How did our galaxy come to be?

Posted on Mar 9, 2022      206

According to modern concepts, the Galaxy formed about 14 billion years ago from a primary, slowly rotating gas cloud that was ten times its size. Initially, this cloud (protogalaxy) comprised 75% hydrogen and 25% helium. For about 3 billion years, the proto-cloud has been shrinking freely under the action of gravitational forces.

This collapse inevitably led to the collapse of the cloud into pieces (fragmentation) and the beginning of star formation. At first, there was a lot of gas, and it was at enormous distances from the plane of rotation. First generation stars emerged, and globular clusters.

A Star Is Born when the center of a compressed cloud reaches densities and temperatures sufficient for efficient thermonuclear reactions to take place. In the bowels of massive stars, thermonuclear fusion of chemical elements heavier than helium took place. These elements entered the primary hydrogen-helium environment in stellar explosions or in the quiescent outflow of matter from the stars. Elements heavier than iron were formed in grandiose supernova explosions. Thus, first-generation stars enriched the primary gas with chemical elements heavier than helium. These stars are the oldest; they comprise hydrogen, helium and a tiny admixture of heavy elements.

The part of the gas that did not become stars continued its process of contraction toward the center of the Galaxy. Because of the conservation of angular momentum, its rotation became faster, a disk was formed, and, in it, the process of star formation began again. Stars of the second generation turned out to be rich in heavy elements, since they were formed from the primary gas already enriched in heavy elements.

The remaining gas was compressed into an even thinner layer. This is how the flat component, the stellar disk, which is the main arena of modern star formation, emerged.

When the compression of the protogalaxy ceased, the kinetic energy of the formed disc stars equaled the energy of the collective gravitational interaction. The conditions for the formation of a spiral structure were created, and the birth of stars occurs already in the spiral branches, in which the gas is sufficiently dense. These are the third generation of stars. This includes our Sun.

Protogalaxies colliding in the young Universe a billion years after the Big Bang. NASA illustration.

Scientists imagine the further evolution of the Galaxy as follows.

The reserves of interstellar gas will gradually deplete and star birth will become less intense. In a few billion years, when all the gas reserves are exhausted, the spiral galaxy will become lenticular, comprising faint red stars and white dwarfs - these are super-dense, small-sized stars that represent one of the last stages of stellar evolution.

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