Hues of Light under Alien Suns

I’ve been much outdoors lately, brainstorming my storyline, making notes and traveling. The idea for this post came after I’ve spent the whole day observing daylight and its hues.

If you had ever wondered what colors of light your world has, I might have an easy answer for you. But I think some would want to see the hues for themselves. Well, I do.

Before moving to the main dish I must say this method only works for earth-type atmospheres (unless you know color temperatures for other atmospheres).

Mars with its blue sunsets is a good example of the difference the atmosphere makes.

I also assume our observer is human (his/her eyes are adapted to solar spectrum which peaks at 501 nm, green-yellow portion of it) and we have a single star.

The distribution of a wide variety of physical, biological, and man-made phenomena follow a power law, including the sizes of earthquakes, craters on the moon and of solar flares, the foraging pattern of various species, the sizes of activity patterns of neuronal populations, the frequencies of words in most languages, frequencies of family names, the sizes of power outages and wars, and many other quantities. And I’m making this mathematical relationship my instrument of choice as well.

So, what we need to do is simply compare two stars. (Don’t frown; it’s easy, assuming you use a spreadsheet program).

First of all we’ll need the properties of our Sun and the properties of the star in question (for our examples I will use two stars of spectral classes FV and KV, the Sun being the GV star). These properties are effective temperatures in Kelvins and masses relative to solar. That’s it. The mass of the star is arguably its most important property. And I’m using it as the main component in my power law equation to get what I want. And what I want is the color temperature of illumination. What we are going to do is not very scientific, but it works well under our condition of earth-type atmosphere.

The Sun’s effective temperature is 5778 K and its mass is 1. The F star will be 7500 K and 1.4 solar masses. The K star will be 3700 K and 0.45 solar masses.

My next step will be determining the power relations between the Sun and each of the two stars. I’m using MS Excel spreadsheet, so my formulas look approximately like this:

Exp = LOG(7500; (5778*1.4)) = 0.9916 for our F star

Exp = LOG(3700; (5778*0.45)) = 1.0449 for our K star

For other masses and effective temperatures you’ll need a recalc, of course.

Now then, let’s move onto the next part to get the hues of daylight.

Color temperatures

Color Temperature is a measurement in degrees Kelvin that indicates the hue of a specific type of light source. Color temperatures attributed to different types of light are correlated to visible colors matching a black body (e.g. a star), and are not the actual temperature. High color temperatures are considered “cold” and low are considered “warm”.

Color temperature would not be such an important factor in color perception if people were not adaptable. Our eyes will adjust to various light sources which have different color temperatures. And even though the power spectrum density of the light is different, we still see the light as white.

Simply put, color temperature is the tendency of different “white light” sources to change our perception of a color. The color temperature of a light source causes the colors of everything illuminated to change, but the change is often barely noticeable.

Color Temperatures
Color Temperatures

The effect of sunlight on the perceived color of an object changes with temperature because of how much air the sunlight has to pass through. If the sun is directly overhead (warmer), it passes through less air than it does when it is low on the horizon (cooler). Colors in northern climates, especially in the winter, look different than they do in southern climates.

The processes of absorption and scattering in the atmosphere are responsible for the apparent colors of the sun and the sky. While sunlight falls only on non-shadowed areas, the light of the sky falls on everything, so shadow hues are determined by sky color. The hue gradient is clearly noticeable when the sun and the sky obviously differ in color, like near the sunrise and sunset, and the difference in colors of shadowed and illuminated parts becomes the clearest on light surfaces such as snow.

The color temperatures for solar illumination (outdoors) are:

  • Sunlight (sunrise or sunset) – 2000 K – 3000 K
  • Sunlight (1 hour after dawn) – 3500 K
  • Sunlight (early morning and late afternoon) – 4300 K
  • Sunlight (average noon, summer, mid-latitudes) – 5000 – 5400 K
  • Daylight (sunlit sky) 5500 – 6500 K
  • Overcast sky / haze – 6000 K
  • Light summer shade – 7100 K
  • Average summer shade / hazy sky – 8000 K
  • Open shade on clear day – 9000 K
  • Heavily overcast sky – 10000 K
  • Sunless blue skies – 11000 -12000 K
  • Open shade in mountains on a really clear day – 20000 K

In order to get our alien hues, we need to do some simple math again. For our example I will take the color temperature for solar sunrise/sunset, 2000 K. The general formula for our stars will be the following:


TCs is the color temperature of stellar light in the atmosphere, CTe is the light color temperature from the hue list above, Mstar is the mass of the star relative to solar, and Exp is what we calculated in the previous step.

For our F star the average sunrise/sunset hue will be 2619 K, brighter and sharper than the Sun’s; for our K star this hue will be around 1221 K, dimmer and softer than the candle light. To see these temperatures in color, you might consider this chromaticity (hue and saturation) table.

Also, check out this page of the The Planetary Habitability Laboratory (PHL) >> Sunset of the Habitable Worlds.

Recommended read: The Physics and Chemistry of Color: The Fifteen Causes of Color by Kurt Nassau, 2nd Ed, 2001.


Have you ever wondered why the sky is blue, or a ruby red? This classic volume studies the physical and chemical origins of color by exploring fifteen separate causes of color and their varied and often subtle occurrences in biology, geology, mineralogy, the atmosphere, technology, and the visual arts. It covers all of the fundamental concepts at work and requires no specialized knowledge.

This is an excellent scientific overview of human perception of color.

The book IS expensive, but if you have access to a good library, it is a good choice.

Stellar Zoo: Meet the Species

This time I’ll focus on stellar classes and some of their most important (and unique) features to help navigate right to your choice. If you are reading this, I assume that you’ve probably looked through some other articles about stellar classes elsewhere (e.g. Wikipedia, Stellar classification) and deliberately search for more detailed info for your future world-to-be. This is exactly what I have in mind, presenting current article on stellar species and their natural habitat.

House of beasts

Our star is a member of a giant cosmic structure known as the Milky Way Galaxy, along with up to 400 billion other stellar folks. It is agreed that the Milky Way is a spiral galaxy, with observations suggesting that it is a barred spiral galaxy. Its stellar disk is approximately 100 000 light-years (30 kiloparsecs, 9*10^17 km) in diameter and is considered to be, on average, about 1 000 ly (0.3 kpc) thick.

The Milky Way is part of the Local Group of galaxies and is one of hundreds of billions galaxies in the observable universe. And that’s a lot of places to reside in, at least in fiction.

So, what is so special about our Galaxy? The answer is simple: we know it has life – us.

Not every galaxy in the universe is suitable for life (as we know it – sorry, I’ll stick with this again since I’m very interested in it being a member of one myself). So, if you are choosing a stellar home from a range of existing galaxies, take a good look at its specs. Galaxies can be distinguished by shapes, sizes, composition and stellar formation activity, and their relationships with neighbors. They are dynamic systems and very violent ones, to that. They live and they die, and they feed on their own kind.

Now, let’s look at the features of our stellar home and how is it suitable for life to exist.

It’s a spiral galaxy with relatively high chemical abundance of interstellar medium, active stellar formation regions and calm places far away from hazardous to life star forming regions.

Two huge bubbles that emit gamma rays have been found billowing from the center of the Milky Way galaxy, extending 25,000 light-years north and south from the galactic core. Gamma rays are the most energetic forms of light, and in space they tend to come from violent events such as supernovae or from extreme objects such as black holes and neutron stars.

These bubbles are made of hot, charged gas that’s releasing the same amount of energy as a hundred thousand exploding stars. Scientists are conducting more analyses to better understand how the structure was formed. One possibility includes a particle jet powered by matter falling toward the supermassive black hole at the galactic center. While there is no evidence the Milky Way’s black hole has such a jet today, it may have in the past. When galactic black holes are actively feeding, they tend to spew jets of particles traveling at nearly the speed of light. Astronomers have found such active galactic nuclei elsewhere in the universe, but have never before seen any convincing proof of this process happening in the Milky Way. The bubbles also may have formed as a result of gas outflows from a burst of star formation, perhaps the one that produced many massive star clusters in the Milky Way’s center several million years ago. If that’s the case, the giants could now be dying together, creating an outbreak of supernovae.

Update Jan 2013: observations tell the bubbles are star-power.

Update Sept 2013: around 2 million years ago, a bright moon-sized smudge was also visible in Earth’s southern sky.

Update August 2014: Despite extensive analysis, Fermi bubbles defy explanation. The mystery remains.

Galaxies are in motion and they crash into one another from every conceivable direction. It is believed that Andromeda and the Milky Way galaxies will collide in just a few billion years. During the collision our Sun and solar system may be stripped away from its present orbital radius and come to reside in Andromeda. If the two galaxies merge they will form an elliptical galaxy.

These images show 6 different snapshots of galaxies at different stages of merging.
These images show 6 different snapshots of galaxies at different stages of merging. Credit: NASA, ESA, the Hubble Heritage Team (STScI/AURA)-ESA/Hubble Collaboration and A. Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University), K. Noll (STScI), and J. Westphal (Caltech)

Galaxies exchange stars. In fact, some of the stars from the Sagittarius Dwarf Elliptical Galaxy (SDEG) may have been captured by the Milky Way billions of years ago (Chou, et al., 2009; Majewski et al., 2003). Moving in a roughly polar orbit around the Milky Way as close as 50,000 ly from its galactic center (Ibata et al., 1997), SDEG comes far too close to the Milky Way for the dwarf to remain intact from the resulting gravitational tides. Here is a dramatic display of galactic cannibalism. Although it may have begun as a ball of stars before falling towards the Milky Way, SDEG is 10,000 times smaller in mass than the Milky Way, so it is getting stretched out, torn apart and gobbled up by our Galaxy. The Canis Major Dwarf Galaxy (CMDG) is even closer (Martin et al., 2004). SDEG and CMDG are both older galaxies and the Milky Way likely stripped millions of stars from them (Chou, et al., 2009; Martin,et al., 2004; Majewski et al., 2003).

Our Galaxy hosts more than a dozen known stellar streams, the remnants of satellite galaxies that were gravitationally torn apart and consumed. Most of the other streams loop around the plane of our disk-shaped galaxy, like octopus tentacles grasping a dinner plate.

The Milky Way has 14 known dwarf galaxies orbiting it, and recent observations have also led astronomers to believe the largest globular cluster in the Milky Way, Omega Centauri, is in fact the core of a dwarf galaxy with a black hole in its center, which was at some time absorbed by the Milky Way.

If colliding, merging, and interacting galaxies and their stars and solar systems harbored life, then not just stars, but living organisms would have also been transferred between galaxies. Therefore, even if life had been fashioned only once, in some ancient galaxy which predates our own, the descendants of this life form could have easily journeyed from planet to planet, from solar system to solar system, and from galaxy to galaxy.

A living planet with an atmosphere, orbiting within the habitable zone of a sun-like star, will be subjected to that star’s solar winds. In the case of Earth, the powerful magnetic field protects the planet from these winds. These winds will periodically increase significantly in strength and eject air-born microbes into space and distribute them not just to neighboring planets, but outside the solar system where they may come to contaminate collections of Oort Cloud stellar objects and passing comets. After hundreds of millions of years survivors may fall upon a planet orbiting a distant star.

However, this does not mean that all this free floating molecular material would have achieved life. It needs the right address and the right time of delivery. Nevertheless, given even the odds of 1 in a trillion, it can be predicted that life could have arisen in multiple galaxies through chance combinations of the necessary ingredients in the nebular clouds.

So, the question is which galaxies are best suited for life formation and evolution?

We may consider galaxies as composed of some “building blocks” whose scale and importance varies from galaxy to galaxy. These include a nucleus, bulge, lens, bar, spheroidal component, disk (with arms, rings, etc), and an unseen halo which is significant for the mass distribution and dynamics. The order revealed by existing classification systems tells us that these are not really independent; disk/bulge correlates overall with arm morphology, for example. These relations hold clues to both the formation and evolution of galaxies, and their suitability for life.

Looking at these classification systems, I can name at least four galactic factors that are vital for our objective: stellar population, chemical abundances in the interstellar medium, star formation history along with integrated stellar spectrum and location of potential life-bearing system in the galaxy.

A spiral galaxy like the Milky Way contains stars, stellar remnants and a diffuse interstellar medium (ISM) of gas and dust. This medium has been chemically enriched by trace amounts of heavier elements that were ejected from stars as they passed beyond the end of their main sequence lifetime. Higher density regions of the interstellar medium form clouds, or diffuse nebulae, where star formation takes place. In irregular galaxies, they may be found throughout the galaxy, but in spirals they are most abundant within the spiral arms. A large spiral galaxy may contain thousands of H II regions.

Together with irregulars, spiral galaxies make up approximately 60% of galaxies in the local Universe. They are mostly found in low-density regions and are rare in the centers of galaxy clusters.

In contrast to spirals, an elliptical galaxy loses the cold component of its interstellar medium within roughly a billion years, which hinders the galaxy from forming diffuse nebulae except through mergers with other galaxies. Gas is the raw building material for stars and the more gas a galaxy contains, the more stars are born.

Most elliptical galaxies are composed of older, low-mass stars, with a sparse interstellar medium. Star formation is minimal or has finished after the initial burst, leaving them to shine with only their aging stars. In general, they appear yellow-red, which is in contrast to the distinct blue tinge of a typical spiral galaxy, a color emanating largely from the young, hot stars in its spiral arms.

Elliptical galaxies are believed to make up approximately 10–15% of galaxies in the local Universe but are not the dominant type of galaxy in the universe overall. They are preferentially found close to the centers of galaxy clusters and are less common in the early Universe.

Despite the prominence of large elliptical and spiral galaxies, most galaxies in the universe appear to be dwarf galaxies. These galaxies are relatively small when compared with other galactic formations, being about one hundredth the size of the Milky Way, containing only a few billion stars. Star formation in dwarf galaxies appears to be complex, with many systems exhibiting multiple episodes of star formation.

Take, for example, the Magellanic Clouds (Large Magellanic Cloud and Small Magellanic Cloud). These dwarfs are members of our Local Group and are orbiting our Galaxy. While both are often considered an irregular type (the NASA Extragalactic Database lists the Hubble sequence type as Irr/SB(s)m), they contain very prominent bars in their centers, suggesting that they may have previously been barred spiral galaxies. Aside from their different structure and lower mass, they differ from our Galaxy in two major ways. First, they are gas-rich; a higher fraction of their mass is hydrogen and helium compared to the Milky Way. They are also more metal-poor than the Milky Way; the youngest stars in the LMC and SMC have a metallicity of 0.5 and 0.25 times solar, respectively. Both are noted for their nebulae and young stellar populations, but as in our own Galaxy their stars range from the very young to the very old, indicating a long stellar formation history.

There are, of course, other morphologies, like ring or lenticular galaxies, for example.

If you have further interest in galaxies, I suggest reading “Galaxy Morphology and Classification” by Sidney van den Bergh.

Stellar populations and their habitat

Stars are the building blocks of galaxies. They are formed continuously in some galaxies, and only in bursts in others. The term stellar population is used to refer to a single generation of stars characterized by a common age and chemical composition. A galaxy can be composed of a large number of individual populations. Stellar populations are not discrete in their properties, but rather have a continuum of characteristics that reflect the changes in star formation with time. Stellar populations are tracers of events in galaxy’s past and formation.

A young (less than 100 Myr) population contains massive stars. These are hot and generate much blue light. An old (more than 3 Gyr) population contains no massive, hot stars, only cool low-mass stars on the main sequence and red giants. It is therefore yellow-red in color. A very young population (less than 10 Myr) produces much ionizing radiation, which creates H II regions in the surrounding gas, whose spectra contain strong emission lines. The dominant emission line of hydrogen gives such regions a characteristic pinkish tint in color images. Thus, the integrated color of a stellar population is a function of its age, and this relation can be used to age-date populations in distant galaxies.

The Hertzsprung-Russell (HR) diagram is the basic tool for analyzing the temperature distributions of evolving stellar systems.

Hertzsprung-Russell diagram

In the HR diagram temperature (increasing to the left) is plotted against instrinsic stellar brightness or luminosity (increasing upward). Most stars fall on the main sequence (MS), which runs diagonally across the HRD. More massive stars on the MS are brighter and hotter, hence bluer. MS stars maintain themselves by burning hydrogen in their cores. But when they run out of H fuel, they evolve off the MS (to the right, or toward lower temperatures, in the diagram). More massive stars evolve faster. A single generation will have stars with a large range in mass, which means that its HR diagram will change as the more massive stars evolve.

You may also enjoy playing with the following HR Diagram Explorer.

There are basically three stellar populations in our Galaxy, corresponding to the three distinct dynamical components to the Galaxy; the disk population, the bulge population and the halo population. The disk population inhabits the rotating, flattened region of our Galaxy. The bulge population is restricted to the rounded, central region of the Galaxy, also rotating. And the halo population inhabits the far outer regions of the Galaxy, on long ellipisodal orbits that takes it into the disk and bulge.

The three components not only have distinct kinematic properties, but the types of objects in them also varied. The disk contains all the gas and young stars, although old stars are also found there. The bulge is dominated by old stars and a violent core. The halo contains very old stars and globular clusters. The reason for this separation of stellar types is a clue to how the Galaxy formed.

Disk and bulge stars tend to be rich in heavy elements (above helium on the periodic table). Halo stars tend to be very poor in heavy elements. Changes in the chemical composition of a star are due to the initial chemical composition of the gas cloud that it was born from. This heavy elements are mostly produced by supernova explosions, gas clouds become enriched by the ejecta of supernova. The larger the number of supernova near a cloud, the richer in heavy elements it will become. As time passes, each of the gas clouds in the Galaxy will increase in the abundance of elements such as carbon, iron, etc. So the more recent a star has been formed, the richer in heavy elements it is. This is a form of dating system for stars and we deduce that halo stars are the oldest stars in the Galaxy since they have the lowest chemical abundances. The disk stars are the youngest since they are the most metal rich.

Distribution of star populations in Milky Way
Distribution of star populations in Milky Way

Thin disk (Population I)

The thin disk of the Milky Way has sustained ongoing star formation for ~10^10 years. Consequently it contains stars with a wide range of ages, and the thin disk may be divided into a series of sub-populations of increasing age.

The spiral-arm population is the youngest in the disk; it appears to trace the spiral pattern of the Milky Way. This population is concentrated very close to the disk plane, with a scale height of ~100 pc. Representative objects include H I and molecular clouds, H II regions, protostars, stars of types O & B, supergiants and classical cepheids. The metallicity of this population is somewhat higher than that of the Sun. As the young stars age, they drift out of the spiral pattern.

The disk population is more smoothly distributed and does not seem to trace the spiral structure. This population may be further subdivided into young, intermediate, and older categories, with ages of ~1, ~5, and ~10 billion years, respectively. The characteristic scale-height of this population increases with age, ranging from ~200 to ~700 pc, while the metallicity declines to perhaps ~20% of the solar value. Representative objects include stars of type A and later, planetary nebulae, and white dwarfs. Our Sun is a member of this group.

Population I stars have regular elliptical orbits around the galactic centre, with a low relative velocity. The high metallicity of Population I stars makes them more likely to possess planetary systems than the other two populations, since planets, particularly terrestrial planets, are thought to be formed by the accretion of metals.

Stellar halo (Population II)

The stellar halo of the Milky Way includes the system of globular clusters, metal-poor high-velocity stars in the solar neighborhood, and metal-rich dwarf stars seen toward the galactic center. While star formation in the outer halo largely ceased more than 10^10 years ago, the situation in the inner kpc of the galaxy is not so clear.

Globular clusters are the classic tracers of the galactic halo. The metal-poor clusters have a nearly-spherical distribution extending to many times the Sun’s distance from the galactic center, while the metal-rich clusters are concentrated towards the center of the galaxy and may have a more flattened distribution.

Metal-poor subdwarfs in the solar neighborhood have large velocities with respect to the Sun and other disk stars. These stars are on highly eccentric orbits about the galactic center; the net rotation of this population is amounts to less than ~40 km/sec, while their random motions are quite large. The metallicity of these stars ranges from ~0.1 to ~10% of solar.

Metal-rich subdwarfs in the central bulge of the galaxy span a wide range of metallicity, from ~0.1 to > 100% of solar. The inner kpc of the bulge also appears to contain stars of type A, implying that some star formation has occurred rather recently.

Thick disk (Intermediate Population II)

Intermediate Population II stars are common in the bulge near the center of our Galaxy; star-counts suggest that this intermediate population is distributed in a thickened disk with a scale height of ~1 to 1.5 kpc. This population accounts for only ~1% of the stars in the vicinity of the sun but dominates the high-altitude tail of the thin disk population at z > 1 kpc.

Kinematic studies imply that the thick disk rotates with a velocity of ~180 km/s, compared to the less than 40 km/s rotation of the metal-poor subdwarfs. This would indicate that the thick disk is more closely associated with the thin disk population, which rotates at ~220 km/s. Metallicity measurements also support the idea that the thick disk is distinct from the stellar halo; the characteristic metal abundance of thick disk stars is ~25% of solar, while the stellar halo is significantly more metal-poor in the solar neighborhood.

If you decide to go further into gory details of galactic kinematics, you should check Astrophysical Formulae, Volume II: Space, Time, Matter and Cosmology, K. R. Lang, 3rd ed., pp 42-47.

Also, you could check this site or this one for rotation curve of the Galaxy.

Taxonomy of dragons

The vast majority of stars are main sequence stars – these are star like the Sun that are burning hydrogen into helium to produce their energy. Most stars spend 90% of their life as main sequence stars.

The spectral types and subclasses represent a temperature sequence, from hotter O stars to cooler M stars, and from hotter subclass 0 to cooler subclass 9. The temperature defines the star’s “color” and surface brightness. The luminosity class designation describes the size (gravitational acceleration in photosphere) of a star from the atmospheric pressure. For larger stars of a given spectral type, the surface gravity decreases relative to what it was on the main sequence, and this decreases the equivalent widths of the absorption lines.

There are several classes of luminosity: 0 – Hypergiants (extremely luminous), Ia – Bright Supergiant Stars (large and luminous), Ib – Supergiants (less luminous than Ia*), II – Bright Giants, III – Giants, IV – Subgiants, V – Main Sequence, sd – Sub-Dwarfs, D -White Dwarfs. *Classes can be categorized like: Ia, Ib, Ic, IIa, IIb, IIc, and so on (sub-a being brighter that sub-b).

The classification of stars
The classification of stars. Credit: Richard Powell

Stars are clearly distinguished according their spectra: O type stars are the brightest and the live the shortest; B type stars are blue white and also burn bright, but not as bright as the O type; A type stars are less bright, a little larger than our Sun, but still a lot hotter; F – Brighter than our Sun and a little hotter; G – Our Sun is a G type star; K – dimmer that our Sun, with longer lifespan because temperature is lower; M – the dimmest stars, will burn for a long time. Each spectral type is divided into 10 subclasses, A0, A1, A2, …A9 etc.

Spectrum features for each class according to element radiation lines must be considered. O stars – strong HeII (He+) lines, in absorption or emission; strong UV continuum; weak HeI, prominent H lines; SiIV, OIII, NIII, CIII. B stars – strong HeI in absorption; HII absent; H lines stronger; MgII, SiII. A stars – H lines at max in A0; HeI & HeII absent; FeII, SiII, MgII at max; some CaII; weak neutral metals. F stars – H lines weaker; CaII H&K stronger; neutral metals stronger. G stars – CaII lines dominate; H very weak; FeI, MnI, CaI become stronger; CH G-band strong. K stars – neutral metals strong; TiO bands appear. M stars – neutral metals and molecules (CH, TiO, etc.)

Element Lines in the Spectra of Stars
Chem Element Lines in the Spectra of Stars

The complete spectral type (e.g., F5V) nearly uniquely places a star in the HR diagram. Classification schemes assume solar abundances and slight changes are needed for metal-poor stars.

Many stars have peculiar features, their spectral types are coded with additional designations.

To explore each stellar class in greater detail, you can look through this data.

Luminosity and temperature are not the only important properties of stars and I will be returning to this theme some time later, in context of habitable worlds.

P.S. I’ve avoided the term Galactic Habitable Zone on purpose for several reasons. One of them being a thought that in galaxies similar to our own, stars such as the Sun can migrate great distances, thus challenging the notion that parts of these galaxies are more conducive to supporting life than other areas. It’s the same as saying some parts of a well built road are better for traffic than others. Accidents happen, just not with everyone during the ride. Also, some circumstances may result in suitable planets outside the defined “habitable zone”. And there’s always the probability of life as we don’t know it.

Set Your Timer: The Star

Stars are amazing. Lots of things depend on them. So, figuratively speaking, they are your local centers of the universe.

A star is a massive, luminous, self-gravitating ball of plasma. And the force that triggers a stellar timer is gravity. So, once it is on, things start to happen. During its lifetime a star undergoes a sequence of changes. These changes affect everything in the vicinity of a star; and maybe a little further than that. Stellar evolution is studied by observing numerous stars at the various points of their life, and by simulating stellar structure with computer models.

Before choosing a star (or a multiple star) for your system, you might consider some options here: life as we know it; life as we don’t know it, or no life at all. And if the latter, what next? Will there be someone arriving or is it just a dead world?

In this article I will stick to the life as we know it scenario.

This leaves me with a limited choice of suitable stellar types from the main sequence range between a late F and an early/mid K. Because these are the ones that are more or less suitable having an earthlike world (be it a planet or a moon) in terms of habitability according to the UV constrains and other accepted criteria of habitability (liquid water on planet’s surface, comparable surface inventories of carbon dioxide, water, dinitrogen and other biogenic elements, an early history allowing chemical evolution that leads to life, and subsequent climatic stability for several billions of years).

Stars earlier than F0 have very short (2 Gyr) main sequence lifetimes and, hence, have a low probability of harboring planets with complex life. Their habitable zones migrate outwards rapidly, so even microbial life might not have time to develop in these systems. Planets orbiting stars later than K5 are likely to become tidally locked, which may be a problem for habitability. Or maybe not, but it is a different case, though no less interesting.

Main sequence stars of spectral class F8V to K2V (or 0.50 to 1.00 in B−V) are similar to the Sun in mass and evolutionary state, and are usually called solar-like stars.

Circumstellar habitable zones

The habitable zones of main sequence stars have traditionally been defined as the range of orbits that intercept the appropriate amount of stellar flux to permit surface water on a planet. The inner HZ boundary is determined by the loss of water via photolysis and hydrogen escape. The outer HZ boundary is determined by the condensation of carbon dioxide crystals out of the atmosphere.

However, in the case of life as we know it, this is not the only factor to consider.

Ultraviolet radiation is known to inhibit photosynthesis, induce DNA destruction and cause damage to a wide variety of proteins and lipids. In particular, UV radiation between 200-300 nm becomes energetically very damaging to most of the terrestrial biological systems. On the other hand, UV radiation is usually one of the most important energy sources on the primitive Earth for the synthesis of many biochemical compounds and, therefore, essential for several biogenesis processes (Buccino et al. 2006, 2007).

Chromospheric UV radiation is increased in young stars in regard to all stellar spectral types. Compelling observational evidence (Gudel et al. 1997) shows that zero-age main sequence (ZAMS) solar-type stars rotate over 10 times faster than today’s Sun. As a consequence of this, young solar-type stars, including the young Sun, have vigorous magnetic dynamos and correspondingly strong high energy emissions.

Flare activity (an important agent for the delivery of highly energetic radiation, especially UVC) is most pronounced in K and M-type stars, which also has the potential of stripping the planetary atmospheres of close-in planets, including planets located in the stellar habitable zone. The amount of chromospheric radiation is, in a statistical sense, directly coupled to the stellar age as well as the presence of significant stellar magnetic fields and dynamo activity.

Studies have shown that chromospherically induced biological damage for planets hosted by F and G-type stars will be insignificant. The situation is, however, decisively different for K and M dwarfs. Any of these stars, including inactive stars, show noticeable chromospheric emission. For stars with basal chromospheric emission, chromosphere induced biological damage only occurs for stars of spectral type mid-K and later, whereas for stars with relatively high chromospheric emission, chromosphere-induced biological damage can also be found in the environments of very late G-type and early K-type stars (Cuntz et al., 2010).

However, photochemical/radiative–convective modeling shows that planets around K2V and F2V stars both exhibit better UV protection than does Earth. See “Ozone concentrations and ultraviolet fluxes on Earth-Like planets around other stars”, Segura et al, Astrobiology, Volume 3, Number 4, 2003.

So, in order to set your world in a right place of your solar system, you need to consider two factors – liquid water HZ and UV HZ.

Both are important factors to determine the location and the size of circumstellar habitable zone.

For life systems to evolve, the planet should be in the habitable zone continuously during a certain time. A common choice for this time is 4 Gyr, the time that was needed on Earth for intelligent life to emerge and evolve into a technological civilization. Henry et al. (1995) and Turnbull and Tarter (2003a) considered 3 Gyr, while Schopf (1993) used the time required for microbial life to emerge 1 Gyr in his definition of the CHZ.

Because main sequence stars become brighter as they age and convert hydrogen into helium and heavier elements, habitable zones tend to migrate outward with time. In order to analyze the evolution of the habitable zones, it is assumed that the UV radiation follows the same pattern as the change in luminosities of F, G and K main sequence stars.

Until an atmospheric protection is built, a planetary surface would be exposed to larger amounts of UV radiation, which could act as one of the main source in the synthesis of bioproducts and, in a certain wavelength, could be damaging for DNA. Both concepts represent the UV boundaries for the UV habzone. To obtain the exact AUs of this “safe circle”, you might consider using formulas for UV habitable zones around host stars presented by Guo et al, 2010.

For traditional luminosity-water habitable zone calculations see Kasting et al.,1993.

An example of liquid water and UV HZs calc
An example of liquid water and UV HZs calc

The traditional and the UV HZ must at least partially overlap for a considerable amount of time in order to host a habitable planet in it.

An example of HZ calculus output in MS Excel
An example of HZ calculus output in MS Excel

Things you’ll need to get started

As I mentioned before, stars evolve. You need evolutionary tracks of the star if you are interested in the long term evolutionary path of your world and its inhabitants, whatever those might be.

First of all, decide on a mass and effective temperature of your star. Those usually correspond to the spectral class (F8V…K2V or later). You might actually want to use an existing star from a catalogue to get all the parameters necessary. And don’t forget to look up star’s actual age. It will be your anchor.

In order to obtain evolutionary tracks of any star you might choose, you don’t have to derive any formulas. It has already been done. You simply can use stellar model grids. Personally, I like to use Stellar models grids I. Z=0.02, M=0.8 to 125 Msol by Claret, 2004, which present stellar models suitable for the mean solar neighborhood, i.e. for Z=0.02 (metallicity) and X=0.70 (hydrogen). The covered mass range is from 0.8 up to 125Msol and the models are followed until the exhaustion of carbon in the core, for the more massive ones. In addition, the effective temperatures of the more massive models are corrected for the effects of stellar winds, while models with lower effective temperatures are computed using a special treatment of the equation of state. You can read a more detailed description here and see the grids here.

The grid model has time steps in years and you can compare it with your selected star’s data (i.e. in luminosity, effective temperature and other things according to age, which is your anchor).

Try to experiment with different grids for other element abundance ratios. They can be found here. (E.g. Grids of stellar models. VIII. From 0.4 to 1.0 Msun at Z=0.020 and Z=0.001, Charbonnel+ 1999, description).

When you’re done, you’ll have a full table of stellar lifetime parameters, which later can be used as a basis for all of your calculations. These include mass, luminosity, effective temperature, central density and temperature, element abundancies and some other stuff like fractional gyration radius at several age steps.

Have fun, and more exciting stuff to follow.

Why Build Worlds?

I love building worlds completely from scratch, just like I love preparing a meal from fresh ingredients instead of using ready-to-cook food. And since both processes require real working physics, we won’t talk about magical fantasies here.

I don’t really know when building worlds became a passion of mine. But I do know that each year’s scientific discoveries make it grow stronger. I didn’t even notice I became swallowed by this amazing thing. I am a science fiction writer and my primary purpose for building worlds came as a necessity. Later, however, I realized that those worlds were much more than just settings, backstory and historical events.

World building itself is a form of science fiction. Just like a novel may draw you in to go on adventures you never had or will never have in real life, a well designed world may provide you with the same experience of escaping reality for a while and enjoy the realm of the unknown and vast universe. And you don’t just create things. You learn about them. You enrich your experience of the world you were born from and are living in.

Also, world building is fun.

You can go anywhere, anytime. And the playground is immensely huge.

And believe me, the commitment is worthy of the sense of accomplishment you’ll get in the process. That’s why world building matters. As any form of fiction does.

Science fiction, to various extents, focuses on the objects within a world. The technology itself is a character, if not the de facto main character. Since most people are people-oriented, an object in a center of attention is a deceit of a sort. And as a matter of fact, the more I know about the current state of scientific and technological research and development, the harder it is for me to read most science fiction. I often find that, rather than being too imaginative, it’s not imaginative enough.

Serious world building, to me, is hard science fiction and it is imaginative enough because it is based on our current scientific understanding and technological developments, not some futuristic predictions about it. This is where past, present and future overlap and are within one’s grasp. Science/technology itself is not the main character but an instrument here, as it should be for a human.

Interacting with your own creation, being the intelligent observer of your own universe, makes it alive and realistic. And, if by any ‘coincidence’ you are a writer, it leaves more options for you to focus on characters. You are, after all, building the whole thing for them to exist. The world and its inhabitants should be inseparable.

There you have it, your borderless sandbox.

P.S. A note to some lazy folks.

I dare to say world building is a form of art. And it follows the same rules as any other art form out there. You don’t draw portraits without getting familiar with the object of drawing (even purely fictional) nor without examining the works of others beforehand. You don’t build worlds without learning how they function and what are they made of. Even sketching out a few quick details requires understanding what you are doing. How you do it – it’s your business. However, it’s your personal work, don’t expect others to do it for you.