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The Sun (Nearest Star to us) Dr. Arvind C. Ranade
The Sun (Nearest Star to us) Dr. Arvind C. Ranade Flow of Talk Our SUN Physical Profile of Sun Sun among other Stars Atmosphere of Sun Photosphere Chromosphere Corona Internal Structure of Sun Core Radiative Zone Convective Zone Magnetic Activities on Sun Sunspot Cycle/Magnetic Cycle Effect on the Earth Possible end of Sun Our SUN The governor of Solar System. Only day time star and closest star to the Earth. It is 13,00,000 times bigger than Earth. The light takes 8.24 minutes to reach Earth. To cover its diameter, 109 Earth will be required. It emits the radiations in all Electromagnetic spectrum (Gamma Rays to Radio waves). Physical Profile of Sun Physical characteristics Mean diameter 1.392×106 km Equatorial radius 6.955×105 km Mass 1.9891×1030 kg Average density 1.408×103 kg/m3 Equatorial surface gravity 274.0 m/s2 Escape velocity 617.7 km/s Temperature 5,778 K Luminosity (Lsol) 3.846×1026 of surface (effective) Rotation characteristics Obliquity 7.25° Sidereal Rotation period 25.38 days (at equator) 25.05 days (at poles) 34.3 days Rotation velocity 7.189×103 km/h (to the ecliptic) (at 16° latitude) (at equator) Observation data Orbital characteristics Mean distance ~2.5×1017 km Galactic period (2.25–2.50) × 108 Velocity ~220 km/s from Milky Way core Mean distance 1.496×108 km Visual brightness (V) −26.74 Absolute magnitude 4.85 Spectral classification G2V Angular size 31.6′ – 32.7′ from Earth 26,000 light-years yrs (orbit around the center of the Galaxy) Sun among other Stars Spectral Class (Temp) O – > 30,000 K B - 30,000- 12,000 K A – 12,000- 9,000 K F – 9,000 – 7,000 K G – 7,000- 5,500 K K – 5,500 – 3,800 K M - < 3,800 K Luminosity Class (Spectral lines) I – Super giants II-III – Giant IV-V - Dwarf Atmosphere of Sun Sun’s atmosphere starts from and above the surface layer named ‘Photosphere’. The layer above Photosphere, with the thickness of ~2,000 KM is ‘Chromosphere’. The outermost layer of the Sun’s atmosphere is known as ‘Corona’, it extends upto millions of kilometer above Chromosphere. Chromosphere and Corona is visible only during Total Solar Eclipse. Photosphere It is less than 500 KM deep. The photoshpere is the layer in the Sun’s atmosphere that is dense enough to emit plenty of light but not so dense that the light can’t escape. It is very-low-density gas. The density is 3400 times less dense than the air we breath. The layer below photosphere is dense and can produce continuous spectra, but atoms in the photosphere absorb photons of specific wavelengths producing absorption lines. Photosphere The layer has a mottled appearance because it is made up of dark-edged regions. It is known as granules and pattern is called as granulation. Granule lasts for 10 to 20 minutes. Spectra shows that centers are a few hundred degrees hotter than the edges, and Doppler shifts reveals that the centers are rising and the edges are sinking at the speed of 1 km/sec. Chromosphere Chromosphere is 1000 times fainter than the photosphere. Characteristic flash seen during the total solar eclipse in pink colour is produced in chromosphere – it is a combination of emission lines in red, blue and violet Balmer line. Chromosphere has lowdensity gas. It is about 108 times less dense than the air we breath. At the bottom of layer temperature is low (4300 K) but then rises rapidly. The region where temperature rises rapidly is called transition region. Chromosphere An H-alpha filtergram reveals that the chromosphere contain complex structure which is invisible in normal photographs. Spicules are characteristic flamelike jets of gas, rise upward into the chromosphere and last for 5 to 15 minutes. Corona Spectrum of corona showed that the temperature is in the order of million degree. It is not bright because it is very low density gas only 1-10 atoms/cm3. The study showed that the light from outer corona is reflected sunlight i.e. outer corona contains dust particles that reflects sunlight in all directions. Corona is the region where high speed winds are getting generated known as Solar Wind. It has the velocity of 300-800 km/sec. The “invisible” corona is seen only when the bright photosphere is masked by moon during total solar eclipse. Alternatively, from space by artificial eclipse, and in xray, UV lights. Temperature profile of the Sun’s atmosphere Opacity on the Sun’s atmosphere corona density temperature Temperature and Density over the outer solar atmospheric layers chromosphere photosphere Internal Structure of the Sun • Helioseismology gives the answers to the internal structure of Sun. • Global Oscillation Network Group (GONG) Core It extends from 0 to 0.25 solar radii. It has the density of 150g/cm3. It has the temperature of 13.6 to 15 million K. It is the fastest rotating part. All of its energy generate in the core through nuclear fusion. 98% is through p-p chain and 2% is through CNO cycle. P-P Chain • In the core of Sun, proton-proton chain occurs 1037 times in each second. • The Sun exhausts 4.26 million metric tons of matter in every second. 383 yottawatts (3.83×1026 W). C-N-O Chain In the CNO cycle, four protons fuse, using carbon, nitrogen and oxygen isotopes as a catalyst, to produce one alpha particle, two positrons and two electron neutrinos. The positrons will almost instantly annihilate with electrons, releasing energy in the form of gamma rays. Radiative Zone It starts from 0.25 to 0.70 solar radii. It is hot and dense which helps in transferring the matter from core to outer part through radiation. Material from bottom to top gets cooler with the altitude (7 million to 2 million K). Density drops to hundred folds (20 g/cm3 to 0.2 g/cm3). Region of random walk problem. The region between the radiative zone and the convection zone is a transition layer called the tacholine. 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. Convective Zone It is the outer layer in the internal structure and it starts from below photosphere down to about 200,000 km. The solar plasma is not dense enough or hot enough to transfer the heat energy of the interior outward via radiation. Thermal convection is the process through which thermal columns carry hot material to the surface (photosphere) of the Sun. It is the region where granules and supergranules originates. The turbulent convection in the outer part gives rise to the small scale dynamo which produces magnetic south and north. Magnetic Activities on the Sun Solar Flares Sunspots Prominences Sunspot Cycle/Magnetic Cycle Sunspot is the easily observable magnetic activity on the Sun’s surface. Because of their cooler temperature region at the magnetic region it appears as a dark spot. The temperature of the sunspot is of the order of 4250K as compared to surrounding of 5700 K. It has a inside dark part known as umbra and outside lighter part known as penumbra. The presence of magnetic filed in the sunspot was first observed by Gorge Ellery Hale in 1908 through the Zeeman Effect. Sunspots Zeeman Effect • Size of the sunspot varies from 1000 km to 80,000 km. • Life of Sunspot is as small as an hour and also can be upto month time. Sunspot Active region on the Sun Maunder Butterfly Diagram Observation of Sunspot Effect on Earth Formation of Aurora in high latitudes near magnetic poles Formation of Sunspot Magnetic field lines in Sunspot Solar Wind and Earth Aurora Life Cycle of Sun/Possible end of Sun