Sun's Energy transport mechanism

Sun's Energy transport mechanism

• 1.
  ENERGY TRANSPORT MECHANISMIN THE SUN ALLAN GUEMO ALAURIN BTTE-1
• 2.
  • Energy generatedin the star’s center must be transported to the surface • Inner layers of the Sun: • Radiation • Energy carried by photons • Outer layers of the Sun: • Convection • Energy carried by convective motion of large masses
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  Energy Transport • Thereare three possible mechanisms for energy transport from one location to another: conduction, radiation, and convection. • Conduction is the transfer of energy within a substance by collisions between atoms and/or molecules. This is not a significant factor in transporting energy within the Sun.
• 4.
  Radiation • The energyproduced by the Sun is carried out by photons emitted at one spot and absorbed at another (radiative diffusion). • The speed at which photons get out from the Sun depends heavily on its opacity – the ability of a substance to stop photons (the opposite of transparency). • In the Sun’s core, the photons typically travel 10-6 m before being reabsorbed. • About 1025 absorptions and reemissions are needed before the energy reaches the Sun’s surface
• 5.
  Convection • Radiative diffusioncarries solar energy out to 70% of the Sun’s radius. • At this distance, the temperature has dropped to 1.5 million K and hydrogen atoms start to from, increasing the opacity. • As a result, the rate of temperature decline becomes steep and convection takes over the energy transport in the outer 30% of the Sun’s radius • Typically, the diffusion of energy from core to surface takes on average 170,000 years.
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  • Radiative zone –This region is comparatively transparent to EM radiation. – energy is carried away from core as electromagnetic radiation (photons) by the radiative diffusion mechanism. – However light has a tough time traveling through this region since the solar material in this region is very dense. – Therefore, it takes light 170,000 years for the energy created at the core to travel through the radiative zone (696,000 km) at a rate of 50cm per hour (20 times slower than a snails pace)
• 8.
  • Convective zone– – In this region the temperature is low enough for nuclei to join with electrons and form hydrogen atoms, and these absorb light very efficiently. – Gases are opaque to light, thus convection is the transportation mechanism. – Therefore, radiative diffusion is not an efficient method of energy transport in this region. – material(gas) convects energy (heat) to surface. – Hot gas goes up & cooler gas comes down.
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  • Conduction –moleculeto molecule within a substance • Convection (and advection) –mass movement of a fluid • Radiation –absorption of electromagnetic waves
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  Energy transport insolar interior • How is energy transported outwards through the Sun? • Three possible mechanisms: 1. Conduction 2. Radiation 3. Convection • Role of conduction: gas pressure >> radiation pressure in the Sun, • so, since the thermal energy of the electrons is greater than that • of the photons, we might expect thermal conduction to be important. • But the mean free path of the electrons is tiny compared with the • dimensions of the solar interior. Thus, thermal conduction is • negligible in the Sun – at most it makes a very small contribution at the solar core (inner 0.2 Rʘ where Rʘ = solar radius).
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  Radiation and convectiontransport in the solar interior Of the 2 remaining transport processes, energy is transported by radiation in the inner part of the solar interior (0.25Rʘ < r < 0.71 Rʘ) and convection in the outer part (r > 0.71 Rʘ). We now calculate how the temperature drops off – the temperature gradient dT/dr – with distance r from the Sun’s centre assuming first radiative energy transport then derive conditions for when convection takes over as the energy transport mechanism.
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  Role of Radiationenergy transfer: Photon transport in the solar interior • The time for a photon to travel from the solar core to the solar photosphere can be estimated from a random walk formula. The photons are continually stopped by scattering off free electrons in the solar interior – Thomson scattering. • Random walk process:
• 14.
  • Core – Wherethe energy is created. – Nuclear reactions burn every second about 700 million tons of hydrogen into helium. • Radiation Zone – Where energy is transported by radiation. – Although the photons travel at the speed of light, they bounce so many times through the dense material that they use about a million years to escape the Sun. • Convection Zone – Energy transported by convection (just like boiling soup) where heat is transported to the photosphere.