The Sun Motion and location within the MW

The Sun is currently traveling through the Local Interstellar Cloud (or Local Fluff), an underdense cloud roughly 30 light years across within the still more underdense Local Bubble, in the inner rim of the Orion Arm of the Milky Way galaxy. It orbits the center of the Milky Way at a distance of approximately 24,000–26,000 light years from the galactic center, completing one clockwise orbit, as viewed from the galactic north pole, in about 225–250 million years (a galactic year), so it is thought to have completed 20–25 orbits during the lifetime of the Sun. The mean distance of the Sun from the Earth is approximately 149.6 million kilometers (1 AU). At this average distance, light travels from the Sun to Earth in about 8 minutes and 19 seconds.

The solar apex is the direction that the Sun travels through space in the Milky Way. The general direction of the Sun's galactic motion is towards the star Vega near the constellation of Hercules, at an angle of roughly 60 sky degrees to the direction of the Galactic Center.

The Sun's orbit around the Galaxy is expected to be roughly elliptical with the addition of perturbations due to the galactic spiral arms and non-uniform mass distributions. In addition the Sun oscillates up and down relative to the galactic plane approximately 2.7 times per orbit. This is very similar to how a simple harmonic oscillator works with no drag force (damping) term. It has been argued that the Sun's passage through the higher density spiral arms often coincides with mass extinctions on Earth, perhaps due to increased impact events.

Main properties The Sun is by far the biggest body in the solar system, comprising about 99.86% of the total mass. From its angular size of about 0.5° and its distance of almost 150 million kilometers, its diameter is determined to be 1,392,000 kilometers. This is equal to 109 Earth diameters and almost 10 times the size of the largest planet, Jupiter. The Sun’s mass is 1.9891×1030 kg, about 333,000 times the Earth's mass and over 1,000 times the mass of Jupiter, resulting in an average density of 1.408 g/cm3. The equatorial surface gravity is 274.0 m/s2 or about 28g. The Sun is a Population I, or heavy element-rich star. The formation of the Sun may have been triggered by shock waves from one or more nearby supernovae. This is suggested by a high abundance of heavy elements in the Solar System, such as gold and uranium, relative to the abundances of these elements in so-called Population II (heavy element-poor) stars. Spectroscopy shows that hydrogen makes up about 94% of the solar material, helium makes up about 6% of the Sun, and all the other elements make up to just 0.13% (oxygen, carbon, and nitrogen, the three most abundant “metals”, they make up to 0.11%). The Sun also has traces of neon, sodium, magnesium, aluminum, silicon, phosphorus, sulfur, potassium, and iron. The percentages quoted here are by the relative number of atoms. If you use the percentage by mass, you find that hydrogen makes up 78.5% of the Sun's mass, helium 19.7%, oxygen 0.86%, carbon 0.4%, iron 0.14%, and the other elements are 0.54%. The metallicity of the Sun is approximately 1.8 percent by mass. Once regarded by astronomers as a small and relatively insignificant star, the Sun is now presumed to be brighter than about 85% of the stars in the Milky Way galaxy, most of which are red dwarfs. The bolometric absolute magnitude of the Sun is +4.83, corresponding to an apparent visual magnitude of –26.74. Its spectral class, G2V, indicates that its surface temperature is of approximately 5,778 K. The Sun is a near-perfect sphere: its polar diameter differs from its equatorial diameter by only 10 km (the ellipticity is 0.00005). As the Sun exists in a plasmatic state and is not solid, it rotates faster at its equator than at its poles. This behavior is known as differential rotation, and is caused by convection in the Sun and the movement of mass, due to the steep temperature gradient from the core outwards. This mass carries a portion of the Sun’s counter-clockwise angular momentum, as viewed from the ecliptic north pole, thus redistributing the angular velocity. The period of its actual rotation is approximately 25.6 days at the equator and 33.5 days at the poles. However, due to our constantly changing vantage point from the Earth as it orbits the Sun, the apparent rotation of the star at its equator is about 28 days.

Structure

Core

The Sun, like most stars, is a main sequence star, and thus generates its energy by nuclear fusion of hydrogen nuclei into helium. In its core, the sun fuses between 430–600 million tons of hydrogen each second.

The core of the Sun is considered to extend from the center to about 0.2 to 0.25 solar radii. It reaches a maximum density larger than 160 g/cm3 (estimated) and a central temperature

ranging from ~13.6 to 15.7×107 K depending on the model. Most of the Sun's energy is produced by nuclear fusion through a series of steps called the p–p (proton–proton) chain. Less than 2% of the helium generated in the Sun comes from the CNO cycle.

The proton-proton chain occurs around 9.2×1037 times each second in the core of the Sun. Since this reaction uses four protons, it converts about 3.7×1038 protons (hydrogen nuclei) to helium nuclei every second (out of a total of ~8.9×1056 free protons in the Sun), or about 6.2×1011 kg per second. Since fusing hydrogen into helium releases around 0.7% of the fused mass as energy, the Sun releases energy at the mass-energy conversion rate of 4.26 million metric tons per second, 3.846×1026 W, or 9.15×1010 megatons of TNT per second*. The Sun’s total power output is called its luminosity. At the center of the sun, fusion power is estimated by models to be about 276.5 W/m3, a power production density which more nearly approximates reptile metabolism than a thermonuclear bomb. The tremendous power output of the Sun is not due to its high fusion power per volume, but instead due to its large size.

* In comparison, gravitational energy (with an efficiency of 1/10000 of one percent) could power the sun for 30 million years, while the typical chemical reactions involved in fuel burning release roughly 10-19 J per atom, so the length of time required to consume the entire Sun by burning would be of only ~104 years.

The high-energy photons (gamma rays) released in fusion reactions at the core are absorbed in only a few millimeters of solar plasma and then re-emitted again in random direction (and at slightly lower energy), so it takes a long time for radiation to reach the Sun's surface. Estimates of the ‘photon travel time’ range between 10,000 and 170,000 years. After a final trip through the convective outer layer to the transparent surface of the photosphere, the photons escape as visible light. Each gamma ray in the Sun's core is converted into several million visible light photons before escaping into space. Neutrinos at a rate of 1038 per second are also released by the fusion reactions in the core, but unlike photons they rarely interact with matter, so almost all are able to escape the Sun immediately. The flux of solar neutrinos at the Earth's surface is on the order of 1011 per square centimeter per second. For many years measurements of the number of neutrinos produced in the Sun were lower than theories predicted by a factor of 3. This discrepancy (known as the solar neutrino problem) was recently resolved through the discovery of the effects of neutrino oscillation: the Sun emits the number of neutrinos predicted by the theory, but neutrino detectors were missing 2⁄3 of them because the neutrinos had changed flavor. Neutrinos should have been massless according to the then-accepted Standard Model of particle physics; this means that the flavor type of neutrino (electron, muon or tau) would be fixed when it was produced. The Sun should emit only electron neutrinos as they are produced by H–He fusion. The solution of the solar neutrino problem required to admit that neutrinos have mass, so they can change from the type that had been expected to be produced in the Sun's interior into the other two types ―the probability of measuring a particular flavor for a neutrino varies periodically as it propagates― that would not be caught by the detectors in use.

Sunlight is Earth's primary source of energy. The solar constant is the amount of power that the Sun deposits per unit area that is directly exposed to sunlight. The solar constant is equal to approximately 1366 W/m2 at a distance of one AU from the Sun. Sunlight on the surface of Earth is attenuated by the Earth's atmosphere so that less power arrives at the surface —closer to 1000 W/m2 in clear conditions when the Sun is near the zenith.

Radiative zone

From about 0.25 to about 0.7 solar radii, solar material is hot and dense enough that thermal radiation is sufficient to transfer the intense heat of the core outward. In this zone there is no thermal convection, the temperature changes from 7,000,000 K to about 2,000,000 K, and the density drops a hundredfold (from 20 g/cm3 to only 0.2 g/cm3) from the bottom to the top.

Between the radiative zone and the outer convective zone is a transition layer at ~0.7 R☼ called

the tachocline. This is a region with a thickness of 0.04 times the solar radius where the sharp regime change between the uniform solid-body rotation of the radiative zone and the differentially rotating outer convective zone results in a large very shear —a condition where

successive horizontal layers slide past one another. Presently, it is hypothesized that a magnetic dynamo within this layer generates the Sun's magnetic field.

Convective zone and photosphere

In the Sun's outer layer, from its surface down to approximately 200,000 km (or the last 30% of the solar radius), the solar plasma is not dense enough or hot enough to transfer the heat energy of the interior outward through radiation (in other words it is opaque enough). As a result, thermal convection occurs as thermal columns carry hot material to the surface (photosphere) of the Sun. Once the material cools off at the surface, it plunges downward to the base of the convection zone, to receive more heat from the top of the radiative zone. At the visible surface of the Sun, the temperature has dropped to less than 5,800 K and the density to only 0.2 g/m3 (about 1/10,000th the density of air at sea level). The turbulent convection of this outer part of the solar interior causes a ‘small-scale’ dynamo that produces magnetic north and south poles all over the surface of the Sun.

The thermal columns in the convection zone form an imprint on the surface of the Sun. The grainy appearance of the solar photosphere is produced by the tops of these convective cells and is called granulation. The rising part of the granules is located in the center where the plasma is hotter. The outer edge of the granules is darker due to the cooler descending plasma. In addition to the visible appearance, Doppler shift measurements of the light from individual granules provide evidence for the convective nature of the granules. A typical granule has a diameter on the order of 1,000 kilometers and lasts 8 to 20 minutes before dissipating. Below the photosphere is a layer of supergranules up to 30,000 kilometers in diameter with lifespans of up to 24 hours.

The photosphere has a particle density of ~1017 cm−3 (this is about 1% of the particle density of Earth's atmosphere at sea level).

Chromosphere

The coolest layer of the Sun is a temperature minimum region about 500 km above the photosphere, with a temperature of ‘only’ about 4,100 K. This part of the Sun is cool enough to support simple molecules such as carbon monoxide and water, which can be detected by their absorption spectra. Above the temperature minimum layer it is a layer about 2,000 km thick, dominated by a spectrum of emission and absorption lines. The chromosphere is visible as a colored flash at the beginning and end of total eclipses of the Sun. The temperature in the chromosphere increases gradually with altitude, ranging up to around 20,000 K near the top. In the upper part of chromosphere helium becomes partially ionized.

The density of the chromosphere decreases from from 2.0×10-7 g/cm³ near the photosphere to 10-14 g/cm³ at its boundary with the corona. The chromosphere is more visually transparent than the photosphere. The name comes from the fact that it has a reddish color, as the visual spectrum of the chromosphere is dominated by the deep red Hα Balmer spectral line of hydrogen (photons from electrons falling from the n=3 level to the n=2 level; λ=656.3 nm). The coloration may be seen directly with the naked eye only during a total solar eclipse, where the chromosphere is briefly visible as a flash of color just as the visible edge of the photosphere disappears behind the Moon.

The chromosphere shows numerous vertical filaments, called spicules, that are rising jets of gas associated with regions of high magnetic flux of about 500 km diameter. A typical spicule raises at the rate of 20 km/s, reaches a height of nearly 10,000 km, then collapses and fades away after 15 minutes or so. Approximately 100,000 spicules exist at any one time, covering about 1% of the Sun’s surface.

Corona

Above the chromosphere there is a thin (about 200 km) transition region in which the temperature rises rapidly from around 20,000 K in the upper chromosphere to coronal temperatures closer to one million Kelvin.

The corona is the extended outer atmosphere of the Sun, which is much larger in volume than the Sun itself. The corona continuously expands into the space forming the solar wind, which fills all the Solar System. The low corona, which is very near the surface of the Sun, has a particle density around 109–1010 cm−3. The average temperature of the corona and solar wind is about 1–2 MK, however, in the hottest regions it is 8–20 MK (and therefore is best observed in X rays and UV light).

The Sun's corona is therefore much hotter (by a factor of nearly 200) than the visible surface of the Sun and 10−12 times as dense as the photosphere, and so produces about one-millionth as much visible light. The exact mechanism by which the corona is heated is still the subject of some debate, but the most likely possibility is induction by the Sun's magnetic field. Magnetic field arches extending tens of thousands of km into the corona carry streamers of electrically charged particles. If two arches come near to each other, their flowing charges can interact to form a gigantic short circuit and releasing a tremendous amount of energy that is more than enough to maintain the corona’s temperature.

The corona is not always evenly distributed across the surface of the sun. During periods of quiet, the corona is more or less confined to the equatorial regions, with coronal holes covering the polar regions. Coronal holes are linked to unipolar concentrations of open magnetic field lines. During solar minimum, coronal holes are mainly found at the Sun's polar regions, but they can be located anywhere on the Sun during solar maximum. The fast-moving component of the solar wind is known to travel along open magnetic field lines that pass through coronal holes (otherwise particles flow out slowly impeded by the Sun’s magnetic field). During the intervals at which coronal holes are formed at low solar latitudes they spray the Earth with high speed plasma streams, like a powerful garden sprinkler, and are responsible for generating space weather storms that recur in intervals of 27 days as the coronal hole rotates back over the limb

of the Sun. These recurrent storms tend to be weaker than storms produced by coronal mass ejections and to be most frequent during the years just following solar maximum.

Solar wind

The Sun's hot corona continuously expands in space creating the solar wind, a hypersonic stream of fast-moving ions that escape the Sun's gravitational attraction moving outward at hundreds of km/s. It extends from approximately 20 solar radii (0.1 AU) to the farthest reaches of the Solar System. The bubble in the interstellar medium formed by the solar wind, the heliosphere, is the largest continuous structure in the Solar System. It is a bubble in space ‘blown’ into the interstellar medium by the solar wind. Although electrically neutral atoms from the interstellar medium can penetrate this bubble, virtually all of the material in the heliosphere emanates from the Sun itself. The region where the solar wind slows down to subsonic speed is the termination shock. This causes compression, heating, and changes in the solar magnetic field. In our solar system the termination shock is believed to be 75 to 90 AU from the Sun. The region where the interstellar medium and solar wind pressures balance is called the heliopause at roughly 100 AU. It is hypothesized that the Sun also has a bow shock* produced as it travels within the interstellar medium. Bow shocks will occur if the interstellar medium is moving supersonically toward the Sun. When the interstellar wind hits the heliosphere it slows down, becomes subsonic and creates a region of turbulence. The solar bow shock may lie at around 230 AU from the Sun.

* This phenomenon has been observed in other stars. For instance, in the red giant star Mira in the constellation Cetus. This star has been shown to have both a cometlike debris tail of ejecta and a distinct bow shock preceding it in the direction of its movement through space (at over 130 km/s).

Solar wind particles passing close to planets with strong magnetic fields are deflected around them, some reaching the planet's magnetic poles. When the charged particles hit the planet's atmosphere, they make its gas particles produce emission spectra —on Earth they are known as the aurora borealis in the north and aurora australis in the south. Red aurorae on the Earth are produced by hydrogren emission at the top of the atmosphere. Green aurorae are produced by oxygen emission lower down but still many tens of kilometers above the surface. During solar maximum the increased number and energy of the solar wind particles produce more extensive auroral displays —the aurorae in the Earth's atmosphere can even be seen at latitudes near 30° N or S; usually, they are seen only above 50° N or 50° S.

The solar magnetic field extends well beyond the Sun itself. The magnetized solar wind plasma carries the Sun's magnetic field into the interplanetary medium. The ‘ballerina-skirt’ shape of the heliospheric or interplanetary current sheet (see figure) results from the influence of the Sun's rotating magnetic field on the plasma in the interplanetary medium. Its surface marks the region where the polarity of the Sun's magnetic field changes from north to south (it has a thickness of ~10,000 km). This field extends throughout the Sun's equatorial plane into the

heliosphere. A small electrical current of about 10−10 A/m² flows within the sheet. The magnetic field at the surface of the Sun is about 10-4 T. If the form of the field were a magnetic dipole, the strength would decrease with the cube of the distance, resulting in about 10-11 T at the Earth's orbit. However, the actual magnetic field at the Earth due to the Sun is 100 times greater.

Solar cycle

The Sun's magnetic field structures its atmosphere and outer layers all the way through the corona and into the solar wind. Its spatiotemporal variations lead to a host of phenomena collectively known as solar activity. All of solar activity is strongly modulated by the solar magnetic cycle, since the latter serves as the energy source and dynamical engine for the former. The differential rotation of the Sun's latitudes causes its magnetic field lines to become twisted together over time, causing magnetic field loops to erupt from the Sun's surface and trigger the formation of sunspots and prominences. This twisting action creates the solar dynamo and an, approximately, 11-year solar cycle of magnetic activity from solar maximum to solar maximum (solarmax).

Sunspots are regions of intense magnetic activity where convection is inhibited by strong magnetic fields, reducing energy transport from the hot interior to the surface. Magnetic field strengths within sunspots range from 2,000 to 4,000 Gauss, and are thousands of times more intense than Earth's average surface field strength of about 0.5 Gauss. The fields within sunspots are also much stronger than the Sun's global average field, which is around 1-2 Gauss. Larger sunspots have higher field strengths.

The intense magnetic fields at sunspots inhibit mixture of hot plasma from the surrounding photosphere into the sunspot regions. Sunspots are thus cooler than their surroundings. According to the Stefan-Boltzmann law

flux from sunspot (umbra)/flux from photosphere = (4300 K/5800 K)4 = 0.30

that is, the sunspot emits only 30% as much light as an equally large patch of undisturbed photosphere, which is why sunspots appear so dark.

The sunspot itself can be divided into two parts:

• The central umbra, which is the darkest part, where the magnetic field is approximately vertical.

• The surrounding penumbra, which is lighter, where the magnetic field lines are inclined.

The Wilson effect (the penumbra and umbra vary in the manner expected by perspective effects if the umbrae of the spots are in fact slight depressions in the surface of the photosphere; the magnitude of the depression may be as large as 1,000 km) tells us that sunspots are actually depressions on the Sun's surface. Observations using the Zeeman effect (the splitting of a spectral line into several components in the presence of a static magnetic field) show that prototypical sunspots come in pairs with opposite magnetic polarity. From cycle to cycle, the polarities of leading and trailing (with respect to the solar rotation) sunspots change from north/south to south/north and back. Sunspots usually appear in groups. The largest sunspots can be tens of thousands of kilometers across. Associated with sunspots are coronal loops, loops of magnetic flux, upwelling from the solar interior.

Sunspots generally appear in pairs with opposite magnetic polarities; one where the bundle of magnetic ‘ropes’ emerges from the solar surface, and the other where it plunges back down through the photosphere. Similar phenomena (starspots) have been observed on nearby stars using both Doppler imaging and spectroscopy.

The number of sunspots visible on the Sun is not constant, but varies over an ~11-year cycle. Differential rotation causes the magnetic field in the photosphere to become wrapped around the Sun’s surface. As a result, the magnetic field then becomes concentrated at high latitudes on either side of the solar equator. Convection in the photosphere causes the concentrated magnetic field to become tangled and ‘kinks’ erupt through the solar surface. Sunspots appear where the magnetic field protrudes through the photosphere. At a typical solar minimum, few sunspots are visible, and occasionally none at all can be seen. Those that do appear are at mid solar latitudes (~30°–35° north and south of the solar equator). As the sunspot cycle progresses, the number of sunspots increases and they move closer to the equator of the Sun, until at the end of the cycle they are virtually all on the solar equator. Sunspots usually exist in groups (of 10 on average) dominated by two large spots with opposite magnetic polarity. The magnetic polarity of all the leading sunspots, which statistically are larger and last longer than the trailing ones, in one solar hemisphere is the same: in the hemisphere where the Sun has its north magnetic pole, the preceding members of all sunspot groups have north magnetic polarity. In the opposite hemisphere, where the Sun has its south magnetic pole, the preceding members all have south magnetic polarity.

Sunspots are located in dipolar active regions. Sunspots may exist anywhere from a few days to a few months. After sunspots decay their underlying active regions remain and keep moving on the Sun’s surface. The preceding parts of these active regions from the two hemispheres travel toward the equator where they cancel each other because of their opposite polarity. On the other hand, the trailing zones of the active regions migrate poleward and accumulate at high solar latitudes, first cancelling out with the opposite polarity of the poles and eventually reversing the Sun’s overall magnetic field about every 11 years. The (di)polar component of the solar magnetic field is observed to reverse polarity around the time of solarmax, and reaches peak strength at the time of solar minimum. Active regions and sunspots, on the other hand, are produced from a strong toroidal (longitudinally-directed) magnetic field within the solar interior. Physically, the solar cycle can be thought of as a regenerative loop where the toroidal component produces a poloidal field, which later produces a new toroidal component of sign such as to reverse the polarity of the original toroidal field, which then produces a new poloidal component of reversed polarity, and so on. The Sun’s magnetic pattern therefore repeats itself only after two sunspots cycles, which is why astronomers speak of a 22-year solar cycle.

The solar cycle has a great influence on space weather, since luminosity varies in phase with the solar magnetic activity, but only a mild influence on the Earth's climate. The variation caused by the sunspot cycle to solar output (and its effects on Earth’s climate) is (are) on the order of 0.1% of the solar constant: a peak-to-trough range of 1.3 W m−2 compared to 1366 W m−2 for

the average solar constant or total solar irradiance (TSI) at 1 AU*. This is less than the ~3 percent variation associated with the seasonal variation of the orbital radius of the Earth. Variations about the average up to −0.3% are caused by large sunspot groups and of +0.05% by large faculae and bright network on a week to 10 day timescale. Yet, TSI variations sustained over the several decades could be a significant forcing for climate change: in the 17th century between 1645 and 1715 very few sunspots were observed for several decades (during one 30-year period astronomers observed only about 50 sunspots, as opposed to a more typical 40,000–50,000 spots in modern times). During this era, which is known as the Maunder minimum or Little Ice Age, Europe experienced very cold temperatures. In total there seem to have been 18 periods of sunspot minima in the last 8,000 years, and studies indicate that the Sun currently spends up to a quarter of its time in these minima.

* TSI is higher at solarmax, even though sunspots are darker (cooler) than the average photosphere. This is caused by magnetized structures other than sunspots during solar maxima, such as faculae and active elements of the 'bright' network, that are brighter (hotter) than the average photosphere. They collectively overcompensate for the irradiance deficit associated with the cooler but less numerous sunspots. The primary driver of TSI changes on solar rotational and sunspot cycle timescales is the varying photospheric coverage of these radiatively active solar magnetic structures.

Solar faculae (literally bright spots) are short-lived convection cells several thousand kilometers across that constantly form and dissipate over timescales of several minutes. Faculae are produced by concentrations of magnetic field lines and are mapped closely by plages in the chromosphere above, but the latter have much larger spatial scales.

A prominence is a large, bright 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 Sun's corona. While the corona consists of extremely hot ionized gases, known as plasma, which do not emit much visible light, prominences contain much cooler plasma, similar in composition to that of the chromosphere. A prominence forms over timescales of about a day, and stable prominences may persist in the corona for several months. Some prominences break apart and give rise to coronal mass ejections. A typical prominence extends over many thousands of kilometers. The mass contained within a prominence is typically of the order of 1014 kg of material.

When a prominence is viewed from a different perspective so that it is against the sun instead of against space, it appears darker than the surrounding background. This formation is instead called a solar filament.

A solar flare is a large explosion in the Sun's atmosphere that can release as much as 6×1025 J. Solar flares affect all layers of the solar atmosphere (photosphere, corona, and chromosphere), heating plasma to tens of millions K and accelerating electrons, protons, and heavier ions to near the speed of light. They produce radiation across the electromagnetic spectrum at all wavelengths, from radio waves to gamma rays. Most flares occur in active regions around sunspots, where intense magnetic fields penetrate the photosphere to link the corona to the solar interior. Flares are powered by the sudden (timescales of minutes to tens of minutes) release of magnetic energy stored in the corona. If a solar flare is exceptionally powerful, it can cause coronal mass ejections (CME). The ejected material is a plasma consisting primarily of electrons and protons (in addition to small quantities of heavier elements such as helium, oxygen, and iron), plus the entraining coronal magnetic field. CMEs range in speed from about 20 km/s to 3,200 km/s. The average mass based on coronagraph images is 1.6×1012 kg. Due to the two-dimensional nature of the coronagraph measurements, these values are lower limits. The frequency of ejections depends on the phase of the solar cycle: from about one every 5 days near solar minimum to about 3 per day near solarmax.

Solar flares and CMEs strongly influence the local space weather of the Earth. They produce streams of highly energetic particles in the solar wind and the Earth's magnetosphere that can present radiation hazards to spacecraft and astronauts. They can interfere with short-wave radio communication and can increase the drag on low orbiting satellites, leading to orbital decay. Energetic particles in the magnetosphere contribute to the auroras.

Solar flares release a cascade of high energy particles known as a proton storm. Protons can pass through the human body, doing biochemical damage. Most proton storms take two or more hours to reach Earth's orbit. A solar flare on January 20, 2005 released the highest concentration of protons ever directly measured, taking only 15 minutes after observation to reach Earth, indicating a velocity of approximately one-half light speed.

Life cycle

The Sun was formed about 4.57 billion years ago when a hydrogen molecular cloud collapsed. The Sun's current main sequence age, determined using computer models of stellar evolution and nucleocosmochronology is in close accord with the radiometric date of the oldest Solar System material, at 4.567 billion years ago.

The Sun is about halfway through its main-sequence evolution, during which nuclear fusion reactions in its core fuse hydrogen into helium. Each second, more than four million metric tons of matter are converted into energy within the Sun's core, producing neutrinos and solar radiation. At this rate, the Sun has so far converted around 100 Earth-masses of matter into energy. The Sun will spend a total of approximately 10 billion years as a main sequence star.

The Sun does not have enough mass to explode as a supernova. Instead, in about 5 billion years, it will enter a red-giant phase, its outer layers expanding as the hydrogen fuel in the core is consumed and the core contracts and heats up. Helium fusion will begin when the core temperature reaches around 100 million K and will produce carbon, entering the asymptotic giant branch phase.

By the time it is an AGB star, the Sun will have lost roughly 30% of its present mass due to stellar winds, so the orbits of the planets will move outward. If it were only for this, Earth would probably be spared, but new research suggests that Earth will be swallowed by the Sun owing to tidal interactions. Even if Earth would escape incineration in the Sun, still all its water will be boiled away and most of its atmosphere would escape into space. Even during its current life in the main sequence, the Sun is gradually becoming more luminous (about 10% every 1 billion years), and its surface temperature is slowly rising. The Sun used to be fainter in the past, which is possibly the reason life on Earth has only existed for about 1 billion years on land. The increase in solar temperatures is such that already in about a billion years the surface of the Earth will become too hot for liquid water to exist, possibly ending all terrestrial life.

Following the red giant phase, intense thermal pulsations will cause the Sun to throw off its outer layers, forming a planetary nebula. The only object that will remain after the outer layers are ejected is the extremely hot stellar core, which will slowly cool and fade as a white dwarf over many billions of years. This stellar evolution scenario is typical of low- to medium-mass stars.

Heat transfer

Conduction is heat transfer by means of molecular agitation within a material without any motion of the material as a whole. If one end of a metal rod is at a higher temperature, then energy will be transferred down the rod toward the colder end because the higher speed particles will collide with the slower ones with a net transfer of energy to the slower ones.

Convection is heat transfer by mass motion of a fluid such as air or water when the heated fluid is caused to move away from the source of heat, carrying energy with it. Convection above a hot surface occurs because hot air expands, becomes less dense, and rises (ideal gas law for constant P: V/T = nR/P = const.). Heated water expands and becomes more buoyant, while cooler, more dense water near the surface descends and patterns of circulation are formed, causing convection currents which transport energy. The granules in the Sun’s photosphere are convection cells which transport heat from the interior of the Sun to the surface.