Now we're going to get into the real nuts-and-bolts part, creating worlds and aliens. Note that you can start this process at either end: begin with the star and work your way through the planet to its life forms and their society, or start with what you need for the story and work backward. Either approach is fine. I'm using the cosmic to domestic progression simply because the steps build on each other more simply.
Real Planets: The simplest way to create a star system or a planet is to let the Universe do the work. Until about 20 years ago we only knew about the worlds of the Solar System, and there was still uncertainty about how common planets actually were. The first exoplanet was cornfirmed in 2002, and since then the floodgates have opened. Nowadays we have basic orbital data and at least an estimate of mass for thousands of planets circling other stars. I know of at least a couple of SF writers who have basically given up on creating their own alien star systems and just pick real ones.
You can find them in a number of places. NASA's Exoplanet Archive is here: https://exoplanetarchive.ipac.caltech.edu, and the European Extrasolar Planets Encyclopaedia is here: http://exoplanet.eu. There's also the Open Exoplanet Catalogue: http://openexoplanetcatalogue.com. And finally Wikipedia's various lists of exoplanets are pretty handy: https://en.wikipedia.org/wiki/Lists_of_exoplanets.
Stars: Unless you want a dark and cold "rogue planet" drifting in interstellar space, your alien world needs a star. We think that planets all form in stellar systems, though some get kicked out by interactions with other planets.* The choice of star is thus very important.
Stars are classified by temperature (color) which roughly correlates to mass, because the bigger a star is, the hotter and brighter it is. The classes range from O (huge and searingly bright blue-white giants) through B, A, F, G (yellow stars like the Sun), K, and M (red dwarfs); plus white dwarfs, brown dwarfs, and black holes.
Note: when I talk about stars here, I'm talking about stars on the "Main Sequence," the time when their energy output is relatively stable, which occupies most of a star's lifespan. Stars leave the Main Sequence as they deplete their fuel, swelling to red giants and then either collapsing or blasting apart in a supernova explosion, depending on how big they are. Those are not good candidates for lifebearing worlds, although one could certainly tell an interesting story of a civilization watching its star enter those last stages.
To have an Earthlike world a star needs to live long enough for planets to form and life to evolve. Big bright stars live fast and die young — which means that most of the stars you've heard of are unlikely to have lifebearing worlds. Star Trek loves to put planets around familiar stars like Rigel, Deneb, Spica, or Canopus — and all of them are bright short-lived stars unlikely to have any planets at all, let alone habitable ones.
Red dwarfs make up the majority of all stars, between 2/3 and 3/4, but they have problems, too: they are so dim that habitable worlds are likely tidally locked, and they tend to be flare stars. These aren't insoluble problems; a large moon orbiting a planet in the habitable zone of a red dwarf would have a day-night cycle, and there have been suggestions that most red dwarf flare activity is along their polar axes, where planets don't typically orbit.
The sweet spot for stars to have worlds capable of supporting life like our own appears to be in the F-G-K range. These stars have masses between 0.5 and 1.5 times that of the Sun, and brightness from about 0.1 to 7 times the Sun's output. They also have a long enough lifespan for solid worlds and complex life to form. Collectively they make up about 20 percent of the stars in the Galaxy.
Note that about a third of all star systems are binary or multi-star systems, with two or more stars orbiting around each other. These range from very close "contact binaries" to stars so distant it's hard to tell if they're actually orbiting or just passing nearby. They can and do have planets, so if you want a Star Wars style double sunset you can have it. For a close binary just treat them as a single star. Distant binaries function as separate star systems which just happen to be near each other.
If you want to use real stars (with or without known exoplanets), the best resource is SolStation.com: http://solstation.com/#sthash.IUQ23ZbY.dpbs. They've got encyclopedic descriptions of stars out to 100 light-years from Earth (http://solstation.com/stars.htm#sthash.htedKjWl.dpbs), plus star maps and very useful articles on planets and the potential for life.
Winchell Chung (his name's going to come up a lot) has not one but three very useful pages. The first is his "Atomic Rockets" discussion of stars and other stellar objects: http://www.projectrho.com/public_html/rocket/spacemaps.php, which includes some extremely useful "delta-V" maps of the Solar System, plus increasing scale maps out to intergalactic scale. His page on "Weird Astronomy" (http://www.projectrho.com/public_html/rocket/weirdastronomy.php) discusses real objects of interest. He also has a separate site for creating star maps, showing where things are in the Solar neighborhood. Very useful if you're creating a game setting or fictional "sandbox" with multiple inhabited star systems. It's at http://www.projectrho.com/public_html/starmaps/, and includes links to other star catalogues I haven't used.
And, again, Wikipedia is surprisingly useful, presumably because there aren't any aliens online engaging in edit wars. Their list of Lists of Stars is a good starting point: https://en.wikipedia.org/wiki/Lists_of_stars.
The most important features of a star are its mass and its luminosity. These are related: the bigger, the brighter. There are any number of sites listing the mass and brightness for different star types. If you want to make up a star rather than use a real one, just pick the type you want.
Brown Dwarfs: In recent years astronomers have gotten better at detecting "brown dwarf" objects — too small to be luminous, but big enough to generate some heat of their own. Basically they're the intermediate step between giant planets like Jupiter and the small end of red dwarf stars.
There's three very interesting things about brown dwarfs. First, there are probably an awful lot of them. In general, stars are more common the smaller they are. I've mentioned that nearly 3/4 of stars are red dwarfs. If this distribution holds true, then there are probably more brown dwarfs in the Galaxy than there are visible stars! Interstellar explorers and colonists might leapfrog from one brown dwarf to another across relatively short distances.
The second is that they do emit heat, and can have moons. A brown dwarf might have icy moons with subsurface liquid oceans, as is suspected of Europa and Enceladus in our own Solar System. That means a lot of places where life might arise.
And third, the atmospheres of smaller brown dwarf objects can be at "Goldilocks" temperatures where liquid water and — potentially — life might exist. As I mentioned, there are probably lots and lots of brown dwarfs, so even if it's unlikely for life to form on one, with enough rolls of the dice the right result might come up.
Wikipedia really has the best list of brown dwarfs I've been able to find, and includes links to full articles on some of them: https://en.wikipedia.org/wiki/List_of_brown_dwarfs.
*Or by the I.A.U.
Next Time: Systems!
Random note about your blog:
I can't seem to access it from my Samsung Internet browser on my phone, or from firefox on Windows sometimes. I get some DNS error. I'll check the details later, but it's bugged me before.
"I know of at least a couple of SF writers who have basically given up on creating their own alien star systems and just pick real ones."
One issue with that might be that, because most of our extrasolar planets are from the Kepler mission's transit method, we've only got some milli-arc-second field of view out into the galaxy where all these detected planets are at. Many of the closer stars to the sun are still unmapped.
Re: The sun being a "yellow star": I was actually working through a bunch of radiometry problems the other day to figure out sky radiance under various conditions. One reason that the sun appears yellow from below the atmosphere is the same reason the sky appears blue: Rayleigh scattering. That short-wavelength energy was stolen from the direct line of sight and smeared out over the whole sky. From space, the blue end of the spectrum is more energetic and the sun appears bluish-white.
You can look up an ISS photo with the sun in view: One is here: https://www.space.com/earth-perihelion-closest-sun-approach-2020.html
In general, hotter stars are "blue-er" than colder stars, but the color balance to the human eye is shifted by our atmosphere.
Aaron
Posted by: MadRocketSci | 06/20/2022 at 11:45 AM
The sun appears red at sunset because the slant path through the atmosphere is longer. Longer wavelengths are being scattered out of the direct path. Also the sky around the sun is greenish at sunset. (Atmospheric conditions wildly vary though.)
By my calculations, the mean free path at 700nm is something like 145.8km, at 550nm, 55.2km, at 450nm 22.7km.
On a clear day in the desert southwest, looking ~30 km or so to a mountain range, all the blue was washed out of the image, and the green was starting to blur. The fine details could only be perceived in the red.
Posted by: MadRocketSci | 06/20/2022 at 11:52 AM
Re: Details of my trouble accessing your blog:
It seems to be some google chrome thing. When entering variations of http://jamescambias.com or https://jamescambias.com, I get the following:
This site can’t be reached. Check if there is a typo in jamescambias.com.
DNS_PROBE_FINISHED_NXDOMAIN
I have to enter (specifically) https://www.jamescambias.com to reach the page.
Posted by: MadRocketSci | 06/20/2022 at 12:14 PM
Not the right post, but I found the other locked. I'm just reading Perelandra for the first time and noticed at the beginning of chapter 3 there is this connection to HP Lovecraft and specifically the colour out of space:
“I think he was
referring to some system of meditation which claimed to make ‘the
form of Life itself’ visible to the inner eye. At any rate Ransom let
himself in for a long cross-examination by failing to conceal the fact
that he attached some very definite idea to this. He even went so far
— under extreme pressure — as to say that life appeared to him, in
that condition, as a ‘coloured shape’. Asked ‘what colour?’, he gave
a curious look and could only say ‘what colours! yes, what colours!’
But then he spoiled it all by adding, ‘of course it wasn’t colour at all
really. I mean, not what we’d call colour,’ and shutting up complete-
ly for the rest of the evening."
In my view this is definitely paying homage to Lovecraft 🙂
Posted by: Laz | 06/26/2022 at 04:48 PM