

Star Types

Though the variety of star types may seem daunting at first, really almost all variation among stars comes down to mass and age. A star with a given starting mass will predictably “evolve” through major stages as it ages, and many important characteristics of stars are directly related to mass. There are some variations due to composition, but they’re too subtle to be worth bothering with here.





When astronomers observe stars, they classify them by luminosity, their total light output, and effective temperature, which determines their color. A Hertzsprung-Russel (H-R) diagram charts stars by these two values, and shows clearly that the vast majority can be grouped together in a prominent diagonal strip. This is the main sequence, where stars spend the majority of their matter-fusing lives, and it accounts for around 90% of stars in the galaxy today. The lightest main sequence stars are at the bottom right and the most massive are at the top left, with a consistent mass gradient between them. That is to say, massive stars are bright, blue, and hot, and lighter stars are dim, red, and cold.





“Cold” being a relative term; the coldest luminous stars are still over 2400 K, but the very hottest push past 50,000 K. Many people tend to exaggerate the color difference; incandescent lightbulbs have a similar temperature to the coldest main sequence stars, and so put out a similar spectrum of light, and candles are even colder. The range of colors seen on a planet orbiting one of these cold stars would be noticeably shifted towards orange, but not uniformly blood red as is sometimes depicted—and human color perception tends to adjust to ambient conditions anyway. The hottest stars can be quite blue, but as we’ll see in a moment we don’t need to worry much about the conditions of planets orbiting them.





Because of the close relationship between mass, luminosity, and effective temperature, we can roughly predict the latter two values for a main sequence star based on mass:







T eff = effective temperature (K)

M = star mass (solar masses)

(unreliable outside ~0.2-20 solar masses)





Estimating luminosity requires different formulas for different mass ranges:







L = luminosity (ratio to sun)

M = star mass (solar masses)





Actual values will vary due to composition, age, and other factors, but these are decent ballpark numbers. A good set of reference values can be found at the ever-informative Atomic Rockets website .

In order from bluest and hottest to coldest, these stars are grouped into the spectral types O, B, A, F, G, K, and M (yeah, it’s a weird sequence, they didn’t know the proper order when they started naming spectral types) with divisions at roughly 30,000, 10,000, 7,500, 6,000, 5,200, and 3,700 K. Each type is subdivided by appending a number from 0 to 9, again in order from hot to cold—you will also sometimes hear researchers refer to “early” or “late” members of a type, in the same order as these numbers. The sun is a G2 star, comfortably in the middle.

But stars don’t sit still. Very young stars are still contracting under gravity, and the extra energy causes them to start briefly brighter and then “descend” onto the main sequence, moving down on the H-R diagram.

Blue numbers are masses in solar masses of stars moving along the blue tracks, red numbers are ages of stars as they pass the red isochrones (lines of equal age). Stahler and Palla 2008

On the main sequence, helium builds up in their cores and causes the cores to contract and grow hotter over time, pushing the outer layers out; the overall effect is to make stars brighter and slightly hotter as they age, shifting them up and left on the H-R diagram. Once they run out of hydrogen fuel, they leave the main sequence and evolve through various post-MS stages. Generally this will push them towards the upper right corner of the H-R diagram, though stars of different masses can pass through a lot of odd detours.





These dying stars can pass through several spectral types as they age, so to distinguish them from the main sequence astronomers also group stars into classes in order of decreasing luminosity (though there’s some luminosity overlap; they only appear distinct on the H-R diagram). Classes 0, Ia, Ib, II, III, and IV are all varieties of post-MS stars, collectively called “giants” because they’re much larger than their MS predecessors. Class V is main sequence stars, which are also called “dwarfs” even though the largest O-type dwarfs can be larger than some giants. Class VI is a subset of MS stars, called “subdwarfs” that are particularly metal-poor and so dimmer than similar-mass class V stars. Class VII, at the lower right corner of the H-R diagram, includes white dwarfs, the final evolutionary stage for many stars once fusion in the core stops entirely.



Spacepotato, Wikimedia

Though many stars share similar evolutionary stages, the timing of stages can vary widely between stars, again based on mass. As I mentioned in the last post , larger stars live dramatically shorter lives. Even though they have a larger fuel supply, the greater pressure within their cores causes them to burn through their fuel much faster. Additionally, larger stars have poorer internal convection, so their cores will run out of fuel even while there’s abundant hydrogen in the outer layers. Large stars can die even while over 4/5 of their mass remains hydrogen .





We can ballpark the time a star spends on the main sequence based on mass and our prior estimate of luminosity, but again it’s a rough estimate and there’s a good deal of individual variation:







t = main sequence lifetime (billions of years)

M = star mass (solar masses)

L = luminosity (ratio to sun)





If we want a planet to have time to develop intelligent life, we’d better be sure its star lasts long enough. The largest O-type stars live a scant few million years; too short to finish forming planets, let alone develop life. If we want a star to last for at least 1 billion years to give life a good chance to appear, the largest it can get is about 2.2 times the sun’s mass, a large A-type star. For a lifetime of at least 4 billion years, about the time for complex life to emerge on Earth, we have to limit ourselves to F-type stars under 1.4 times the sun’s mass.





habitable zone (HZ)—the range of orbital distances where a planet can have liquid water on the surface—moves outward. Excepting an especially fortuitous case of planetary migration, a single planet can only remain within this zone for so long. The presence of life on a world may actually help But the window of opportunity for life to appear on a planet is not the same as the star’s lifetime. As stars brighten with age, the(HZ)—the range of orbital distances where a planet can have liquid water on the surface—moves outward. Excepting an especially fortuitous case of planetary migration, a single planet can only remain within this zone for so long. The presence of life on a world may actually help increase this habitable period , but only to an extent .



Source

So even though the sun has another 6 billion years of life left in it, within 1 or 2 billion years the Earth’s atmosphere will be stripped away, the oceans boiled off, and the planet left a lifeless husk. Because larger stars have larger habitable zones, the maximum habitable time is a larger proportion of their total lifetime, but to get a habitable time comparable to the current lifetime of Earth, our limit is a midrange F-type star with 1.2 solar masses . Fortunately, the longer lifetimes of smaller stars also makes them more common (they also form more often today), meaning that roughly 98% of main sequence stars in the galaxy today are below this mass limit.





Now that we’ve situated ourselves within the range of star types, let’s explore the differences between stars of different masses and how hospitable they may be to life. Note that I’ve chosen these categories more to reflect post-MS evolution than main sequence habitability, though I’ll discuss that as well.

0.08-0.25 Solar Masses; Late M





But even if the atmosphere survives, any life that develops at this early stage could be in trouble. The XUV output of a young red dwarf could cause harsh surface radiation for planets with oxygen-poor atmospheres , but as mentioned these planets are likely to form oxygen-rich atmospheres anyway even before life appears. But this oxygen could itself be an issue; though we think of it as vital now, to early life free oxygen was toxic, and the first appearance of a few centibars of oxygen in the atmosphere caused a mass extinction (you might argue that oxygen was toxic to early life only because it developed without it, but the presence of oxygen also blocks some chemical pathways that may have been vital to the development of earlier life—it’s a subject we’ll discuss in more detail in a later post). So though an oxygenated atmosphere may be beneficial to life in the long term, it may inhibit the appearance of life in the first place. Various mineral sinks may be able to absorb the excess oxygen, but that just gets us back around to the issue of excessive stellar radiation.





The inconsistency of the XUV radiation may also be problematic; early life will have difficulty adapting to months or years of low radiation punctuated by brief, intense flares. And once the star quiets down, radiation in the HZ may actually become too low , reducing the surface production of organic molecules necessary for life . But presumably there’s an intermediate stage of ideal conditions in there, and any life that formed before then could have survived underground or in the deep ocean. Even if life didn’t appear until the end of the star’s active stage, a delay of a few billion years is insignificant next to a habitable era tens or hundreds of billions of years long.





Visible light too is lower in the HZ, and shifted towards lower-energy red wavelengths, but it should still be sufficient to support photosynthesis . Some authors have speculated that plant life in a red dwarf system should develop to absorb as much of the little visible light as possible, so it may appear black to human eyes (The exact mechanics behind why plant life on Earth uses the spectrum of light it does is a bit complicated, so I’ll tackle it in more depth in a later post regarding biochemistry).





Even if a planet survives past the early bright period, a further danger comes from tidal forces. Just as the moon’s gravity causes tides on Earth, the sun causes tides as well; the sun’s gravity is stronger on the day side of the planet than near its center, so a bulge of gas, water, and even rock forms on the point facing the sun; similarly, the sun’s gravity is weaker on the night side of the planet than near the center, so a second bulge is formed on the point furthest from the sun of material “left behind” as the planet is accelerated towards the sun. As the planet rotates it moves these bulges out of position, but the sun’s gravity drags them back, producing internal friction that both heats the planet and slows its rotation.

Source

For the Earth, tidal forces from the sun aren’t too important. The moon’s influence is stronger, and has slowed Earth’s rotation—lengthening the day from 4 to 24 hours. But a planet orbiting in the HZ of an M-type star will experience much greater tidal forces from the star. It’s likely that shortly after formation most such planets will be tidal-locked to their star, causing them to enter synchronous rotation ; their day is exactly as long as their year, meaning that one side—the dayside —permanently faces the star and the other side—the nightside —is in eternal darkness. This eliminates most tidal friction, though axial tilt (obliquity) and change in orbital distance (eccentricity) can still cause some, and will tend to be dampened over time as well. Given enough time and no competing influences, tidal forces will lock a planet into a perfectly circular orbit with no obliquity, and both the substellar point —the point in the middle of the dayside directly facing the star—and the terminator —the line dividing the dayside and nightside—standing completely stationary on the surface.

We used to think that this was the end of the story, with the nightside locking away any but the thickest atmospheres in ice and the dayside left a barren landscape by intense sunlight—leaving, at best, a thin, barely habitable twilight zone constantly wracked by intense winds. But once again, recent modelling has been more optimistic. Atmospheric and ocean circulation should keep the nightside above 240 K on average and limit ice formation to a ~10 m layer over water and ~1 km layer over continents insolation (light intensity from the star on the surface) even with much thinner atmospheres and smaller oceans compared to Earth. This means the distribution of continents on the nightside could alter sea level on the dayside, but is unlikely to leave it completely desiccated except on the driest worlds. On the dayside, a feedback effect of increasing cloud cover for increasing(light intensity from the star on the surface) could stabilize the climate , increasing both the habitable area on the surface and size of the star’s HZ .



Yang et al. 2014

For a perfectly tidal-locked world, the star would remain motionless in the sky. Shadows would be more-or-less permanent, and even in temperate or fairly warm regions there may be frozen valleys that never receive direct sunlight. Plants may experience fierce competition for access to sunlight, which may alter the sort of forest ecosystems that could develop—undergrowth may be in trouble if it’s easier for tree canopies to completely monopolize the light.

But it’s also possible for slight imperfections in the planet’s orbit to cause the star to appear to shift slightly in the sky. Obliquity will cause the star to oscillate north and south over the course of the year, and eccentricity will cause it to oscillate east and west. This can cause strips of land to cross over between the day side and night side, and other regions will experience an odd sort of seasonality due to the sun getting higher and lower in the sky. The two effects in combination could even cause “summer” to circle around the perimeter of the day side, with “winter” on the opposite side of the planet. Even a planet with very strong tidal forces is likely to develop these orbital imperfections if there are other planets in the system exerting a gravitational influence. But if they grow too large, the tidal heating may produce extreme volcanic activity that prevents life from developing on the surface.

Libration of the moon from Earth's perspective. Tomruen, Wikimedia

But tidal-locking doesn’t inevitably lead to 1:1 synchronous rotation; planets with thick atmospheres could experience uneven heating that will lead to tidal effects altering their day length

3:2 spin-orbit resonance like Mercury in our solar system, completing 3 revolutions about its axis for every 2 orbits around the star, which means that 1 solar day lasts for 2 orbital periods . A planet with some eccentricity (~0.15-0.3) may also settle into like Mercury in our solar system, completing 3 revolutions about its axis for every 2 orbits around the star, which means that 1 solar day lasts for 2 orbital periods

. For a world orbiting in the HZ of a minimum-mass red dwarf, these days can be as short as 10 Earth days—but tidal forces in the HZ are strongest for such a star, strong enough to either preclude such an orbit from forming or producing enough heat in the planet’s interior to induce extreme volcanic activity. For a world orbiting a larger star, nights would be long but still shorter than the seasonal nights of the poles on Earth, and with much brighter days. We can easily imagine all life on such a world developing a hibernation-boom lifecycle like we see in the taiga regions of Earth.





more eccentric planet may also be in a 2:1 resonance (0.3-0.4 eccentricity, 1 solar day/orbit) or a 5:2 resonance (0.4-0.47 eccentricity, 2/3 solar days/orbit)

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blue dwarf, both much hotter and much brighter than before. Stars of about 0.2 solar masses and above can even become brighter than our sun is now. For the most part this marks the end of habitable conditions for that system; planets that used to be in the HZ will be baked dry, and for most blue dwarfs the luminosity—and thus position of the HZ—changes too quickly for any other planets to be habitable for long. Every red dwarf in the universe today is still quite young, and none will leave the main sequence for a long time, but theoretical modelling of their interiors gives us a pretty good idea of what will happen when they do. When a red dwarf eventually runs low on hydrogen fuel, the core contracts and warms, and the star spends several billion years as a, both much hotter and much brighter than before. Stars of about 0.2 solar masses and above can even become brighter than our sun is now. For the most part this marks the end of habitable conditions for that system; planets that used to be in the HZ will be baked dry, and for most blue dwarfs the luminosity—and thus position of the HZ—changes too quickly for any other planets to be habitable for long. The one exception is for stars close to 0.16 solar masses, which will stabilize at about 1/3 as bright as the current sun and then stay there for 5 billion years . This means that planets that had been cold, frozen, and dead for the near 3-trillion year main sequence lifetime of the star could thaw and form life, and that life could plausibly develop to the point of intelligence, all within this “short” episode at the end of the star’s life.



Evolutionary tracks of low-mass stars, starting from the middle right. Adams et al. 2005

Well, some planets might do this. Ice is very reflective, so the light required to melt a frozen planet is higher than that required to keep an already-warm planet stable. Exactly how much higher depends on the spectrum of light; ice reflects less infrared than visible light. So planets melted by a main-sequence M-type star as it brightens over its lifetime could be habitable afterwards, but by the time a K, G, or F-type star—or a blue dwarf—manages to melt a planet, it will be producing enough light to push that planet past habitable conditions to something more resembling Venus . However, tidal-locked worlds are also easier to melt, and a planet orbiting a star for trillions of years will probably become tidal-locked, so habitability around a blue dwarf is still plausible.



Numbers are surface temperatures in kelvin. Yang et al. 2017



Finally, when 99% of the star’s initial hydrogen fuel has been consumed, fusion in the core ceases and the star collapses inwards until it is stopped by electron degeneracy pressure, a quantum mechanical effect that prevents electrons from being packed too closely together. The remnant white dwarf (a D-type star) retains a great deal of heat, and so continues to radiate light with gradually decreasing luminosity and effective temperature for at least 1015 years, possibly much longer depending on the nature of dark matter and proton decay. Once it has cooled to the temperature of background radiation (which, mind you, will be much lower in the far future) it will be a black dwarf.





So even though fusion has stopped, there’s still some light to work with if we want to extend this star’s habitable period even further. Initially the white dwarf is too hot, producing intense XUV radiation, but within a few billion years it can cool enough to form a habitable zone in an extremely close orbit (~0.01 AU) that a planet could remain within for as much as 8 billion years .

However, we have to ask how a potentially habitable planet would end up in such a position. If it had formed there initially it would have been baked under intense light for trillions of years (or, in the case of larger stars that also form white dwarfs, swallowed during the red giant phase). A planet could migrate inwards from a safe position in the outer solar system into the habitable zone, but in doing so would have to pass through a period of high eccentricity, during which it would experience intense tidal heating that would boil off any oceans or atmosphere and potentially even melt the surface .



Perhaps such a world could later be replenished with water by later comet impacts, or a new planet could form in place after some cataclysmic event. But in such close orbits, even a tiny amount of orbital eccentricity can cause intense tidal heating. At the prime period of habitability, 5 billion years after the white dwarf forms, an eccentricity of more than 0.0001 would be sufficient to cause a runaway greenhouse effect for planets in the HZ, rendering them uninhabitable . Venus, the least eccentric planet in our solar system, has an eccentricity of 0.0068. In a small, tight system, any other planets would likely induce eccentricities greater than this even as extreme tidal forces were working to reduce them. Perhaps it could be possible for a combination of tidal heating and light to warm a planet outside the traditional habitable zone to liquid water temperatures, but for reasons we’ll explore in the next part this is less likely to maintain a stable climate in the long term. It’s also worth noting that tidal effects tend to push close-orbiting planets outwards—as our Moon is moving outwards from the Earth—contrary to the inward movement of the habitable zone.





So an intelligent spacefaring civilization could potentially find some use for the light from a white dwarf, but it’s unlikely to do less advanced life much good.

0.25-0.8 Solar Masses; Early M to K





The next group includes more red dwarfs as well as orange dwarfs, with MS lifetimes ranging from 20 billion to 1 trillion years. They’re another large group including about 47% of nearby stars, 3/4 of which are M-types below 0.45 solar masses.

In terms of main sequence habitability, the low end of this group in terms of mass has much the same challenges as the previous one, and the high end is similar to the sun and probably just about as habitable. Some scholars consider K-type stars the ideal for life, as they have longer lifetimes than the sun and so provide longer habitable periods for their planets, but are still large enough to avoid many of the drawbacks of M-type stars.





What unites this group is their post-MS evolution. Similar to late M-types, none of these stars have yet left the main sequence, but when they do they’re expected to follow much the same evolutionary path as the sun and similar stars up to 8 solar masses. Unlike the previous group, interior convection is poorer and the core may exhaust its hydrogen fuel while some hydrogen remains in the outer layers of the star. When this happens, the interior contracts and temperature rises until fusion of hydrogen begins in a shell around the core. As the mass of the core increases with the products from this reaction, pressure in the shell builds and the rate of fusion increases, pushing the outer layers of the star outward and causing the star to become larger, brighter, and redder. This is the red giant phase of stellar evolution.

astronomical units (AU, the average distance between the Earth and sun) As a star develops into a red giant, the habitable zone sweeps outward through the solar system at an increasing speed, so a planet around a sunlike star with an orbital radius of 2(AU, the average distance between the Earth and sun) could expect to remain in the habitable zone for over 3.5 billion years across the late main-sequence and red giant stages, whereas one 3 AU out would only have 250 million years . As always, smaller stars develop slower and provide longer habitable periods , as much as 9 billion years for an ideally-placed planet after the red giant stage begins for an M1 star .





If life had already developed on a planet closer to the star earlier in the system’s history, it’s possible it could be carried to the outer system by meteorites in this stage, giving these planets a head-start—or they could have developed their own life in a subsurface ocean—but below a billion years or so of habitability it’s doubtful that any could proceed through all the evolutionary stages of forming an oxygen-rich atmosphere and complex surface life.





A world orbiting in the HZ of a red giant would find their star to be massive in the sky, but fairly dim nonetheless. The spectrum would resemble an M-type red dwarf, and because these are necessarily old systems it’s more likely that planets will be tidal-locked to the star. So in many respects these are similar to red dwarf systems.





Eventually, the hydrogen in the shell around the core is exhausted, and the outer layers collapse back inward, forming a white dwarf.

0.8-8 Solar Masses; G to Late B

Image of the sun in UV. NASA/SDO (AIA)

And now we come to familiar ground, the yellow dwarfs—though many will appear white, but “white dwarf” is already taken. This is pretty broad group, with MS lifetimes from a mere 10 million years to 20 billion. Altogether they account for about 11.9% of nearby stars, and again it’s a skewed distribution with 9/10 being G and F-type stars below 1.4 solar masses.





As I already discussed, only G and late F-type stars could keep a planet in the habitable zone long enough to plausibly form complex life. Early F and late A-type stars might host simple life, but probably nothing more. Don’t expect to see life around B-type stars.

These stars will form red giants like the last group, and the smallest of these stars could plausibly support life during this stage. But unlike smaller stars, the core of one of these stars will eventually become dense enough to fuse helium into carbon and oxygen. The expansion of the outer layers ceases and reverses, and the star becomes a hotter and dimmer horizontal branch star (the name derives from the distribution of stars on the H-R diagram).

asymptotic giant branch (AGB) stage, with fusion once again restricted to shells around the core—now an inner helium-burning shell and outer hydrogen-burning shell. The outer layers reverse course again and continue to expand, brighten, and cool to a greater extent than during the red giant phase. The helium shell rapidly consumes its fuel and ceases fusing, but helium produced in the hydrogen shell causes it to reignite every 10,000 to 100,000 years. Each time, luminosity spikes and some of the star’s outer layers are completely blown off into space, which can strip away significant portions of the atmosphere of earthlike planets After a couple hundred million years at most, the helium in the core is exhausted and the star moves into the(AGB) stage, with fusion once again restricted to shells around the core—now an inner helium-burning shell and outer hydrogen-burning shell. The outer layers reverse course again and continue to expand, brighten, and cool to a greater extent than during the red giant phase. The helium shell rapidly consumes its fuel and ceases fusing, but helium produced in the hydrogen shell causes it to reignite every 10,000 to 100,000 years. Each time, luminosity spikes and some of the star’s outer layers are completely blown off into space, which can strip away significant portions of the atmosphere of earthlike planets as far away as the Kuiper belt . In combination, these two effects make AGB systems effectively uninhabitable.





After a few million years the star has lost over half its mass and what remains is too deficient in hydrogen for fusion to continue, so the core collapses inwards as some of the remaining outer layers continue to be lost to space. What remains forms a white dwarf, though with a different composition from those formed by less massive stars.

8-30 Solar Masses; Early B to Late O

Technically we could call these “blue dwarfs” but again the name is taken so, whatever, they’re big stars. They have short MS lifetimes of only 500,000 to 10 million years and account for most of the last 0.1% of nearby stars, but are so bright that they’re prominent in the night sky. There’s no main sequence habitability to speak of—the most massive don’t even last long enough to form planets—so we’ll jump straight past that to their final days.





A star this massive begins fusing helium in its core almost as soon as hydrogen fusion ceases, which prevents core collapse and the associated brightening when the outer layers expand and cool. It may blow off its outer layers of hydrogen during this stage, leaving the bare core as a Wolf-Rayet star (W-type) with an effective temperature as high as 100,000 K.

Meanwhile, it will continue to fuse heavier elements in its core past carbon and oxygen to neon, sodium, and magnesium for stars of 8-10 masses, and onwards in heavier stars to silicon, sulfur, argon, calcium, and finally iron (or rather, unstable nickel that soon decays to iron). Smaller stars cannot fuse past magnesium, and for larger stars fusing iron (or nickel) consumes more energy than it produces. In either case, the core no longer produces outward pressure by fusion, and is held up only by electron degeneracy pressure, as with a white dwarf. But fusion continues in shells, feeding more material into the core.

Distribution of elements in an evolved massive star (not to scale). Rursus, Wikimedia

Once the core reaches 1.4 solar masses, electron degeneracy pressure is finally overcome and protons and electrons are forced to merge together into neutrons. The core collapses inwards at astounding speed until it is halted by neutron degeneracy pressure. The combined shockwave produced by the sudden halt and energy produced by nuclear reactions and compression in the core produces a massive explosion, termed a supernova, which blows away the outer layers of the star and either destroys orbiting planets or at least strips away their atmospheres and outer layers.

What remains of the core is now an extraordinarily dense neutron star, containing as much as twice the sun’s mass in an object less than 30 km in diameter. It’s largely composed of compacted neutrons—a material called neutronium—acting in effect as a single immense atomic nucleus, but distinct atoms can exist in the outer layers in highly compressed, degenerate forms.





In terms of habitability, neutron stars combine many of the worst aspects of other stars. They necessarily form from a supernova, so no habitable worlds could remain from their previous lives as main sequence stars; they’d have to reform atmospheres, if they survived at all, or new planets would have to form or be captured from elsewhere. Though more massive than white dwarves, the nature of neutronium gives neutron stars a much lower heat capacity, so lacking an additional energy source they’ll cool too rapidly to support a long-term habitable zone.





But many neutron stars do have an extra energy source in the form of angular momentum. This momentum is conserved as an immense giant star collapses into a city-sized neutron star, so they are born rapidly spinning, typically completing a revolution every 0.01-10 seconds. Over time interactions with the star’s magnetic field will cause this energy to be lost as radiation, slowing the spin rate and creating intense beams of light from the magnetic poles all across the electromagnetic spectrum. The magnetic poles do not necessarily align with the axis of spin, so these beams can sweep across the sky. From a distance, these stars appear to emit regular radio pulses, hence the name pulsars. Most neutron stars form with too little angular momentum to keep producing light this way for more than a billion years, but neutron stars that form in binary systems can gain more energy by accreting gasses from their partner, reaching angular speeds as high as 1 revolution per millisecond, and thus remain productive for much longer.





NASA

Whether or not a planet encounters these beams depends on the particular geometry of its orbit and the star’s spin and magnetic poles. If it doesn’t encounter them, to get enough light to stay warm it would have to orbit extremely close to the neutron star and so experience intense tidal forces. If it does encounter the beams, it will be blasted by gamma rays, X-rays, UV light, and energetic charged particles that could strip away the atmosphere and sterilize the surface.



Even here, though, there may be a chance for life: Some researchers believe that if a large rocky planet had a particularly thick atmosphere—as much as 30% of the total mass, for a planet several times as massive as Earth—then a sufficient portion of that atmosphere to protect the surface may be able to survive for billions of years .



The atmosphere would block not only harmful radiation, but visible light as well; so though the surface would be warm enough for liquid water, any life would have to rely on geothermal or chemical energy sources. It would be much like life on the deep sea of Earth, but without the benefit of food dropping from a more productive ecosystem above. Vast, lifeless deserts would separate pockets of simple life living around volcanic hotspots. The chances for complex or intelligent life under these circumstances would seem remote, much less a civilization that could produce spacecraft to breach the thick atmosphere and then withstand the harsh conditions outside.

It’s possible there is a category of star denser than a neutron star, a quark star electroweak star, where the conversion of quarks to leptons in the deepest core provides enough energy to hold back further collapse for upwards of 10 million years where the internal pressure is great enough to push past neutron degeneracy pressure and decompose neutrons into their constituent quarks, but still not great enough to completely collapse the star to a black hole. There may even be another, still denser category of star, an, where the conversion of quarks to leptons in the deepest core provides enough energy to hold back further collapse for upwards of 10 million years quark-nova, which is . Some stars may begin as rapidly-rotating neutron stars after they supernova, and then convert to quark stars after they’ve lost much of their angular momentum, producing an enormous amount of energy in the form of a, which is one possible source of gamma ray bursts .





However, there have been no confirmed observations of such stars (though there are some candidates) and we know so little about the bizarre materials they’re made of that we can’t say much about their likely properties, other than that they’re probably broadly similar to neutron stars from the outside.

>30 Solar Masses; Early O

Finally, we come to the very end of the main sequence, a small category of short-lived, massive stars. Though common in the early universe, they’re now vanishingly rare, accounting for less than 0.01% of nearby stars. The most massive known star, R136a1 quasistars up to 10,000 solar masses that outwardly appeared to be gigantic stars but were in fact clouds of gas falling into black holes formed by the compression in their cores , is 315 solar masses, and it seems unlikely that any star could ever get much bigger before the light produced by the core was great enough to blow away the outer layers. Lower metallicity in the past may have raised this limit, and in the very early universe there may have been up to 10,000 solar masses that outwardly appeared to be gigantic stars but were in fact clouds of gas falling into black holes formed by the compression in their cores .





Note that despite being the most massive star, R136a1 is fairly small in size (~35 solar radii) compared to the largest red giants (~10 solar masses, ~1708 solar radii) and both are dwarfed by quasistars (~7200 solar radii). Sauropodomorph, Wikimedia



black holes. After one of these stars undergoes a supernova, enough mass remains in the resulting neutron star—about 3 or more solar masses—that its gravity overcomes even neutron degeneracy pressure (and whatever barriers may remain beyond that) and it continues to collapse, perhaps without limit. Compressing so much mass into so small a space creates a gravity field so intense the geometry of space around such an object only allows for travel towards the center, not away; so no light or anything else can escape. Rare though high-mass stars are, they’re the only ones large enough to produce the objects that will eventually come to dominate the universe:. After one of these stars undergoes a supernova, enough mass remains in the resulting neutron star—about 3 or more solar masses—that its gravity overcomes even neutron degeneracy pressure (and whatever barriers may remain beyond that) and it continues to collapse, perhaps without limit. Compressing so much mass into so small a space creates a gravity field so intense the geometry of space around such an object only allows for travel towards the center, not away; so no light or anything else can escape.





Especially massive (>40 solar masses) and metal-poor stars experience a collapse so forceful that it cannot be even briefly halted and the star collapses directly to a black hole without a supernova. Even more massive (130-250 solar masses) and metal-poor stars can experience a pair-instability supernova, which occurs when quantum mechanical processes in the core consume some of the star’s light output, which reduces outwards pressure, which causes the core to partially collapse inwards, which rapidly accelerates fusion, and the star can be completely blown apart with no remnant left.



Black holes themselves produce almost no light. Quantum mechanical effects at a black hole’s boundary can cause energy loss by Hawking radiation, but the total luminosity is inverse to the black hole’s mass, so that it does not emit enough light to be realistically capable of warming a planet to habitable temperatures until the last minutes of its life (which, for any current stellar-mass black hole, will not occur until long past the point when all stars and planets are long gone).





accretion disk, where tidal forces and friction can heat it enough to emit light, and in rare cases However, when matter falls into a black hole it is stretched into an, where tidal forces and friction can heat it enough to emit light, and in rare cases even undergo fusion . A stellar-mass black hole can form an accretion disk by consuming the mass of a binary partner, but such a disk would emit high amounts of XUV radiation, have an inconsistent total light output, and last for only tens of millions of years. A supermassive black hole can sustain a large but cooler disk for longer periods of time, but the disk is so bright that a theoretical habitable zone would be a light-year out or more. Any such planet here would be likely be perturbed from its orbit by the frequently passing stars that would necessarily have to be falling into the black hole to feed its accretion disk (as I mentioned in part 1, much the same issue applies even to planets orbiting stars near the galaxy’s center that might be receiving extra light from the accretion disk).

There is, however, one bizarre proposal for how life might survive near a more quiescent black hole with no accretion disk. A planet in a very tight orbit around a rotating black hole will see the cosmic microwave background blueshifted (the inverse of redshift), both due to gravitational time dilation and the Doppler effect due to its immense orbital velocity. This blueshift could be enough to raise the background temperature above 273 K, just as in the early universe . This light would be concentrated on the side of the planet facing the direction of the planet’s travel, while the black hole itself—occupying a quarter of the sky—would pull heat away from the planet, creating a temperature gradient that could aid in the development of life.





However, the researchers proposing this concept did not calculate the potential tidal effects on such a planet, nor the length of time that this planet would stay habitable considering the continuing redshift of the CMB and time dilation on the surface. Even were such a planet to form, life there would have to contend with issues similar to that of the earlier pulsar-orbiting planet; little available energy, and a deep gravity well separating them from the rest of the universe.





As I said in Part 1, black holes may ultimately host the majority of the universe’s life across its entire lifetime, but such life would have to develop elsewhere first and then reach these black holes.

0.013-0.08 Solar Masses; Y to Very Late M

To finish off our tour of stars, let’s leave supermassive stars and their remnants behind and return to the least massive members of the star family, brown dwarfs. At this point it’s more convenient to state their mass in relation to Jupiter, which is 1/1,047 times as massive as the sun; so these bodies are near to 13 to 80 Jupiter masses (the upper boundary is a bit fuzzy and dependent on metallicity). It’s hard to say exactly how common they are because they’re so dim, but they may be comparable in number to all main sequence stars put together.





Brown dwarfs are sometimes called “failed stars” because the pressure in their cores is too low to fuse ordinary lone-proton hydrogen into helium. However, they can fuse deuterium and—for brown dwarfs over 65 Jupiter masses—lithium. This is sufficient to initially warm the dwarf to the temperature of a late M-type star, but the abundances of deuterium and lithium are so low (about 20 parts per million and 1 part per billion, respectively, in undisturbed gas in our galaxy) that fusion tapers off after a few 100 million years at most—only 10s of millions of years in the smallest dwarfs .

Concept of 2MASS J22282889–4310262. NASA/JPL-Caltech

L-type. After 100 million to 10 billion years they enter the T-type when they cool past around 1300 K, low enough for methane gas to form in their atmosphere. After their initial red, this blue methane turns most dwarfs magenta, with some variation by composition; few brown dwarfs are actually brown. Though the surface is still hot enough for They gradually cool off thereafter, exiting the M spectral type for the recently-coined. After 100 million to 10 billion years they enter thewhen they cool past around 1300 K, low enough for methane gas to form in their atmosphere. After their initial red, this blue methane turns most dwarfs magenta, with some variation by composition; few brown dwarfs are actually brown. Though the surface is still hot enough for phenomena like iron rain , these dwarfs emit very little visible light. Finally, dwarves below around 500 K are considered Y-type, a category that also includes objects below 13 Jupiter masses—conceivably as small as 1 Jupiter mass—that form from collapsing gas clouds like stars but are too small to ever experience any type of fusion.



NASA/JPL-Caltech

In terms of habitability, brown dwarfs suffer from many of the same pitfalls as white dwarfs. The habitable zone sweeps inwards over a period of a few billion years, and from the start is close enough to the dwarf for tidal heating to be an issue for even moderately eccentric planets. Within a billion years the eccentricity constraints are even more stringent than for a white dwarf and the habitable zone is approaching the Roche limit for an Earth-sized planet; the orbital radius at which tidal forces will overcome the planet’s gravity and tear it apart into a ring of debris . Even during the best period for habitability, the peak light output from the dwarf is far into the infrared, making photosynthesis difficult if not impossible.



Brown dwarfs may be of some interest to interstellar civilizations simply because they’re so common and could provide convenient sources of resources, but they’re unlikely to host any complex life of their own.

However, note that all of this only applies to brown dwarfs as they exist today. In the far future, when average metallicity of new stars is several times higher than today, brown dwarfs may be able sustain protonic fusion , making them proper main sequence stars with extremely long lifetimes (incidentally this also means there are some high-mass brown dwarfs around today that were too metal-poor when they formed to enter the main sequence, up to a theoretical maximum of 0.092 solar masses with zero metallicity ). But these stars would also have very low effective temperatures, as low as 273 K for a star of 0.04 solar masses. So in terms of habitability they’re mostly no better than current brown dwarfs, but there may still be a range of masses too low to form MS stars now, but high enough that when they do, they’ll produce just enough light to possibly support life.

Multiple Stars Concept of HD 98800. NASA/JPL-Caltech/T. Pyle (SSC)

multiple-star systems, but current research shows that the exact frequency depends greatly on mass. That about wraps up our tour of star varieties, but stars don’t always appear alone. Many stars form in pairs, or in groups observed to be as large as 7. We used to believe that the majority of all types of stars are found in, but current research shows that the exact frequency depends greatly on mass. Among nearby stars within the galactic disk, over 80% of systems with an O-type primary star have companion stars, while the same is true for only 44% of sunlike star systems and less than 25% of red dwarf or brown dwarf systems (note that this is a count of systems, not stars, which means that the total proportion of individual red dwarfs that are in multiple-star systems is higher) . Higher-mass multiple star systems are also more likely to include more companion stars within each system. Age also plays a role, as stars that form with companions may lose them over time due to gravitational interactions with other stars. Old Population II stars off the galactic disk seem to particularly lack companion stars, but whether because they’ve had more time to lose their companions or their lower metallicity caused them to form with fewer companions in the first place is not yet clear.



Barring any brief upsets, the 2 stars in a binary system both settle into stable elliptical orbits around their common barycenter —center of mass for the system—with equal eccentricities and orbital periods, such that a straight line between the two stars always passes through the barycenter. If they’re of unequal mass, the more massive star orbits closer to the barycenter; for very unequal masses the barycenter may be inside the more massive star.

There are no particular constraints on the distance separating the two stars; they can be light-years apart or so close that they share a common atmosphere. But in the former case it will be much easier for another passing star to pull the binary apart, and in the latter case angular momentum will be lost to friction over time and the two stars will collide and merge. Current observational data indicates that sunlike stars tend to be separated by 10s to 100s of AU, with orbital periods of decades to centuries, while red dwarfs peak an order of magnitude lower on both counts.





Star systems with more than two stars are arranged hierarchically: A trinary system will have one binary pair, and then the barycenter of that pair will be in a binary relationship with another, more distant star. For a 4-star system you can replace that third star with the barycenter of a binary pair, or have the common barycenter of the whole trinary in a pair with another star. And then you could replace that star with a binary, or have two trinaries paired, and so on. A good analogy for these hierarchies is a child’s mobile; each rod is balanced on the center of mass of everything hanging from it, just as the barycenter of every pair in a multiple star system is the center of mass for everything involved in the pair.





Newborn stars may briefly exist in chaotic non-hierarchical associations, but before long these will either settle into a hierarchy or eject extra stars out of the system until they do. Non-hierarchical stable trinary systems are theoretically possible, but have never been observed and require such specific starting conditions that I wouldn’t hold my breath.





So, can life exist in these multiple-star systems? Planets can form in binary systems in two main types of orbits: S-type, where they orbit closely to one star with a much smaller orbital radius than its distant companion star; and P-type or “circumbinary”, where they orbit distantly around the system barycenter with a much wider radius than either of the stars.





S-Type

For a widely separated binary, S-type planets can form just as frequently as planets around a lone star, and with the same properties. If the companion star is dim, the inhabitants of such planets may not even realize they are in a binary system until they develop astronomy and sufficiently powerful telescopes. But if the companion star orbits closer, its gravitational influence could interfere with the formation of planets or knock them out of their orbits after they form.





Modelling of binary systems indicates that Earthlike S-type planets should still be possible with companion stars as close as 5 AU , but while planets have been observed in such systems, there is a generally a paucity of S-type planets for systems with separations less than 100 AU and debris disks—which imply the presence of solid bodies like planets—are similarly less common below 50 AU of separation .

Note that these numbers are most appropriate for sunlike stars. More generally, theoretical modelling of these systems indicates that the extreme limit of long-term stability for the semimajor axis (average orbital radius, roughly) of an S-type orbit is about 1/4 the semimajor axis of the binary pair for equal-mass stars. If one of the stars is more massive than the other, the stability zone increases for planets orbiting the more massive star and decreases for planets orbiting the other star. Eccentricity of the binary orbit will shrink the stability zones for both stars. Altogether, the maximum stable orbit for a planet in a binary system can be estimated from the binary system’s eccentricity and the ratio of the mass of the companion star (the star the planet is not orbiting) to the total mass of the binary (see also table 3 here ; there have been many attempts to model this for various configurations, some producing smaller limits, but this seems to be a typical result).

a max = maximum stable semimajor axis for a S-type planet (any unit so long as a bin is the same)

a bin = semimajor axis of the binary

m = (mass of the companion star) / (total mass of the binary)

e = eccentricity of the binary





In most cases the companion star does not contribute enough light to affect the habitability of an S-type planet . However, were there a planet orbiting a red dwarf with 0.1 solar masses at 0.1 AU (outside the typical habitable zone) and the dwarf was itself orbiting a sunlike star at 1 AU, the planet would receive about 11 times as much light from the larger star, putting it back in the HZ. In such a case it might even be better to treat the planet like a moon of a gas giant in terms of habitability, though the presence of constant dim light on one side of the planet even at “night” (as it will likely still be tidal-locked to the smaller star) and variance in distance from the primary star of 0.1 AU could both lead to interesting climatological effects.





This calculator can give you good estimates for HZs and stable regions for S-type (and P-type) orbits, but note that we’ll discuss the wrinkles of habitable zones in more depth in the next post Roughly half of all binary systems should be widely separated enough to support habitable Earthlike planets in stable S-type orbits (and 10% should be close enough for such worlds in stable P-type orbits) .

P-Type





Once the stars are within a few AU of each other, they can start to form circumbinary planetary systems similar to those for lone stars, and sunlike stars separated by less than about 0.2 AU should be able to form P-type planets in the habitable zone. Several planets have been observed in P-type orbits . Curiously enough, these planets seem more likely to form near the inner limit of a stable orbit—though this may be in some part due to biases in current detection methods—and are less likely to form around binaries with very small separations and orbital periods less than 10 days—perhaps due to tidal effects ejecting planets in the early evolution of such systems .



As before, these values can vary with different stars and configurations, with the inner limit of a stable planetary semimajor axis being around 2.3 times the binary semimajor axis for equal-mass, low eccentricity stars. But in this case, the difference in masses has little effect and the eccentricity is much more important. The minimum stable orbit for a planet can again be estimated based on the balance of mass and eccentricity of the binary: See table 7 here)





a min = minimum stable semimajor axis for a P-type planet

a bin = semimajor axis of the binary

m = (mass of the less massive star) / (total mass of the binary)

e = eccentricity of the binary





Intriguingly, retrograde orbits may actually be more stable , though for large planets so close to the system barycenter they’ll also be exceedingly rare. A captured planet could end up in such an orbit, but then its history as a rogue planet could negatively affect its habitability in the present.





Generally speaking, for all purposes of habitability the star pair in such a system can be treated as a single star with a luminosity equal to the sum of the luminosities of the two stars. There will be some climate variation on a P-type planet’s surface over the period of the pair’s orbital period, but not enough to significantly impact habitability . If the planet’s orbit is in the same plane as that of the binary, one of the stars may eclipse the other, but the change in surface light would be comparable to seasonal changes on Earth, and much briefer.





If the two stars are of different masses, then they will evolve in different ways, which may or may not be beneficial for the long-term habitability of the planet depending on the particular configuration. And if one of the stars in a close-orbiting binary becomes a white dwarf, there is a chance it could accrete gas from its companion, resulting in a nova—wherein enough gas accumulates to undergo fusion in a shell above the degenerate white dwarf core, producing a period of intense light and bombarding the system with radiation and highly accelerated gas—or a type Ia supernova—wherein the white dwarf surpasses 1.4 solar masses and collapses into a neutron star, triggering rapid fusion in the surrounding mass and thus outputting enough energy to destroy the system entirely and eject the companion star.

T-Type

As a third option, some authors have proposed Trojan planets, in the L4 or L5 Lagrange point of one of a binary pair—that is, occupying the same orbit as one of the paired stars but offset by 60 degrees ahead or behind, where the gravitational influence of the stars will tend to keep it stably positioned. We might call this a “

” orbit.





Motion of the Lagrange points for a large body (yellow) with a smaller companion (blue); points 4 and 5 are stable. Anynobody, Wikimedia





However, for such an orbit to be stable in the long term , the mass ratio between the two stars would have to be greater than 25

. If the smaller star is a minimum-mass M8 star—with 0.08 solar masses—then the larger star cannot be smaller than 2 solar masses, making it a late A-type with a lifetime of about 1.5 billion years, too short for a plausible shot at complex life. If you do decide to include such a system in a story—say, if it is visited by an interstellar traveler—then note that, because the stars and planet will all be equidistant from each other, the larger star will be about 47,000 times brighter than the smaller one, or 4.7 million times brighter if only visible light is counted. If the planet were receiving the same amount of light from the primary star as the Earth did from the sun, the secondary would be only 1/12 as bright as the moon in Earth’s night sky.





However, if the secondary in this system is a brown dwarf or gas giant below 0.05 solar masses, then the primary star can be small enough for this to be a plausible home for complex life. The system should be stable so long as the inclination is below ~60° relative to the primary’s equator and eccentricity is fairly low . Indeed, some models indicate such planets should be common, though so far none have been observed .

T rinary

Concept of surface of Gliese 667Cd. ESO/M. Kornmesser





Finally, it is possible for planets to appear in systems of 3 or more stars, and a handful of such planets have been observed. Typically these planets will only orbit one or a pair of stars in such systems in the S- and P-type orbits we’ve discussed, with the distant third star having little impact. But it should be possible for a habitable planet to orbit around three stars, if quite rare.





Theoretical modelling indicates that, for three stars of equal mass, the orbital period of the distant third star with the binary pair must be 4.37 times greater than the orbital period of the binary . This ratio of orbital periods can be as low as 2.37 for greater mass ratios, though that case would require the largest star to be 98 times the smallest.





If we use 3 equal-mass sun-sized stars, and put 2 of them in a binary with a period of 1 day (0.016 AU semimajor axis, probably near the limit before they join atmospheres and merge) with the third orbiting on a comfortable 5-day orbit (0.039 AU semimajor axis between this star and the barycenter of the binary) then a planet would receive insolation similar to Earth at a distance of 1.73 AU, over 10 times the semimajor axis of the trinary system--probably a safe distance.



What the stability constraints are for systems with 4 or more stars is something I haven’t seen any theoretical work on, so I can’t speak to the potential habitability of planets in such a system.

Back Home

Taking it all in, it seems we can order all the candidates for stars that might support life as ideal (G- and K-type main sequence stars), challenged (F- and M-type stars, red giants, blue dwarfs), marginally plausible or short-lived (A-types, white dwarfs, brown dwarfs, pulsars) and essentially impossible (black holes). I would only expect the development of complex and intelligent life to be plausible on planets orbiting the first two categories. Part of this perception may be due to our bias comparing against Earth life with biochemistry optimized for the environment of a G-type star, but on the other hand it would make sense that intelligent life—possibly the first intelligent life in this region of the universe—developed under the most hospitable conditions available.





Teacup”. Given the option, we might as well choose a star that will give us a longer habitable periods and closer-spaced planets that will make interplanetary travel easier, so let’s make Teacup A a K5V orange dwarf modeled after With that in mind, let’s put our example species in an ideal system. We don’t know yet what the eventual language of this species will be like, so for the interim let’s just call the system “”. Given the option, we might as well choose a star that will give us a longer habitable periods and closer-spaced planets that will make interplanetary travel easier, so let’s makea K5V orange dwarf modeled after 61 Cygni A . Compared to our sun it has 70% the mass, 66% the radius, and 15% the luminosity. After an initial warm period of 100 million years, it goes through a couple more variations and then after about a billion years settles into the main stage of its life with an effective temperature of around 4,500 K. A planet comfortably within the habitable zone at this point can expect to spend about 20 billion years there, and potentially twice as much if very well placed. After 44 billion years Teacup A will transition to a red giant, expanding over 3 billion years to a radius of 9 AU and a luminosity 2,500 times that of the sun, then collapse into a helium white dwarf.

Our reference star, 61 Cygni A (lower middle) compared to the sun (left) (upper right is 61 Cygni B, a K7V star). RJHall, Wikimedia





Teacup B, and make it an M3V star modeled after To spice things up just a little, we’ll give this star a companion,, and make it an M3V star modeled after Gliese 581

. So that’s 31% the mass, 30% the radius, and 1.3% the luminosity of the sun (44%, 45%, and 8.7% respectively with regards to Teacup A) though most of that luminosity is in the infrared given its effective temperature of ~3,500 K, leaving it with just 0.2% the visible light output of the sun. Teacup B is sufficiently large that by the time any intelligent life arises on Teacup A, it will have quieted down and settled into the main sequence, where it will remain for about 300 billion years.

Gliese 581 (right) compared to the sun (left). RJHall, Wikimedia