Binary Stars

[Christopher]

1. I'd like the star to be blue, and noticeably blue in the night sky of a planet orbiting the other star. Fairly prominent. Maybe 1/8 of the moon's size is quite large, perhaps considerably less could still work, but as long as it's more noticeable than your average star, and blue in appearance if that's possible.

As StarryEyed said, a blue star is out of the question due to the shortness of their lifespan. And as I mentioned earlier, the star is going to be by far the brightest star in the night sky, even visible in the daytime sky, even if it's a yellow dwarf like the Sun and even if it's just a point source. So it's undoubtedly going to be more noticeable than any other star (or planet, most likely) in the planet's sky, aside from the planet's primary star ("sun") itself.

2. Around 2 months to get there travelling at .4c.

Is there any reason the speed has to be that enormous? Surely the travel time is what matters for the story. And 2 months at 0.4c translates to 4200 AUs, which is preposterously far for your purposes.

And for what it's worth, speeds that high are a pretty bad idea for in-system travel, since the density of dust, micrometeoroids, and the like is a lot higher than in interstellar space, so the odds of a catastrophic collision are relatively high. 0.4c is fast enough that being hit by a micrometeoroid would be like getting hit by a nuclear warhead.

 

[Meredith]

What fun it will be when we find out we live in a binary system consisting of our sun and a much much further out Brownish Red Dwarf star....

 

[Bones2]

Yeah, actually, cutting down the speed would seem to make a lot of sense. Though I did have an idea incorporated into the ship design to act as a sort of literal deflector shield, to account for the...particle thing.

As blue is out of the question, is there such a thing as green stars (outside BOTF)?

 

[Christopher]

As blue is out of the question, is there such a thing as green stars (outside BOTF)?

Outside what now?

There are stars whose peak wavelength is in the green part of the spectrum. Our own Sun is one of them. However, the human eye does not perceive them as green. A blackbody curve that peaks in the green is going to have a pretty even distribution of other colors, including red and blue. So when we look at the Sun, all three of the color pigments in our eyes (red, green, and blue) are triggered, and the mind interprets that as white light.

http://blogs.discovermagazine.com/badastronomy/2008/07/29/why-are-there-no-green-stars/

Antares B, the type-B dwarf companion of the red giant Antares, is often perceived as green by observers, but this is believed to be a contrast effect; it's actually white, but if you look at a bright red light (like Antares) long enough and then look at something white, it'll seem green to you (or rather cyan) because your red-sensing cells are tired out and you're only seeing the green and blue parts of the white light. However, many observers report that Antares B looks green even when Antares A is eclipsed. So maybe its apparent green color is a consequence of the nebulosity it's engulfed in.

(And Antares would definitely not be a suitable binary system for your story. Given its mass, it can't be more than a few million years old, and it's already swollen into a red supergiant bigger in radius than the orbit of Mars.)

 

[Bones2]

BOTF - Birth of the Federation. Still the finest Trek game of all time, I maintain. I remember the stars in that were various pretty colours.

Now that I think of it, any star large enough to appear as significantly large as I was thinking would look white because it would be so bright. Being a star. That's right isn't it? Which is why all stars we see from earth look white. Whereas, when viewed from space, they look more like the colours...err...

Yeah, I'm no expert. But the main thing is, any star visible in a night sky would appear white, right? At least, unless it was so big that it wouldn't be night if you could see it.

Again, I'm no expert, and I'm sleep deprived.

 

[Asbo Zaprudder]

Longer expositions of Christopher said -- why we don't see green stars:

http://blogs.discovermagazine.com/badastronomy/2008/07/29/why-are-there-no-green-stars/

http://www.universetoday.com/guide-to-space/stars/are-there-green-stars/

... and I agree BOTF is the finest Trek game, but, like most popular sci-fi, it isn't red-hot on scientific accuracy.

 

[O'Dib]

Couldn't a rogue blue star have wandered into the system?

 

[Christopher]

^Blue stars are pretty massive. Such a close rendezvous would probably disrupt the orbits of the system's planets, or at least cause a catastrophic surge of cometary bombardments.

Blue stars are also short-lived and rare, so the odds of such an encounter would be fairly low.

Now that I think of it, any star large enough to appear as significantly large as I was thinking would look white because it would be so bright. Being a star. That's right isn't it? Which is why all stars we see from earth look white. Whereas, when viewed from space, they look more like the colours...err...

Actually lots of stars in the night sky have visible colors, at least to those with sensitive eyes or telescopes. But if they're up close, like actually in your system, then yes, they'll probably appear essentially white.

The filament in an incandescent light bulb typically has a temperature of around 2500-3500 K, equivalent to the surface temperature of a type-M red dwarf star. And yet it looks white if you look right at it. And the light it fills a room with looks white to us because human color perception is contextual; the way we see a color is influenced by the colors around it. (Some optical illusions are based on this principle -- two dots that are the same color and shade appear different when their surroundings are different.) We may have a subliminal perception that room lighting is different in quality than direct sunlight, but it's hard to notice the actual color difference.

 

[StarryEyed]

Your other star should be M-type (red), K-type (orange), G-type (yellow) or F-type (yellow-white) if you're looking for realism. It's also worth noting that an F-type is going to be more massive than your your planet's star so it would be the primary (closer to the orbital center of the system).

 

[Colonel Midnight]

Unless it's detrimental to your plot already... why not make the existence of an "old" blue star (i.e. one that's been around long enough for a civilization to arise) be the reason your ship is going there in the first place?

Cheers,
-CM-

 

[Bones2]

Well, when a star is blue, how long does it have left anyway?

 

[Christopher]

Unless it's detrimental to your plot already... why not make the existence of an "old" blue star (i.e. one that's been around long enough for a civilization to arise) be the reason your ship is going there in the first place?

You mean, to investigate the apparent impossibility of a blue star living thousands of times longer than it should?

That's probably a completely different story from what he has in mind, though. I gather the main requirement is simply that the star be prominent in the planet's sky. And as I've said, a binary companion a few dozen or hundred AU away would definitely be the brightest object in the sky other than the planet's sun, as long as it wasn't a really dim M star or brown dwarf.

 

[StarryEyed]

Well, when a star is blue, how long does it have left anyway?

It depends on its mass. Even the smallest blue star will exist for well under a billion years - typically under 100 million years. As I said earlier, that may not even be enough time for rocky planets to cool to the point where they are not molten.

You should Google "Stellar Lifetimes."

 

[StarryEyed]

If you REALLY want a blue star in your system, place your primary close to a young, open star cluster like the Pliedes (only 450 light years away). A blue star could easily be ejected from the cluster and become gravitationally bound with your older system. In that case, you would want to have the blue star far away (so as not to cause too much gravitational disturbance), say several hundred A.U. It would still be very prominent in your night sky. Your planet's night sky would be pretty spectacular as it is, being so close to the open cluster.

 

[Christopher]

Except the formational throes of that cluster could be hostile to life on any nearby systems. The planet would be exposed to bursts of radiation, or might suffer from nebular gases passing through the system and either a) blocking light from the sun and cooling the planet, b) trapping heat from the sun and overheating the planet, c) eroding the planet's atmosphere away, or d) chemically altering the planet's atmosphere in harmful ways.

 

[Red Ranger]

^BTW, I don't think the Pleiades cluster is in the Milky Way's Galactic Habitable Zone, but I'm not sure. The GHZ (more info here) theory holds that complex life like ours can only develop in a portion of a galaxy, a Goldilocks zone where the conditions are just right for life as we know it to evolve. At present, life as we know it can only evolve in a habitable zone around a star much like the Sun's, that would live long enough for a planet to form and for intelligent life to evolve as on Earth. -- RR

 

[Christopher]

^I hate the term "Galactic Habitable Zone," because it grossly overstates the case. Yes, there's reason to believe conditions in our region of the galaxy are more conducive to the formation and survival of planetary biospheres than in regions nearer the core (where the radiation and turbulence are greater) or nearer the rim (where the availability of metallic elements is lower). But it's a gross exaggeration to say that those other regions aren't even habitable, especially since the orbits of stars around the galactic core often move them from one zone to another. I'm also not so quick to assume that life developing in harsher conditions wouldn't be able to evolve adaptations to those conditions. Just because Earthly life couldn't thrive there doesn't mean nothing could; it just means Earthly life hasn't needed to evolve the necessary adaptations. So at best, it's a galactic temperate zone.

Anyway, the Pleiades are only 440 light-years away, which is way, way too close to be outside the temperate zone (if such a thing even exists). As I said, the cluster itself could create a problem for habitability in the immediate region around it, but that's something operating on a much smaller scale (though still mind-bogglingly huge by human standards) than any factors that might define a galactic habitable or temperate zone. If you're right next to an erupting volcano, it doesn't really matter to your survival whether that volcano is in the tropics or the Arctic.

 

[Argus Skyhawk]

There's no way. One-eighth the apparent diameter of the full Moon is 1/16 of a degree, or about 450 arcseconds. 1000 AUs is 149.6 billion kilometers. By the small angle formula, the star would have to have an actual size of (450)(1.496x10^11 km)/206265 = 326.4 million kilometers. That's 234 times the diameter of the Sun, making it a supergiant star. Such a star would have a life expectancy in the millions of years, so there's no way it could exist in a system old enough to have an inhabited planet.

Wait a sec, when I was younger I often read that in 4 to 5 billion years our sun was expected to become a red giant with a diameter that would engulf Earth's orbit. Wouldn't that be about as large as what you are describing? Couldn't the star in Bones' story be one that had been a smaller sun-like star for billions of years and became a red giant in the last few million?

 

[Christopher]

Wait a sec, when I was younger I often read that in 4 to 5 billion years our sun was expected to become a red giant with a diameter that would engulf Earth's orbit. Wouldn't that be about as large as what you are describing? Couldn't the star in Bones' story be one that had been a smaller sun-like star for billions of years and became a red giant in the last few million?

If it were indeed a red star, rather than a blue star, then I suppose that would be possible.

Still, I don't see the necessity. As Bones has said, his ultimate goal is simply that the star be the most prominent object in the sky (other than the planet's own sun). And an ordinary main-sequence binary companion would definitely be that, even without a visible disk or an unusual color, simply due to its much greater brightness than any other star or planet in the night sky.