The Milky Way has already perished once, and we exist in the era of its "second life", the researchers are sure, whose calculations have revealed previously unknown details about the Galaxy.
Stars were formed in different ways during the two eras. There was a long period of calm in between when star formation was interrupted. The history of the Milky Way turned out to be much more dramatic than astronomers initially assumed.
Masafumi Noguchi of Tohoku University has calculated the evolution of the Milky Way over ten billion years, including "cold stream accretion": a new idea, proposed by Avishai Dekel of the Hebrew University of Jerusalem and his colleagues, that during the formation of the galaxy the surrounding gas. Despite the fact that Yuval Birnboim of the Hebrew University proposed a two-stage formation for more massive galaxies, Noguchi was able to prove that this model is applicable to our Galaxy.
The history of the Milky Way is recorded in the composition of stars, as their elements inherit the composition of the gas from which they are formed. They literally remember the elemental content of gases at their birth.
There are two groups of stars located near the Sun, and both of them differ in their composition. One is rich in alpha elements such as oxygen, magnesium and silicon. The other contains a lot of iron. Recent observations by Misha Haywood of the Paris Observatory and his colleagues have shown that this phenomenon is prevalent in a large part of the Milky Way. However, the origin of this dichotomy was unclear. And model Noguchi solves this riddle.
Model prediction of three different regions of the Milky Way. Contours drawn from observations by APOGEE / M. Noguchi, M. Haywood et al., ESO
The description of the development of the Milky Way in the Noguchi model begins from the moment when streams of cold gas rushed into the Galaxy (accretion of a cold stream) and stars were formed from this gas. During the same period, alpha elements appeared in the gas, emitted by explosions of short-lived type II supernovae. This is why there are so many alpha elements in first generation stars.
Shock waves, which appeared seven billion years ago, heated the gas to high temperatures. He stopped falling into the Galaxy, and stars were no longer formed. At the same time, belated explosions of long-lived type Ia supernovae filled the gas with iron and changed its composition. As it emitted, the gas cooled and began to re-enter the Galaxy about five billion years ago, leading to the formation of second-generation iron-rich stars. Our Sun was among them.
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According to Benjamin Williams of the University of Washington, the neighboring galaxy, Andromeda, also formed stars during two separate eras. Noguchi's model predicts that massive spiral galaxies like the Milky Way and Andromeda experienced a starburst rupture, while smaller galaxies continually spawned new stars. Noguchi expects that future observations of nearby galaxies can revolutionize our view of the origin and evolution of galaxies in the universe.
Vladimir Guillen