Here's something strange: The Milky Way is made up of two types of stars, and scientists haven't been sure why that is. One type is made up of what are known as α ("alpha") elements, such as oxygen, magnesium, and silicon. The other type is rich in iron. That suggests that they formed in different ways — but how?

To answer that, Noguchi relied on a concept proposed in 2006 called cold-flow accretion, which describes how stars form in galaxies via the influx of cold streams of gas. Noguchi came up with a model of the Milky Way's evolution over a 10-billion-year period, starting with a period where cold gas streams flowed into the baby galaxy and made it possible for new stars to form. Over time, these stars began to explode in type II supernovae — the kind of death that happens when a star runs out of fuel and collapses under its own gravity. Those type II supernovae gave off lots of α-elements, thereby giving rise to even more stars rich in α-elements.

But around 7 billion years ago, those cold gas flows collided with other clouds of gas and produced shock waves that heated the whole shebang to a much higher temperature. At that point, gas stopped flowing into the galaxy and stars stopped forming — but they kept on dying. This time, more mature stars exploded in type Ia supernovae. These supernovae only happen when a star is in a tight binary orbit with a white dwarf, which pulls gas from its larger companion until it's compressed to such a degree that it triggers a runaway nuclear reaction. This cataclysmic explosion produces — you guessed it — iron, which is jettisoned into the gas of the galaxy.

Finally, around 5 billion years ago, the gas in the Milky Way emitted enough radiation to start cooling down again. Cold gas began flowing back into the galaxy again, and new stars began forming once more. But this time, they used the surrounding iron in their formation. (Shout-out to our sun! It formed during this period.) That explains why some stars in our galaxy are rich in iron and some aren't.

So there you have it: There was a 2-billion-year period where the Milky Way completely stopped making stars, and we're currently on its comeback tour. Even cooler, this appears to be true of a whole lot of galaxies: Andromeda, our closest neighbor, also formed stars in two separate periods, and the same is true of other galaxies that are much more massive than ours. But this is the first time we've confirmed that a galaxy of our size formed this way. Noguchi's model doesn't extend to all galaxies — smaller galaxies appear to make stars continuously. But with this information under our belts, we can learn even more from watching other galaxies. "... future observations of nearby galaxies may revolutionize our view about galaxy formation," he says.

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