Colliding galaxies add matter and make a central supermassive black hole {active galactic nucleus} (AGN), whose infalling matter makes light, forms accretion disk, and/or sends out plasma jets. Quasars are active galactic nuclei. AGN are less than one light-year diameter, down to light-minutes.
Large and small regions {Bok globule} have cool interstellar gas and dust.
Hot gas {galactic corona} surrounds galaxies.
Giant stars heat and ionize interstellar gas, which rises out of galactic plane, cools, and falls in again {galactic fountain}.
Galaxies have 10^10 stars {galactic halo} outside disk.
Galaxies have central spherical star concentrations {galactic nucleus}|, 20,000 light-years diameter, with many exploding stars and massive black holes.
When galaxies collide, groups {globular cluster}| of millions of same-age stars form. Milky Way has 200 groups of 10^6 stars each, with diameter 100 light-years.
Particles {interstellar dust}| are between stars. Interstellar dust is 1% of interstellar matter, which is 99% hydrogen and helium. 10% is 0.005-nm carbon and hydrogen particles. 80% is 0.35-nm mostly iron, carbon, and silicate particles with organic mantles. Mantles can be simple organic molecules, like formaldehyde, or complex organics {polycyclic aromatic hydrocarbons} (PAH), caused by different ultraviolet-radiation levels. 10% is 0.002-nm polycyclic aromatic hydrocarbons.
source
Most interstellar dust came from red-giant nova atmospheres.
gas
In dense clouds, which are colder, water and ammonia stick to dust. Otherwise, radiation evaporates gas from surfaces.
comets
Comets are dust grains stuck together.
Gas {interstellar gas}| can be 88% hydrogen and 12% helium, because most helium is still in stars. Formaldehyde absorption, at wavelength 6 centimeter, shows dense hydrogen locations.
Gas flows {Magellanic Stream} around galaxy in Magellanic-Cloud orbits.
Gaseous nebulae {nebula}| include Coal Sack, Crab Nebula, Great Nebula in Orion, Horsehead, Lagoon Nebula in Sagittarius, Monoceros, Nebula in Serpens, North America, Rosette Nebula, Trifid, and Veil Nebula or Cygnus Loop.
Galaxy-like objects {quasar}| can emit 10,000 times normal-galaxy energy in infrared or radio waves, have diameter 1000 times solar-system diameter, last only 10^9 years, and number 14,000,000.
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Date Modified: 2022.0225