The Science of Existence (14-6) Galaxy Dynamics

Galaxy Dynamics

Galaxy formation history may be telling us something about the places in the universe where life can form. ~ Swedish astrophysicist Kambiz Fathi

At different scales, accretion disks of dust and gas are the cradles of galaxies, stars, and planets. Accretion disks are common because the coalescing force of gravitation is offset by angular momentum, forming a disk.

The growth of galaxies in the early universe was vigorous. Within just 3 billion years of galactic formation, there were already thousands of galaxy clusters.

The nascent universe was much smaller. Star density was 10 times greater than today. Each galaxy cluster contained hundreds of thousands of galaxies. Some were massive galaxies, with several hundred billion stars, formed by collisions of smaller galaxies.

Most of the earliest galaxies were elliptical, having many stars, but insufficient dust and gas to fuel organic expansion. The most massive galaxies are giant ellipticals.

The mass of a galaxy directly relates to the mass of its central black hole. Mass determines how fast a galaxy spins. Spin slows as a galaxy grows.

Regardless of whether a galaxy is very big or very small, if you could sit on the extreme edge of its disk as it spins, it would take you about a billion years to go all the way round. ~ American astrophysicist Gerhardt Meurer

Just 3 billion years after galaxies got going there were already spent elliptical galaxies: no longer forming new stars. In contrast, spiral galaxies, like the Milky Way, contain much material for star formation.

Young galaxies furiously create stars. The more gas a galaxy has, the more sparkling stars are ignited. A galaxy’s magnetic field nudges huge clouds of gas and dust into pregnant concentrations that give birth to stars.

Through self-excitation, a magnetic field is created from virtually nothing, whereby the complex movement of the conductive plasma serves as an energy source. ~ German physicist Frank Stefani

Massive cosmic magnetic fields pervade the universe and persist for billions of years. Small-scale fluctuations of astrophysical plasma create large-scale, persistent magnetic fields which shape the material dynamics of galaxies. Like rivers of energy, plasmas flow in a certain direction. There are also plasmatic counter-streams.

Like water, plasmas have abiding internal structures which have been observed during star formation and star death. Supernovas are tremendous plasma producers.

Plasmas also pulse on a galactic scale. The coherent self-organizing of plasma among seeming chaos produces the energetic seeds from which galaxies and star systems are born.

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A massive black hole forms and builds a galaxy from its quasar emissions. The mass of a black hole in a galaxy’s center typically ranges between a million and a billion times that of the Sun.

Galactic formation dynamics act as a thermal gyre: pulling in dense, cold gas, and ejecting hot gas back into intergalactic space. A galaxy ends up with a fraction of the raw material it processes.

Galaxies grow from the inside out. Most galaxies have a bulge at their center, as does the Milky Way.

Black hole growth and star formation typically go together. If the nearby environment of a black hole is gas poor, gas accretion is slow. Radiation emission is correspondingly low.

Black holes at the heart of a galaxy not only spin, they also move across their host galaxy, altering galactic dynamics. The speed at which a black hole spins distinctively affects the spacetime around it: another factor in the gyre of a black hole.

Galactic gyres follow fluid dynamics, with viscosity something of a mystery. The level and nature of turbulence determine what stays and what flies away. In a galactic butterfly effect, small disturbances can affect stabilities and mass transfers at a much larger scale.

The structures and sizes of galaxies vary. Galaxy range from dwarfs of 10 million (107) stars to giants with a hundred trillion (1014) stellar lights.

Galaxies typically spread from 1,000 to 100,000 parsecs in diameter, separated by millions of parsecs (megaparsecs) of intergalactic space. The space between galaxies is a tenuous gas, with less than 1 atom per cubic meter.

A parsec is an astronomical length unit: about 3.26 light-years, just under 31 trillion (3.1 x 1013) kilometers (km). A light-year is ~9.461 trillion km: how far light can travel in a vacuum in 1 Julian year (365.25 days).

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Big galaxies are crashing into other big galaxies to make even bigger galaxies. ~ American astrophysicist Adam Bolton

Galaxies are attracted to each other under the influence of their gravity. Galaxies may collide, merge, or pass through each other. Large galaxies grow by absorbing smaller ones.

Even with no major collisions, the interstellar medium of gas and dust interact, triggering bursts of star formation. Collisions gas up galaxies, further triggering star birth bursts.

Stellar collisions can severely distort the galaxies involved, forming oddly shaped galactic artifacts, such as tail-like structures. Stellar orbits about a galaxy can be thrown off course.

Relatively passive pass-throughs between galaxies can leave lasting connections. Tendrils of cold hydrogen gas can be pulled from one galaxy toward another, creating a tenuous bridge between the two.