Astronomers have identified the Milky Way’s first two building blocks

Help comes from fundamental physics. When galaxies collide and their stellar populations mix, most stars retain the most basic properties directly related to the speed and direction of motion of the galaxy in which they originated.
“Stars from the same galaxy before the merger have similar values in both their energy and what physicists call angular momentum – the momentum associated with orbital motion or rotation. For stars moving in the galaxy’s gravitational field, both energy and angular momentum are conserved: they remain constant over time. Look for large groups of stars with similar, unusual values of energy and angular momentum – and you are likely to find remnants of a merger,” the study authors noted.