THE SUN
●The Sun is the Star at the center of the Solar System. It is a nearly perfect sphere of hot Plasam with internal Convective motion that generates a Magnetic Field via a Dynamo Process.
●It is by far the most important source of Energy for life on Earth.
●Its diameter is about 1.39 million kilometers (864,000 miles), or 109 times that of Earth, and It's Mass is about 330,000 times that of Earth. ●
●It accounts for about 99.86% of the total mass of the Solar System.
●Roughly three quarters of the Sun's mass consists of hydrogen (~73%); the rest is mostly helium (~25%), with much smaller quantities of heavier elements, including oxygen, carbon, neon, and Iron.
- ●The Sun is a G-type main-sequence star (G2V) based on its spectral class. As such, it is informally and not completely accurately referred to as a yellow dwarf (its light is closer to white than yellow). It formed approximately 4.6 billion years ago from the gravitational collapse of matter within a region of a large molecular cloud. Most of this matter gathered in the center, whereas the rest flattened into an orbiting disk that became the Solar System. The central mass became so hot and dense that it eventually initiated nuclear fusion in its core. It is thought that almost all stars form by this process.
●The Sun currently fuses about 600 million tons of hydrogen into helium every second, converting 4 million tons of matter into energy every second as a result. This energy, which can take between 10,000 and 170,000 years to escape from its core, is the source of the Sun's light and heat. When hydrogen fusion in its core has diminished to the point at which the Sun is no longer in hydrostatic equilibrium, its core will undergo a marked increase in density and temperature while its outer layers expand, eventually transforming the Sun into a red giant. It is calculated that the Sun will become sufficiently large to engulf the current orbits of Mercury and Venus, and render Earth uninhabitable – but not for about five billion years. After this, it will shed its outer layers and become a dense type of cooling star known as a white dwarf, and no longer produce energy by fusion, but still glow and give off heat from its previous fusion.
●The enormous effect of the Sun on Earth has been recognized since prehistoric times, and the Sun has been regarded by some cultures as a deity. The synodic rotation of Earth and its orbit around the Sun are the basis of solar calendars, one of which is the predominantcalendar in use today.
The structure of the Sun contains the following layers:
◆ CORE ◆
●The innermost 20–25% of the Sun's radius, where temperature (energies) and pressure are sufficient for nuclear fusion to occur.
●Hydrogen fuses into helium (which cannot currently be fused at this point in the Sun's life). The fusion process releases energy, and the helium gradually accumulates to form an inner core of helium within the core itself.
◆ RADIATIVE ZONE ◆
●Convection cannot occur until much nearer the surface of the Sun. Therefore, between about 20–25% of the radius, and 70% of the radius, there is a "radiative zone" in which energy transfer occurs by means of radiation (photons) rather than by convection.
◆Tachocline ◆
●The boundary region between the radiative and convective zones.
◆ CONVECTION ZONE ◆
●Between about 70% of the Sun's radius and a point close to the visible surface, the Sun is cool and diffuse enough for convection to occur, and this becomes the primary means of outward heat transfer, similar to weather cells which form in the earth's atmosphere.
Photosphere –
●The deepest part of the Sun which we can directly observe with visible light. Because the Sun is a gaseous object, it does not have a clearly defined surface; its visible parts are usually divided into a 'photosphere' and 'atmosphere'.Atmosphere – a gaseous 'halo' surrounding the Sun, comprising the chromosphere, solar transition region, corona and heliosphere. These can be seen when the main part of the Sun is hidden, for example, during a solar eclipse.
SOLAR CORE :-
●The core of the Sun extends from the center to about 20–25% of the solar radius.
●It has a density of up to 150 g/cm3 (about 150 times the density of water)
and a temperature of close to 15.7 million kelvins (K).
●By contrast, the Sun's surface temperature is approximately 5,800 K.
●Recent analysis of SOHO mission data favors a faster rotation rate in the core than in the radiative zone above.
●Through most of the Sun's life, energy has been produced by nuclear fusion in the core region through a series of nuclear reactions called the p–p (proton–proton) chain; this process converts hydrogen into helium.
●Only 0.8% of the energy generated in the Sun comes from another sequence of fusion reactions called the CNO cycle, though this proportion is expected to increase as the Sun becomes older.
●The core is the only region in the Sun that produces an appreciable amount of thermal energy through fusion; 99% of the power is generated within 24% of the Sun's radius, and by 30% of the radius, fusion has stopped nearly entirely.
●The remainder of the Sun is heated by this energy as it is transferred outwards through many successive layers, finally to the solar photosphere where it escapes into space through radiation (photons) or advection (massive particles).
●The proton–proton chain occurs around 9.2×1037 times each second in the core, converting about 3.7×1038 protons into alpha particles (helium nuclei) every second (out of a total of ~8.9×1056 free protons in the Sun), or about 6.2×1011 kg/s.
●Fusing four free protons(hydrogen nuclei) into a single alpha particle (helium nucleus) releases around 0.7% of the fused mass as energy, so the Sun releases energy at the mass–energy conversion rate of 4.26 million metric tons per second (which requires 600 metric megatons of hydrogen ), for 384.6 yottawatts (3.846×1026 W), or 9.192×1010 megatons of TNT per second.
● The large power output of the Sun is mainly due to the huge size and density of its core (compared to Earth and objects on Earth), with only a fairly small amount of power being generated per cubic metre.
●Theoretical models of the Sun's interior indicate a maximum power density, or energy production, of approximately 276.5 watts per cubic metre at the center of the core,which is about the same rate of power production as takes place in reptile metabolism or a compost pile.
●The fusion rate in the core is in a self-correcting equilibrium: a slightly higher rate of fusion would cause the core to heat up more and expand slightly against the weight of the outer layers, reducing the density and hence the fusion rate and correcting the perturbation; and a slightly lower rate would cause the core to cool and shrink slightly, increasing the density and increasing the fusion rate and again reverting it to its present rate.
◆ RADIATIVE ZONE ◆
●From the core out to about 0.7 solar radii, thermal radiation is the primary means of energy transfer.
●The temperature drops from approximately 7 million to 2 million kelvins with increasing distance from the core.
●This temperature gradient is less than the value of the adiabatic lapse rate and hence cannot drive convection, which explains why the transfer of energy through this zone is by radiation instead of thermal convection.
●Ions of hydrogenand helium emit photons, which travel only a brief distance before being reabsorbed by other ions.
●The density drops a hundredfold (from 20 g/cm3 to 0.2 g/cm3) from 0.25 solar radii to the 0.7 radii, the top of the radiative zone.
◆ TACHOCLINE ◆
●The radiative zone and the convective zone are separated by a transition layer, the tachocline.
●This is a region where the sharp regime change between the uniform rotation of the radiative zone and the differential rotation of the convection zone results in a large shearbetween the two—a condition where successive horizontal layers slide past one another.
●Presently, it is hypothesized (see Solar dynamo) that a magnetic dynamo within this layer generates the Sun's magnetic field.
◆ CONVECTION ZONE ◆
●The Sun's convection zone extends from 0.7 solar radii (500,000 km) to near the surface.
●In this layer, the solar plasma is not dense enough or hot enough to transfer the heat energy of the interior outward via radiation.
●Instead, the density of the plasma is low enough to allow convective currents to develop and move the Sun's energy outward towards its surface.
●Material heated at the tachocline picks up heat and expands, thereby reducing its density and allowing it to rise.
● As a result, an orderly motion of the mass develops into thermal cellsthat carry the majority of the heat outward to the Sun's photosphere above.
●Once the material diffusively and radiatively cools just beneath the photospheric surface, its density increases, and it sinks to the base of the convection zone, where it again picks up heat from the top of the radiative zone and the convective cycle continues. At the photosphere, the temperature has dropped to 5,700 K and the density to only 0.2 g/m3 (about 1/6,000 the density of air at sea level).
●The thermal columns of the convection zone form an imprint on the surface of the Sun giving it a granular appearance called the solar granulation at the smallest scale and supergranulation at larger scales.
● Turbulent convection in this outer part of the solar interior sustains "small-scale" dynamo action over the near-surface volume of the Sun.
●The Sun's thermal columns are Bénard cells and take the shape of hexagonal prisms.
◆ PHOTOSPHERE ◆
●The effective temperature, or black body temperature, of the Sun (5,777 K) is the temperature a black body of the same size must have to yield the same total emissive power.
●The visible surface of the Sun, the photosphere, is the layer below which the Sun becomes opaque to visible light.
●Photons produced in this layer escape the Sun through the transparent solar atmosphere above it and become solar radiation, sunlight.
●The change in opacity is due to the decreasing amount of H− ions, which absorb visible light easily.
●Conversely, the visible light we see is produced as electrons react with hydrogen atoms to produce H− ions.
●The photosphere is tens to hundreds of kilometers thick, and is slightly less opaque than air on Earth.
●Because the upper part of the photosphere is cooler than the lower part, an image of the Sun appears brighter in the center than on the edge or limb of the solar disk, in a phenomenon known as limb darkening.
●The spectrum of sunlight has approximately the spectrum of a black-body radiating at 5,777 K, interspersed with atomic absorption lines from the tenuous layers above the photosphere.
●The photosphere has a particle density of ~1023 m−3 (about 0.37% of the particle number per volume of Earth's atmosphere at sea level).
●The photosphere is not fully ionized—the extent of ionization is about 3%, leaving almost all of the hydrogen in atomic form.
●During early studies of the optical spectrum of the photosphere, some absorption lines were found that did not correspond to any chemical elements then known on Earth.
●In 1868, Norman Lockyer hypothesized that these absorption lines were caused by a new element that he dubbed helium, after the Greek Sun god Helios.
●Twenty-five years later, helium was isolated on Earth.
◆ ATMOSPHERE ◆
See also: Solar corona and Coronal loop
●During a total solar eclipse, the solar corona can be seen with the naked eye, during the brief period of totality.
●During a total solar eclipse, when the disk of the Sun is covered by that of the Moon, parts of the Sun's surrounding atmosphere can be seen.
●It is composed of four distinct parts: the chromosphere, the transition region, the corona and the heliosphere.
●The coolest layer of the Sun is a temperature minimum region extending to about 500 kmabove the photosphere, and has a temperature of about 4,100 K.
●This part of the Sun is cool enough to allow the existence of simple molecules such as carbon monoxide and water, which can be detected via their absorption spectra.
●The chromosphere, transition region, and corona are much hotter than the surface of the Sun.
●The reason is not well understood, but evidence suggests that Alfvén waves may have enough energy to heat the corona.
●Above the temperature minimum layer is a layer about 2,000 km thick, dominated by a spectrum of emission and absorption lines.[82]
●It is called the chromosphere from the Greek root chroma, meaning color, because the chromosphere is visible as a colored flash at the beginning and end of total solar eclipses.
●The temperature of the chromosphere increases gradually with altitude, ranging up to around 20,000 K near the top.
●In the upper part of the chromosphere helium becomes partially ionized.
●Above the chromosphere, in a thin (about 200 km) transition region, the temperature rises rapidly from around 20,000 K in the upper chromosphere to coronal temperatures closer to 1,000,000 K.
●The temperature increase is facilitated by the full ionization of helium in the transition region, which significantly reduces radiative cooling of the plasma.
●The transition region does not occur at a well-defined altitude. Rather, it forms a kind of nimbusaround chromospheric features such as spicules and filaments, and is in constant, chaotic motion.
●The transition region is not easily visible from Earth's surface, but is readily observable from space by instruments sensitive to the
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