Sun
The Sun is the nearest star to Earth and the central body of the Solar System. Its gravity governs the motion of the planets, while its radiation supplies Earth with energy. The Sun is a main-sequence star: hydrogen is converted into helium in its core, sustaining its luminosity for billions of years. Because it is so close, its surface, atmosphere and magnetic fields can be studied in far greater detail than those of any other star.[1]
Size and physical characteristics
The Sun is about 1.39 million kilometres in diameter, wide enough to fit roughly 109 Earths side by side. A car travelling continuously at 100 km/h would take almost 19 months to cover the same distance. Its mass is approximately 1.99 × 1030 kg, equivalent to about 333,000 Earths.[2]
Despite its enormous mass, the Sun has an average density of about 1.4 g/cm³, nearly four times lower than Earth's. Its matter is not distributed evenly: density rises to roughly 150 g/cm³ in the central region, while the outer layers are far more rarefied.[2][1]
The Sun's effective temperature is about 5,772 K (5,500 °C), whereas the core reaches approximately 15 million degrees.[3][5] Its luminosity is about 3.83 × 1026 watts. This energy spreads in every direction, and Earth intercepts only a very small fraction of it.[3]
Composition and source of energy
The Sun consists primarily of hydrogen and helium. At its high temperatures, much of this matter exists as plasma: electrons are separated from atomic nuclei, allowing the material to interact strongly with magnetic fields.[1]
Energy is produced in the core through nuclear fusion. In a chain of reactions, hydrogen nuclei ultimately combine to form helium. The products have slightly less mass than the original particles, and the difference is released as energy. The Sun therefore shines through nuclear reactions rather than chemical combustion, which would require oxygen.[6]
The central region reaches a temperature of about 15 million degrees. Solar gravity compresses matter to a very high density, while the resulting internal pressure opposes further contraction. This balance allows the star to maintain a relatively stable size over long periods.[5][1]
Internal structure
The Sun's internal regions are distinguished mainly by the way energy moves through them:
- The core is the central region where fusion occurs.
- The radiative zone is the layer in which energy travels mainly as radiation. Repeated interactions between radiation and matter make this process extremely slow.
- The convective zone forms the outer part of the solar interior. Heated plasma rises, releases energy and sinks again as it cools. This circulation is called convection.[5]
Once energy reaches the visible layers, it can leave the star and travel into space. Light takes about 8 minutes 20 seconds to cross the distance from the Sun to Earth. This interval is determined by the distance between the two bodies; energy transport inside the Sun is a separate and much slower process.[5][7]
Photosphere and solar atmosphere
The visible disc is formed by the photosphere, the layer from which most observable visible light escapes. The Sun has no solid surface on which a spacecraft could land: hot stellar matter continues beneath the apparent boundary.[5]
Above the photosphere lie the chromosphere, the transition region and the corona. The corona is extremely tenuous, yet its temperature reaches one million degrees or more—far hotter than the photosphere. Solar physics studies how energy carried by magnetic fields and moving plasma sustains this intense heating.[8]
During a total solar eclipse, the Moon covers the bright disc and the corona becomes visible as a pale glow around it. At other times it is observed with instruments that block the light of the disc, as well as at wavelengths in which hot plasma is especially prominent.[8]
Spectrum and colour
Sunlight contains a broad range of visible colours that combine to appear nearly white. The common description “yellow dwarf” is an astronomical classification and does not mean that the Sun emits only yellow light.[9]
When sunlight is dispersed into a spectrum, it contains dark lines where material in the solar atmosphere absorbs radiation at particular wavelengths. These lines reveal chemical composition and physical conditions in the emitting and absorbing layers, allowing the star's matter to be studied without bringing a sample to a terrestrial laboratory.[9]
Rotation and magnetic activity
Different solar latitudes rotate at different rates: equatorial regions complete a rotation in about 25 days, while regions near the poles take roughly 36 days. This differential rotation is possible because the Sun is a body of plasma rather than a solid sphere.[1]
The motion of this material continually reshapes the star's magnetic field. Where the field is especially strong, sunspots appear as regions of the photosphere that are cooler and therefore darker than their surroundings. Their number generally rises and falls over a cycle of about 11 years. Individual cycles vary in length and intensity, so this is an approximate rhythm. Because the large-scale magnetic field reverses polarity, a full magnetic cycle lasts about 22 years.[4]
Magnetic activity also produces solar flares and coronal mass ejections. A flare is a rapid release of energy observed as a sudden increase in radiation. A coronal mass ejection sends a cloud of plasma and magnetic field into interplanetary space. The two phenomena may occur together, but they are physically distinct events.[10]
Solar wind and effects on Earth
The Sun continuously emits the solar wind, a stream of charged particles consisting mainly of protons and electrons. It travels between the planets and, together with the solar magnetic field, forms the heliosphere: the region dominated by the Sun within the surrounding interstellar medium.[11]
Earth's magnetic field deflects most of the incoming flow, although the interaction transfers energy into the magnetosphere. Some of that energy accelerates particles into the upper atmosphere, where collisions with atoms and molecules produce auroras.[12]
Strong disturbances can affect satellites, radio communication and navigation systems. In extreme cases they can also disrupt power grids on the ground. Solar observations therefore serve a practical purpose: they support forecasts of space weather and help assess the effects of solar activity on technology.[13]
Formation and future evolution
The Sun formed about 4.6 billion years ago as a cloud of gas and dust contracted under gravity. Most of the material accumulated in the central star, while the planets and smaller bodies of the Solar System later formed from the surrounding disc.[1]
Its present state is a long-lived stage of stellar evolution. As hydrogen in the core is consumed, the internal structure will change. In about 5–6 billion years the Sun will become a red giant and expand far beyond its current size. These timescales are derived from models of stellar evolution tested against observations of stars at different ages.[14]
During its later evolution, the Sun will shed its outer layers. The dense remnant of its core will become a white dwarf and gradually cool. The familiar star will therefore not retain its present size, colour and luminosity indefinitely.[14]
Exploration
The Sun is studied by ground-based observatories and spacecraft. Different methods complement one another: images track changes across the disc, spectra reveal the properties of plasma, and instruments in interplanetary space directly measure particles and magnetic fields.[15]
The Solar Dynamics Observatory monitors the solar atmosphere and magnetic activity. The Parker Solar Probe investigates the near-Sun environment and the processes that form the solar wind. Solar Orbiter, an ESA mission conducted with NASA participation, combines remote observations of the Sun with measurements of the space surrounding the spacecraft. Together, these missions connect events on the star with their consequences across the Solar System.[15]
See also
References
- ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 Sun: Facts // NASA Science.
- ↑ 2.0 2.1 2.2 2.3 2.4 2.5 Sun Fact Sheet // NASA, National Space Science Data Center.
- ↑ 3.0 3.1 3.2 3.3 3.4 Mamajek E. E. et al. IAU 2015 Resolution B3 on Recommended Nominal Conversion Constants for Selected Solar and Planetary Properties. International Astronomical Union, 2015.
- ↑ 4.0 4.1 Magnetic Field // National Solar Observatory.
- ↑ 5.0 5.1 5.2 5.3 5.4 Anatomy of our Sun // European Space Agency. 7 October 2019.
- ↑ Where Does the Sun's Energy Come From? // NASA Space Place.
- ↑ Chapter 1: The Solar System // NASA Science.
- ↑ 8.0 8.1 What Is the Sun's Corona? // NASA Space Place.
- ↑ 9.0 9.1 The Sun and Its Missing Colors // NASA Science, Astronomy Picture of the Day. 11 June 2023.
- ↑ Solar Storms and Flares // NASA Science.
- ↑ What Is the Solar Wind? // NASA Science.
- ↑ Auroras // NASA Science.
- ↑ What We Study // NASA Science, Heliophysics.
- ↑ 14.0 14.1 The Sun's future // European Space Agency. 2022.
- ↑ 15.0 15.1 Missions Exploring our Sun // NASA Science.