Sun

A solar filter dimmed true-color image of the visible photosphere of the Sun photographed on May 8, 2019. Sunspots AR2740 (to the right) and AR2741 (to the left) visible. Some other interesting features include faculae, white spots near AR2741, limb darkening, and tiny convection cells called granules all across its surface. Credit: Matúš Motlo/Wikimedia Commons

Main characteristics (click to enlarge)

   The Sun is the star at the center of the Solar System, around which all the planets, dwarf planets and small bodies of this system revolve. The Sun is a G2 V spectral type main-sequence star that is bigger and hotter than about 95% of all stars in the Milky Way Galaxy. It is a nearly perfect sphere of hot plasma, heated by nuclear fusion reactions in its core, radiating the energy mainly as visible light and infrared radiation. It is by far the most important source of energy for life on Earth. Its diameter is about 1.39 million kilometres (864,000 miles), or 109 times that of Earth. At 1.9885 × 10³⁰ kilograms, or about 333,000 times the mass of the Earth, it contains about 99.86% of the total mass of the Solar System. Because of its enormous mass, the Sun dominates the gravitational field of the Solar System. The motion of everything within a few light years of the Sun is dominated by the effect of the solar mass. Roughly three quarters of the Sun's mass consists of hydrogen (~73%) and the rest is mostly helium (~25%), with much smaller quantities (1,69%) of heavier elements, including oxygen, carbon, neon and iron.

Diagram of the structure of the Sun. Credit: Kelvin Song/Wikimedia Commons

   The Sun's gravity creates extreme pressures and temperatures within its core, sustaining thermonuclear reactions fusing hydrogen nuclei and producing helium nuclei. The core has a density of up to 150 g/cm³ and a temperature of close to 15.7 million K. It extends from the center to about 20–25% of the solar radius. The Sun's core fuses about 600 million tons of hydrogen into helium every second, converting 4.26 million tons of matter into energy every second as a result. About 99% of the energy is produced through a series of nuclear reactions called the proton–proton (p–p) chain and about 1% by the carbon–nitrogen–oxygen (CNO) cycle. The Sun produces energy at the rate of 3.846 × 10²⁶ W. About 98% of the energy produced by the fusion reactions is radiated away from the Sun by electromagnetic radiation and about 2% is carried by the neutrinos. The mass of the Sun is so large that during its whole main sequence lifetime less than 0.1% of its mass is turned into energy. Since the formation, about 4.6 billion years ago, only about 5% of the hydrogen in the Sun has been turned into helium, but in the central core only 40% of the matter is now hydrogen, because of the nuclear fusion.

   The energy in the form of electromagnetic radiation can take between 10,000 and 170,000 years to escape the core. Above the core, between about 0.2–0.25 solar radii to about 0.7 solar radii, is the radiative zone, where the primary means of energy transfer is thermal radiation. Matter in a radiative zone is so dense that photons can travel only a short distance before they are absorbed or scattered by another particle. In this zone the temperature drops from approximately 7 million to 2 million K and the density from 20 g/cm³ down to only 0.2 g/cm³ with increasing distance from the core. Above about 0.7 solar radii to near the surface extends the convective zone. In this layer, the density and the temperature of the plasma is low enough to allow convective currents to develop and move the Sun's energy outward towards its surface. The radiative zone and the convective zone are separated by a transition layer, the tachocline. There is a sharp change between the uniform rotation of the radiative zone and the differential rotation of the outer convection zone. It is now believed that the Sun's magnetic field is generated by a magnetic dynamo in this transition layer.

   The innermost layer of the Sun atmosphere is the photosphere, which is about 300–500 km thick. The photosphere is the visible surface of the Sun, where the density rapidly increases inwards, hiding the interior from sight. The temperature falls from about 6600 K at its base to about 4400 K at its upper region, and the gas pressure changes from 125 mb at the bottom to 0.868 mb at the top of the photosphere. The effective temperature of the photosphere is 5772 K. The convective transport of energy from below gives rise to solar granulations visible in the photosphere, that are about 1000 km across. Each granulation cell last about 5–10 min as hot gas, having risen from below the surface radiates energy away, cools and sinks down again. Grouping the typical granules are supergranulations up to 30,000 kilometers in diameter with lifespans of up to 24 hours.

  Sunspots are temporary phenomena on the Sun's photosphere that appear as spots darker than the surrounding areas. They are regions of reduced surface temperature caused by concentrations of magnetic field flux that inhibit convection. Sunspots usually appear in pairs of opposite magnetic polarity. The surface temperature in a sunspot is about 1500 K below that of its surroundings. The diameter of a typical sunspot is about 10,000 km and its lifetime is from a few days to several months. In the interior of the spot there is a dark umbra and around it, a less dark penumbra. The temperature of the umbra is about 3,000–4,500 K and the penumbra is about 5,780 K. The number of sunspots varies according to the approximately 11-year solar cycle (actual period may be between 7 and 17 years).

   Above the photosphere lies the chromosphere, which is about 2500 km thick. The density of the chromosphere is only 10⁻⁴ times that of the photosphere and the temperature increases from about 4,400 K at the top of the photosphere to about 25,000 K. The chromosphere is normally invisible and it can be seen only during a total solar eclipse, where its pink and reddish color is revealed. The structure of the chromosphere is very complex and dynamic. The flamelike jets of gas, called spicules, are rising up to 10,000 km above the chromosphere, and lasting for about 5 to 15 minutes. There are also horizontal wisps of gas called fibrils.

   Above the chromosphere is a thin (about 200 km) transition region, in which the temperature rises very rapidly to about 1 million K. The transition region leads into the outermost part of the Sun’s atmosphere called the corona, where temperatures reach in excess of 2 million K. The low corona, near the surface of the Sun, has a particle density around 10¹⁵ m⁻³ to 10¹⁶ m⁻³. The inner part of the corona, the K corona, has a continuous spectrum formed by the scattering of the photospheric light by electrons. The outer part, the F corona, a few solar radii from the surface, has a spectrum showing Fraunhofer absorption lines, and its light is sunlight scattered by dust. The corona extends beyond 20 solar radii, but its form and extent depends strongly on the solar activity that varies through the solar cycle. The corona can only be observed during a total eclipse of the Sun, or by using coronagraphs, that can block the light from the solar disc.

   The gas in the corona is constantly streaming outwards, gradually becoming a solar wind, which carries a flux of particles away from the Sun. The solar wind consists mainly of electrons and protons with some nuclei of helium atoms and trace amounts of heavy ions and atomic nuclei. At the distance of the Earth there are typically 5–10 particles per cubic centimetre, with the velocity about 300 to 800 km/s. With the solar wind the Sun is losing about 2–3 × 10^-14 of its mass per year (about 1.3–1.9 million tonnes per second).

   A coronal hole is a patch of the Sun’s atmosphere with much lower density than elsewhere. In ultraviolet views of the Sun, coronal holes appear as dark splotches. These are regions where the Sun’s magnetic field lines are connected directly to interplanetary space, allowing solar material to  escape out in a high-speed stream of solar wind, leaving a dark “hole” near the surface of the Sun. Coronal holes appear throughout the solar cycle, but can last for much longer periods of time during solar minimums, when the Sun is less active. 

   A prominence is a large and bright, gaseous feature extending outward from the Sun's surface, often in a loop shape. Prominences are anchored to the Sun's surface in the photosphere, and extend outwards into the solar corona. There are several types of prominences: the quiescent prominences, where the gas is slowly sinking along the magnetic field lines; loop prominences, connected with magnetic field loops in sunspots; and eruptive prominences, where gas is violently thrown outwards.The prominence plasma is typically a hundred times cooler (10,000–20,000 K), more luminous and dense than the coronal plasma. A typical prominence extends over thousands of kilometers; the largest on record was estimated at over 800,000 km long. A prominence forms over timescales of about a day and may persist for weeks or months. When a prominence is viewed against the solar disk, instead of space, it appears darker than the background and is called a solar filament.

  A solar flare is a sudden flash of increased brightness on the Sun, with an energy release of up to 6 × 10²⁵ joules, lasting from one second to just under an hour. Flares are usually observed near the Sun's surface and in close proximity to a sunspot group. They occur when accelerated charged particles, mainly electrons, interact with the plasma medium. Evidence suggests that the magnetic reconnection leads to this extreme acceleration of charged particles. Powerful flares are often, but not always, accompanied by a coronal mass ejection.

   Coronal mass ejections (CMEs) are large expulsions of plasma and accompanying magnetic field from the solar corona. They can eject billions of tons of coronal material into the solar wind with velocities ranging from 20 km/s to 3,200 km/s. Coronal mass ejections are often associated with solar flares and prominence eruptions but they can also occur in the absence of either of these processes. The frequency of CMEs varies with the sunspot cycle from about one CME a week at solar minimum to an average of 2 to 3 CMEs per day near solar maximum. The flares and coronal mass ejections give rise to disturbances on the Earth. The X-rays cause changes in the ionosphere, which affect shortwave radio communications. The particles give rise to strong auroras when they enter the Earth’s magnetic field a few days after the outburst. They can also damage satellites and even electric networks.

On 2 July 2019, a total solar eclipse passed over ESO’s La Silla Observatory in Chile. This image combines a photograph from La Silla in visible light, with ultraviolet data from SOHO's EIT instruments (284 Å) and four filters from Solar Dynamics Observatory's AIA instrument (193 Å, 211 Å, 304 Å and 171 Å). The combination of these different data all captured close to totality shows the complexity of the solar magnetic field during the eclipse. A large, but not powerful active region can be seen in the middle of the solar disk. The poles of the Sun are relatively dark and with more outflow of ions. Credit: ESO/P. Horálek/SOHO (NASA&ESA)/SDO (NASA)

Coronal Mass Ejection (CME) with stunning, bright details in the ejected material. In this composite image from SOHO observatory, an EIT image of the Sun in extreme UV light, taken at about the same time (January 4, 2002), was enlarged and superimposed on LASCO C2. In coronagraph images, direct sunlight is blocked by an occulter (covered by the Sun here) to reveal the surrounding faint corona. Credit: SOHO (ESA & NASA)

The image of the Sun taken with neutrinos, through the Earth at night, at the Super-Kamiokande detector (Japan). The image has been obtained with a 503 days exposure, by registering neutrinos emitted from the solar core and detected in a 50 000-ton water pool located 1 km underground. At night, neutrinos were transparently traversing the whole earth before being registered in this image. Centered on the Sun's position, the picture covers a significant fraction of the sky (90x90 degrees in R.A. and Dec.). Brighter colors represent a larger flux of neutrinos. Credit: R. Svoboda and K. Gordan (LSU)

A high-resolution image of solar sunspots captured by the Inouye Solar Telescope on May 11, 2021. The data leading to this image were acquired with the Visible Broadband Imager blue channel at a wavelength of 450 nanometers. Credit: NSO/AURA/NSF

The highest-resolution image of the Sun’s surface (photosphere) ever captured, taken at 416 nm by the Inouye Solar Telescope. It reveals deformed boundaries of magnetic elements and ultra-fine scale stripes, both associated with Kelvin-Helmholtz instability. Credit: NSF/NSO/AURA/MPS

When a filament is seen off the edge of the sun, as shown here, it is called a prominence. The reason for the different name is historical; originally, prominences were seen during total eclipses of the sun, long before modern techniques allowed filaments to be observed on the solar disk. This particular prominence is a hedgerow type, so-called because of its shape. Credit: NSO/AURA/NSF

References:

Abuter, R.; Amorim, A.; Bauböck, M.; et al., "A geometric distance measurement to the Galactic center black hole with 0.3% uncertainty". Astronomy & Astrophysics, Volume 625 (May 2019). doi: 10.1051/0004-6361/201935656
Hathaway, D.H.,"Solar Physics". NASA Marshall Space Flight Center
Hunt, J.A.S.; Bovy, J.; Carlberg, R.G., "Detection of a dearth of stars with zero angular momentum in the solar neighborhood". The Astrophysical Journal Letters, Volume 832, Number 2 (2016). doi: 10.3847/2041-8205/832/2/L25
Karttunen, H., Kröger, P., Oja, H., Poutanen, M., Donner, K.J.: Fundamental astronomy. Sixth Edition. Springer-Verlag, 2017
Mamajek, E.E.; Prsa, A.; Torres, G.; et al. (2015), "IAU 2015 Resolution B3 on Recommended Nominal Conversion Constants for Selected Solar and Planetary Properties", arXiv:1510.07674
Williams, D.R., "Sun Fact Sheet". NASA Goddard Space Flight Center

© 2026, Andrew Mirecki

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