Nascent Galaxies

Nascent Galaxies

Within the nascent galaxies, much smaller clouds are also experiencing gravitational collapse; interior temperatures become very high, thermonuclear reactions are thus initiated, and the first stars turn on! The hot, massive young stars evolve rapidly, profligates carelessly spending their capital of hydrogen fuel, soon ending their lives in brilliant supernova explosions, returning thermonuclear ash – helium, carbon, oxygen and heavier elements – to the interstellar gas for subsequent generations of star formation.

Supernova explosions or massive early stars produced successive overlapping shock waves in the adjacent gas, compressing the intergalactic medium and accelerating the generation of clusters of galaxies. Gravity is opportunistic, amplifying even small condensations of matter. Supernova shock waves may have contributed to accretions of matter at every scale. The epic of cosmic evolution had begun, a hierarchy in the condensation of matter from the gas of the Big Bang – clusters of galaxies, galaxies, stars, planets and, eventually, life and an intelligence able to understand a little of the elegant process responsible for its origin.

Clusters of galaxies fill the universe today. Image: Another World © by Elena

Some clusters of galaxies are insignificant, paltry collections of a few dozen galaxies. The affectionately tilted Local Group contains only two large galaxies of any size, both spirals : the Milky Way and M31. Other clusters run to immense hordes of thousands of galaxies in mutual gravitational embrace. There is some hint that the Virgo cluster contains tens of thousands of galaxies.

On the largest scale we inhabit a universe of galaxies, perhaps a hundred billion exquisite examples of cosmic architecture and decay, with order and disorder equally evident: normal spirals, turned at various angles to our earthly line of sight (face-on we see the spiral arms, edge-on, the central lanes of gas and dust and stars running through the center, connecting the spiral arms on opposite sides; stately giant elliptical galaxies containing more than a trillion stars which have grown so large because they have swallowed and merged with other galaxies; a plethora of dwarf ellipticals, the galactic midges, each containing some paltry millions of suns ; an immense variety of mysterious irregulars, indications that in the world of galaxies there are places where something has gone ominously wrong; and galaxies orbiting each other so closely that their edges are bent by the gravity of their companions and in some cases streamers of gas and stars are drawn out gravitationally, a bridge between the galaxies.

(By Carl Sagan, Cosmos)