Stars {blue dwarf} with mass less than 0.25 Sun mass can burn all hydrogen and then become white dwarfs.
Stars {brown dwarf} can have mass 13 to 80 times Jupiter mass or 7% Sun mass. Color is red, because it is ten times hotter than Jupiter. It burns deuterium but not hydrogen. It cannot burn lithium but has methane. Galaxy has 100,000,000,000 brown dwarfs. Objects {sub-brown dwarf} with mass less than 13 times Jupiter mass do not burn but only have heat from gravitational collapse.
In red-giant stars with mass more than 1.5 times, but less than 3 times, Sun mass, helium nuclear fusion to carbon at 10^8 K causes explosion. Stars {Cepheid variable star} {yellow supergiant} have a carbon center, helium layer, hydrogen-fusion layer, and hydrogen layer. Old stars in globular clusters and galactic nuclei are Cepheid variables {type-two Cepheid variable}.
65% of all stars are pairs {double star} or higher multiples. Large star typically has smaller stars circling it. Probably all stars have companion stars, black-dwarf companions, or planets. 15% of stars have black-dwarf companions. Perhaps, galaxies have 3000 x-ray-emitting double stars {x-ray star} with white-dwarf, pulsar, or black-hole companions.
Stars {dwarf star} can have less than 10% Sun mass.
Black holes {gravastar} can contain dark energy, which has negative gravity. Surface is ordinary matter.
Almost black holes {gray hole} can let light out, but then it falls back in.
Short-duration gamma-ray flares can come from stars {magnetar}, as star quakes disrupt magnetic field.
Dwarf stars {massive compact halo object} (MACHO) can be dark matter.
65% of all stars are pairs or higher multiples {multiple star}. Large stars typically have smaller stars circling. Probably all stars have companion stars, black-dwarf companions, or planets. 15% of stars have black-dwarf companions.
Stars {neutron star}| can be mostly neutrons, with density the same as atomic nuclei.
layers
Neutron stars have heavy-particle nucleus, neutron layer, elements up to atomic mass 140, and gas layer.
size
Neutron stars are 10,000-meter diameter and have mass the same as Sun mass.
temperature
Neutron stars have high temperature, because gravitational collapse turns potential energy into random kinetic energy.
spin
Neutron stars spin at 1 Hz to 30 Hz, because any initial rotation increases as diameter decreases.
magnetism
Few rotating neutron stars are magnetic, with magnetic field 10^12 times Earth field.
process
Neutron star starts with mass between 1.4 and 2.5 Sun mass. If mass is more, it becomes black hole rather than neutron star. If mass is less, it becomes white dwarf. After fusion forms iron at center, and nuclear reactions stop, heat and pressure decrease, and gravitational attraction causes a Type-Ib, Type-Ic, or Type-II supernova. Supernova explosions are typically asymmetric and push star through space at up to 1000 km/s. After supernova, gravitation is so great that it overcomes electron degeneracy pressure, and atoms collapse, leaving only neutrons, to keep total charge zero. Neutron stars balance gravity by neutron degeneracy pressure.
bursts
Once every 10 million years in galaxy, neutron stars collide and emit gamma-ray bursts {gamma-ray burst} (GRB). Neutron-star collisions make 95% of nuclei heavier than iron.
In red-giant stars with mass less than 1.25 Sun mass {Chandrasekhar limit, nova}, outer layers expand away by explosions {nova}|. For several months, nova radiates energy 10^6 times Sun energy. At any moment, galaxy has 1000 novas.
White dwarf remains. White dwarfs are 5% to 10% of all stars. White dwarfs fuse all hydrogen into helium, and then fuse all helium into carbon. Helium fusion is hotter than hydrogen fusion, so star is white. White dwarfs keep contracting and exploding. White dwarfs in binary star systems explode off outer layers every 30 to 50 years.
Young stars {population I star}, like Sun, have many metals, so they can have rocky planets.
Old stars {population II star} are in globular clusters, galactic halo, and galactic nucleus and have only hydrogen and helium, so they have no rocky planets.
Neutron stars {pulsar}| can emit radio waves with 1000 times greater intensity than Sun radiation. From them, Earth observers receive dozens of microwave pulses per second. Galaxy has million pulsars.
accretion-powered pulsar
Pulsars {accretion-powered pulsar} can accrete matter from companion stars and have matter-accretion disks that spin almost as fast as pulsar. Disk charge acceleration emits x-rays, not radio waves, because gravitational force is very high.
magnetar
Neutron-star magnetars can have magnetic field 10^10 tesla. Strong magnetic field accelerates charges, emitting x-rays constantly.
rotation-powered pulsar
Most pulsars {rotation-powered pulsar} emit microwave radiation by magnetic-field rotation. Such pulsars have magnetic fields 10^12 times Earth magnetic field and spin dozens of times per second. Pulsar radiation causes rotation-powered pulsars to spin slower as rotational energy is lost.
Rotation-powered pulsars can have companion stars and can accrete matter, typically increasing pulsar spin but weakening magnetic field. These pulsars spin tens or hundreds of times each second.
Rotation-powered pulsars {strong-field pulsar} can have stronger 10^8-tesla magnetic fields and spin once each second.
rotation-powered pulsar: poles
Magnetic poles typically do not align with spin axis. Magnetic field rotates at angle to pulsar rotation, causing electric fields. Electric fields accelerate charges from pulsar surface. Magnetic field aligns accelerated charges along magnetic poles. Accelerated charges have almost light speed and emit synchrotron-radiation microwaves. Synchrotron radiation lowers relativistic mass, to keep charges below light speed.
When charges have almost light speed, special relativity causes light waves not to radiate in all directions but form a beam in motion direction. Synchrotron radiation aligns along magnetic-pole axis. Microwave beams continuously radiate from both poles, in opposite directions. See Figure 1.
Because pulsars spin, magnetic-pole axis rotates. Axis can point toward Earth once each rotation. For most pulsars, magnetic-pole axis never points toward Earth. Because pulsars rotate, magnetic-pole axis can never constantly point at Earth.
Stars {pulsating star}, like RR Lyrae stars, can be average-size stars that can double brightness over hours or years, as they expand and contract up to 30%. Ion and helium layers change depth and cause brightness changes. Pulsating stars are less than 1% of stars.
After white dwarf is mostly carbon, it cools, first to yellow and then to red {red dwarf}. Eventually, it becomes dark.
Main-Sequence stars accumulate helium at center, as nuclear fusion turns hydrogen into helium. In stars with average mass, 30% of stars, when helium becomes more than hydrogen, helium fuses to make carbon. This makes star hotter, so hydrogen nuclear fusion becomes faster. Outer hydrogen layer expands. After 10^9 years, outer layer is thousands of times bigger and is cooler, so star {red giant}| is red.
Massive stars have extreme novas {supernova}|. For several months, star radiation is 10^9 times Sun radiation. Only 1% of stars become supernovas, so one galaxy has one supernova every 50 years. Sumerians saw a supernova (Mul Nun-ki) [-3000] in Vela. A supernova [185] lasted 20 months and was as bright as Moon. Supernova [393]. A supernova [1006] lasted years and was as bright as Moon. A type-2 supernova [1054] formed the Crab nebula. Supernova [1181]. Tycho observed a Type-1a supernova [1572], as bright as Venus. Kepler observed a supernova [1604].
Supernovas make all titanium through iron nuclei, mostly carbon, oxygen, silicon, magnesium, and iron. Supernovas also make five percent of elements heavier than iron.
Very young stars {T-Tauri star} have gas and dust around them.
Starting with hydrogen, stars first make helium, then carbon and oxygen, then heavier nuclei, such as silicon, sulfur, and calcium. Heat dissipates, and star stops fusion. Star still has high temperature and turbulence. Helium rises to surface, and heavier nuclei go to core. White-dwarf stars have no hydrogen and are small. White dwarfs orbiting stars with larger diameters but smaller masses can become supernovas {Type 1a supernova} {supernova 1a}. White dwarfs accrete gas from other star, until gas has enough matter to pressure star core to restart nuclear reactions. From carbon and oxygen, chain reactions produce nickel, iron, and cobalt and, after several seconds, explode star.
Stars with mass 8 to 25 times Sun mass start with hydrogen and first make helium, then carbon and oxygen, then heavier nuclei, such as silicon, magnesium, and iron. Such massive stars can fuse nuclei to make elements up to iron, which requires temperature 10^12 K. Such stars have turbulent hydrogen, helium, carbon, oxygen, silicon, magnesium, and iron layers, from surface to core, respectively. Star diameter is more than four million kilometers. Iron cannot fuse to anything else, so core becomes cooler and has less pressure. At critical pressure, gravitation collapses iron nuclei in one second to make neutron star. Gravitation continues to pull matter inward, and nuclei bounce off neutron star turbulently at supersonic speed, making shock waves. Nuclei stream inward between shock-wave sides. Neutrinos heat shock-wave gas, which keeps expanding. Shock waves explode star {Type 1b supernova} {Type 1c supernova} {Type 2 supernova} {supernova 2}. Asymmetrical explosion pushes neutron star to speeds up to 1000 km/s. Explosion makes high temperature and pressure and can make elements higher than iron. Perhaps, higher-element making involves antineutrinos. Higher-element making uses energy rather than making energy, absorbs heat, and cools core. Expansion weakens, and gravitational collapse takes several minutes.
In red-giant stars with mass less than 1.25 Sun mass {Chandrasekhar limit, white dwarf}, as outer layers expand away by nova explosions, Earth-size star {white dwarf} remains. Electron-degeneracy pressure counterbalances gravity, so atoms do not collapse. White dwarfs are 5% to 10% of all stars.
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Date Modified: 2022.0225