Worldbuilding Guide Part 2: Galaxies and Nebulae

Link to part 1
Previously, we created a bunch of different types of stars. In Worldbuilding Guide Part 2: Galaxies and Nebulae we will create the objects which surround the stars, and make them not float in empty space. In this part we will be creating and discussing Galaxies, Nebulae, and Star Clusters.
== Galaxies ==
Galaxies will be the largest contiguous objects in pretty much any worldbuilding project, and hugely vary in size, ranging from 1,000 to 100 trillion stars. They also hugely vary in appearance, with some being featureless yellow-white globes, and others having beautiful spiral structures. 
=== What is a galaxy? ===
To begin, we will ask the question, what is a galaxy? In short, a galaxy is a huge system of stars, gas, dust, and dark matter, orbiting a common center of mass (usually a black hole), bound together by gravity. Galaxies come in many different shapes and sizes. The three main types of galaxies are Elliptical, Spiral, and Irregular. However, many other types exist, such as Dwarf Galaxies, Peculiar Galaxies, and Ultra-Diffuse Galaxies. 
==== Dark Matter ====
Back when astronomers first started weighing galaxies, they noticed that there was nowhere near enough visible matter to hold the galaxy together. They had no explanation for this until a dude named Fritz Zwicky came along and proposed Dark Matter, a strange substance which doesn't interact with any of the fundamental forces except for Gravity and maybe the Weak Nuclear Force.
Dark Matter isn't something you really ever have to mention that much. But if you want to note down how much of it your galaxy has, a general rule is that most galaxies are ~85% dark matter, but you can vary this about 10% either way if you wish.
==== Distribution ====
Galaxies are not distributed randomly. They tend to form galactic clusters or groups. Galaxies in these groups are gravitationally bound and tend to influence one another. These groups also tend to gravitationally attract each other, forming massive structures referred to as Galactic Superclusters. Collections of superclusters can form even larger structures referred to as Filaments, which are visible at the scale of the entire observable universe.
Because of the distribution of galaxies, galaxies can collide relatively often. When two or more galaxies collide, they tend to trigger rapid star formation, one of the mechanisms by which a so-called Starburst Galaxy can form.
=== Spiral Galaxies ===
A spiral galaxy is the most common large galaxy type in the modern universe. A spiral galaxy is defined by its large density waves, causing large amounts of star formation and making the areas around the waves appear blue. Spiral galaxies are usually considered more habitable galaxies than other types, but this is debatable. 
==== Structure ====
Spiral galaxies consist of a central bulge, a disk, large spiral arms (overdensities within the disk), and a sparse halo extending to intergalactic space. The bulge is generally considered not very habitable because of the higher densities, but this is again debatable. Similarly, the densest portions of the spiral arms are not considered very habitable because of the higher densities and much higher supernova rates. 
==== Barred Spiral Galaxies ====
The most common subtype of spiral galaxies is the barred spiral galaxy. These are very similar to spiral galaxies, the difference being that the bulge is elongated into a long bar, with the spiral arms emanating from the ends. That's basically the only difference.
==== Lenticular Galaxies ====
Another common subtype of a spiral galaxy is a lenticular galaxy. This is a galaxy which is mostly starved of gas and dust. Lenticulars lack the prominent spiral arms and blue hue of normal spiral galaxies, and most planets in them are relatively old.
==== Math ====
Spiral Galaxies are often considered to have a "Galactic Habitable Zone", a region in which life is most likely to develop. This is usually at 0.47-0.6 times the radius in light-years. 
The number of stars can be approximated by calculating the volume of a cylinder with the radius of the galaxy and a height of 1/100 the radius in light-years and multiplying by 0.004 for the disk. Then, add the volume of the bulge, which can be approximated as an ellipsoid and multiply by 0.0346. Once you add these together with your calculator you'll have the full number of stars in your galaxy. 
=== Elliptical Galaxies ===
Elliptical Galaxies are spherical or elliptical in shape. They lack the gasses required to form new stars, so they are almost entirely composed of old stars (Stellar Population|Population II), giving them a reddish/yellowish color in contrast to the blue of spiral galaxies. Large elliptical galaxies usually have extensive systems of globular clusters surrounding them. Elliptical galaxies are thought to be the result of galactic collisions. 15% of galaxies are elliptical.
The smaller cousin of the Elliptical galaxy is the Dwarf Elliptical Galaxy: something in between an elliptical galaxy and a globular cluster. These dwarfs are often found as a satellite galaxy to a bigger galaxy.
The stars within the galaxy don’t orbit on a single plane, as in disk-shaped galaxies, but rather orbit randomly at varying inclinations. Profiles such as the Sersic Profile and Einasto profile can mathematically calculate the intensity and density of the galactic bulge, but these are rather complex and require you to work out things such as the scale height and central intensity of your elliptical galaxy, which is overkill and not very relevant overall. All that you really need to know is that the stellar density of the galaxy increases as you get closer to the galactic bulge.
==== Classification ====
The Hubble Classification System denotes ellipticals with the letter E, and dwarf ellipticals with dE. A number from 0 to 7 follows this, and describes the shape of the elliptical galaxy, with 0 describing a spherical galaxy and 7 describing a cigar-shaped galaxy. If you know the semi-major and semi-minor axes of the elliptical, you can calculate its "E number" as En where:
n = 10 • (1-b/a)
where a is the semi-major axis and b is the semi-minor axis of the ellipse.
About 1/10 of ellipticals have shell structures, where stars in the galaxy’s halos are arranged in concentric shells. This is a property exhibited only by ellipticals.
====Math====
The star count of elliptical galaxies can be approximated by finding the volume of the galaxy in cubic light-years (they're ellipsoids) and multiplying by 0.006 or something near that. 
===Irregular Galaxies===
Irregular Galaxies are the most common of galaxies, and are mostly found in the form of dwarf satellites. Irregular galaxies are generally highly active galaxies, with high rates of star formation. Irregular galaxies, being shapeless blobs, are rather difficult to classify aside from this. Irregular galaxies are very rare to find larger than dwarf size.
====Subtypes====
Irregular galaxies have many subtypes. The most common of them is the Magellanic Spiral, which is a type of irregular galaxy which used to be a spiral. The most well-known real-world example of such a galaxy is the Large Magellanic Cloud, which has a single stubby spiral arm extending out from it. 
===Peculiar Galaxies===
Peculiar Galaxies are essentially a drop box of all the random galaxy types which do not fit in the three main categories. As such, no general principles can be said about them.
====Ultra-Diffuse Galaxies====
Ultra-Diffuse galaxies are galaxies which have stars spread out in an extremely sparse fashion. These galaxies are extremely difficult to see, and aren't very useful in worldbuilding projects.
====Interacting Galaxies====
Interacting Galaxies are pairs (or larger numbers) of galaxies close enough to gravitationally affect each other. These galaxies can be practically any class except ultra-diffuse. Interacting galaxies generally are going through a starburst phase, with huge amounts of star formation.
====Ring Galaxies====
Ring Galaxies are very interesting. Instead of a disk, they have a large ring, separated from the core by a transparent gap with few stars in it. An example of this galaxy type is Hoag's Object. 
==Nebulae==
A nebula is a large structure in the interstellar medium composed of interstellar gas. There are many types, and can serve as a very interesting backdrop for worldbuilding. They come in several types.
===Planetary Nebulae===
Planetary nebulae are the remnants of the final stages of stellar evolution for mid-mass stars(varying in size between 0.5-~8 solar masses). Evolved asymptotic giant branch stars expel their outer layers outwards due to strong stellar winds, thus forming gaseous shells, while leaving behind the star's core in the form of a white dwarf. Radiation from the hot white dwarf excites the expelled gases, producing emission nebulae with spectra similar to those of emission nebulae found in star formation regions. They are H II regions, because mostly hydrogen is ionized, but planetary are denser and more compact than nebulae found in star formation regions.
Planetary nebulae were given their name by the first astronomical observers who were initially unable to distinguish them from planets, and who tended to confuse them with planets, which were of more interest to them. Our Sun is expected to spawn a planetary nebula about 12 billion years after its formation. 
Generally, one can make mature planetary nebulae mathematically by placing a sphere of gas around a white dwarf and picking a radius. It seems that the mean true expansion velocity of the nebular edge of a planetary nebula is approximately 42 km/s (source), so simply multiply the radius of the nebula by ~7138 to find the approximate age. If you don't want to do that, instead you can take an existing nebula (preferably a lesser-known one) and copypaste it. Thats basically what everyone does and nobody cares.
====Protoplanetary Nebulae====
A protoplanetary nebula (PPN) is an astronomical object at the short-lived episode during a star's rapid stellar evolution between the late asymptotic giant branch (LAGB) phase and the following planetary nebula (PN) phase. During the AGB phase, the star undergoes mass loss, emitting a circumstellar shell of hydrogen gas. When this phase comes to an end, the star enters the PPN phase.
The PPN is energized by the central star, causing it to emit strong infrared radiation and become a reflection nebula. Collimated stellar winds from the central star shape and shock the shell into an axially symmetric form, while producing a fast moving molecular wind. The exact point when a PPN becomes a planetary nebula (PN) is defined by the temperature of the central star. The PPN phase continues until the central star reaches a temperature of 30,000 K, after which it is hot enough to ionize the surrounding gas.
Protoplanetary nebulae are really complicated, and are ridiculously hard to make from scratch. Here your best bet is to just yeet a lesser-known one.
===Supernova Remnants===
A supernova occurs when a high-mass star reaches the end of its life. When nuclear fusion in the core of the star stops, the star collapses. The gas falling inward either rebounds or gets so strongly heated that it expands outwards from the core, thus causing the star to explode. The expanding shell of gas forms a supernova remnant, a special diffuse nebula. Although much of the optical and X-ray emission from supernova remnants originates from ionized gas, a great amount of the radio emission is a form of non-thermal emission called synchrotron emission. This emission originates from high-velocity electrons oscillating within magnetic fields.
Supernova remnants again have really complicated structures, despite the fact that they should be just planetary nebulae but speed. Just go grab one from the real universe (not the crab nebula that one is so overused).
===Ejecta Nebulae===
An Ejecta Nebula is similar to a planetary nebula. It consists of large amounts of material ejected from high-mass stars before the end of their life. They are actually rather common and surround a decent fraction of high-mass stars, but they're mostly ignored in discussion about nebulae. They aren't even mentioned in the wikipedia page about nebulae! 
To create one, you have two options: either grab one from the real world (including central star or it doesn't make sense) or describe it as a shell of a certain size around a late-stage star of your choice. Some ejecta nebulae have more than one shell!
===Molecular Clouds===
Higher density regions of the interstellar medium form clouds, or diffuse nebulae, where star formation takes place. In contrast to spirals, an elliptical galaxy loses the cold component of its interstellar medium within roughly a billion years, which hinders the galaxy from forming diffuse nebulae except through mergers with other galaxies.
In the dense nebulae where stars are produced, much of the hydrogen is in the molecular (H2) form, so these nebulae are called molecular clouds. Dense molecular filaments, which are central to the star formation process, will fragment into gravitationally bound cores, most of which will evolve into stars. Observations indicate that the coldest clouds tend to form low-mass stars, observed first in the infrared inside the clouds, then in visible light at their surface when the clouds dissipate, while giant molecular clouds, which are generally warmer, produce stars of all masses. These giant molecular clouds have typical densities of 100 particles per cm3, diameters of 100 light-years (9.5×1014 km), masses of up to 6 million solar masses (M☉), and an average interior temperature of 10 K. About half the total mass of the galactic ISM is found in molecular clouds and in The Silky Way there are an estimated 6,000 molecular clouds, each with more than 100,000 M☉.
A more compact site of star formation is the opaque clouds of dense gas and dust known as Bok globules, so named after the astronomer Bart Bok. These can form in association with collapsing molecular clouds or possibly independently. The Bok globules are typically up to a light year across and contain a few solar masses. They can be observed as dark clouds silhouetted against bright emission nebulae or background stars. Over half the known Bok globules have been found to contain newly forming stars. These are useful for hiding big secret things in worldbuilding. 
These are even more insanely complicated, so the best way to go is to say "there's a big cloud of gas here" and maybe make a list of nebula names that fit for small objects, to allow for the existence of smaller nebulae within it without actually having to make them exist. Or of course you could clone a real one. Just don't do tarantula or orion please. 
==Open Clusters==
An open cluster is a type of star cluster made of up to a few thousand stars that were formed from the same giant molecular cloud and have roughly the same age. They are loosely bound by mutual gravitational attraction and become disrupted by close encounters with other clusters and clouds of gas as they orbit the galactic center. This can result in a migration to the main body of the galaxy and a loss of cluster members through internal close encounters. Open clusters generally survive for a few hundred million years, with the most massive ones surviving for a few billion years. In contrast, the more massive globular clusters of stars exert a stronger gravitational attraction on their members, and can survive for longer. Open clusters have been found only in spiral and irregular galaxies, in which active star formation is occurring.
Young open clusters may be contained within the molecular cloud from which they formed, illuminating it to create an H II region. Over time, radiation pressure from the cluster will disperse the molecular cloud. Typically, about 10% of the mass of a gas cloud will coalesce into stars before radiation pressure drives the rest of the gas away.
To create them in worldbuilding projects, one must create the most massive stars within them and say that they are located in a region of space x light-years across. One can also describe the cloud of gas that may be surrounding it. Open Clusters are one of the most simple and most common of deep space objects, and therefore are among the most tedious to make.
==Globular Clusters==
Globular Clusters are very dense, spherical collections of stars which orbit a galaxy as a satellite. They are found in the galactic halo or bulge, and galaxies can host from hundreds to thousands of them. The Milky Way hosts 150 to 158 confirmed globular clusters, while Andromeda may host as many as 500 clusters. Giant elliptical galaxies at the centres of galaxy clusters are by far the most abundant source of globular clusters: M87 hosts about 13,000 globular clusters.
Globular clusters are far denser than star clusters in the galaxy itself, and may have around 0.04 stars per cubic parsec in the outer envelope of the cluster, to as many as 100 to 1000 stars per parsec in the dense, globular core. Globular clusters consist primarily of old, low-metallicity population II stars. They range in size from a radius of about 10 parsecs to 50 parsecs.
To design one, basically say "its this big and has this many stars and is located here" and thats it for the overall structure. Obviously, its a good idea to write about the underlying lore or its pretty pointless otherwise.
==Stellar Neighborhoods==
A Stellar Neighborhood is defined as all stars within a certain radius of a given system. This is generally useful if you want to write about some sort of interstellar empire. Unless you have a lot of time on your hands, it isn't generally necessary to write about EVERY single star in your main star's Stellar Neighborhood, but it IS useful to know the general proportions of star types. 
The most useful size for a Stellar Neighborhood in my opinion is ~10 parsecs. Within this distance of our sun, we have:
*0 O or B-type stars
*4 A-type stars
*8 F-type stars
*18 G-type stars (like the sun)
*38 K-type stars (considered best for habitability)
*249 M-type stars
*5 L-type brown dwarfs
*30 T-type brown dwarfs
*11 Y-type brown dwarfs
Although the last three numbers are almost certainly a huge underestimate. When making your own stellar neighborhood, it is best to keep these rough proportions although you should feel free to fudge them a little. If you happen to have an O or B-type star within 10 parsecs of you, well SOMEONE has to have an O or B-type star within 10 parsecs of them! Don't fudge things too much, though.
Once this is done, arrange them randomly in a 10 parsec sphere, pick which ones will be the interesting ones, and write about them! All there is to it!
==Conclusion==
Now that we have mapped our universe at the largest scales, it is time to look at the smaller scales. It is time to design our solar systems in Worldbuilding Guide Part 3: Planetary Systems!
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