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.
Emission at wavelength 21 centimeters {21-cm line} {21-centimeter line} shows hydrogen gas. Neutral hydrogen atoms have electron spins that can align or anti-align. Aligned spins have higher energy than anti-aligned spins.
At intermediate radius {corotation circle}, stars orbits precess at same rate.
Galaxy-recession velocity is galaxy distance times universe expansion rate {Hubble constant}| {Hubble's law}.
Galaxy matter contracts to make pressure. At equilibrium {hydrostatic equilibrium}, gravitation inward balances pressure outward.
At larger and smaller radius, star orbits synchronize precession {Lindblad resonance} (Bertil Lindblad), so precession and star always meet at same orbit place, as planetary rings do.
Sun is in a galaxy {Milky Way galaxy}.
properties
Diameter is 100,000 light-years, 10^29 meters. Thickness is 3000 light-years. Mass is 10^44 grams. Speed is 60 km/s toward Hydra constellation, where mass of 10^49 grams at 2.5 x 10^8 light-years causes this motion.
properties: shape
Barred-spiral disk has two large arms, Sagittarius and Perseus.
properties: age
Age is 12,000,000,000 years.
stars
Galaxy has 10^11 stars and 10^8 stars with planets. Inner stars revolve faster. Dozens of absorbed satellite galaxies cause galaxy star layers.
stars: cluster
300 groups {open cluster}, of 1000 stars each, with diameter 10 light-years, have many young, blue-white stars.
neighbors
Near galaxy are two irregularly shaped galaxies of 10^9 stars each, Large Magellanic Cloud and Small Magellanic Cloud. Eleven satellite galaxies include Sagittarius dwarf galaxy, opposite Sun on other galaxy side.
Originally, universe had density variations in which all wavelengths superimposed {pink noise}.
Dark matter never collides, so particles only exchange potential and kinetic energy with gravitational fields. Fields change as particles move and eventually settle into equilibrium {virial equilibrium}, so dark matter forms sphere.
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.
Small galaxies {dwarf galaxy} with irregular shapes contain primordial matter, often atomic hydrogen. Dwarf galaxies do not form stars unless they undergo collision and have starbursts. After many collisions and starbursts, they become globular clusters.
Galaxies have groups {galactic cluster}|. Cluster has diameter 10 million light-years, has thousand galaxies, and has even dark-matter and hot-gas distribution. Hot gas radiates x-rays. In early universe, radiation energy was high compared to gravity, so mass fluctuations were small. Only later did mass differences increase with time. Over distances of 1,000,000,000 light-years, galaxy density varies by 1/10. At larger volumes, grouping decreases, so universe is homogeneous. Early galaxies clustered more than dark matter.
For x-ray emitting galactic clusters, temperature varies directly with mass.
Closest spiral galaxy is Andromeda galaxy or Messier 31, 2 x 10^6 light-years away and twice as large as Milky Way. Milky Way Galaxy and Andromeda Galaxy are in Local Group {galaxy group}, which contains 40 dwarf galaxies. Local Group is near edge of Virgo Cluster, which contains hundreds of galaxies. The Great Attractor is another large galaxy group, 200,000,000 light years diameter.
Colliding galaxies {radio galaxy}| can emit three times more radio waves than light waves.
Regular-size spiral galaxies {Seyfert galaxy} can emit 100 times more infrared radiation than normal galaxies, from small central nuclei filled with fast moving ions. They last only 10^8 years.
Galactic clusters form groups {supercluster}, with diameter 30,000,000 light-years. Superclusters form groups {wall}, with diameter 300,000,000 light-years. Walls do not appear to have groups. Great Wall is 200,000,000 light years away.
Luminous infrared galaxies {ultraluminous infrared galaxy} (ULIRG) were abundant among early galaxies.
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Date Modified: 2022.0225