How Galaxies Are Formed: Gravity is the Key Force, from Protogalaxies to the Hubble Classification

How Galaxies Are Formed: Gravity is the Key Force, from Protogalaxies to the Hubble Classification
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Formation of the First Galaxies

The origin of the first galaxies can be traced back to just 300,000 years after the Big Bang, during a period in the evolution of the Universe called the “Dark Ages”. At this time, the Universe was filled with atoms of hydrogen and helium, and was essentially homogeneous; that is, conditions were the same everywhere and there were no large structures in existence.

At a very small scale, however, the Universe was not entirely formless. Microscopic variations in the density of the hydrogen and helium atoms (known by cosmologists as anisotropic irregularities) led to regions of greater-than-average gravitational attraction. Within these regions, matter began to accrete, leading to greater densities still and further increases in gravity.

In the currently favored model of galaxy formation, called the “bottom-up” or “hierarchical” scenario, these accretions of matter were the “seeds” of the progenitors of galaxies, or protogalaxies. As the protogalaxies increased in size and density, short-lived Population III stars were born, then rapidly extinguished as supernovae. The resulting release of heavy elements led to bursts of star formation, producing metal-rich Population II stars. Over the next few billion years, continuing enrichment of the intergalactic medium with heavy elements lead to the appearance of the first planets.

How Hubble Class Galaxies are Formed

Almost all observed galaxies can be placed into one of the four basic Hubble classes – lenticular, spiral, elliptical or irregular. It was formerly thought that galaxies evolved through the Hubble classes, from elliptical to spiral. Now, it is believed that a galaxy’s class depends on the environment in which it formed, and the mergers and interactions with other galaxies that it has undergone during its life.

Lenticular Galaxy NGC 5866

According to the hierarchical scenario, the dense central bulge of the disk galaxies (lenticulars and spirals) is the consequence of the merger of clusters of stars under their mutual gravitational attraction. Accretion of further gas and dust produces the thin rotating disk surrounding the bulge.

The origin of the arms of spiral galaxies is not well understood. Mathematical models suggest that, in a smooth rotating disk of matter, any small variations in density can become exaggerated and self-propagating. The result, known as a “spiral density wave”, sweeps around the disk indefinitely, compressing the matter ahead of it as it progresses. Applied to a disk galaxy, this model produces spiral arms containing many stars, with fewer stars in the spaces between the arms – as seen in astronomers’ observations.

Two Spiral Galaxies Merging (NGC 6050 and IC 1179, Arp 272)

Two Galaxies Distorted by an Ongoing Interaction (ESO 69-6, ESO 069-IG006, AM 1633-682)

Elliptical galaxies are thought to be produced by mergers between galaxies. Theories indicate that mergers always produce an elliptical galaxy, even when both of the merging galaxies are disks. The stars in a disk galaxy all orbit the galactic centre in the same direction, but during a merger these orbits become disrupted by gravitational forces. The result is a galaxy full of stars orbiting their common center of gravity in random directions; that is, an elliptical galaxy. Through repeated mergers, elliptical galaxies can become extremely large. M87 in the Virgo Cluster is thought to contain 200 times the mass of the Milky Way (a typical spiral galaxy) and extends up to 500,000 light-years across, compared with the Milky Way’s 100,000 light-years.

Irregular galaxies are thought to arise through interactions between other types of galaxy. Such encounters are not close enough to result in a merger, but the gravitational forces involved can cause massive distortions of the galaxies’ shape, sometimes eliminating all traces of their original Hubble class.

Are Galaxies Still Forming Today?

It was once thought that galaxy formation had slowed throughout the observable Universe and that few new galaxies were being created. In 2007, however, NASA’s Galaxy Evolution Explorer detected over thirty young galaxies aged only 100 million to one billion years (the Milky Way is 10 billion years old) within two to four billion light-years from Earth. This discovery suggests that, in parts of the Universe at least, galaxies continue to be born, providing cosmologists with new opportunities to study how galaxies are formed.

Image Credits

Anisotropy: NASA,

Lenticular Galaxy: NASA/ESA,

Merging Galaxies: NASA/ESA,

Interacting Galaxies: NASA/ESA,