It Turns Out We're Living in a 'Zombie' Galaxy

Our galaxy has "died" once before and we are now in its second life, according to a hypothesis put forward by a Japanese researcher.

In much of the Milky Way, the stars can be divided into two groups. Stars in the first group are rich in so-called "alpha" elements such as oxygen, magnesium and silicon, while those in the other are abundant in iron.

The reasons for this difference have long remained a mystery, but now, astronomer Masafumi Noguchi from Tohoku University, in Sendai, Japan, has provided a model that may explain what's going on.

Noguchi calculated the evolution of the galaxy over a 10-billion-year period based on a theory called "cold flow accretion." This theory was originally intended to describe much bigger galaxies than our own, but now Noguchi has shown that it also applies to the Milky Way.

According to the model, outlined in a study published in the journal Nature, the two groups of stars correspond to two periods of star formation, separated by a long dormant period in between when the process came to a halt.

He was able to demonstrate this because the chemical composition of stars is dependent on the gases from which they are formed.

In the initial stage of the model, cold gas streams (cold flow accretion) flowed into the galaxy from outside and the first stars formed from this gas. After a few million years, some of these short-lived stars exploded as supernovas, producing large amounts of the alpha elements, which made their way into the gas and subsequently became integrated into other stars.

But about 7 billion years ago, shock waves appeared and heated the gas to high temperatures, stopping its flow into the galaxy and thus halting the process of star formation.

During this 2-billion year "dormant" period, further supernova explosions of long-lived stars injected iron into the gas, thus changing its composition. Then about 5 billion years ago, the gas cooled sufficiently to start forming a second generation of stars—like our sun—this time with much higher proportions of iron.

Previous research has indicated that our neighbor galaxy Andromeda also produced stars in two different epochs with a dormant period in between, which would be consistent with Noguchi's latest model.

The results suggest that it might be normal for massive spiral galaxies, like Andromeda and the Milky Way, to experience a gap in star formation, while smaller galaxies produce stars continuously.

"Future observations of nearby galaxies may revolutionize our view about galaxy formation," Noguchi said in a statement.