Lecture arc
A galaxy is not only a present-day equilibrium: disrupted satellites and chemically tagged stars preserve its assembly record. Start by asking students what the telescope actually measures. Then introduce streams, accretion, and time-dependent structure; delay the equation until the geometry and assumptions are visible. Use the visual as an argument: students should identify what is data, what is a model, and what is inferred.
The lecture should return to the one-dimensional worked calculation and ask whether the answer is stable under a plausible change in distance, tracer selection, or model family.
Terms Developed in Context
Stellar stream
A coherent, elongated structure of stars stripped from a cluster or dwarf galaxy along its orbit. Streams preserve phase-space evidence of accretion and probe the Galactic potential. Their track, distance, velocities, and width are compared with orbit or disruption models. In this lecture, use this term while reasoning about streams, accretion, and time-dependent structure, and distinguish the measured quantity from the model-dependent quantity in \(t_{\rm lookback}=t_0-t_{\rm formation}\). A stream is not exactly one orbit because stripping occurs over time and stars have finite dispersion. This boundary is important when comparing tracers or carrying the result into the worked example.
Accretion event
The incorporation and tidal disruption of a smaller stellar system by the Milky Way. It contributes stars, clusters, and dark matter while leaving kinematic and chemical substructure. Common integrals of motion and abundance patterns can associate debris with a progenitor. In this lecture, use this term while reasoning about streams, accretion, and time-dependent structure, and distinguish the measured quantity from the model-dependent quantity in \(t_{\rm lookback}=t_0-t_{\rm formation}\). A named debris structure may combine multiple events or overlap in projected phase space. This boundary is important when comparing tracers or carrying the result into the worked example.
Chemical tagging
The probabilistic association of stars through multidimensional elemental-abundance patterns. It can connect dispersed stars to shared formation environments after spatial coherence is lost. Abundance vectors complement ages and orbital information in archaeology. In this lecture, use this term while reasoning about streams, accretion, and time-dependent structure, and distinguish the measured quantity from the model-dependent quantity in \(t_{\rm lookback}=t_0-t_{\rm formation}\). Similar chemistry is not a unique birth certificate because enrichment pathways overlap. This boundary is important when comparing tracers or carrying the result into the worked example.
Radial migration
A lasting change in a disk star's guiding-center radius, often driven by resonant interactions. It mixes stars born at different radii and blurs present-day chemical gradients. Migration connects spiral or bar resonances to the age-metallicity-radius distribution. In this lecture, use this term while reasoning about streams, accretion, and time-dependent structure, and distinguish the measured quantity from the model-dependent quantity in \(t_{\rm lookback}=t_0-t_{\rm formation}\). It differs from epicyclic blurring, which changes instantaneous radius without shifting the guiding center. This boundary is important when comparing tracers or carrying the result into the worked example.
Lookback time
The elapsed time between emitted light or a past formation event and the present epoch. It places stellar ages and assembly episodes on a common temporal axis. For a formation time \(t_{\rm formation}\), \(t_{\rm lookback}=t_0-t_{\rm formation}\). In this lecture, use this term while reasoning about streams, accretion, and time-dependent structure, and distinguish the measured quantity from the model-dependent quantity in \(t_{\rm lookback}=t_0-t_{\rm formation}\). For Galactic stars it is usually an age inference, not a cosmological light-travel measurement. This boundary is important when comparing tracers or carrying the result into the worked example.