Astrophysics for authors
This prompted by recently seen errors in books.
If you are writing SF involving interstellar travel, you need to know a few things about stellar types.
First off, the "colors" of stars used in descriptions by astronomers are *not* the actual colors. "Red stars" have more red in the spectrum, but are not what normal people would call red.
Instead, the sunlight from a red dwarf (or red giant) is pretty much the same color as light from an old style incandescent bulb. This makes sense when you think about as the surface temperatures of those stars is about the same as the temperature of the filaments in those bulbs.
so you might notice that some things have a different color indoors and outdoors, but it won't be a lot of things and it won't be by much. A good way to get a feel for this is to grab some gumdrops and look at the outside on a sunny day, and then inside under different sorts of light.
You'll find that it's had to tell the difference between some colors (usually orange & yellow or orange &red).
Likewise "yellow" stars are much the same color as normal sunlight (Sol is a "yellow dwarf"). and blue & blue white stars *are* bluer than our sun, but not actually blue. But they will be apt to be blinding and have more UV.
Next biggie is that you have to consider not just stellar type (OBAFGKM) when deciding where to put a habitable planet, but also "size" (dwarf, giant, etc)
Main sequence giant stars aren't on the main sequence long enough to have habitable planets, heck most of them aren't there long enough for planets to *form* much less become habitable. Less than a million years. (you can look up the details, but you don't really need them in most cases)
Red Giant stars are what happens after a star moves off the main sequence. They are no longer burning hydrogen but have moved on to heavier elements. This require higher temperatures and pressures in the core, so the star expands and contrary to what you might think, the hotter core results in a cooler surface.
Once this stage starts, the one time habitable zone around the star moves outwards. The planets that may have been habitable before are now too hot and may even wind up getting absorbed by the expanding surface of the star.
Alas, the planets farther out may become warm enough for life, but they won't have time for it to evolve. Between the fact that the star will keep expanding slowly as it ages (thus moving the habitable zone farther and farther out and the fact that the burning of heavier elements provides less and less energy as you move up the line and get used up faster as well. a red giant isn't long for this world.
If it isn't too massive (less than 1.2 times the mass of the sun) it will no longer be able to maintain fusion in the core, and it will start cooling down after fusion stops. This will result in the (surviving) planets all cooling off as the star slowly contracts until it becomes a white dwarf.
White dwarves are less than 100 miles in diameter (don't quote me on that figure) and they are intensely hot on the surface. But because they are so small it'll take billions or even trillions of years to cool off.
But the the planets are out of luck as none of them will maintain a habitable temp from long enough for life to evolve. Even if seeded or terraformed, it won't last long (centuries to millenia)
Heavier stars last long in the red giant stage. Until you get ones that are massive enough that the core eventually tries to fuse iron. But fusing iron nuclei to heavier element doesn't *release* energy, it consumes it.
so all of a sudden the core isn't generating to photons that have been supporting to overlying layers against gravity. So the core collapses. And the layers above it fall in on it and slam to a stop after compressing it enough to produce some elements heavier than iron.
This implosion spreads outwards as more layers hit the core. And in a very short time (minutes?Hours?) the star explodes in a supernova. Which pretty much wipes out all life in the system as well as most of the planets.
The *neutrino* flux from a supernova is strong enough to give you a lethal dose at *Jupiter's* distance from the sun. Of course, that's only a worry if you are hiding behind something like Jupiter because otherwise the various forms of electromagnetic radiation will have vaporiized you before the blast wave arrives.
What's left after the supernova is a neutron star. Likely a pulsar. If the star was *really* massive, you get a black hole instead.
Either way, not a neighborhood you want to be in. :-)
So stick to "dwarf" stars in the F, G, K and M classes. And watch out for the red dwarfs. They have a bunch of common properties that make living around one "interesting" enough that you'll want to do extra research.
Now we get to orbits. Both planetary and those of moons.
First off, not that the phase of a moon depends *solely* on the angle between it and the sun. So, a "new moon" happens when the moon is in a position near the sun in the sky, which means it is rising or setting near the time the sun does.
Likewise a full moon has the moon on the *opposite* side of the sky, so it is rising a bit before or after the sun sets, and setting a bit before or after the sun rises.
And the quarters have the moon at 90 degrees to the sun so it will be highest in the sky at sunrise or sunset depending on which quarter it is,.
I mention this because at least one well known author had a full moon high in the sky at sunset!! Oooops.
Multiple moons? Again, their phase depends on the angle between that moon and the sun. So with multiple moons in different parts of the sky, they'll likely be in different phases.
Heck, if they are close enough to the planet, they may be changing phases in the course of the night!
Which brings up the Roche limit. This is the distance from the planet where the tidal forces are such that a rock on the near side of the moon and one on the far side, will be experiencing forces string enough to neutralize the moon's gravity and they'll cheerfully float off in seperate orbits.
It's roughly 1.4 times the diameter of the planet. Exact distance depends on things like what the moon is made of.
No moons closer than that unless they are both really small and made of something really strong. Artificial satellites of earth are bot strong (metal) and really small.
Planets can have ring systems inside the Roche limit.
Which brings up the matter of orbits and there relation to each other. You may have noticed that Saturn's rings have gaps. You may have even hear that the rings of other planets and even the asteroid belt have gaps as well.
These are caused by orbital resonance. Basically if two objects orbiting around a planet (or a star) have orbital periods with a simple numerical ratio, they'll affect each other's orbit.
Let's take the simplest ratio 2:1.
so we've got a with a period of X and B with a period of 2X.
Let's call the point where the two are at their closest approach 0 degrees.
At time zero both A & B are at 0 degrees in their orbits.
At time X, A is at 0 again, and B is at 180.
At 2X both are at 0 again.
So every time they are both at 0, gravity tugs them towards each other a little. Because of the synchronization, this adds up. and it'll change their orbits. If one is a lot more massive than the other, it'll get changed more.
But this effect eventually results in changing their orbits so they no longer have a simple ratio between them.
This is what causes the gaps in the asteroid belt (mostly due to Jupiter tugging on asteroids) and the gaps in Saturn's rings (various moons pulling and pushing on the ring particles).
Why does this matter to you the author? Because it means you can have moons (or planets) with simple ratios between their periods.
Thus, you can't have moon Alpha orbit in a week, and moon Beta orbit in two weeks. It isn't stable. and would have changed long ago.
Yeah, it makes designing your calendars and things harder. Thems the breaks, folks.
This entry was originally posted at https://kengr.dreamwidth.org/1148415.html. Please comment there using OpenID.
If you are writing SF involving interstellar travel, you need to know a few things about stellar types.
First off, the "colors" of stars used in descriptions by astronomers are *not* the actual colors. "Red stars" have more red in the spectrum, but are not what normal people would call red.
Instead, the sunlight from a red dwarf (or red giant) is pretty much the same color as light from an old style incandescent bulb. This makes sense when you think about as the surface temperatures of those stars is about the same as the temperature of the filaments in those bulbs.
so you might notice that some things have a different color indoors and outdoors, but it won't be a lot of things and it won't be by much. A good way to get a feel for this is to grab some gumdrops and look at the outside on a sunny day, and then inside under different sorts of light.
You'll find that it's had to tell the difference between some colors (usually orange & yellow or orange &red).
Likewise "yellow" stars are much the same color as normal sunlight (Sol is a "yellow dwarf"). and blue & blue white stars *are* bluer than our sun, but not actually blue. But they will be apt to be blinding and have more UV.
Next biggie is that you have to consider not just stellar type (OBAFGKM) when deciding where to put a habitable planet, but also "size" (dwarf, giant, etc)
Main sequence giant stars aren't on the main sequence long enough to have habitable planets, heck most of them aren't there long enough for planets to *form* much less become habitable. Less than a million years. (you can look up the details, but you don't really need them in most cases)
Red Giant stars are what happens after a star moves off the main sequence. They are no longer burning hydrogen but have moved on to heavier elements. This require higher temperatures and pressures in the core, so the star expands and contrary to what you might think, the hotter core results in a cooler surface.
Once this stage starts, the one time habitable zone around the star moves outwards. The planets that may have been habitable before are now too hot and may even wind up getting absorbed by the expanding surface of the star.
Alas, the planets farther out may become warm enough for life, but they won't have time for it to evolve. Between the fact that the star will keep expanding slowly as it ages (thus moving the habitable zone farther and farther out and the fact that the burning of heavier elements provides less and less energy as you move up the line and get used up faster as well. a red giant isn't long for this world.
If it isn't too massive (less than 1.2 times the mass of the sun) it will no longer be able to maintain fusion in the core, and it will start cooling down after fusion stops. This will result in the (surviving) planets all cooling off as the star slowly contracts until it becomes a white dwarf.
White dwarves are less than 100 miles in diameter (don't quote me on that figure) and they are intensely hot on the surface. But because they are so small it'll take billions or even trillions of years to cool off.
But the the planets are out of luck as none of them will maintain a habitable temp from long enough for life to evolve. Even if seeded or terraformed, it won't last long (centuries to millenia)
Heavier stars last long in the red giant stage. Until you get ones that are massive enough that the core eventually tries to fuse iron. But fusing iron nuclei to heavier element doesn't *release* energy, it consumes it.
so all of a sudden the core isn't generating to photons that have been supporting to overlying layers against gravity. So the core collapses. And the layers above it fall in on it and slam to a stop after compressing it enough to produce some elements heavier than iron.
This implosion spreads outwards as more layers hit the core. And in a very short time (minutes?Hours?) the star explodes in a supernova. Which pretty much wipes out all life in the system as well as most of the planets.
The *neutrino* flux from a supernova is strong enough to give you a lethal dose at *Jupiter's* distance from the sun. Of course, that's only a worry if you are hiding behind something like Jupiter because otherwise the various forms of electromagnetic radiation will have vaporiized you before the blast wave arrives.
What's left after the supernova is a neutron star. Likely a pulsar. If the star was *really* massive, you get a black hole instead.
Either way, not a neighborhood you want to be in. :-)
So stick to "dwarf" stars in the F, G, K and M classes. And watch out for the red dwarfs. They have a bunch of common properties that make living around one "interesting" enough that you'll want to do extra research.
Now we get to orbits. Both planetary and those of moons.
First off, not that the phase of a moon depends *solely* on the angle between it and the sun. So, a "new moon" happens when the moon is in a position near the sun in the sky, which means it is rising or setting near the time the sun does.
Likewise a full moon has the moon on the *opposite* side of the sky, so it is rising a bit before or after the sun sets, and setting a bit before or after the sun rises.
And the quarters have the moon at 90 degrees to the sun so it will be highest in the sky at sunrise or sunset depending on which quarter it is,.
I mention this because at least one well known author had a full moon high in the sky at sunset!! Oooops.
Multiple moons? Again, their phase depends on the angle between that moon and the sun. So with multiple moons in different parts of the sky, they'll likely be in different phases.
Heck, if they are close enough to the planet, they may be changing phases in the course of the night!
Which brings up the Roche limit. This is the distance from the planet where the tidal forces are such that a rock on the near side of the moon and one on the far side, will be experiencing forces string enough to neutralize the moon's gravity and they'll cheerfully float off in seperate orbits.
It's roughly 1.4 times the diameter of the planet. Exact distance depends on things like what the moon is made of.
No moons closer than that unless they are both really small and made of something really strong. Artificial satellites of earth are bot strong (metal) and really small.
Planets can have ring systems inside the Roche limit.
Which brings up the matter of orbits and there relation to each other. You may have noticed that Saturn's rings have gaps. You may have even hear that the rings of other planets and even the asteroid belt have gaps as well.
These are caused by orbital resonance. Basically if two objects orbiting around a planet (or a star) have orbital periods with a simple numerical ratio, they'll affect each other's orbit.
Let's take the simplest ratio 2:1.
so we've got a with a period of X and B with a period of 2X.
Let's call the point where the two are at their closest approach 0 degrees.
At time zero both A & B are at 0 degrees in their orbits.
At time X, A is at 0 again, and B is at 180.
At 2X both are at 0 again.
So every time they are both at 0, gravity tugs them towards each other a little. Because of the synchronization, this adds up. and it'll change their orbits. If one is a lot more massive than the other, it'll get changed more.
But this effect eventually results in changing their orbits so they no longer have a simple ratio between them.
This is what causes the gaps in the asteroid belt (mostly due to Jupiter tugging on asteroids) and the gaps in Saturn's rings (various moons pulling and pushing on the ring particles).
Why does this matter to you the author? Because it means you can have moons (or planets) with simple ratios between their periods.
Thus, you can't have moon Alpha orbit in a week, and moon Beta orbit in two weeks. It isn't stable. and would have changed long ago.
Yeah, it makes designing your calendars and things harder. Thems the breaks, folks.
This entry was originally posted at https://kengr.dreamwidth.org/1148415.html. Please comment there using OpenID.