1) The sky will appear the color which has the shortest wavelength, while still both a) being emitted by the star 2) visible to the observer. (For more info on this, see here
) If you wanted to make the sky green, you'd have to stop the sun emitting blue light entirely, whereas it might appear soft-UV if your eye can see that.
2) Can't help you, sorry.
edited 27th Dec '10 4:04:46 AM by Yej
Excellent answer, but not really what I was looking for. The star is a real star, so it'll produce a (somewhat modified) black-body spectrum. The only free parameter is the peak wavelength. So, "stop [...] blue light entirely" is not an option. As I understand it, our atmosphere scatters green light too, but the sky appears blue because it scatters blue light a lot more. What I should really be asking is whether, by changing the peak wavelength red-wards, it's possible to end up with a star for which the ratio of the luminosities in the green and blue bands is so high that the preferential scattering of the blue component wouldn't be sufficient to compensate, thus making the sky appear green.
And I hadn't even considered the observer-side of this question, shame on me. Let's just say their visual spectrum is the same as hours, for the time being, to keep things simple.
Soon the Cold One took flight, yielded Goddess and field to the victor: The Lord of the Light.Indeed. You'll have to fiddle with the numbers a lot to get what you want, though, because blue light (such as in a blu-ray laser) scatters 37 times more than red (DVD laser) light does in the same circumstances. You'd have to adjust the graph so that the red light output is >40 times greater than the blue light output.
Also, if you're going the Shown Their Work-level of hard sci-fi, keep in mind that any native creature will adapt to see light the star's putting out, not what Sol's putting out.
edited 27th Dec '10 5:07:12 AM by Yej
Of course, you're right! And if the visible spectrum is centered on the peak wavelength, it follows directly that the sky will appear "blue-ish", where "blue-ish" is the designation of the blue-wards tail of that visible spectrum. I think my whole question was based on an insufficiently broad notion of colours and colour-terms.
Thanks!
But if any Earth organism gets to the planet, it won't appear anything like Sol-blue. Also, visible light is sat in a rather odd energy level, and alien!blue might have some properties visible light doesn't. For instance, if "blue" (or even "red") corresponds to what we call infra-red, warm things glow. If "blue" is actually hard UV, sunburn ahoy.
edited 27th Dec '10 5:44:31 AM by Yej
True, but not a concern, since I'm writing Otherworld
fantasy without any form of interplanetary/-dimensional/-whathaveyou travel. Since I'm nowhere near creative enough to invent an entire eco-system from scratch, the fauna and flora is going to be identical to Earth's except for species-level details. And I was already planning on using English approximations rather than fictional names for those species. For example, there might be two types of pantherines, one which is solitary and one which is gregarious. Those would be called "leopards" and "lions", even though they probably don't have spots and a mane, respectively. By extension, the visual spectrum will definitely span "red" to "blue", no matter what the corresponding wavelengths are.
More good points! The star is several billion years old, to allow for a reasonable evolutionary timescale (read: about as old as Earth), so anything far to the blue of Sol isn't worth considering in any case. But the other direction might be worth thinking about. There are no Earth-animals that can see body-heat, are there? I only know about e.g. snakes which have a separate IR-sensing organ. How far into the IR-range would I have to extend the visible spectrum to make warm bodies (300 K, say) glow faintly but perceptibly?
ETA: To clarify, going less far doesn't seem very interesting. Fire/embers are hot enough to be in the solar-visual spectrum, and there just isn't much in between, temperature-wise, it seems to me. For hot water/food we have steam as a tell-tale, after all.
edited 27th Dec '10 6:50:14 AM by kassyopeia
Soon the Cold One took flight, yielded Goddess and field to the victor: The Lord of the Light.Quoth Wikipedia's article on black body radiation
.
There is also the actual equation for radiation emission on that page, but it's rather... complicated.
Okay, lemme see if I can figure this out. From the article on incandescence
, "[...] substances start to glow around 470°C (about 750K), visible in the dark, with a very dull red color [...] Their incandescence does not vanish below that temperature, but it is too weak in the visible spectrum to be perceivable."
Using Planck's law (simplistically) to require a similar intensity at the red end of the visible spectrum for a "substance" at 300K, designating the temperatures T_fire and T_body, respectively:
lambda_solar^5 * (exp((h c)/(lambda_solar k T_fire)) - 1) = lambda_alien^5 * (exp((h c)/(lambda_alien k T_body)) - 1)
Which yields a value for "red" of about 2,500 nm, or about three times the solar value. Which, in turn, corresponds to a stellar temperature of below 2,000 K, which places it outside the conventional classification scheme (Class L in the extended scheme). Doesn't sound promising for "Earth-like life", unfortunately... *sulk*
Soon the Cold One took flight, yielded Goddess and field to the victor: The Lord of the Light.Recycling this thread for a new question now:
3) What sorts of interesting things could plausibly show up in the Alien Night Sky of my planet? The planet has neither moons nor rings, nor even inner planets which would appear as "morning star"/"evening star" analogues. Thus, anything outside the solar system in addition to a "milky way" and a bunch of ordinary stars would be nice. Fresh supernova remnants are very pretty, but I can't place one nearby without wiping out all the life on the planet in the nova itself, IIRC. So, that's out. Maybe something in that vein, though, some sort of extended region of dust and gas that's internally lit?
Soon the Cold One took flight, yielded Goddess and field to the victor: The Lord of the Light.Unfortunately, most other things i can think of would also be a real threat to life, the requirement of being highly visible means high degrees of radiation which makes it dangerous.
Well, one thing. Not very realistic, but at least in theory you could have a lot of twin star systems in the neighbourhood, providing a large amount of novae, so a constantly shifting night-sky. Not visually impressive immediately, but given enough novae with periods of a couple of years, you could see change even within a couple of days. Although it wouldn't be stable over any length of time of course, sooner or later all the fuel is gone.
edited 22nd Nov '12 4:26:11 AM by McKitten
A nearby protogalaxy sounds perfect on several counts, but the universe can't be all that young. For rocky planets to exist, the stellar population must have matured quite a bit already; for sentient life to evolve there, add another few billions of years.
That sounds annoyingly plausible. Also, extended regions of gas are where star formation happens, and active star formation again lead to the nearby supernova problem in short order. Bah.
Perhaps I could have a moderately active galactic nucleus which lights up "stuff" in the galactic halo? There's no direct line of sight to the nucleus, if the galactic disk is anything like ours, which should assuage the radiation problem.
Soon the Cold One took flight, yielded Goddess and field to the victor: The Lord of the Light.Perhaps I could have a moderately active galactic nucleus which lights up "stuff" in the galactic halo? There's no direct line of sight to the nucleus, if the galactic disk is anything like ours, which should assuage the radiation problem.
It's perhaps not quite what you're asking for, but might a set of captured comets work? They'd presumably lose all of their lovely trail-making ice-and-dust eventually, so perhaps you might have another star system nearby which has a large comet cloud from which comets are regularly ejected through some quirk of gravitational sling-shotting, sending them in a long stream towards your solar system. Once there, they circle a few times before falling into the local star, creating a never-ending, ever-shifting march of wispy shapes in the sky...
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Actually, what I meant before was that the planet's galaxy itself would have the active nucleus and would be lighting up its own halo. But your take is much more interesting, I think: How about if the galaxy is in the early stages of colliding with another galaxy, starting at the far side. As you say, seeing another galaxy close-up would already be impressive in itself, and on top of that there'd be a lot of highly luminous activity going on in the collision region, but none of it close enough to the planet's system to cause trouble.
If the collision region has already advanced to include either or both nuclei, those would become significantly more active, as I earlier suggested, and the energetic polar jets could in turn light up ordinarily inert stuff in the other's disk.
Of course, a few tens or hundreds million years hence, the planet's system would being to be negatively affected by all this as well... but that's hardly my concern, is it.
Perhaps meteor showers rather than comets? The planet is in a highly eccentric orbit about its (binary) star, so I could actually have it pass through a circular debris belt twice a year. That ought to produce some rather impressive displays. I'd just have to come up with an explanation for why none of the chunks are big enough to result in disastrous impacts...
Well, realistically, a galactic collision would certainly make the display more chaotic, but there's not actually that many real "collisions" going on. Star systems mostly get flung around, maybe a couple merge as twin systems, but real stars colliding due to a galactic collision is still exceedingly rare.
Still i'd say it's plausible enough that creative license can cover the remaining distance.
On another note, let's do some back-of-envelope calculations. If we're talking about a milky-way sized galaxy, that means ~100klys in size, and roughly circular when seen top-down. So if it's a bit more thn 100kly distant, that'd mean it's 60° in the sky. That's a third of the sky on a flat plain, pretty damn impressive.
However, that also means the inhabited solar system would need to belong to some very small galaxy or be a rouge star system, otherwise, it's right in the middle of a galactic collision itself, or at least will be soon.
Yeah, no star-on-star collisions, but lots of gas-cloud-on-gas-cloud compaction, which turns up the star formation rate, which is perhaps not quite as spectacular as what would happen if the naive idea of "galaxies collide like cars collide" were true, but still a lot more spectacular than a quiescent galaxy!
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As you say, the solar system will be in the middle of the collision "soon", but in this case soon is a relative term. 100 klys times less than 1% c collision speed equals more than 10 Mys.
And as you also said, it's not like the collision is actually going to eradicate the star or dislodge the planet or anything like that; and for an "ordinary" extinction-level event, that timescale is perfectly acceptable.
Soon the Cold One took flight, yielded Goddess and field to the victor: The Lord of the Light.Well by the time Andromeda collides with the Milky Way Sol (our Sun) is long since expected to have left the main sequence part of a solar life cycle and become (or finished being) a red giant. At this point assuming Earth is not vaporized by increasing solar luminosity (can't say the same for Mercury and Venus, all predictions indicate they die when the Sun goes red giant), our planet will be one big ball of molten iron or otherwise extremely hot.
At that point in time the only solid objects potentially capable of supporting life are Mars (likely extremophiles such as certain bacteria owing to the fact that like Earth it will be extremely hot at red giant phase), the Jovian Moons, the Saturnine Moons and beyond.
Oh, the planet definitely isn't the Earth and neither of its binary suns is the Sun, but I suppose there is no real reason for its galaxy not to be the Galaxy. That quite simply never occurred to me before now.
Soon the Cold One took flight, yielded Goddess and field to the victor: The Lord of the Light.Well, I was going to recommend an emission nebulae
, or maybe a globular cluster, but obviously colliding galaxies have those beat in the drama dept hands down...
Some kind of particulate nebula is what I originally had in mind, but as far as I can tell those are invariably associated with star formation, which in turn invariably means that some of the formed stars will be big enough to go supernova after relatively short time. Can't have that.
Thanks for the link, anyway; we seem to have pretty much exhausted the options listed there, which is reassuring.
The combination of what I have now should provide enough of a vista to make up for the lack of a moon: There are globally visible aurorae when the planet is near perihelion, due to the increased solar wind flux. As a counterpoint to which I'm thinking the meteor showers from post #15 would be nice near aphelion. During the rest of the year, the colliding galaxies will just have to do.

Two questions:
1) Is the colour of a planet's sky dependent on the colour of the planet's sun, given that its atmosphere is Earth-like? To put it another way, if you had the power to change both the Sun's luminosity and spectral type, but nothing else, could you turn Earth's sky red or green, or would brighter blues on the one hand and a night-time-like sky on the other be the extents of what you could achieve?
2) How bright does a celestial object have to be in order to be visible right next to the Sun? Is it sufficient to be brighter than the day-time sky, or do I have to add an extra margin for "solar glare" or some such?
Since none of the familiar celestial objects clearly qualify, historical accounts of supernovae might be a good place to look, thought I. After a bit of digging and figuring, it turns out that the famous Crab supernova, aka SN 1054
, was indeed fairly close to the Sun during its maximum: It occurred in early July at about RA 5h 30' Dec +22°. At the summer solstice, i.e. just a week earlier, the Sun is at exactly RA 6h Dec +23.5° (this follows directly from the definition of the celestial coordinate system, unless I messed up). And voila, I found this passage in a related paper
:
Now, this is helpful but not quite what I had in mind. The limit the article addresses is what one is able to discern if one tries really hard. The limit I'm wondering about is what one can't help but see if one happens to look in the right direction. Venus certainly meets that criterion just before sunrise and just after sunset, but it just as certainly doesn't meet it at noon. If there were such a thing as a full moon close to the Sun, it might qualify; I'm not sure.
Any help?
edited 27th Dec '10 3:16:51 AM by kassyopeia
Soon the Cold One took flight, yielded Goddess and field to the victor: The Lord of the Light.