s temperature, magnetic activity, and atmosphere. By observing the Sun in multiple wavelengths, scientists can piece together a complete picture of how it works.

Visible Light

Visible light is the small portion of the spectrum that human eyes can detect. It comes from the photosphere, the Sun’s “surface,” and shows us features like sunspots and granulation. This is the traditional view of the Sun, but it represents only a tiny fraction of the story.

Ultraviolet (UV) Light

Ultraviolet light comes from hotter, higher layers of the Sun’s atmosphere, especially the chromosphere and transition region. UV images reveal thin jets, bright patches, waves, and the early stages of flares. Many of the Sun’s most dynamic features are visible only in UV.

Extreme Ultraviolet (EUV)

Extreme ultraviolet wavelengths show million-degree plasma in the corona. These images highlight hot loops, active regions, coronal holes, and the structure of the magnetic field. EUV observations are essential for tracking the sources of solar wind and eruptions.

X-rays

X-rays reveal the hottest and most energetic parts of the Sun’s atmosphere. They are produced during strong magnetic activity, such as flares and rapidly changing coronal structures. X-ray observations help scientists understand how magnetic energy is built up and released.

Infrared

Infrared light comes from cooler regions lower in the atmosphere and can penetrate dust more easily. It is useful for studying sunspot structure, flows of plasma beneath the surface, and some deeper atmospheric layers. Infrared also helps measure the Sun’s oscillations for helioseismology.

Radio Waves

Radio emission comes from several processes, including energetic particles, shock waves, and magnetic disturbances. During solar flares or CMEs, the Sun can produce strong radio bursts that travel across the solar system. Radio observations help track how disturbances move outward through space.

Why Multiple Wavelengths Matter

Each wavelength highlights different temperatures, regions, and physical processes. The Sun looks completely different depending on which type of light you observe:

> Visible light shows the photosphere.
> UV shows the dynamic chromosphere.
> EUV and X-rays reveal the million-degree corona.
> Radio waves trace shocks and particle acceleration.
> Infrared gives insights into deeper layers and magnetic structure.

Studying the Sun across the spectrum allows scientists to monitor its magnetic field, understand how flares start, follow eruptions, and predict space-weather effects on Earth.

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The solar wind is a continuous flow of charged particles that streams outward from the Sun in all directions. It is made primarily of electrons and protons, along with traces of heavier ions, all moving fast enough to escape the Sun’s gravity. This outflow fills the entire solar system and shapes the magnetic environment around every planet.

Where the Solar Wind Comes From

The solar wind originates in the Sun’s outer atmosphere, the corona, where temperatures reach more than a million degrees. At these temperatures, particles move so quickly that they are no longer held in by the Sun’s gravity. They stream outward into space, carrying the Sun’s magnetic field with them.

The wind is not uniform. Some regions of the corona release a slower, denser wind, while others, especially coronal holes, produce a faster, more rarefied outflow. These differences create changes in space weather throughout the solar system.

How It Travels Through Space

As the solar wind moves outward, the Sun’s rotation causes the magnetic field embedded in the wind to form a spiral pattern known as the Parker spiral. This structure influences how particles and solar disturbances move through space and how they reach planets.

The solar wind travels at hundreds of kilometres per second, reaching Earth in just a few days. Far beyond the planets, it eventually slows down and interacts with the interstellar medium, forming the boundary of the heliosphere.

Effects on Earth and Other Planets

The solar wind interacts with planetary magnetic fields and atmospheres in different ways. At Earth, it shapes the magnetosphere, the protective magnetic bubble surrounding the planet. Changes in solar wind speed and density can compress or stretch this bubble and create disturbances inside it.

When the solar wind carries stronger magnetic fields or is accompanied by solar eruptions, it can transfer energy into Earth’s magnetosphere. This process drives auroras, disturbs satellite orbits, and can affect radio and GPS signals. Planets without strong magnetic fields, such as Mars, are more directly exposed to the solar wind, which can strip material from their atmospheres over long periods of time.

Why the Solar Wind Matters

The solar wind is a key part of the Sun’s influence on the solar system. It shapes the heliosphere, interacts with planetary environments, and carries the Sun’s magnetic field far beyond Pluto. It also provides the background conditions for space weather – the constant environment in which flares and coronal mass ejections travel.

Understanding the solar wind helps scientists predict how solar activity will affect Earth and supports the safety of satellites, spacecraft, and astronauts.

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The Sun produces a wide range of eruptive activity, from rapid bursts of radiation to large clouds of plasma launched into space. These events are driven by changes in the Sun’s magnetic field and can vary from small, quiet releases of material to major disturbances that affect the entire solar system. Although “eruption” can sound dramatic, many of these processes happen regularly and are part of the Sun’s normal behavior.

Below are the main types of solar eruptions and how they differ. More information on solar eruptions can also be found in the ‘Space Weather’ book.

Solar Flares

A sudden release of electromagnetic energy.

Solar flares occur when magnetic fields in an active region rapidly change shape. Magnetic reconnection releases energy in the form of X-rays, ultraviolet light, and radiation across the electromagnetic spectrum. Solar flares:

> Happen in the Sun’s atmosphere
> Are mainly seen as brightening in UV and X-rays
> Can last from minutes to hours
> Affect Earth almost immediately because radiation travels at the speed of light

Flares themselves do not throw significant mass into space, but they are often associated with larger eruptions occurring nearby.

Coronal Mass Ejections (CMEs)

A large cloud of plasma and magnetic field launched into space.

A CME is a massive bubble of charged particles and embedded magnetic field that erupts from the corona. CMEs can take one to three days to reach Earth, depending on their speed, and are responsible for the most significant geomagnetic storms. Key features of CMEs:

> They contain actual solar material (plasma).
> They carry a strong magnetic field.
> Their effects depend on the direction of the magnetic field inside the CME when it arrives at Earth.

Not all flares have CMEs, and not all CMEs have strong flares, the two phenomena are related but distinct.

Solar Energetic Particle (SEP) Events

High-speed particles accelerated to near-relativistic energies.

SEP events occur when particles, mainly protons and electrons, are accelerated to high speeds by either:

> Shock waves driven by fast CMEs, or
> Reconnection sites in very strong flares.

SEPs can arrive at Earth in less than an hour and can pose radiation hazards to astronauts and satellites. These events do not require a CME or flare to be Earth-directed; particles can follow magnetic field lines that curve through space.

Ground Level Enhancements (GLEs)

Rare SEP events detected at Earth’s surface.

GLEs are a subset of SEP events where particle energies are high enough that radiation increases can be measured by ground-based neutron monitors. These events are uncommon and typically associated with major, fast CMEs and strong shock acceleration. They are important for understanding extreme solar activity but occur only once every few years or less.

Prominences and Filaments

Large structures of cooler plasma supported by magnetic fields.

Prominences are bright, looping structures seen at the Sun’s limb, while filaments are the same structures seen against the solar disk, appearing darker by contrast. They can remain stable for days or weeks, suspended above the surface by magnetic fields.

Sometimes a prominence or filament becomes unstable and erupts, often forming the core of a CME. These eruptions can contribute to strong space-weather events if directed toward Earth.

Coronal Holes

Regions of open magnetic field where solar wind escapes more easily.

Coronal holes appear dark in extreme ultraviolet images and mark areas where magnetic field lines open out into space rather than looping back to the surface. These open lines allow high-speed solar wind streams to flow outward. Coronal holes:

> Are common during declining phases of the solar cycle
> Can persist for many rotations
> Produce high-speed solar wind streams (HSS) that interact with slower wind ahead of them

These interactions form corotating interaction regions (CIRs), which can cause geomagnetic disturbances and long-lasting auroras.

High-Speed Streams & CIRs

Recurring space-weather drivers from persistent coronal holes.

High-speed solar wind streams from coronal holes can produce recurring effects on Earth roughly every 27 days (one solar rotation). When fast wind overtakes slower wind, the compression forms a CIR. CIRs can:

> Disturb Earth’s magnetic field
> Produce moderate geomagnetic storms
> Create steady auroras over multiple days
> Accelerate particles in the heliosphere

These are less dramatic than CMEs but are an important part of everyday space weather.

How These Eruptions Fit Together

Although all of these phenomena come from the Sun’s magnetic field, they are not the same thing:

> Flares release radiation.
> CMEs expel plasma and magnetic field into space.
> SEP events involve high-energy particles travelling along magnetic field lines.
> Prominences/filaments are cooler plasma structures held in place until they erupt.
> Coronal holes release steady high-speed solar wind, not explosive eruptions.

Together, they make up the full spectrum of the Sun’s eruptive behaviour.