🏛️ Lecture 01 — Concept & History

Summary

Galactic archaeology uses the chemical composition, kinematics, position, and age of stars as “fossils” to reconstruct the Milky Way’s formation history. This lecture covers the simple stellar population concept and the history — from Herschel to Searle & Zinn — that led to identifying populations I and II.

Lecture info

Course: Galactic Archaeology and Stellar Populations Institution: National Observatory (ON), Brazil Professor: Hélio Dotto Perottoni Prerequisite: Stellar Evolution

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🎯 The concept and scope of Galactic Archaeology

The term is frequently applied to research on the formation and history of the Milky Way and its stellar populations, combining dynamics, chemistry, age, and position to reconstruct that history [Helmi 2020, Streams, Substructures, and the Early History of the Milky Way; Freeman & Bland-Hawthorn 2002, The New Galaxy: Signatures of Its Formation].

The core premise: stars carry a long-term memory of their origin in their chemical composition and (sometimes) their dynamics. Their position and age carry additional information. The field’s four working axes are therefore:

• Positions (where stars are today)
• Chemistry (what their atmospheres reveal about the gas they formed from)
• Motions (kinematics/orbital dynamics)
• Ages

Data such as Gaia DR2 (~1.8 billion stars) and APOGEE DR16 (chemistry + dynamics combined) form the modern observational basis for this reconstruction.

🌌 The Milky Way’s history, in broad strokes

The current picture (e.g., Xiao+2025, Semenov+2024, Naidu+2021, Helmi+2018) organizes the Galaxy’s formation into approximate stages:

Stage	Epoch	Description
Disk formation	$z\sim 7$, $>11$ Gyr	Proto-Milky Way + spin-up
Gaia Sausage-Enceladus	$z>2$, $\sim$11–9 Gyr	Heated disk/splash + starburst + inner halo + possible warp
Sagittarius dSph	$2>z>0.3$, $\sim$5 Gyr	Disk stellar overdensities + increase in star formation rate
Magellanic Clouds	$z<0.3$, $>\sim 3.5$ Gyr	Halo “wakes up” with first infall

This roadmap is reconstructed precisely with the tools this course develops — orbits, chemistry, ages — and will be revisited in detail in Units 3, 4, 6, and 7 of the syllabus.

⭐ The concept of a stellar population

The field’s central goal is to understand galaxy formation and evolution through the properties of their constituents. To do so, a galaxy is assumed decomposable into populations: groups of stars, clusters, and gas sharing common properties.

The concept is greatly simplified by the Simple Stellar Population (SSP) approximation: a group of stars that is

1. Coeval (same age);
2. Chemically homogeneous;
3. Sharing similar kinematic properties.

A typical galaxy can then be written as a sum of composite populations, each itself a sum of SSPs:

$\text{Galaxy}={\sum }_n{\text{population}}_n,\text{population}={\sum }_j{\text{SSP}}_j$

The bulge, halo, and galactic disk are examples of composite populations. Each represents a complex grouping of stars with its own metallicity, velocity, and age distributions — presumably the result of mixing several SSPs. The fewer SSPs that make up a population, the more easily its history can be reconstructed; identifying SSPs in large galaxies is therefore a complex task, one that simplifies when an SSP has characteristic patterns (well-defined spatial distribution, kinematics, chemistry, and age).

The SSP concept also allows the integrated spectrum of a galaxy to be reconstructed as a weighted sum of the spectra of its stellar populations [Conroy 2013].

📜 History of galactic studies

Structure of the Galaxy (18th–20th centuries)

• William and Caroline Herschel (1785): first systematic mapping of the Galaxy’s shape, by counting stars in different sky directions.
• Hubble (1925): uses the Cepheid period-luminosity relation (discovered by Henrietta Leavitt and Edward Pickering, 1912) to measure distances and place the Milky Way in the context of other galaxies.

Taxonomic classifications (early 20th century)

Early 20th-century Galactic Astronomy focused on classifying groups of stars with similar properties: radial velocities and proper motions, stellar types (color, brightness, variability, spectral type), and location (spiral arms, field, clusters/associations, bulge, halo). The main names of this era are Kapteyn, Jeans, Smart, Strömberg, Oort, and Lindblad — studies of stellar kinematics already suggested the existence of “stellar streams” in the solar neighborhood, linking position in the Galaxy with the structure of the stellar distribution.

Nucleosynthesis and chemical composition

• Sandage & Schwarzschild (1952) and the stellar nucleosynthesis theory of Burbidge, Burbidge, Fowler & Hoyle (1957) made it possible to understand how chemical elements form as a function of stellar evolution, and to measure ages from surface changes in stars.
• Chamberlain & Aller (1951): primitive concept of an age-metallicity relation — chemistry indicates a star’s relative age.
• Roman (1950, 1952): points out correlations between stellar kinematics and spectroscopic patterns — the first explicit link between chemistry and kinematics.

Identifying populations I and II

The work of Oort (1926) and Baade (1944) led to the recognition of the main stellar populations, each with distinct kinematics, chemical composition, and color-magnitude diagram. From studying stellar colors in Andromeda, Baade proposed:

Population II — K giants brighter than Pop I ones; no red or blue supergiants; shorter-period RR Lyrae; high velocity relative to the Sun; weak spectral lines; typical of globular clusters and the outer parts of the Galaxy (halo and bulge). Can be found in systems without Pop I.

Population I — typical of open clusters; includes O and B stars; typical of the solar neighborhood; low velocity relative to the Sun; strong spectral lines. Only found in systems that also contain Population II.

Comparing the Milky Way with Andromeda allowed inferring the corresponding structural components:

• Pop I = disk = low-velocity stars
• Pop II = bulge and halo = high-velocity stars

The 1957 Vatican Conference

In 1957, a historic conference of the Pontifical Academy of Sciences and the Vatican Observatory proposed a new division of the Galaxy’s stellar populations — but the scheme, despite consensus at the time, fell out of use by the end of the following decade.

🧩 The first model and the accretion signature

The Monolithic Model [Eggen, Lynden-Bell & Sandage 1962] was the first quantitative attempt to explain the Galaxy’s formation, proposing a fast, relatively uniform collapse of a primordial cloud.

Searle & Zinn (1978) found the first crack in the model: halo clusters show an age spread larger than the free-fall timescale — indicating that independent galactic sub-fragments possibly fell onto the Galaxy after the bulk of the system had already formed. Isobe (1974), Saio & Yoshii (1979), and Mihalas & Binney (1980) had already pointed out selection effects in the Eggen-Lynden-Bell-Sandage (ELS) methodology: the monolithic model is too simplistic to account for populations such as retrograde stars. The halo’s formation must therefore have been slower and more complex than the original proposal.

This is the historical seed of the modern hierarchical-formation paradigm that underlies much of what galactic archaeology investigates today (see Lecture 07 for chemical evolution, and the merger roadmap — Gaia Sausage-Enceladus, Sagittarius, Magellanic Clouds — introduced above).

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📌 Key concepts

• SSP (Simple Stellar Population): a coeval, chemically homogeneous group with similar kinematics — the “building block” of any composite population (bulge, disk, halo).
• Population I / Population II: Baade’s classification by age, metallicity, kinematics, and structural association (disk vs. bulge/halo).
• Monolithic Model → hierarchical formation: Searle & Zinn (1978) was the first observational hint that the halo formed by accretion of fragments, not a single collapse.

🔗 References and related

• Helmi (2020) — Streams, Substructures, and the Early History of the Milky Way, ARA&A 58, 205
• Freeman & Bland-Hawthorn (2002) — The New Galaxy: Signatures of Its Formation
• Conroy (2013) — simple stellar population synthesis (ARA&A 51, 393)
• Baade (1944) — identification of populations I and II
• Eggen, Lynden-Bell & Sandage (1962) — Monolithic Model
• Searle & Zinn (1978) — accretion signature in the halo
• CursoON — overview
• Lecture 02 — HR Diagram & Star Clusters
• Winter School — Galactic Archaeology — sibling minicourse, shorter in scope, on the same general topic (Portuguese only)
• Anomaly Detection in Gaia Data — my research uses exactly the stellar population data introduced here