Sun

### **Composition**

-Hydrogen (73.6%)-Helium (24.9%)-Oxygen (0.77%)-Carbon (0.29%)-Iron (0.16%)-Neon (0.12%)-Nitrogen (0.09%)-Silicon (0.07%)-Magnesium (0.05%)-Sulfur (0.04%)

## Kurzgesagt

### **Latest appearance**

Why We Should NOT Look For Aliens - The Dark Forest

## General information

The Sun is the star at the center of the solar system. It is around 99.86% of the solar system's mass, and can fit 1 million earths inside. Three-quarters of the star is hydrogen, while the rest is helium with neon, carbon, oxygen, and iron

The Sun is a main-sequence star, or a yellow dwarf. However, the Sun is closer to white in color. The sun formed from the Solar Nebula, which was formed from gases and dusts from two neutron stars. Its energy comes from the nuclear fusion in the core. This causes it to glow and produce radiation, which in high amounts can give animals cancer. It would also shoot out its plasma, causing solar winds.

About 600 million tons of hydrogen and 4 million tons of matter are fused into helium in the Sun's core every second, producing energy. The Sun's heat and light come from this energy, which can take anywhere from 10,000 to 170,000 years to exit the core. The Sun will ultimately become a red giant when hydrogen fusion in its core is reduced to the point that the Sun is no longer in hydrostatic equilibrium. This will cause the Sun's core to significantly increase in density and warmth, forcing its outer layers to expand. In five billion years, this process will enlarge the Sun to the point that it is no longer livable for humans. Following this, the Sun will lose its outer layers and transform into a white dwarf, a dense kind of cooling star that still glows and emits heat from its prior fusion but no longer produces energy through fusion.

Since ancient times, humanity have acknowledged the Sun's immense influence on Earth, and some societies have even revered it as a deity. The foundation of certain solar calendars is the Earth's synodic rotation and its orbit around the Sun. The most widely used calendar in use today is the Gregorian calendar, which is based on the common understanding from the 16th century that the movement of the Sun is actually moving.

### Properties

The Sun, a main-sequence star of the G type, accounts for 99.86% of the solar system's mass. With an estimated absolute magnitude of +4.83, the Sun is brighter than around 85% of the Milky Way's stars, the majority of which are red dwarfs. A Population I star, or one rich in heavy elements, is the Sun. It might have formed as a result of shock waves from one or more neighboring supernovae. This is shown by the fact that the Solar System has a higher abundance of heavy elements than so-called Population II, heavy-element-poor, stars, such as uranium and gold. Most likely, endothermic nuclear processes during a supernova or transmutation through neutron absorption generated the heavy elements.

The Sun, with an apparent magnitude of -26.74, is by far the brightest object in the Earth's sky. Compared to Sirius, the next brightest star, with an apparent magnitude of −1.46, this is almost 13 billion times brighter.

The mean distance between the center of the Sun and the center of Earth is one astronomical unit; however, this distance fluctuates as Earth travels from perihelion, or around January 3, to aphelion, or roughly July 4. The range of extreme values is 147,083,346 km to 152,112,126 km, while the range of distances is 147,098,074 km to 152,097,701 km. Light from the nearest points of the Sun and Earth takes around two seconds shorter to reach their horizons than light from the Sun, which travels an average distance of eight minutes and twenty seconds. Through photosynthesis, solar energy sustains nearly all life on Earth and influences the planet's temperature and weather patterns.

The density of the Sun falls exponentially with increasing height above the photosphere, although the Sun lacks a clear border. The distance between the Sun's core and the photosphere's edge—that is, its apparent visible surface—is used to calculate the Sun's radius.39]. Based on this metric, the Sun is a nearly perfect sphere with an estimated 9 millionths oblateness. The planets have a minor tidal influence that has little effect on the Sun's form. At its equator, the Sun revolves more quickly than at its poles. Convective motion from heat transfer and the Coriolis force from the rotation of the Sun are the two factors causing this differential rotation. The rotating period, in a frame of reference determined by the stars, is around 25.6 days near the equator and 33.5 days at the poles. The apparent rotational period of the Sun near its equator, as observed from Earth as it circles the Sun, is approximately 28 days. The Sun revolves around its axis of spin in a counterclockwise direction when viewed from a location above its north pole.

#### Composition

Hydrogen and helium are the main chemical components that make up the Sun. They make up 74.9% and 23.8%, respectively, of the Sun's mass in the photosphere at this point in its existence. Less than 2% of the mass is made up of all the heavier elements, known as metals in astronomy, with oxygen, carbon, neon, and iron being the most prevalent.

The abundance of each element is most frequently expressed in dex, a scaled logarithmic unit, in solar research. According to definition, the abundance of hydrogen is 12, whereas the abundance of helium ranges between about 10.3 and 10.5 based on the solar cycle's phase. Iron is 7.62, carbon is 8.47, nitrogen is 8.29, and oxygen is 7.69.

The interstellar medium, from which the Sun evolved, left behind its initial chemical makeup. Its original composition would have been 1.5% heavier elements, 27.4% helium, and around 71.1% hydrogen. The heavier elements were created by earlier star generations before the Sun formed and spread into the interstellar medium during the final stages of stellar life and by events like supernovae. The hydrogen and most of the helium in the Sun were created by Big Bang nucleosynthesis in the first 20 minutes of the universe.

The primary process of fusion ever since the Sun's formation has been the fusion of hydrogen and helium. The amount of helium and its position within the Sun have progressively altered during the last 4.6 billion years. Due to fusion, the amount of helium in the core has grown from around 24% to roughly 60%, and gravity has caused some of the heavy elements and helium to settle from the photosphere closer to the Sun's center. The heavier element ratios remain unaltered. The fusion products are not lifted outward by heat because heat is carried outward from the Sun's core by radiation rather than convection. Instead, they stay in the core, where over time an inner core of helium has started to develop that cannot be fused due to the Sun's core not being hot or dense enough to fuse helium at this time. Helium proportion is lower in the present photosphere, and metallicity is only 84% of the protostellar phase (before to the onset of nuclear fusion in the core). Helium will keep building up in the Sun's core in the future, and after around 5 billion years, this slow accumulation will force the Sun to emerge from the main sequence and transform into a red giant.

It is generally accepted that the photosphere's chemical makeup serves as a proxy for the solar system's initial composition. The above-discussed methods of detecting solar heavy-element abundances include spectroscopy of the photosphere of the Sun and measurements of abundances in meteorites that have not been cooked to melting temperatures. It is believed that the makeup of these meteorites is preserved from the protostellar Sun, meaning that the settling of heavy elements does not impact them. Overall, the two approaches accord rather well.

##### Core

The Sun's core reaches a distance of around 20–25% around its center. It has a temperature of over 15.7 million Kelvin and a density of up to 150 g/cm3. The surface temperature of the Sun, in comparison, is around 5800 K. A more rapid rotation rate is suggested in the core than in the radiative zone above by a recent study of data from the SOHO spacecraft. Hydrogen has been converted to helium for the majority of the Sun's existence by nuclear fusion in the core area via the proton-proton chain. Only 0.8% of the energy produced by the Sun now originates from the CNO cycle, a different series of fusion processes.

Only the Sun's core generates a significant quantity of heat energy through fusion; within 24% of the Sun's radius, 99% of the power is produced, and by 30% of the radius, practically all fusion has halted. This energy is transported outward through several subsequent layers, heating the remaining portion of the Sun before escaping into space by radiation or advection in the solar photosphere.

##### Radiative Zone

At 0.45 solar radii, the radiative zone is the sun's thickest layer. Thermal radiation is the main energy transmission mechanism from the core out to around 0.7 solar radii. The temperature decreases with increasing distance from the core, from around 7 million to 2 million kelvins. Because the temperature gradient is less than the adiabatic lapse rate, convection cannot occur, which is why radiation rather than thermal convection is used to transport energy across this zone. Hydrogen and helium ions release photons, which are quickly reabsorbed by other ions after they have traveled a short distance. At the peak of the radiative zone, at 0.7 solar radii, the density decreases by a factor of 100.

##### Tachocline

The tachocline, a transition layer, divides the radiative zone from the convective zone. This is the area where there is a significant shear between the convection zone and the radiative zone due to the abrupt regime shift between the two, causing consecutive horizontal layers to glide past one another. The current theory states that the magnetic field of the Sun is produced by a magnetic dynamo located in this layer.

##### Convective Zone

The convective zone of the Sun reaches from 0.7 solar radii to the vicinity of the surface. The solar plasma in this layer is not hot enough or thick enough to radiate the heat energy from the inside outward. Rather, the plasma's density is low enough to let convection currents form and transfer the Sun's energy from its inside to the exterior. When material is heated near the tachocline, it absorbs heat, expands, and loses density, which permits it to rise. Consequently, the mass moves in an organized fashion to form thermal cells, which radiate the majority of the heat upward into the Sun's photosphere. The material's density rises as it descends to the base as it diffusively and radiatively cools just below the photospheric surface.

At smaller sizes, the solar granulation and at larger scales, the supergranulation, are the granular appearances left by the heat columns of the convection zone imprinting the surface of the Sun. Through turbulent convection, "small-scale" dynamo activity is maintained over the Sun's near-surface volume in this outer region of the solar interior. The Bénard cells that make up the Sun's thermal columns resemble approximately hexagonal prisms.

##### Photosphere

The layer under which the Sun turns opaque to visible light is known as the photosphere, or visible surface. Through the clear solar atmosphere above it, photons created in this layer escape the Sun and transform into solar radiation, or sunshine. The decrease in H- ions, which readily absorb visible light, is the cause of the shift in opacity. On the other hand, electrons reacting with hydrogen atoms to form H− ions provide the visible light that humans perceive.

The photosphere is somewhat less opaque than Earth's atmosphere, with a thickness ranging from tens to hundreds of kilometers. Legma darkening is the phenomena where an image of the Sun looks brighter in the center than on the edge or limb of the solar disk due to the fact that the top half of the photosphere is colder than the lower section. Sunlight's spectrum is roughly that of a black body emitting at 5,777 K, with atomic absorption lines from the thin layers above the photosphere scattered throughout. Particle density in the photosphere is approximately 1023 m−3. Approximately 3 percent of the photosphere is ionized, meaning that nearly all of the hydrogen is still in its atomic form.

Early investigations into the photosphere's optical spectrum revealed some absorption lines that had no known chemical element correlation. After the Greek Sun deity Helios, Norman Lockyer postulated in 1868 that a new element known as helium was responsible for these absorption lines. After 25 years, helium was found alone on Earth.

##### Atmosphere

The photosphere, chromosphere, transition area, corona, and heliosphere are the four components that make up the Sun's atmosphere. The photosphere is obscured during a total solar eclipse, revealing the corona.

A temperature minimum zone that reaches around 500 km above the photosphere and has a temperature of 4,100 K is the coldest layer of the Sun. Simple molecules like carbon monoxide and water may live in this region of the Sun because it is sufficiently cold, as seen by their absorption spectra. Compared to the Sun's surface, the chromosphere, transition area, and corona are far hotter. Evidence implies that Alfvén waves may have sufficient energy to heat the corona, while the exact cause is unknown.

There is a layer approximately 2,000 km thick above the temperature minimum layer, which is characterized by a spectrum of emission and absorption lines. Because the chromosphere appears as a colorful flash at the start and finish of total solar eclipses, the term "chromosphere" comes from the Greek word "chroma," which means color. The chromosphere's temperature rises steadily with altitude, reaching a peak temperature of about 20,000 K. Helium partly ions in the highest region of the chromosphere.

Around 20,000 K in the upper chromosphere and closer to 1,000,000 K in the corona, the temperature increases quickly in a narrow transition area above the chromosphere. Full helium ionization in the transition area greatly lowers radiative cooling of the plasma, hence facilitating the temperature increase. The transition zone doesn't happen at a certain height. Instead, it constantly moves in an erratic manner, forming a type of nimbus surrounding chromospheric structures like filaments and spicules. Though it is not clearly visible from Earth's surface, devices sensitive to the extreme ultraviolet part of the spectrum may easily see the transition area from space.

The Sun's next layer is called the corona. The solar wind and corona have an average temperature of between 1,000,000 and 2,000,000 K, but in the hottest places, it may reach 8,000,000 to 20,000,000 K. At least part of the heat in the corona is known to originate from magnetic reconnection, despite the lack of a comprehensive hypothesis to explain the temperature of the region. The Sun's extended atmosphere, known as the corona, is substantially greater than the area that the photosphere of the Sun encloses. The solar wind is an outflow of plasma from the Sun into interplanetary space.

Solar wind plasma fills the heliosphere, the thin outermost atmosphere of the Sun. This outermost layer of the Sun is characterized as starting at around 20 solar radii, which is the distance at which the solar wind flow becomes superalfvénic, or faster than the speed of Alfvén waves. Since information can only move at the speed of Alfvén waves, turbulence and dynamic forces within the heliosphere cannot alter the structure of the solar corona within. Till it reaches the heliopause more than 50 AU from the Sun, the solar wind constantly moves outward through the heliosphere, shaping the solar magnetic field into a spiral pattern. The Voyager 1 probe traveled through a shock front that is believed to be a component of the heliopause in December 2004. Voyager 1 observed a significant decrease in solar wind particles with lower energy and a marked increase in cosmic ray collisions towards the end of 2012. This indicated that the probe had indeed entered the interstellar medium and passed through the heliopause on August 25, 2012, when it was roughly 122 astronomical units from the Sun. Because to the Sun's velocity, a heliotail extends behind the heliosphere.