Introduction
The Sun is the star at the centre of our Solar System in the Universe. It is a gigantic sphere of hot, electrically charged plasma whose gravity governs the motion of the planets, asteroids, comets and other objects orbiting it.
The Sun is a main-sequence star of spectral type G2 V. It formed approximately 4.6 billion years ago and contains about 99.8% of the mass of the Solar System.
Energy generated by nuclear fusion in the Sun's core ultimately powers nearly all of the natural energy received by Earth.
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Key Facts About the Sun
| Property | Approximate Value |
|---|---|
| Age | About 4.6 billion years |
| Type | G2 V main-sequence star |
| Distance from Earth | About 150 million km (1 astronomical unit) |
| Diameter | About 1.39 million km |
| Radius | About 700,000 km |
| Mass | About 333,000 Earth masses |
| Photosphere temperature | About 5,500 °C |
| Core temperature | About 15 million °C |
| Equatorial rotation period | About 25 Earth days |
| Polar rotation period | About 36 Earth days |
NASA gives the Sun's diameter as approximately 1.4 million km and its average Earth distance as approximately 150 million km.
```The Sun's Importance to Earth
The Sun provides the energy that drives many of the processes occurring on Earth.
Light
Solar radiation provides the primary source of visible light reaching Earth's surface.
Climate
Solar energy is a fundamental driver of Earth's climate system and atmospheric circulation.
Photosynthesis
Plants use solar energy to convert carbon dioxide and water into chemical energy.
Space Weather
Solar activity can influence Earth's magnetosphere, satellites, communications and power infrastructure.
Structure of the Sun
The Sun does not possess a solid surface like Earth. It is composed primarily of plasma and can be divided into several major regions.
| Region | Description |
|---|---|
| Core | The central region where nuclear fusion converts hydrogen into helium. |
| Radiative Zone | Energy is transported mainly through the interaction of radiation with matter. |
| Convection Zone | Hot plasma rises and cooler plasma sinks, transporting energy by convection. |
| Photosphere | The visible layer from which most of the Sun's observable light escapes. |
| Chromosphere | A relatively thin atmospheric layer above the photosphere. |
| Corona | The extremely hot, extended outer atmosphere of the Sun. |
Nuclear Fusion in the Sun
The Sun shines because of nuclear fusion occurring in its core. Under the extreme temperature and pressure there, hydrogen nuclei undergo a sequence of reactions that ultimately produce helium.
The principal mechanism in the Sun is the proton–proton chain.
The mass of the resulting helium nucleus is slightly less than the combined mass of the original hydrogen nuclei. The missing mass is converted into energy according to Einstein's famous equation:
ESA estimates that the Sun converts approximately four million tonnes of mass into energy every second.
```Hydrostatic Equilibrium
A star remains stable because its inward gravitational attraction is balanced by pressure acting outward.
This equation describes hydrostatic equilibrium in a spherically symmetric star.
In the Sun, energy produced by nuclear fusion supplies the thermal and radiation pressure needed to maintain the stellar structure against gravitational collapse.
```Energy Transport
Radiative Transport
In the radiative zone, photons interact repeatedly with matter. Energy therefore takes an extremely long and indirect path through this region.
Convective Transport
Farther from the centre, the plasma becomes sufficiently opaque that convection becomes an important mechanism for transporting energy.
The Solar Atmosphere
Above the photosphere are the chromosphere, transition region and corona.
Photosphere
The photosphere is the visible layer commonly described as the "surface" of the Sun. Its temperature is approximately 5,500 °C.
Chromosphere
The chromosphere lies above the photosphere and is particularly prominent during observations at certain wavelengths and during solar eclipses.
Corona
The corona is the Sun's extended outer atmosphere. One of the major unsolved problems in solar physics is why the corona reaches temperatures of millions of degrees, much hotter than the photosphere.
Solar Activity
The Sun is not a static object. Its magnetic field produces a wide range of constantly changing phenomena.
Sunspots
Darker, relatively cooler regions of the photosphere associated with strong magnetic fields.
Solar Flares
Powerful bursts of electromagnetic radiation produced by sudden releases of magnetic energy.
Prominences
Large structures of relatively cool plasma suspended in the solar atmosphere by magnetic fields.
Coronal Mass Ejections
Huge eruptions of magnetised plasma expelled from the solar corona.
Sunspots
Sunspots appear dark because they are cooler than the surrounding photosphere. They are associated with concentrated magnetic fields emerging through the solar surface.
Individual sunspots can range from thousands to tens of thousands of kilometres across and can persist for days or months.
The number and distribution of sunspots change during the solar activity cycle.
```Solar Flares
A solar flare is a sudden release of magnetic energy in the solar atmosphere.
Flares can produce intense radiation across a broad range of the electromagnetic spectrum, including ultraviolet and X-rays.
Coronal Mass Ejections
Coronal mass ejections, commonly abbreviated as CMEs, are enormous eruptions of magnetised plasma from the Sun.
If a CME is directed towards Earth, its interaction with Earth's magnetic field can produce a geomagnetic storm.
Such storms can produce spectacular auroras but can also disturb satellites, communications and electrical infrastructure.
```The Solar Cycle
Solar activity varies approximately over an 11-year cycle.
During solar minimum, the Sun generally has fewer sunspots and fewer major eruptions. During solar maximum, magnetic activity increases and sunspots, flares and coronal mass ejections become more frequent.
The magnetic polarity of the Sun reverses approximately every solar cycle, giving an approximately 22-year magnetic cycle for a complete return to the original polarity.
```The Solar Wind
The solar corona continuously releases electrically charged particles into space. This outflow is called the solar wind.
The solar wind consists primarily of electrons and ions and travels throughout the Solar System.
The Heliosphere
The solar wind creates a gigantic bubble around the Sun called the heliosphere.
The heliosphere extends far beyond the orbit of the planets and provides a region in which the solar wind dominates the local interplanetary environment.
NASA describes the heliosphere as the magnetic bubble created by the solar wind. Voyager 1 and Voyager 2 have crossed the heliopause, the boundary between the heliosphere and interstellar space.
```Space Weather
Space weather describes changes in the space environment caused primarily by solar activity.
| Solar Phenomenon | Possible Effect at Earth |
|---|---|
| Solar Flare | Can produce radio blackouts and enhanced radiation. |
| Coronal Mass Ejection | Can produce geomagnetic storms and auroras. |
| High-Speed Solar Wind | Can disturb Earth's magnetosphere. |
| Geomagnetic Storm | Can affect satellites, navigation systems, radio communications and power infrastructure. |
Solar activity can therefore have technological consequences far beyond the immediate vicinity of the Sun.
```The Sun's Magnetic Field
The Sun's plasma is electrically conducting, allowing fluid motions within the star to generate and modify magnetic fields.
The Sun rotates differentially: material near the equator rotates faster than material near the poles. This differential rotation contributes to the complex evolution of the solar magnetic field.
Magnetic fields can become strongly concentrated and twisted, producing many of the explosive phenomena observed in the solar atmosphere.
```Differential Rotation
Unlike a solid object, the Sun does not rotate as a single rigid body.
| Region | Approximate Rotation Period |
|---|---|
| Equator | About 25 Earth days |
| Mid-latitudes | Intermediate values |
| Poles | About 36 Earth days |
This differential rotation is an important component of solar dynamo theory and the generation of large-scale magnetic activity.
```Solar Eclipses
A solar eclipse occurs when the Moon passes between Earth and the Sun and, from a particular location, blocks some or all of the Sun's visible disk.
Partial Eclipse
Only part of the Sun's disk is obscured by the Moon.
Total Eclipse
The Moon completely covers the visible solar disk for observers within the path of totality.
Annular Eclipse
The Moon appears slightly smaller than the Sun, leaving a bright ring around its silhouette.
Studying the Sun
Scientists study the Sun using ground-based observatories and spacecraft observing it across many wavelengths.
Solar Orbiter
An ESA mission with strong NASA participation studying the Sun and its polar regions.
Parker Solar Probe
A NASA mission designed to investigate the solar corona and solar wind at very close range.
SOHO
The Solar and Heliospheric Observatory has provided long-term observations of the Sun and solar atmosphere.
Solar Dynamics Observatory
Provides continuous, high-resolution observations of solar activity across multiple wavelengths.
ESA's Solar Orbiter combines remote observations of the Sun with in-situ measurements of the solar wind and is designed to study the Sun from higher heliographic latitudes.
```The Sun and Other stars
The Sun is classified as a G2 V star. Although it is the most important star for Earth, it is not an exceptionally unusual star in the wider Galaxy.
Studying the Sun in detail provides astronomers with an opportunity to understand stellar physics using an object close enough to observe at very high spatial and temporal resolution.
The Sun's Place in the Milky Way
The Sun is located in the Orion Spur, a relatively small structure between the major Sagittarius and Perseus spiral arms of the Milky Way.
The entire Solar System orbits the centre of the Milky Way. One complete galactic orbit takes approximately 230 million years.
The Future of the Sun
The Sun is currently approximately halfway through its main-sequence lifetime.
As hydrogen in its core is gradually depleted, the structure of the Sun will change dramatically.
Red Giant Phase
In roughly five billion years, the Sun is expected to leave the main sequence and expand into a red giant.
Planetary Nebula and White Dwarf
After losing its outer layers, the remaining stellar core will become a white dwarf.
NASA estimates that the Sun has roughly another five billion years before the major red-giant phase of its evolution.
```Historical Timeline
Major Areas of Solar Physics
| Research Area | Main Question |
|---|---|
| Solar Interior | How are energy, matter and magnetic fields transported through the Sun? |
| Helioseismology | What can oscillations of the Sun reveal about its internal structure? |
| Solar Dynamo | How does the Sun generate and maintain its magnetic field? |
| Coronal Physics | Why is the corona millions of degrees hotter than the visible photosphere? |
| Solar Wind | How is the solar wind accelerated through the corona? |
| Space Weather | How can solar eruptions and magnetic storms be predicted? |
Conclusion
The Sun is the central astronomical object of the Solar System and the nearest example of a typical main-sequence star.
Its energy comes from nuclear fusion in the core, while its complex magnetic field produces sunspots, flares, prominences and coronal mass ejections. The solar wind extends the Sun's influence throughout the heliosphere.
Understanding the Sun is important not only for astronomy but also for understanding Earth's environment and protecting modern technological systems from severe space-weather events.
The Sun will continue shining as a main-sequence star for several billion more years before evolving into a red giant and ultimately leaving behind a white dwarf.