Galaxies {galaxy} evolve and have shapes.
number
Universe has 10^11 galaxies, in clusters of 5 to 10,000. Galaxies average 10^7 light-years apart. Between galaxies is pure hydrogen.
names
Famous galaxies are Milky Way Galaxy with Sun, Andromeda or Messier 23, Whirlpool or Messier 51, Centaurus A, and Sombrero.
age
Oldest galaxy is 12 x 10^9 light-years away.
evolution
Originally, universe had density variations in which all wavelengths superimposed (pink noise). Smaller regions had higher densities. As universe expanded, differences exaggerated, because high-density regions became less dense more slowly. As gravity became stronger than expansion, such regions attracted more matter and dark matter and began to collapse, to make sphere with high density at center and low density at edge.
Infalling matter became hot, as potential energy became kinetic energy, and collisions randomized motions. Matter also radiated and cooled. At hydrostatic equilibrium, gravitation inward balanced pressure outward, as in Earth atmosphere. Matter sphere contracted and increased angular speed, becoming disk.
evolution: waves
In galaxies, stars have elliptical orbits, but gravity among stars makes orbits precess, because galaxies have no large central mass. Orbits can align and make galactic waves. Waves cause higher and lower gas-and-star-concentration regions. Waves transport angular momentum.
evolution: shapes
Galaxy shape depends on rotation rate. Slow rate makes elliptical galaxies, which are 20% of galaxies, are large, and have old stars. Medium rate makes spiral galaxies, which are 50% of galaxies, with old stars in center and new stars in or out of arms, created by gravitational effects. Spiral is tight for slower rotation and open for faster. Fast rotation rate makes barred-spiral galaxies, which are 30% of galaxies.
Interstellar star and gas waves, gravitational contraction, radiative cooling, relaxation to equilibrium, and galaxy interactions cause galaxy shapes. Waves make barred, barred-spiral, and spiral galaxies and change shapes continuously. If star orbits shift, spiral galaxies form. Spiral galaxies have a central black hole, which eventually adds no more matter. If star orbits align, barred galaxies form. If central star orbits align but outer star orbits shift, barred spiral galaxies form. Irregularly shaped galaxies are small, form near regularly shaped galaxies, and form after explosions or collisions.
collisions
Galaxy gravitational pull causes tides in small galaxies that pass by and pulls them apart, so they spiral into larger galaxy. If equal-size galaxies collide, larger elliptical galaxy forms. Elliptical galaxies can have outer disk.
collisions: starburst
Colliding galaxies can make many new stars {starburst} in ten million years.
collisions: stellar stream
In galaxies, stars can move as groups {stellar stream}, because they came from absorbed dwarf galaxies, such as Sagittarius dwarf galaxy and Canis Major dwarf galaxy.
matter
In galaxies, 90% of matter is in stars, with mostly hydrogen and helium gas between stars. More than 50 molecules are in space. Main ones are hydrogen, methane, hydrogen cyanide, water, ammonia, and cyanogen. Space carbon molecules have no rings. Strong infrared radiation makes molecules in dust clouds or ionized hydrogen gas.
matter: gas
Hydrogen spheres, with density three atoms per liter, surround galaxies. Hydrogen masses {intermediate velocity clouds} (IVC) {medium velocity clouds} surround galaxies and move toward galactic plane by galactic-fountain mechanism. Hydrogen masses {high velocity cloud} (HVC), 10% of galactic mass, surround galaxies and move toward galactic plane from intergalactic space.
center
Large galaxies have central supermassive black hole, which sends hot-gas jets out both poles, using inflowing gas from equator. Outflowing hot gas prevents surrounding cluster hot gas from cooling. Cluster gas heats as it gravitationally collapses. Sound waves spread energy.
Physical Sciences>Astronomy>Galaxy
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Date Modified: 2022.0224