Galaxies are considered alive or dead depending on the rate at which they form stars. Young galaxies are bright and produce stars very quickly but the older, more massive ones tend to fall behind, often being described as red and dead. Sometimes, galaxies can “come back from the dead” if they steal enough material. It appears that the Milky Way is one of these born-again galaxies.
The study was conducted by Masafumi Noguchi from Japan's Tohoku University and published in the journal Nature. It focused on the chemical composition of stars in our galaxy. According to Noguchi's calculations, the stars around the Sun formed in two generations, separated by 5 billion years. No mechanism could create such a local discrepancy without having far-reaching effects.
The model used by Noguchi suggests that stars first formed when cold intergalactic gas began flowing into the primordial Milky Way. This generation of stars ends up as spectacular supernovae, which has two consequences. The first is that the supernovae produce heavy elements, spreading them far and wide. This enriches the interstellar gas and changes its chemical composition.
The other consequence is heat. Supernovae create powerful shockwaves that raise the temperature of the gas from which stars form. It seems counterintuitive but despite being very hot, stars need to form from cold gas. If the gas is not cold, it cannot condense enough for gravity to make it collapse and form a star.
Noguchi’s model highlights how different types of supernovae release different chemical elements in different quantities. The ones from short-lived stars, known as Type II supernovae, are rich in alpha-elements (such as oxygen, magnesium, and silicon) and happen quickly. This explains why the older generation is indeed richer in these elements. Then the star-formation hiatus happened. Eventually, the more long-lived supernovae, called Type I, exploded and released iron. The second generation of stars is richer in iron. Our Sun is among these stars.
The idea of two distinct periods of intense star formation has already been proposed to explain features of more massive galaxies, so it is interesting to see it applied to an average-sized one like the Milky Way. Some researchers think that Andromeda, our galaxy’s equal-sized neighbor, also had a similar formation. Future observations might clarify if this is the case and maybe change what we know about how most galaxies evolve.