hydrostatic equilibrium
Definition: The local force balance in which an outward pressure gradient exactly opposes inward gravity, so a layer has no net radial acceleration.
Why it matters here: A static, spherically symmetric star is supported against its own gravity by a pressure gradient: dP/dr = -G M(r) rho(r) / r^2 (hydrostatic equilibrium), which underlies every layer of the Sun’s structure from the core to the photosphere.
Relationship: Use this term alongside scale height; the lecture's quantitative model is summarized below.
\[ \frac{dP}{dr} = -\frac{GM(r)\rho(r)}{r^2}, \qquad H = \frac{kT}{\mu m_H g}\ \text{(isothermal scale height)} \]
Boundary or common confusion: Assuming the Sun’s energy-generating core extends nearly to its surface. Nuclear fusion is confined to the innermost ~25% of the solar radius (where density and temperature are high enough), while the outer three-quarters of the Sun’s radius carries that energy outward without generating any of its own -- confusing "where energy is generated" with "how far the Sun’s structure extends" is a common error.
scale height
Definition: The characteristic vertical distance over which pressure or density falls by a factor of e in an isothermal atmosphere with approximately constant gravity.
Why it matters here: For an isothermal atmosphere in a uniform gravitational field g, this integrates to an exponential pressure/density falloff with a characteristic scale height H = kT/(mu m_H g), where mu is the mean molecular weight of the gas; this is a direct, simplified consequence of hydrostatic equilibrium applied locally near the surface where g is nearly constant over the relevant height range.
Relationship: Use this term alongside radiative zone; the lecture's quantitative model is summarized below.
\[ \frac{dP}{dr} = -\frac{GM(r)\rho(r)}{r^2}, \qquad H = \frac{kT}{\mu m_H g}\ \text{(isothermal scale height)} \]
Boundary or common confusion: Assuming the Sun’s energy-generating core extends nearly to its surface. Nuclear fusion is confined to the innermost ~25% of the solar radius (where density and temperature are high enough), while the outer three-quarters of the Sun’s radius carries that energy outward without generating any of its own -- confusing "where energy is generated" with "how far the Sun’s structure extends" is a common error.
radiative zone
Definition: The solar-interior region where energy is carried mainly by photons diffusing through optically thick plasma rather than by bulk fluid motion.
Why it matters here: Energy generated in the core is transported outward first by radiation (photons randomly diffusing outward through the opaque radiative zone) and then, once the temperature gradient becomes too steep for radiation alone to carry the flux (opacity rises sharply as temperature falls), by convection -- bulk fluid motion that is directly visible at the photosphere as granulation.
Relationship: Use this term alongside convective zone; the lecture's quantitative model is summarized below.
\[ \frac{dP}{dr} = -\frac{GM(r)\rho(r)}{r^2}, \qquad H = \frac{kT}{\mu m_H g}\ \text{(isothermal scale height)} \]
Boundary or common confusion: Assuming the Sun’s energy-generating core extends nearly to its surface. Nuclear fusion is confined to the innermost ~25% of the solar radius (where density and temperature are high enough), while the outer three-quarters of the Sun’s radius carries that energy outward without generating any of its own -- confusing "where energy is generated" with "how far the Sun’s structure extends" is a common error.
convective zone
Definition: The outer solar-interior region where buoyant rising material and sinking cooler material transport most of the energy flux.
Why it matters here: The Sun generates energy in a core occupying only the innermost 0.25 of its radius, at a temperature of 1.57e+07 K -- yet by the time that energy reaches the visible photosphere, the temperature has fallen by nearly three orders of magnitude to T_eff=5772 K, and the structure has transitioned from radiative to convective energy transport at r=0.71 R_sun, a boundary now measured precisely by helioseismology (Unit 5).
Relationship: Use this term alongside photosphere; the lecture's quantitative model is summarized below.
\[ \frac{dP}{dr} = -\frac{GM(r)\rho(r)}{r^2}, \qquad H = \frac{kT}{\mu m_H g}\ \text{(isothermal scale height)} \]
Boundary or common confusion: Assuming the Sun’s energy-generating core extends nearly to its surface. Nuclear fusion is confined to the innermost ~25% of the solar radius (where density and temperature are high enough), while the outer three-quarters of the Sun’s radius carries that energy outward without generating any of its own -- confusing "where energy is generated" with "how far the Sun’s structure extends" is a common error.
photosphere
Definition: The thin visible layer from which most solar optical photons escape, conventionally associated with continuum optical depth near unity.
Why it matters here: The Sun generates energy in a core occupying only the innermost 0.25 of its radius, at a temperature of 1.57e+07 K -- yet by the time that energy reaches the visible photosphere, the temperature has fallen by nearly three orders of magnitude to T_eff=5772 K, and the structure has transitioned from radiative to convective energy transport at r=0.71 R_sun, a boundary now measured precisely by helioseismology (Unit 5).
Relationship: Use this term alongside effective temperature; the lecture's quantitative model is summarized below.
\[ \frac{dP}{dr} = -\frac{GM(r)\rho(r)}{r^2}, \qquad H = \frac{kT}{\mu m_H g}\ \text{(isothermal scale height)} \]
Boundary or common confusion: Assuming the Sun’s energy-generating core extends nearly to its surface. Nuclear fusion is confined to the innermost ~25% of the solar radius (where density and temperature are high enough), while the outer three-quarters of the Sun’s radius carries that energy outward without generating any of its own -- confusing "where energy is generated" with "how far the Sun’s structure extends" is a common error.
effective temperature
Definition: The blackbody-equivalent temperature defined by luminosity and radius through L = 4 pi R squared sigma T_eff to the fourth power.
Why it matters here: The photospheric effective temperature (T_eff=5772 K, the IAU nominal value derived from the Sun’s measured luminosity and radius via the Stefan-Boltzmann law) sets essentially every subsequent atmospheric and space-weather quantity used in this course.
Relationship: Use this term alongside energy transport; the lecture's quantitative model is summarized below.
\[ \frac{dP}{dr} = -\frac{GM(r)\rho(r)}{r^2}, \qquad H = \frac{kT}{\mu m_H g}\ \text{(isothermal scale height)} \]
Boundary or common confusion: Assuming the Sun’s energy-generating core extends nearly to its surface. Nuclear fusion is confined to the innermost ~25% of the solar radius (where density and temperature are high enough), while the outer three-quarters of the Sun’s radius carries that energy outward without generating any of its own -- confusing "where energy is generated" with "how far the Sun’s structure extends" is a common error.
energy transport
Definition: The physical transfer of the Sun's luminosity outward by radiation, convection, conduction, waves, or bulk plasma flows.
Why it matters here: The Sun generates energy in a core occupying only the innermost 0.25 of its radius, at a temperature of 1.57e+07 K -- yet by the time that energy reaches the visible photosphere, the temperature has fallen by nearly three orders of magnitude to T_eff=5772 K, and the structure has transitioned from radiative to convective energy transport at r=0.71 R_sun, a boundary now measured precisely by helioseismology (Unit 5).
Relationship: Use this term alongside hydrostatic equilibrium; the lecture's quantitative model is summarized below.
\[ \frac{dP}{dr} = -\frac{GM(r)\rho(r)}{r^2}, \qquad H = \frac{kT}{\mu m_H g}\ \text{(isothermal scale height)} \]
Boundary or common confusion: Assuming the Sun’s energy-generating core extends nearly to its surface. Nuclear fusion is confined to the innermost ~25% of the solar radius (where density and temperature are high enough), while the outer three-quarters of the Sun’s radius carries that energy outward without generating any of its own -- confusing "where energy is generated" with "how far the Sun’s structure extends" is a common error.