2nd World
The second world is easiest described as the Universe itself, or atleast, the planes in which its built upon, that is to say, every Galaxy, Solar System, Planet, or even Atom that is apart of the Universe is part of the Second World, however it is most commonly referred too as the "Human Realm" or "Overworld", ah how narcissistic Humans can be. Well, I suppose any creature that can think would believe themselves to be the center of the universe. But that is beside the point.
Necessities
"Space-Time" a Universe typically functions under a 3D construct system. In physics, spacetime is any mathematical model that fuses the three dimensions of space and the one dimension of time into a single four-dimensional continuum. It should be noted that different observers perceive where and when events occur differently. Time Flows with Space so that Planets spin, Stars Shine, *almost* motion in the Universe is from Energy mixed with Space-Time.
"Matter" Matter is a substance made up of various types of particles that occupies physical space and has inertia, the most familiar examples of material particles are the electron, the proton and the neutron.
"Mass" is a coherent, typically large body of matter with no definite shape. Its what really takes up space in the Universe.
"Atoms" An atom is a particle of matter that uniquely defines a chemical element. An atom consists of a central nucleus that is surrounded by one or more negatively charged electrons. The nucleus is positively charged and contains one or more relatively heavy particles known as protons and neutrons. Its said they make up everything but I trust them.
"Subatomic" is something smaller than the size of an atom, while not relatively needed, sometimes these can act as entirely seperate layers of what appears to be reality, for example what if every creature above the size of an atom didn't know that there was an entire other universe within the subatomic plane? Another world built similarly or completely different than their own just smaller to them but the same size for us. Us? Are we just subatomic particles under a much larger mechanism?...
"Dark Matter" Unlike normal matter, dark matter does not interact with the electromagnetic force. This means it does not absorb, reflect or emit light. It is very important as if there were only ordinary matter in the universe, there would not have been enough time for density perturbations to grow into the galaxies and clusters currently seen. Dark matter provides a solution to this problem because it is unaffected by radiation. Therefore, its density perturbations can grow first. dark matter is not baryonic at all, but that it is made up of other, more exotic particles like axions or WIMPS.
"Dark Energy" Think of dark energy as the "opposite" to gravity-an "anti-gravity" force providing a negative pressure that fills the universe and stretches the very fabric of spacetime. As it does so dark energy drives cosmic objects apart at an increasingly rapid rate rather than drawing them together as gravity does. Which is also a very important component if you wish your Universe to continue to expand farther than the normal boundaries set for it.
Typically Found within Universes
This is not under necessity because its even more rare for there to not he possibilities of these not existing within the infinite multiverse of 2nd World's. While true other worlds may lack some components under the 'Necessity' tab, these are still secondary or even last upon the Creation of a Universe.
"Galaxy" A galaxy is a system of stars, stellar remnants, interstellar gas, dust, and dark matter bound together by gravity. It usually has hundreds of billions of stars, enough gas and dust to make billions more stars, and at least ten times as much dark matter as all the stars and gas put together. And it's all held together by gravity.
"Nebula" A nebula is a distinct luminescent part of interstellar medium, which can consist of ionized, neutral, or molecular hydrogen and also cosmic dust. Nebulae are often star-forming regions
"Solar System" The Solar System is a system of a Sun and the objects that move around it. The center is typically a star which generates heat and a gravitational pull that keeps the planetary bodies moving around it.
"Star" A star is a luminous spheroid of plasma held together by self-gravity. Usually they are giant balls of hot gas, mostly hydrogen, with some helium and small amounts of other elements. Every star has its own life cycle, ranging from a few million to trillions of years, and its properties change as it ages. Stars are born within the clouds of dust and scattered throughout most galaxies. A familiar example of such as a dust cloud is the Orion Nebula. Turbulence deep within these clouds gives rise to knots with sufficient mass that the gas and dust can begin to collapse under its own gravitational attraction. As the cloud collapses, the material at the center begins to heat up. Known as a protostar, it is this hot core at the heart of the collapsing cloud that will one day become a star. Three-dimensional computer models of star formation predict that the spinning clouds of collapsing gas and dust may break up into two or three blobs. As the cloud collapses, a dense, hot core forms and begins gathering dust and gas. Not all of this material ends up as part of a star the remaining dust can become planets, asteroids, or comets or may remain as dust. Stars are fueled by the nuclear fusion of hydrogen to form helium deep in their interiors. The outflow of energy from the central regions of the star provides the pressure necessary to keep the star from collapsing under its own weight, and the energy by which it shines. In general, the larger a star, the shorter its life, although all but the most massive stars live for billions of years. When a star has fused all the hydrogen in its core, nuclear reactions cease. Deprived of the energy production needed to support it, the core begins to collapse into itself and becomes much hotter. Hydrogen is still available outside the core, so hydrogen fusion continues in a shell surrounding the core. The increasingly hot core also pushes the outer layers of the star outward, causing them to expand and cool, transforming the star into a red giant.
If the star is sufficiently massive, the collapsing core may become hot enough to support more exotic nuclear reactions that consume helium and produce a variety of heavier elements up to iron. However, such reactions offer only a temporary reprieve. Gradually, the star's internal nuclear fires become increasingly unstable - sometimes burning furiously, other times dying down. These variations cause the star to pulsate and throw off its outer layers, enshrouding itself in a cocoon of gas and dust. What happens next depends on the size of the core.
Quick Star Subsection-
Average Stars Become White Dwarfs-
For average stars like the Sun, the process of ejecting its outer layers continues until the stellar core is exposed. This dead, but still ferociously hot stellar cinder is called a White Dwarf. Pressure from fast moving electrons keeps these stars from collapsing. The more massive the core, the denser the white dwarf that is formed. Thus, the smaller a white dwarf is in diameter, the larger it is in mass! White dwarfs are intrinsically very faint because they are so small and, lacking a source of energy production, they fade into oblivion as they gradually cool down. This fate awaits only those stars with a mass up to about 1.4 times the mass of our Sun. Above that mass, electron pressure cannot support the core against further collapse. Such stars suffer a different fate...
White Dwarfs May Become Nova- If a white dwarf forms in a binary or multiple star system, it may experience a more eventful demise as a nova. they are in fact, very old stars - white dwarfs. If a white dwarf is close enough to a companion star, its gravity may drag matter - mostly hydrogen - from the outer layers of that star onto itself, building up its surface layer. When enough hydrogen has accumulated on the surface, a burst of nuclear fusion occurs, causing the white dwarf to brighten substantially and expel the remaining material. Within a few days, the glow subsides and the cycle starts again. Sometimes, particularly massive white dwarfs may accrete so much mass in the manner that they collapse and explode completely, becoming what is known as a supernova.
Supernova Leave Behind Neutron Stars or Black Holes
Main sequence stars over eight solar masses are destined to die in a titanic explosion called a supernova. A supernova is not merely a bigger nova. In a nova, only the star's surface explodes. In a supernova, the star's core collapses and then explodes. In massive stars, a complex series of nuclear reactions leads to the production of iron in the core. Having achieved iron, the star has wrung all the energy it can out of nuclear fusion - fusion reactions that form elements heavier than iron actually consume energy rather than produce it. The star no longer has any way to support its own mass, and the iron core collapses. In just a matter of seconds the core shrinks from roughly 5000 miles across to just a dozen, and the temperature spikes 100 billion degrees or more. The outer layers of the star initially begin to collapse along with the core, but rebound with the enormous release of energy and are thrown violently outward. Supernovae release an almost unimaginable amount of energy. For a period of days to weeks, a supernova may outshine an entire galaxy. Likewise, all the naturally occurring elements and a rich array of subatomic particles are produced in these explosions
Neutron Stars-
If the collapsing stellar core at the center of a supernova contains between about 1.4 and 3 solar masses, the collapse continues until electrons and protons combine to form neutrons, producing a neutron star. Neutron stars are incredibly dense - similar to the density of an atomic nucleus. Because it contains so much mass packed into such a small volume, the gravitation at the surface of a neutron star is immense. Like the White Dwarf stars above, if a neutron star forms in a multiple star system it can accrete gas by stripping it off any nearby companions. Neutron stars also have powerful magnetic fields which can accelerate atomic particles around its magnetic poles producing powerful beams of radiation. Those beams sweep around like massive searchlight beams as the star rotates. If it was to be observed it would be seen as regular pulses of radiation that occur whenever the magnetic pole sweeps past the line of sight. In this case, the neutron star is known as a pulsar.
Black Hole- If the collapsed stellar core is larger than three solar masses, it collapses completely to form a black hole: an infinitely dense object whose gravity is so strong that nothing can escape its immediate proximity, not even light. Since photons are what our instruments are designed to see, black holes can only be detected indirectly. Indirect observations are possible because the gravitational field of a black hole is so powerful that any nearby material - often the outer layers of a companion star - is caught up and dragged in. As matter spirals into a black hole, it forms a disk that is heated to enormous temperatures, emitting copious quantities of X-rays and Gamma-rays that indicate the presence of the underlying hidden companion.
From the Remains, New Stars Arise-
The dust and debris left behind by nova and supernova eventually blend with the surrounding interstellar gas and dust, enriching it with the heavy elements and chemical compounds produced during stellar death. Eventually, those materials are recycled, providing the building blocks for a new generation of stars and accompanying planetary systems.
"Planets" A planet is a large, rounded astronomical body that is neither a star nor its remnant. An interstellar cloud collapses out of a nebula to create a young protostar orbited by a protoplanetary disk. They typically have sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium. Heat on planets, shapes, sizes, materials its made out of, consciousness or lack there of, species it may hold, structural characteristics, speed it rotates at, place in a solar system if it has one, and other factors are dependent on planet to planet.
"Moons" are naturally formed bodies that orbit planets are called moons, or planetary satellites. Most planetary moons probably formed from the discs of gas and dust circulating around planets in the early solar system, though some are captured objects that formed elsewhere and fell into orbit around larger worlds.
"Asteroid" Asteroids, sometimes called minor planets, are rocky remnants left over from the early formation of solar systems. Most asteroids are irregularly shaped, though a few are nearly spherical, and they are often pitted or cratered. As they revolve around a star in elliptical orbits, the asteroids also rotate, sometimes quite erratically, tumbling as they go. Many asteroids are known to have a small companion moon or moons as some have two moons. There are also binary asteroids, in which two rocky bodies of roughly equal size orbit each other, as well as triple asteroid systems. They can also simply be seen just as rocks floating in space.
Other Planes
Depending on the setting, other various planes may exist within the Universe, whether this be formless shapeless energies fueling magic most commonly known as "Mana" or more mental forces at work where consciousness, dreams, and more resource from known as the "Astral Plane" or The Mental Plane.
Many times these planes may have many to little or even no rules at all! In some worlds Mana may only exist in living creatures. in others, Mana may exist in everything even if of varying quantities.
There are infinite numbers of extra planes that may be added on or branched off from these 2 planes, for example, in a world where certain people can control things with their mind despite the energy being everywhere, this may be a branch off from the Astral Plane in which's domain is the mind being brought into the "reality" of the 2nd World.