chapter 16.3; the solar interior; theory & 16.4; the solar interior; observations

sun is plasma

• the sun is so bot all the material inside is in the form of an ionised gas

• plasma acts much like a hot gas

stability of the sun

• in a condition of hydrostatic equilibrium

• all forces within the Sun are balanced

  • at each point of the Sun the temperature, pressure, density etc are at constant values

• Calculations show that, in order to exert enough pressure to prevent the Sun from collapsing due to the force of gravity, the gases at its center must be maintained at a temperature of 15 million K.

conduction

• atoms or molecules pass on their energy by colliding with others nearby

convection

• currents of warm material ruse, carrying their energy with them into cooler layers

• stellar convection: occurs as currents of hot has flow up and down through the star

radiation

• energetic photons move away from hot material and are absorbed by some material to which they convey some or all of their energy

• not an efficient means of energy transport ins tars because gases in stellar interiors are very opaque, that is, a photon does not go far before it is absorbed

solar pulsations

• Sun pulsates (expands and contracts)

• can be detected by measuring the radial velocity of the solar surface (the speed of which it moves towards or away from us)

• The typical velocity of one of the oscillating regions on the Sun is only a few hundred meters per second, and it takes about 5 minutes to complete a full cycle from maximum to minimum velocity and back again.

• The change in the size of the Sun measured at any given point is no more than a few kilometers.

• study of solar oscillations: helioseismology

  • convection extends inward from the surface 30% of the way toward the center

• Pulsation measurements also show that the differential rotation that we see at the Sun’s surface, with the fastest rotation occurring at the equator, persists down through the convection zone.

  • Below the convection zone, however, the Sun, even though it is gaseous throughout, rotates as if it were a solid body like a bowling ball.

sunspot structure

• The downward-flowing cool material acts as a kind of plug that block the upward flow of hot material, which is then diverted sideways and eventually reaches the solar surface in the region around the sunspot.

• This outward flow of hot material accounts for the paradox that the Sun emits slightly more energy when more of its surface is covered by cool sunspots.

solar neutrinos

• 3% of total energy generated by nuclear fusion in the Sun is carried away by neutrinos

• 35 million billion (3.5 × 10¹⁶) solar neutrinos pass through each square meter of Earth’s surface every second

• neutrinos change to a different type during their journey from the center of the Sun through space to Earth in a process called neutrino oscillation