What is galactic archaeology? Explain how the age, chemistry and kinematics of stars help reconstruct the merger history of the Milky Way.

Galactic archaeology works out how a galaxy was built by reading the fossil record held in its stars: their age, chemistry and motion. It has now traced the Milky Way's history back about 12 billion years [1]. Its main finding is that our galaxy grew hierarchically, by absorbing smaller galaxies.

Galactic archaeology: the core idea

• Dwarf galaxies that fall in are torn apart by the Milky Way's tidal forces. They leave stellar streams and clusters behind, the "ghost stars" of the galaxies that were absorbed [1].
• This is still happening: the Large and Small Magellanic Clouds are being absorbed today [1].

Kinematics: which stars are foreign?

• Stars that came from other galaxies do not move in lockstep with stars born in the Milky Way [1].
• Precise star positions and motions from the Gaia satellite showed a group of stars on stretched-out, retrograde orbits. These are debris from Gaia-Enceladus (Gaia-Sausage) [2].
• Limitation: motion identifies a group but does not date it, and orbits mix together over billions of years.

Chemistry: which stars were born together?

• Stars carry the chemical fingerprint of the gas they formed from. This lets astronomers group stars that were born together and estimate their rough ages [1].
• This "chemical tagging" links widely scattered stars back to one parent galaxy.

Age: when did each merger happen?

• Ages put the GSE merger at about 10 billion years ago. It built much of the inner halo and helped thicken the disc [2].
• When globular cluster ages are compared with their metal content, the clusters form separate groups. One group is older than the GSE clusters but younger than the native ones, which points to an even earlier merger [3].

Kinematics → identify foreign debris
Chemistry  → link debris to one parent galaxy
Age        → date the merger
        ⇓
  Dated merger timeline

Fig: Three stellar properties used together

Why the three are used together

• No single property is enough. When independent methods agree, confidence rises. GSE debris dominates the inner halo, but cluster ages show an earlier event as well, so the halo was not built by a single merger [2][3].

Together, motion, chemistry and age turn scattered stars into a dated record of the galaxy's collisions. India can deepen its role by combining AstroSat's UVIT ultraviolet images of the Magellanic Clouds [5], taken by India's first multi-wavelength space observatory [4], with open Gaia data. That would build the scientific temper of Article 51A(h) through frontier research.