Stars are ubiquitous throughout the universe and play a key role in many events happening in it, including the development of life and the facilitation of civilisations. But they are more interesting than just that. Here we will explore all kinds of stars conceivable and/or have been discovered/identified, exploring them from initial mass and evolution, as well as describing their parameters, the environment they give to nearby planets, their surroundings, where they likely reside in their respective galaxy and any prospects of habitability and/or usage by a civilisation.

Contents

Formation

Conventional stars form inside molecular clouds - huge, very cold dark clouds composed of molecular hydrogen and helium. Over hundreds of thousands of years, the clouds slowly collapse via gravity. The central part condenses into a big ball and heats up, and becomes a protostar. These are usually much larger and cooler than the main-sequence stars that they will form. As the protostar forms, the remaining cloud flattens into a disk, where dust accumulates and gets bigger.

A million years after its formation, the star likely has some near fully-fledged planets (Unless if its solar wind blows away the protoplanetary disk first, which can happen with high-mass stars) disk before, which should be detectable via gaps they cause in the star's protoplanetary disk. This has been done many times to reveal planets inside the disks of slowly maturing stars. One such example is the finding of a planet in TW Hydrae's planetary disk.

A few million years after the formation of the star, it starts to spew material in bipolar jets and becomes a T-Tauri star, or pre-main sequence star. These are cooler, larger and dimmer than the main-sequence stars that they will mature into. The mass does not significantly decrease from the bipolar jets' spewing out.

Around 10-15 million years after the star's formation, nuclear fusion finally begins in its core (That is if it is massive enough). The star finally shrinks to its expected size and temperature, and starts something called the solar wind. The solar wind is a mass of radiation emanating from a star. Usually the larger and the hotter the star, the more powerful the solar wind. This clears the protoplanetary disk and makes a bubble that gets rid of dangerous cosmic rays. This bubble, sometimes known as the heliopause, is helpful in facilitating life on planets orbiting the star.

Initial mass: <0.0124 MSol

OTS 44, a typical young sub-brown dwarf. It still is surrounded by a dust disk and is forming "planets" out of it.

Anything that forms under this limit is too small to be any kind of fusor, and thus, will be incapable of nuclear fusion. Instead, it will be classified as a sub-brown dwarf and quickly cool off, going through the M, L, T and finally Y brown dwarf classes to become a sort of rogue planet. (The cooling off phenomena also applies for young similarly massive gas giants orbiting stars) These objects can only really be used as refueling stations and any life in these systems is likely to only be subglacial life in vast underground oceans, capped by a seemingly impenetrable wall of ice that leads to the frozen surfaces of I-class worlds. It would be excruciatingly difficult for any life to ever realise that there was a giant universe above the icy dome surrounding them and their ocean, which is a saddening prospect for anyone observing them.

From afar, the only thing that would distinguish young sub-brown dwarfs from young brown dwarfs is their size (they are bigger because they are younger, hotter and less massive), their small mass and the possible existence of protoplanetary disks which would also signal their youth. Old sub-brown dwarfs would be mostly indistinguishable from massive rogue planets, except for possibly larger moons or more extensive systems, as true rogue planets' lunar systems would likely be smaller due to disruptions from other planets that would have made the planet go rogue.

An example of a sub-brown dwarf is OTS 44, a young L-class sub-brown dwarf that weighs a mere 11.5 Jupiter Masses. It is 35% the size of the Sun, making it considerably larger than most brown dwarfs, and so it has comparatively low gravity. Such objects are first classified as brown dwarfs with a γ (Gamma) suffix, which indicates low gravity. Young, glowing sub-brown dwarfs like these are mostly found in molecular cloud complexes, owing to their short time between birth in the complexes and cooling beyond visibility.

Initial mass: Between 0.0124 and 0.07-0.075 MSol

Between these mass limits we have the brown dwarfs. They are less massive than stars, and so have less pressure in their cores which prevents conventional core fusion. As a result, brown dwarfs are quite a bit dimmer than stars. Brown dwarfs are still classified as fusors though, as they are capable of deuterium fusion, which fuses deuterium into helium. But after large amounts of time, the reaction slows and the brown dwarf cools to the same temperature as an exoplanet.

Brown dwarfs start out at their hottest and brightest. Over the next few million years, tens of millions of years or even hundreds of millions of years (depends on their mass, the more massive a brown dwarf is the longer it will stay visible and hot), the brown dwarf will cool, passing through several different spectral types (M, L, T, Y).

SCART representation of an M-type brown dwarf

M class

The M class (The same class used for red dwarfs) applies to very young brown dwarfs, and older brown dwarfs as well if they are considerably massive. They have temperatures similar to the smallest red dwarfs and appear similar, with spectral types usually M8 to M9. M class brown dwarfs are usually found in or near nebulae, but they can also be found by themselves. An example of an M-class brown dwarf is Amirani, also known as DEN 1048-3956, under UFSS control. M-class brown dwarfs look similar to small red dwarfs, often causing them to be mistaken for red dwarfs from a distance.

M-class brown dwarfs can be flare stars, and some have been known to produce X-ray emissions, for example the brown dwarf LP 944-20, also under UFSS control.

LHS 5166 B, an L-class brown dwarf, pictured by SCART.

L Class

The next coolest class is the L class, which is defined by lithium in the atmosphere, an orange to dull red glow and temperatures of 2200 to 1300 Kelvins. Early L-class (L0-3) brown dwarfs look like darker versions of M-class red dwarfs, as they are still massive enough to be convective, whereas late L-class brown dwarfs (L7-9) look like glowing, red-hot gas giants. Mid L-class (L4-6) brown dwarfs look like a mix of the two. L-class brown dwarfs are ubiquitous throughout stellar populations, so don't be surprised if you find one. L-class brown dwarfs usually have planetary systems, which normally comprise of rocky planets, but gas giants can sometimes occur around brown dwarfs.

An example of an L-class brown dwarf is Luhman 16 A, Archas Minor, Lux (In the Castro-Lux system) or Amphion (Also known as DEN 0255-4700).

A regular T-class brown dwarf

T Class

Similarly to the L class brown dwarfs, T class brown dwarfs are commonplace around the universe, albeit to a slightly lesser extent than L class brown dwarfs. The T class is defined by a temperature of 1300 to 550 Kelvins, a very vibrant red or hot pink (sometimes even a magenta-ish colour, due to the abundance of methane in T class brown dwarfs) to a deeper red for cooler T class brown dwarfs, and as mentioned above, an abundance of methane. T-class brown dwarfs resemble red or dark red gas giants, and many have planets orbiting them. An example of an early T-class brown dwarf is Luhman 16 B or the body that Kagnus orbits, and an example of a late T-class brown dwarf is Novad, in the Ambrosia Galaxy.

Your typical Y class brown dwarf. Note the extreme darkness of the object.

Y Class

The coolest of all of the brown dwarf types, the Y class is determined by temperatures lower than about 550 Kelvins, formation of clouds of several volatiles such as water. Y class brown dwarfs are also common throughout the galaxy, to a similar extent to T class brown dwarfs. Because they are so cold, most Y class brown dwarfs emit no visible light at all, and as a result they can often be misclassified as regular rogue planets or sub brown dwarfs.

Due to the sheer dimness of Y class brown dwarfs, it is incredibly hard to find them if you're using anything other than an infrared telescope, and even then it is still considerably harder to find a Y dwarf than for example, finding a T dwarf. An example of a Y dwarf is WISE 1405+5534, a Y class brown dwarf located around 25 light years from Sol. Any prospects of habitability around a Y dwarf are small and any life evolving around such a type of star would likely be subglacial life, just like sub-brown dwarfs.

Initial mass: Between 0.07-0.075 MSol and 0.35 MSol

Stars in these range are fully convective stars and do not have a radiative layer, and as a result do not expand when they leave the main-sequence. All main-sequence stars in this range will be red dwarfs, ranging from M9 to M2-3. Any main-sequence star above this limit has a radiative zone inside it, including Sol.

A typical red dwarf.

Main Sequence

As mentioned above, all main-sequence stars in this range will be red dwarfs. Red dwarfs are the most common type of star, accounting for about 75% of the stellar population. These are low-temperature and low-luminosity stars, producing anywhere from 5% Sol's energy down to 0.0001%. The dimmest of these stars are known as "ultra-cool dwarves." As a result of such a low luminosity, the habitable zones of these red dwarfs are very close to them. It is commonly 0.05 to 0.5 AU from the star. Systems with larger Red Dwarf Stars are often no different from typical stars except for in scale.

A red dwarf during a typical stellar flare.

General Properties

Most red dwarfs are so-called flare stars (also known as BY Draconis variables or UV Ceti variables. This means that they irregularly throw off large amounts of their mass (typically not enough to perturb the orbits of their planets) in giant coronal mass ejections, powered by their magnetic fields. Because of this, life around red dwarf stars is somewhat rare. These coronal mass ejections are often enough to strip planets of their atmospheres. They are even dangerous for technological civilizations, as smaller ones cause electronics and other delicate technologies to become inoperable. Only planets with the strongest of magnetic fields can resist coronal mass ejections, especially when they have to be so close to these dim stars to harbor life.

Red dwarfs under around 3000 Kelvins can actually host chemical substances in their atmospheres, which show up in their spectra. One such example is Wolf 359.

Habitability

The issues addressed above makes the prospect of terrestrial life forming on a planet in these systems very unlikely. Not only does the planet need a somewhat strong magnetic field, it also needs to be within the habitable zone. What's more is that the planets need to be so close to these dim stars that any habitable planet will be tidally locked, cutting down on the available room for life as well as usually scorching one side. Life can evolve to survive in these conditions, however, only a fairly narrow set of conditions allows life to actually form. As such, the life on these planets need to evolve in the Terminator Zone, the area straddling the hot and cold sides of the planet. Of course, less room for life to form makes life less common on these worlds.

Even so, the average red dwarf has a comparatively high metallicity, making the formation of planets rich in the chemicals needed for life common. The most common forms of life around red dwarfs is subglacial life, as the thick ice shells will protect the planet from the mass ejections. These subglacial worlds can also exist anywhere around a red dwarf, provided that they are beyond the habitable zone.

Red dwarfs are so common that even though they are less likely to harbor life, there are still more red dwarfs with life around them than there are other stars that harbor life. Life around a red dwarf is only relatively rare when compared to how many of them there are.

Non-main sequence

A typical blue dwarf.

After a few hundred billion years to a few trillion years after its birth, the red dwarf starts to heat up as its hydrogen fuel wanes. As a result, the habitable zone extends further out and worlds once completely frozen up, on the outskirts of their systems could warm up and be able to host life. On the other hand, planets once in the star's habitable zone might experience a runaway greenhouse effect and become inhospitable towards life, just like what happened to Venus in the distant past.

For example, a red dwarf 15% the mass and size of Sol with a temperature might increase its luminosity 30 or more times over the course of its non-main sequence lifetime as what is known as a "blue dwarf".

A low-mass, quickly cooling white dwarf, the final product of a red dwarf's life.

Blue dwarf stage and beyond

After the red dwarf has extinguished most or all its fuel reserves, it will heat up, going through from red to orange, yellow, white and finally blue. It will also shrink in size as its temperature goes up, shining its last few tens of billions of years as a hot blue dwarf, resembling a small blue subdwarf. Ultimately, at the very very end of the life of this kind of star, it shrivels up into a tiny helium white dwarf and quickly cools down to become a black dwarf. After this, there is basically no hope left for life to take place here.

Initial mass: Between 0.35 MSol and 0.5 MSol

Stars between this range are the larger red dwarfs, from M0 to M2-3. They and their evolutionary paths resemble scaled-down and simplified versions of Sol's evolutionary path. They spend 60-150 billion years on the main sequence, before evolving off it and expanding into red subgiants, and as they are too light, they cannot fuse helium and go straight to blowing off their outer layers (after 10-20 billion years on the red subgiant stage) into a very small, helium white dwarf, like the end product of a fully convective red dwarf. Prospects of habitability are similar to the smaller, fully convective red dwarfs, except for the very largest of these stars.

Initial mass: Between 0.5 MSol and 2.5 MSol

A typical K type main-sequencestar.

Stars between this limit encompass a large part of the main-sequence star types, specifically 4 of the 7 types listed in the Harvard Main-Sequence Classification System (HMSPS), specifically K, G, F and A.

Main Sequence

K Class

The K class is the least massive of the four classes that is encompassed in this range. K-type main-sequence stars resemble smaller versions of Sol. They are 50-90% the mass of Sol (though G-types, the next hottest type, can be less massive), 60-90% the size of Sol and 3900 to 5000 K in temperature. K-type stars are often described as a cross between M type stars and G type stars, being less active than M stars but still more active than G stars. Most K stars are quite calm though, which makes them good candidates for habitable worlds. One of the qualifications of a so-called human "superhabitable planet" (a planet more suited for humans than Earth) is that it orbits a K type star, like Haven. K class stars last from 15 to 70 billion years, and an example of a K-type star is Eytinore's star.

G Class

A typical G type main-sequence star.

The G class is the second least massive of the four classes in this range. It is not very distinct relative to the other three classes, other than being the class that Sol, the parent star of Earth, the homeworld of Humanity (one of the most important species in the CoB). It ranges from 80-105% the mass of Sol, 80-115% the size of Sol and 5000 to 6000 K in temperature. Along with the K class, it is the best class to host life. G class stars last from 7 to 15 billion years.

A typical F type main-sequence star. This one is slightly oblate.

F Class

The F class is the second most massive of the four classes. It ranges from 105% to 140% the mass of Sol, 115-160% the size of Sol and is 6000 to 7500 Kelvins in temperature. It does not live as long as some of the other classes, and hence life around an F-class star is rarer than life around a K class star or a G class star. Still, major examples exist, such as Saykya, the parent star of Lowrokira (homeworld of the Zythyns). F class stars last from 3 to 7 billion years.

A typical A type main-sequence star.

A Class

The A class is the most massive of the four classes here. The mass ranges from 140 to 250% the mass of Sol, 140-230% the size and 7500 to 10500 K in temperature, and lasts for 800 million years to 3 billion years. Life is unlikely to emerge around an A class star, due to its comparatively short lifetime compared to other stars in this range. An example is the primary star in the Sirius system.

A type stats are especially good candidates for hosting Dyson swarms as they are bright, stable and last a relatively long time.

Non-main sequence

A typical subgiant star.

Subgiant phase

After the star has finished all its fuel, it will slowly heat up and expand into a subgiant. An early subgiant star (Like Procyon A) of a certain mass will be larger, generally slightly hotter and brighter than the main sequence star it evolved from. The subgiant phase lasts around 100 million years to a few billion years, depending on the initial mass of the star.

A typical red giant star.

Early red giant phase

After the subgiant phase has ended, the star cools and rapidly expands (Because their cores have shrunk and heated up from not being able to burn more hydrogen, and this has the effect of swelling the outer layers), likely becoming an orange giant (Like Pollux) and then an early red giant, like Arcturus. These are relatively small (20-60 RSol) relatively hot red giants that are burning helium.

A typical Pre-AGB star.

Pre-AGB phase

After some time, the red giant starts to heat up again, and shrinks to become a yellow giant/yellow supergiant. This stage precedes the AGB phase and so is also called the Pre-AGB phase, or the Horizontal Branch. At this time, many stars start to pulsate, forming a few kinds of variable stars. This is also the point where stars begin to fuse helium into carbon and other elements, but this phase only lasts 1 million years. An example of a pre-AGB star is Caph, or Beta Cassiopeiae.

RR Lyrae Variables

RR Lyrae variables are pre-AGB stars that have a mass of 0.5 to 1 solar mass. They are commonly found in Globular clusters, and there are many in the Viana cluster. These variables do not follow a strict period-luminosity relationship at visual wavelengths, which differs them from Cepheid variables. The prototype star for this is RR Lyrae.

Type 2 Cepheid Variables

Another kind of variable that makes up a relatively significant population of pre-AGB stars are the type 2 Cepheid variables, although they occur more in the post-AGB phase. These are old, relatively light (0.4 to 1 solar mass), population II yellow giants, similar in characteristics to RR Lyrae variables, that have similar pulsations to type 1 Cepheids (the conventional cepheids). These stars have low metallicities compared to other stars. There are several subtypes of these stars, such as W Virginis variables, RV Tauri variables, XX Virginis Variables and BL Herculis variables. Examples of these stars include W Virginis, Kappa Pavonis and RV Tauri.

R Coronae Borealis Variables

A small population of these stars become what are called R Coronae Borealis variables. They are similar to the two variable types already mentioned (low-mass, old, low metallicity, population II) eruptive variable stars, usually having 2 variable modes. The first one is a short period low-amplitude variability in brightness, while the second one is irregular, unpredictable fading from 1 to 9 magnitudes, due to build up of carbon dust in the atmospheres of these stars.

This is because these stars are very carbon rich (because there is a lot of carbon in their atmospheres, formed by fusion of helium. Owing to this, they are often also classified as carbon stars, and rightly so. The abundance of carbon in their atmospheres turns into dark, opaque dust which can dim the star by a lot, and this mechanism causes the large-scale dimming.

R Coronae Borealis variables are very rare, with only around 2000 identified with SCART in the Milky Way (As of 15305 CE). Most are old yellow supergiants, but some are much hotter, such as the star DY Centauri, which is similar to a B type giant.

Another way to generate these kinds of stars is by colliding two intermediate-mass white dwarfs.

It is unlikely that life-bearing worlds will arise around such stars, due to them changing brightness often and their instability.

A typical AGB star.

AGB Stage

During the AGB (Asymptotic Giant Branch) stage is when a star will grow biggest and brightest over its entire lifetime. AGB stars are true red giants, normally hundreds of times the size of Sol. (The range is from ~100 to more than 1000 RSol) Stars here are also quite cool, regularly ranging from 2000 to 3500 Kelvins. This is when stars truly start to swallow their planets. During the AGB stage, the mass loss rate is very high and many stars start to pulsate. Events known as "dredge-up" happen and can turn a significant amount of these stars into so-called carbon stars, or zirconium stars.

Carbon stars

Carbon stars are AGB stars that have more carbon than oxygen in their atmospheres. They have distinctive spectral features and were first identified by Angelo Seechi, in the 1860s CE. As much as half (or more) of a carbon star's mass maybe ejected over the course of its time as a carbon star, through powerful stellar winds and a low gravity, causing them to be relatively bad candidates for life-bearing worlds. The ejected stellar matter becomes part of the interstellar medium as graphite-like dust, and could pave the way for new generations of stars and planetary systems in the far future. A classic example of a carbon star is R Leporis, under UFSS control.

Carbon stars are formed from AGB stars by events called "dredge-ups", which is when a star's convective zone extends down to the layers where material has undergone nuclear fusion, therefore distributing fusion products such as carbon and oxygen throughout the star. The spectral type of carbon stars are either C, N or R.

Zirconium stars

Another subtype of AGB stars are zirconium stars, denoted by the spectral type S. These are similar to carbon stars, but have other enriched substances, such as zirconium oxide (their defining feature), and other s-process elements and/or their compounds. These stars have approximately equal amounts of carbon and oxygen, and will generally be younger than their carbon counterparts. Zirconium stars produce of copious amounts of the elements near zirconium. An example of an S class star is Chi Cygni.

Mira Variables

Just like other stages, the AGB phase is ridden with variability. A prominent and very common variable type of variable star in this stage is the Mira variable, named after Mira A, its famous prototype. They pulsate over a very long period, usually over 100 days, up to 1000 days. Mira variables can be characterised by their very red colour, and a variability amplitude of over 2.5 magnitudes. They are one of the most common types of variable star, with tens of millions in the Milky Way alone.

A typical post-AGB star.

Post-AGB stage

After the AGB stage has passed, the last stage before death is the post-AGB stage. This is when the star gradually shrinks and heats up. This stage, more than any other, is the site of many pulsations and rapid mass loss. We get more type 2 Cepheids and RV Tauri variables, among other pulsating variables, and a nebula will normally form around the post-AGB star. At around 8000 Kelvins in temperature, the star finally throws off its outer layers and dies.

Artist's impression of a typical blue subdwarf.

Blue Subdwarf stage

Some stars go through this stage before they die. Blue subdwarfs are small (0.05-0.8 RSol), hot stars, similar to miniature Wolf-Rayet stars. They have extreme sunspot activity and often release many flares. Their temperature usually ranges from 10000 to 50000 Kelvins,

WIP

A typical central star of a planetary nebula.

Death

The star is now a small core surrounded by a giant planetary nebula. The central star will possibly behave like a wolf-rayet star for some time, shining at thousands of solar luminosities and being extremely hot (150000 to 250000 Kelvins). This has the effect of upholding the ionisation of the planetary nebula, keeping it glowing for longer. Slowly, the nebula disperses and the central star, in the so-called CSPN stage (Central Star of Planetary Nebula), cools. When the nebula finally disperses, the star is now a white dwarf.

White dwarf stage

A white dwarf is a shrunken stellar corpse made of degenerate matter. The white dwarf preserves just 20-60% of the original star's initial mass.

A typical white dwarf. This one is quite cool in temperature.

As a result of the white dwarf being made of degenerate matter, it is extremely dense - around 1*106g/cm^3. This makes the gravity extremely high, from around 30000 Gees to over 700000 Gees, higher than almost any kind of star. The small size of the star allows planets to exist close to it, but planets too close get ripped apart by tidal forces. This is a common phenomenon, and results in the white dwarf's atmosphere getting "polluted" by asteroid material.

A white dwarf's mass will usually be around 0.5 to 1.1 solar masses, although they can get as massive as 1.44 solar masses if the initial star's mass is above the upper threshold of this section. White dwarfs cool slowly over time, but at different rates. The less massive the white dwarf, the quicker it cools.

Owing to their small size, white dwarfs are very dim, which allows their planets to be close in and still be habitable. Due to being very stable as light sources, any planets in their habitable zones are bound to enjoy stable climates (except if the white dwarf has a companion star), and so many habitable planets can be found around white dwarfs. But this habitable zone would only last 3 billion years.

A white dwarf's spectral type depends mostly on its composition. These are the seven white dwarf spectral types:

  • DA: Hydrogen lines present, i.e. hydrogen atmosphere. Most (80%) white dwarfs belong to this class.
  • DB: Helium lines present, i.e. helium-rich, but still hydrogen-dominated atmosphere. 16% of white dwarfs belong to this class.
  • DQ: Carbon-dominated atmosphere shown by carbon lines. Hot ones above 15000 Kelvins are very rare. Only 0.1% of white dwarfs belong to the hot class, while cool DQs are more common.
  • DC: Helium-dominated atmosphere, but no ionisation, indicating a relatively low temperature for most DCs.
  • DZ: More metal-rich than other types, but still mostly helium-dominated atmosphere.
  • DO: Helium-dominated atmosphere, a lot of ionisation indicating a high temperature. Usually DOs are 100000 K to 45000 K in temperature.
  • DX: Unclear spectrum.
Variability

Just like other star types, white dwarfs can be variable.

DAV Class/DBV Class

The DAV class is when a white dwarf of the DA class experiences non-radial gravity wave pulsations, similar to ripples in a body of water. The same thing happens to some white dwarfs of the DB class.

GW Virginis stars

GW Virginis stars are pulsating CSPNs, that have carbon, helium or oxygen-dominated atmospheres. They are not strictly speaking, white dwarfs, but rather pre-white dwarfs. These variables all exhibit small (1%–30%) variations in light output, due to multiple gravity waves with periods of hundreds to thousands of seconds, They give insight to the interiors of white dwarfs.

Black dwarf stage

A quickly cooling black dwarf, barely visible to the naked eye.

When a white dwarf cools enough, it will become what is called a black dwarf. Black dwarfs are practically the ashes of stars, no longer able to support any kind of life around them. They are so cold (Less than 500ºC) that they barely emit anymore heat, or light. It is impossible for any kind of life to evolve on a black dwarf, unless if it is so cold that the electrons on the surface pair up to possibly make inorganic life possible. No black dwarfs have so far been discovered, due to the vast timescales needed to make these objects. However, a few have been artificially created.

In time, black dwarfs will slowly evaporate via proton decay, and by 10^40 CE, there will be no more black dwarfs left in the entire universe.

Initial mass: Between 2.5 MSol and 8 MSol

A typical B-type star, or blue-white star. This star is not destined to go supernova though.

Stars in this range will be B-type stars, but there are also B-types outside this range.

B-type stars, also known as blue-white stars, are massive stars that have a relatively short lifespan. The B type stars in this range have a lifespan of 35 to 800 million years, a temperature of 11000 to 20000 K, and a size of 2 to 6 solar radii. They are blue-white in colour, and especially the early B types usually appear in or near nebulous regions, as their short lifespan does not allow them to travel very far from their birthplaces. The B types in this range do not in fact go supernova, but expand into red giants and die as white dwarfs. The more massive B types become red bright giants instead of red giants. Young B type stars are commonly subject to variability. An example of a B type main sequence star like this is Pleione A, a B8V main sequence star in a binary, in the Pleiades cluster.

The CNO cycle

Just like F and A type stars (And any star above 1.1 solar masses), the high pressure in the cores of B type stars allows them to use another kind of fusion alongside conventional hydrogen fusion - the CNO cycle. This is where a carbon atom fuses with some protons and becomes nitrogen, then oxygen. Then it goes back to carbon, along with releasing energy. This likely becomes more common over the star's life as more heavy elements build up in its core.

Bellatrix, a blue giant, about to leave the main sequence and fuse helium.

As B type stars age, they cool and expand, leading to some of the older B type stars to be classed as giant stars. An example of that is Bellatrix.

Non Main Sequence

As B type stars of this range fall off the main sequence, they expand into white giants or white supergiants, stars the same temperature or hotter than the sun, but much bigger, 10 to 50 solar radii. These expand off into yellow giants or supergiants and commonly pulsate, as type 1 Cepheids. The term "Cepheid" is a reference to the prototype of these stars, Delta Cephei.

Delta Cephei, a pulsating type 1 Cepheid and a yellow supergiant.

Type 1 Cepheids

These have similar pulsations to the type 2 cepheids mentioned earlier, but are very different stars. Type 1 Cepheids are yellow giants/supergiants (Sometimes white) that pulsate similarly to type 2 Cepheids, but are younger, more massive Population 1 stars that have just evolved off the main sequence and have not reached the giant stage yet. Type 2 Cepheids have already evolved off the red giant stage and are moving towards the white dwarf stage.

Once this stage is complete, the now big yellow stars move towards their respective red giant stage. The more massive B type stars in this range become bright red giants, while the less massive B types go through the same stages as the K, G, F and A stars do, but on a slightly larger scale. Once the star has exhausted its supply of hydrogen and helium, it may go through the post-AGB stage as a particularly large type 2 Cepheid, then it will expel its outer layers and die, going through the CSPN stage as a [WO] star (A CSPN, but with oxygen Wolf-Rayet characteristics) and becomes a rare neon-oxygen white dwarf. These white dwarfs are very massive and approach the Chandrasekhar limit, which is 1.44 solar masses. Thus, these white dwarfs will stay glowing for the longest and will be the last refuges for any exotic civilisation living in the extremely far future where the Stelliferous era has ended, and the Degenerate era has taken hold of the entire universe, transforming most white dwarfs into black dwarfs.

AE Aurigae, a rare, massive O-type star, lighting up the Flaming Star nebula. It is also a hypervelocity star, once ejected from the Orion Nebula.

Initial mass: Between 8 MSol and 25 MSol

Now we are approaching the truly massive stars in the universe. This range includes all the early B type stars (B2.5V-B0V) and some of the late O type stars(O9.9-O8V) as well. O type stars are the hottest main sequence stars that have ever existed. The stars in this range are the least massive stars to be able to go supernova. Two examples of stars in this range are Paome, a B1.5V type star with exotic energy based life and AE Aurigae, an O9.5V type star that lights up the Flaming Star Nebula, but in fact is a runaway star that originated in the Orion Nebula. (Like Mu Columbae).

These stars have the pressure needed to fuse heavy elements after oxygen, such as silicon and iron. They are the main seeders of important chemicals needed for life when they die as supernovae. Stars in this range are very hot, ranging from 20000 K to 35000 K and are 3 to 8 solar radii usually, and range from 1200 LSol to around 86000 LSol. O-type stars are nearly always found in or near stellar nurseries, unless they are runaway, like AE Aurigae and Mu Columbae.

O type stars have little hydrogen lines in their spectra, as the temperatures on their surfaces ionise all the hydrogen present.

Non Main Sequence

Stars in this range live very quick lives, between 12 and 30 million years. So they evolve very quickly, leaving the main sequence very quickly. They expand into blue giants, stars cooler, but larger and brighter than their main-sequence counterparts. Blue giants are susceptible to variability, especially Beta Cephei variability. This is when the star exhibits mall rapid variations in their brightness due to pulsations of the star's surface, thought due to the unusual properties of iron at temperatures of 200,000 K in its interior. Beta Cephei variables sometimes have multiple variability periods, which is the case for Beta Canis Majoris, also known as Mirzam, under UFSS territory in the Milky Way.

They exhaust their hydrogen supply, and then the core shrinks and heats up, allowing for helium fusion and expanding the star in question. They expand into white supergiants (50 to 250 Solar Radii), then yellow supergiants which are even larger, and finally into red supergiants.

A typical red supergiant. Note the corona and the contrasting bright and dark spots.

Red Supergiant stage

Red supergiants are infamous stars, for their size, their ability to make heavy elements, and their spectacular yet extremely violent deaths as supernovae. Red supergiants go through a shitton of mass loss during their time as red supergiants, which normally reduce their mass to a mere 10 solar masses or less by the time they die. They are relatively common throughout space compared to other types of evolved high-mass star. A red supergiant is normally 500 to 1500 Solar Radii in size, though many red supergiants are known to be larger, for example WOH G64, in the small magellanic cloud. Red supergiants are normally between 3000 and 4000 Kelvins in temperature. They are normally found near H II regions, though you could find them on their own. This is a bit rarer though.

Just like other stars, they can go hypervelocity. Hypervelocity red giants usually have bow shocks (visible comet tail-like formations), which can extend out to a few lightyears from them. Lower mass giants can also create these structures. IRC -10414 (RAFGL 2139) is a red supergiant with such a structure, while Mira A (Omicron Ceti A) is an AGB red giant in a binary that has created a similar structure, extending for more than 10 light years.

Blue Loops

A red supergiant can sometimes perform a blue loop. This is when the red supergiant shrinks and heats up for some time, and then goes back to being a red supergiant. This happens frequently and produces stars such as yellow hypergiants and blue supergiants. (Like Rigel)

A red supergiant stays super big by fusing more and more heavy elements in its core. It can fuse carbon, nitrogen, oxygen, neon, sodium, magnesium, nearly every element up to iron. These elements accumulate in "shells" around the central core of the star, but the time of fusing each element differs by a few magnitudes sometimes.

Helium fusion - 650000 years

Carbon/Oxygen fusion - 600 years

Neon/Sodium/Magnesium fusion - 0.01 to 5 years

Silicon fusion - 1 day

Death

As soon as the core starts fusing iron, the star is doomed. Fusing iron takes in more energy than it gives out, which depletes the core of energy if it tries to fuse energy. After merely 20-30 hours of fusing iron, the star's core has too little energy to support itself, and implodes, taking the rest of the inner layers with it. Everything rebounds, leaving the iron core as a tiny, city-sized ball sitting in the centre of the star. The inner layers travel out from the core at significant speeds, sometimes a substantial fraction of the speed of light. While this drama is happening, the outer layers haven't noticed yet, and are just sitting there. But as soon as the inner layers get to the outer layers, it takes them too, and the entire star blows apart in a titanic explosion known as a supernova. The gaseous layers propagate through space and become what is called a supernova remnant, which is a type of nebula. Supernova remnants are responsible for seeding newly-formed star systems with the ingredients needed for life.

A typical neutron star, the dense, dead remains of a supernova explosion.

Neutron Stars

Stars with masses in this range leave behind, in the centre of the supernova remnant, a small, enigmatic star known as a neutron star. These are city-sized balls of free neutrons, with a thin crust of iron on the top. As they are so massive, the surface gravity is extremely large, leading to neutron stars being able to bend the fabric of space-time around them so much that gravitational lensing is visible around them.

As we go deeper and deeper into the neutron star, we can encounter "nuclear pasta", a type of extremely dense degenerate material organised into structures, found in neutron stars. It is the strongest material in the universe, with a density of approximately 100 billion kg per centimetre^2, which is stupidly dense. Although being extremely robust, this material is rarely used, as it quickly decays when not exposed to extremely high pressures such as those found in neutron stars.

The cores of neutron stars are so dense that the neutrons themselves break down into quarks and gluons, known as quark-gluon plasma. This is thought to be one of the main reasons for the neutron stars' powerful magnetic fields. Sometimes, the neutron star is so massive that its entire mass is made up of quark-gluon plasma, and this is a quark star. Quark stars also cool down much quicker than regular neutron stars, which is one way to tell them apart from neutron stars. They are also visibly smaller than their neutron star cousins. A prime example of a quark star is 3C 58.

Occasionally, the quarks in a quark star can be converted from up/down quarks to strange quarks, which results in a strange star. The physics of these stars are still not very known (circa 2000 CE), although they are thought to be very similar to quark stars. Only around 150 strange stars have been documented in the local group.

Initial mass: Between 25 MSol and 45 MSol

Stars in this mass range will be late-O type stars (O8V to O5.5V). These are the smallest stars able to produce stellar-mass black holes through their death, and are the largest stars able to expand enough to become red supergiants. They will evolve into Wolf-Rayet stars at the end of their lives, and are the least massive stars to do so. (They evolve into WC wolf-rayet stars, but are not able to evolve into WOs)

A typical late-mid O star, between 25 and 45 solar masses. This one resides in Andromeda.

Main Sequence

These stars are extremely rare - only around 0.001% of stars fall into this class, yet they seem so common because they are so bright, anywhere from 40000 solar to a quarter of a million solar luminosities. They can be from 35000 to 42000 Kelvins and 5 to 9 times the size of the sun. As you can see, these are quite extreme stars, shining for less than 12 million years. Often, when they are first found, they are already on the main sequence (and have been for some time), rushing through their hydrogen fuel. In only a few million years, their hydrogen fuel is exhausted, and they rapidly expand.

Non Main Sequence

A typical big red supergiant, between 25 and 45 solar masses. This one is located in the Andromeda Galaxy.

Once their hydrogen fuel is depleted, these stars immediately expand into blue supergiants, only slightly larger, brighter and cooler than their main-sequence counterparts. They are sometimes variable as well. After that, and spending some time as a white supergiant, they quickly balloon into red supergiants.

Red Supergiant stage

The red supergiants in this mass range become even larger and brighter than the ones in the 8 to 25 MSol mass range, possibly ballooning to over 2000 solar radii and increasing mass loss so much that the outer envelopes of the star almost completely dissipate after 2 to 3 million years. During this time, a red supergiant might perform a blue loop, turning into a rare kind of star known as a yellow hypergiant.

A typical yellow hypergiant, looking similar to to a scaled-up version of the Sun. This one is located in the Andromeda Galaxy.

Yellow Hypergiant stage

Yellow hypergiants are one of the rarest kinds of stars known - as of 45000 CE, only 47 have been detected in the Milky Way, with 6 candidate yellow hypergiants. They are extremely bright F, G or K stars, meaning these stars have similar temperatures to our sun, but are vastly different stars. The yellow hypergiants range from 200 to over 1000 solar radii and measure 4100 to 7500 Kelvins in temperature, meaning that they shine from 40,000 solar luminosities all the way to over 2 million solar luminosities in the extreme.

An example of a hot yellow hypergiant is Thomsen's star, in the Andromeda galaxy. An example of a cooler kind of YHG is Rho Cassiopeiae, in the Milky Way. The two stars are 517,000 and 416,000 solar luminosities respectively.

LBV stage

Often, red supergiants in blue loops heat up further into luminous blue variables (LBVs). These are subsets of certain supergiant or hypergiant types (usually blue and white hypergiants) that pulsate over a long period. LBVs compose some of the brightest stars ever known, with examples such as Eta Carinae, with a luminosity of 4.6 million times solar, and Caphitis, a LBV with signs of nitrogen (leading to a spectrum similar to that of a Wolf-Rayet star) with an absolutely stupid luminosity of 27 million solar luminosities on average.

A typical LBV.

Luminous Blue Variables exhibit extreme solar winds and very high mass loss, often on the order of tens of millions to even billions of times quicker than the Sun. As a result their hydrogen content is often lower than what is considered normal from a main-sequence star, similar to a Wolf-Rayet star. The mass loss is deposited into interstellar space, sometimes in the form of a small nebula surrounding the star. LBVs can also undergo great supernova-like eruptions from time to time, throwing off a lot of mass (sometimes several solar masses) into space and definitely producing a nebula. During these outbursts, the star's overall luminosity does not increase, but since its temperature falls, more of its brightness is emitted in the visible part of the spectrum, and so it visually increases in brightness.

Note: LBVs are not always pulsating. They can be quiescent and mostly stable, and when they are, they are known as Ofpe/WN9 stars, stars with both O and WN characteristics and are very bright.

As a result of this mass loss, LBVs quickly expel all their outer hydrogen envelopes and start to reveal their hot cores, the to-be Wolf-Rayet stars.

A typical low-mass Wolf-Rayet star, which will progress from the WNE sequence into the WC sequence.

Wolf-Rayet stage

Wolf-Rayet stars are the small, exposed, blue-hot cores of ancient red supergiants. They are very bright, but since they are so extremely hot nearly all of their radiation is emitted in more energetic wavelengths. For example, at the time of its discovery, only 0.26% of the energy that once emanated from the surface of the evolved Wolf-Rayet star WR 2 (HIP 5100) was emitted in the visible range, due to a hot, 141000 Kelvins surface covering a ball only 89% the size of Sol. (circa 2000 CE)

Though, this is not to say that Wolf-Rayet stars are in any way dim. They are some of the brightest stars discovered, with the average Wolf-Rayet star having a whopping luminosity of 200k-300k times that of the Sun! WR 2, for example, has a luminosity of 282000 LSol, though in the visible range, that dwindles down to a mere 802 LSol. (Still quite bright, though)

Evolution in this mass range

Wolf-Rayets that started in this mass range now only have between 5 and 30 solar masses left, due to all the mass loss that has occurred in the past stages. They will start in the WNE sequence, then move into the WC sequence and explode. The more massive the Wolf-Rayet star, the hotter it will be when it dies.

The WN sequence of Wolf-Rayet stars (Not to be confused with the WNh sequence) is the sequence where ionised nitrogen and helium is visible in the spectrum of said Wolf-Rayet star. Examples include WR 1(WN4), WR 2(WN2), WR 3(WN3), EZ Canis Majoris A(WN5), WR 46(WN3), WR 134(WN6) and Cygnus X-3 A(WN4-6). (There is no such thing as a WN0, or a WC0 or a WC1)WN stars are the most common type of Wolf-Rayet stars, commonly outnumbering the other two types (WC and WO) put together. The less massive the star, the quicker it enters the WC sequence.

The WC sequence is characterised by lines on a spectrum of ionised helium and carbon, and the more ionised the elements are, the hotter the star. WCs have a similar temperature range to WNs, but a subtype of WCs, the WOs (WCs with lines of ionised oxygen, and less carbon) are especially hot, the upper bound of the WC/WNs slightly above the lower bound of the WOs. Originally, WCs and WOs were all categorised as WCs (Before there were enough WO stars discovered to be able to declare the WO class a separate WR class). WC-type stars can also be found in the cores of planetary nebulae, as pre-white dwarfs enriched with carbon.

WC stars are also notable for their high dust production. As their solar winds reach out into space, the carbon in the solar winds condenses into dust which can obscure the star. In fact, a significant amount of carbon present in the local group likely originates from the carbon-rich stellar winds of WC stars, similarly to how they can come from carbon stars.

Over the Wolf-Rayet stage, which usually lasts just 3-700,000 years, a star will gradually shrink and heat up, losing more and more mass through a strong stellar wind. As a result, the hottest Wolf-Rayet stars are the closest to their death. For example, at the top of the WC cosmic thermometer lies WR 114, a WC3.5 star with a temperature of over 130,000 K. It only has 18,000 years left before it finally blows apart. Though, at the time of its discovery, around 2000 CE, it was a WC5 with a temperature of just 79,000 K.

A typical black hole. This one is near the Wolf-Rayet star WR 136, which is the blue star in the bottom.

Death

Once a Wolf-Rayet star finally starts to die, its temperature likely exceeds 150,000 Kelvins, and it starts to become unstable. Finally, when the core, now enriched with iron, collapses, the star, now less than the size of Sol, falls on itself. If the outer layers are light enough, they rebound from the core and produce a type Ib/Ic supernova (Likely Ic, because WO stars do not have high surface helium fractions). These supernovae lack hydrogen lines (Type Ic lacks helium lines) which are consistent with Wolf-Rayet compositions. This turned out to be the case, with the earliest example of this being SN3151 (Supernova of WR 102), being a type Ic supernova.

Once the supernova is over, a small supernova remnant forms, enriched with the fusion products the massive star produced over its life. At the centre lies the final remnant of the star - a black hole. Describable by only two characteristics, it will serve as a reminder of the great big cosmic furnace that once existed here, whose fusion products will propagate ever further into the cosmos and serve as building blocks of what gives the universe its meaning - life.

Initial mass: Between 45 MSol and 65 MSol

The stars in this mass range are the mid-O stars - O6-O4V, sometimes O3. They are extremely bright stars by nature, ranging from over 100,000 LSol to over 400,000 LSol, and have temperatures of 42000 to 47000 Kelvins. The evolutionary path of these stars are very similar to the last ones, save for a few differences.

Main Sequence

The main sequence is quite similar to the main-sequence in the previous range, with a lifespan of less than 7 million years (comparable to the proposed lifetime of the previous range, 12 million years or less) and characteristics similar to the MS in the previous range, just slightly hotter and larger but substantially brighter.

Non Main Sequence

The Non Main Sequence is also similar, with the blue stars expanding slightly after 5 million years into slightly cooler but brighter blue supergiants, and likely topping 500 thousand solar luminosities. An example of a blue supergiant in this range is Naos, or Zeta Puppis, with 56.1 solar masses inside its bulk. (These may also be classified as blue giants).

Yellow Hypergiant stage

Now here comes the first major difference - the stars, now blue supergiants, are too heavy to expamd into red supergiants. Instead, they may become mere yellow hypergiants, shining with a similar radiance to their blue supergiant predecessors. Extreme mass loss happens in this phase, the yellow hypergiants likely creating a giant shell of expelled matter around them, as is the case for the yellow hypergiants HD 178921 and Hen 3-1379, which usually weighs a few solar masses. The yellow hypergiants sometimes perform a blue loop to become blue hypergiants, and sometimes even Luminous Blue Variables if they pulsate.

More Blue Loops

As the star enters its blue loop, it will shrink and heat up, likely brightening in the process. This stage is riddled with mass loss as the star is extremely unstable. The increased luminosity also makes the star sometimes susceptible to ionising the shell it once created as a yellow hypergiant (using its powerful solar wind), creating a wind-blown bubble nebula. But the majority of wind-blown bubble surrounding LBVs are created by a giant eruption like Eta Carinae's nebula, the Homunculus Nebula. Another example of this was V4998 Sagittarii, an astoundingly bright (around 4 million solar luminosities) LBV in the famed Quintuplet Cluster (It's not an LBV anymore), that had a large wind-blown bubble nebula (referred to as its ejection nebula) measuring over 2.6 light years across. Its origin is likely due to a giant eruption that occurred somewhere around 5000 BCE. (Note: The LBV may explode here as well, if it does it likely produces a type II supernova, rich in hydrogen)

WNh stage

Alternatively, the more massive stars in this mass range, or the ones that rotate quickly directly become Wolf-Rayet stars (and not yellow hypergiants), but not the conventional, hydrogen-free ones. They become what are called WNh stars, or hydrogen-rich Wolf-Rayet stars, shining hundreds of thousands of times or even millions of times the luminosity of the Sun. Their surface hydrogen fractions are substantially higher than the ones on hydrogen-free Wolf-Rayet stars, but still lower than regular stars. Though they may be less exotic than their hydrogen-free cousins, these stars are notorious for losing their mass at an even quicker rate than the conventional WRs. For example, R136a1, a hot WNh star with a mass of 203 solar masses, has shed 48 solar masses since its birth (Its initial mass had been 251 solar masses), and 12 solar masses in just 98,000 years, and is still losing 0.00013 solar masses a year!

However, WNh stars still do have the tendency to evolve into LBVs and back, losing even more mass in the process. A WNh star usually transitions to a hydrogen-free WN star.

Wolf-Rayet stage

The hypergiants had extreme mass loss, and this led to the exposure of their hot cores to space as the outer layers were lifted into the vacuum by powerful stellar winds and the cores became WN stars. They stay on this path for quite long and become quite hot when they finally divert from the WN sequence. The WRs that originated from this mass range are even brighter than their less massive cousins and manage to become extremely hot, over 200000 Kelvins, when they do explode in a type Ib/Ic supernova as WO stars, a very rare kind of WC star.

WO Stars

WO stars (Also known as Oxygen-sequence Wolf-Rayet stars) are definitely one of the rarest star types in the universe. On average, 0.000000004% of a galaxy's population is comprised of WO stars, an incomprehensibly small fraction. And even in the extreme world of stars, they are really, really extreme. And when I say extreme, I really mean it. These bad boys are one of the few pinnacles of extreme stellar evolution, being able to reach searing temperatures of 130,000 Kelvins and over, and even sometimes topping 200,000 Kelvins. Usually their masses are small compared to their initial, due to the intense solar winds present all over the lifetimes of these stars. For example, take Salgare, a WO star located in the Crown Cluster, in the Lareas Galaxy. It currently weighs just 18.6 Solar Masses, but its initial mass was much higher, at around 56 solar masses, and due to a very strong stellar wind, Salgare has managed to lift 67% of its weight into space. (yes I know I'm biased about these stars because I'm obsessed about them, but still they are cool)

As the WR finally stars to run out of fuel, they start to collapse on themselves and heat up. The outer layers rush in at possibly near the speed of light. And if the WR is massive enough, all that matter just falls in into the nascent black hole. If it is not too massive, the outer layers may rebound to create a spectacular supernova, or even a Gamma-Ray Burst if the star is spinning quickly. (Like GRB-18579, the death of WR 2)

And so ends another of the great stellar cycles. The supernova produced spews out many of the building blocks of life, such as carbon and oxygen and such. Some even heavier elements such as copper may also have been made.

Initial mass: Between 65 and 120 MSol

This is one of the most massive mass ranges, and it covers one of the most diverse star ranges, as we will see. Again, the stars in this mass range start as O types on the ZAMS (zero-age main sequence) of spectral types from O5 to O1, depending on metallicity.

Main Sequence

Temperatures range from 43,000 to 51,000 K, and luminosities can range from 140,000 solar luminosities to over 800,000 solar luminosities, and will put some of these giants in the list of brightest stars. For example, SR 253 (Sira-Renali 253), a star located in the Crown Cluster, is of O1.5V type, and is truly extreme, with over 100 solar masses inside ball shining with 930,000 solar luminosities. This yields a radius of around 12.2 solar radii, very large for something so close to the ZAMS, and a surface temperature of a whopping 51,300 Kelvins, which makes SR 253 one of the hottest main sequence stars ever observed.

Non Main Sequence

In just 3-5 million years after birth, the massive O type star has exhausted much of its original hydrogen supply and is already starting to burn helium. The star expands slightly, and may gain a giant or supergiant luminosity class. As the helium is fused in the core, convection brings copious amounts of helium and nitrogen to the surface of the star. The star gains an 'f' at the end of its spectral classification, indicating that it is evolving. After some time, so much nitrogen is in the star that it can be classed as a nitrogen-sequence Wolf-Rayet star, albeit a young, hydrogen-rich one. The stellar wind increases, bringing a lot of stellar material into space, sometimes as much as a solar mass every 100,000 years. Over time, the hydrogen content slowly goes down.

When the hydrogen fuel is just gone, the star expands into a hypergiant star, or an Ofpe/WN9 while also cooling in temperature. An example of a massive Ofpe/WN9 is the Peony Star. It is located in the galactic centre and is very bright, with a luminosity of 3.2 million LSol.

WIP

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