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Edge-on, the bright thin galactic disk forms a narrow band, with a central bulge protruding above and below it and a thinner gas-and-dust lane along the midplane. Young stars and H II regions occupy a vertical scale of a few tenths of a kiloparsec or less; a broader old thick disk has scale of order a kiloparsec. These are approximate scale heights, not sharp boundaries. The old stellar halo and globular clusters form a much larger, low-surface-brightness spheroidal component, extending to many tens of kiloparsecs or more. The dark matter halo is inferred dynamically rather than directly visible in such a sketch. The preceding figure is a structural diagram, not an external photograph or a reconstruction of a particular survey.

Consider a steady Keplerian disk around a central mass $M$, with positive inward accretion rate $\dot{M}$ and zero viscous torque in an accretion disk at $R_{\mathrm{in}}$. Angular momentum conservation gives the outward internal torqueDifferential rotation dissipates energy at the rate $\mathcal{T}(-d\Omega /dR)dR$ in an annulus. Dividing by its projected area $2\pi RdR$ and using $\Omega =\sqrt{GM/R^3}$ gives the flux summed over both faces:The energy flux through one face is $F_{\mathrm{diss}}/2$. In a locally radiating, optically thick disk, one therefore sets $\sigma T_{\mathrm{eff}}^4=F_{\mathrm{diss}}/2$, rather than equating a single-face blackbody flux to the two-face expression.^[ref]

The bracket enforces the zero-torque inner boundary. Far outside it, $F_{\mathrm{diss}}\propto R^{-3}$. Internal torque transports energy between annuli as well as transporting angular momentum, so local dissipation is not simply the local loss of orbital binding energy. A nonzero inner torque, strong advective transport or a mass outflow changes this model.

A geometrically thin, optically thick disk has a radially varying effective temperature. Relativistic transport and an atmosphere model convert its thermal spectrum into an inferred emitting area and inner radius. Black-hole mass, distance, inclination and spectral hardening are important external constraints. Selecting thermal-state observations with moderate luminosity limits contamination and the breakdown of the thin-disk approximation. Relating the fitted inner radius to the innermost stable circular orbit remains a physical modeling assumption.