Questions about the Alpha Centauri Star System

[snabald]

Say you were on a planet within the "habitable" zone around Alpha Centauri A, what would Alpha Centauri B look like at nigh? How bright would it be?

Then Let's reverse that would would Alpha Centauri A look like from a habitable planet orbiting Alpha Centauri B?

[granthutchison]

Very bright - brighter than anything we have in the night sky of Earth. Since the two stars can be as close as 11AU and as far apart as 35AU, the brightness varies. At closest approach, A shines at magnitude -22.0, and B at magnitude -20.6; at farthest separation, A shines at -20.4, B at -19.0.
This is roughly 1/500th as bright as the Sun in Earth's sky, and about as bright as the Sun appears from Uranus.

Grant

[Evil Dr Ganymede]

This is roughly 1/500th as bright as the Sun in Earth's sky, and about as bright as the Sun appears from Uranus.

See, I can't really picture this... (500 times less bright than the sun to me still sounds bright enough to burn your eyes out ). I've always imagined it as a REALLY bright star, that's so bright that it actually turns the sky blue around it (through scattered light in the atmosphere, like it was daylight). Does that sound remotely accurate, or is that overkill?

[snabald]

This is roughly 1/500th as bright as the Sun in Earth's sky, and about as bright as the Sun appears from Uranus.

See, I can't really picture this... (500 times less bright than the sun to me still sounds bright enough to burn your eyes out ). I've always imagined it as a REALLY bright star, that's so bright that it actually turns the sky blue around it (through scattered light in the atmosphere, like it was daylight). Does that sound remotely accurate, or is that overkill?

That's what I was wondering, if it would be bright enough to make part of the sky blue, I also wonder how big it would appear in the sky...

[granthutchison]

Well, if we assume an Earth-like atmosphere, then light is going to scatter in exactly the same way, and so blue light will be spread over the whole vault of the sky, but at 1/500th of its intensity in Earth's daytime sky.
To be visible during the day on Earth, a star or planet needs to have a magnitude of -2.7, in order to stand out against the luminance of the daytime sky (we're talking about overhead, BTW, not at the horizon). Since the luminance of the sky in our Centauri system is being reduced 500-fold, we can reduce the visibility threshold by the same amount: equivalent to 6.75 magnitudes. So at night, when the companion star is visible, we'd be able to pick out magnitude 4 stars if there were no haze, and assuming we could block the light of the companion star and the reflected light from the landscape. That's a pretty black sky, if you consider how long after sunset you get this level of seeing.
Of course, in practice, the illumination from the companion star would constrict our pupils very dramatically: we'd be walking through a landscape as well illuminated as bright indoor lighting, and I doubt if we'd be able to pick out any but the brightest stars (like looking out the window at night with the room lights on).
So we're talking a black sky above a brightly illuminated landscape, I think.

At their most distant, both stars are going to show no visible disc: A is just 1.1 minutes of arc across, and B 0.8 minutes. At close approach, there's going to be, potentially, a tiny disc visible: 3.5 minutes and 2.5 minutes. In practice, it's difficult to know if anyone would be conscious of the disc: you're going to have a tiny fleck in the sky with a surface brightness equal to the Sun - you'll get a retinal burn if you look at it directly.
I don't know enough about the eye's protective reflexes to say whether you'd get a blink reflex or not - do we blink in response to high illumination (the total incoming light) or high luminance (the surface brightness of the object)? If it's the former, then you might find you were able to look directly at Alpha Cen A without discomfort, while burning a hole in your retina (oops) - there's really no reason why our Earth-bound reflexes should have evolved to give us any protection from such tiny intense light sources.

Grant

[Evil Dr Ganymede]

Well, if we assume an Earth-like atmosphere, then light is going to scatter in exactly the same way, and so blue light will be spread over the whole vault of the sky, but at 1/500th of its intensity in Earth's daytime sky.
To be visible during the day on Earth, a star or planet needs to have a magnitude of -2.7, in order to stand out against the luminance of the daytime sky (we're talking about overhead, BTW, not at the horizon). Since the luminance of the sky in our Centauri system is being reduced 500-fold, we can reduce the visibility threshold by the same amount: equivalent to 6.75 magnitudes. So at night, when the companion star is visible, we'd be able to pick out magnitude 4 stars if there were no haze, and assuming we could block the light of the companion star and the reflected light from the landscape. That's a pretty black sky, if you consider how long after sunset you get this level of seeing.
Of course, in practice, the illumination from the companion star would constrict our pupils very dramatically: we'd be walking through a landscape as well illuminated as bright indoor lighting, and I doubt if we'd be able to pick out any but the brightest stars (like looking out the window at night with the room lights on).
So we're talking a black sky above a brightly illuminated landscape, I think.

Kooky. That'd be much more illuminated than full moon on a clear night on earth, right? So there really would be no true 'night' until the companion was below the horizon.

*snip*
there's really no reason why our Earth-bound reflexes should have evolved to give us any protection from such tiny intense light sources.

Heh. Nice. I bet that little gem gets forgotten about in most sci-fi books/rpgs .

[granthutchison]

That'd be much more illuminated than full moon on a clear night on earth, right?

A thousand times more - you'd have no problems with colour discrimination, you'd be able to read fine print ... more or less indistinguishable from daylight unless you were trying to do some very fine work. And all under a black sky with just a few stars and planets visible ...

Heh. Nice. I bet that little gem gets forgotten about in most sci-fi books/rpgs :).

Only person I remember commenting particularly was Larry Niven, in "Grendel", when he made the eye-threatening luminance of CY Aqr a plot element.

Grant

[Guest]

So, something like this...

[snabald]

Oh shoot... forgot to add my username in the last post... (I made that picture after reading the descriptions) Thank you!!!

[Evil Dr Ganymede]

Oh shoot... forgot to add my username in the last post... (I made that picture after reading the descriptions) Thank you!!!

Nice. Was that done in Terragen?

[ajtribick]

Some further questions:

Suppose we place a hypothetical planet in the region around Alf Cen A where the planet can support Earthlike conditions in terms of atmosphere and liquid water.

During the daytime, would Alf Cen B be visible and if so, would it cast visible shadows?

Two: We place our habitable planet around Alf Cen B. Would Alf Cen A be visible during the daytime and would it cast visible shadows?

In addition: Alf Cen B is a K-type star therefore I think it radiates more infrared compared to visible light than Sol - would this mean that the visible light intensity of Alf Cen B from a planet in the habitable zone at the same temperature as Earth would be less than the light intensity Earth receives from Sol?

In addition, Alf Cen B is redder than Sol so would this colour difference be noticeable or would the retinas be saturated and the difference not noticed?

Oh, and just for fun, what would be the combined light intensity from the A-B pair of the Alf Cen system from Proxima?

[snabald]

Oh shoot... forgot to add my username in the last post... (I made that picture after reading the descriptions) Thank you!!!

Nice. Was that done in Terragen?

Yes

[granthutchison]

During the daytime, would Alf Cen B be visible and if so, would it cast visible shadows?

It would be clearly visible - the calculated magnitude is much higher than required to be visible through the scattered blue of an Earth-like sky. I don't think you'd see a shadow, though. If we stick with my rough estimate of 1/500th of the light output of the Sun as seen from Earth, and assume that Alpha Cen A isn't going to be far off Sun-like illumination levels, then the general level of illumination in daytime is going to be 501 Alpha Cen B equivalents. Shadows cast by Alpha Cen A will be illuminated by Alpha Cen B, and so will have a light level of 1/501 of the ambient illumination; shadows cast by Alpha Cen B will by illuminated by A, and will have a light level of 500/501 of the ambient illumination. I dunno for sure, but it seems to me that 0.2% contrast is going to be more or less undetectable, although the shadow would be very hard-edged because of the nearly point source of light.

Two: We place our habitable planet around Alf Cen B. Would Alf Cen A be visible during the daytime and would it cast visible shadows?

The same answer as above. The light levels are going to be the same within a factor of two, so the big determinant is the large difference in illumination between the two sources.

Alf Cen B is a K-type star therefore I think it radiates more infrared compared to visible light than Sol - would this mean that the visible light intensity of Alf Cen B from a planet in the habitable zone at the same temperature as Earth would be less than the light intensity Earth receives from Sol?

Yes indeed. The Sun sits very near the maximum of luminous efficacy for a black body radiator - for a given output, it pretty close to maximizes the sensation of illumination perceived by human eyes. So stars that are either cooler or hotter than Sol will shed less light in their habitable zones. There has been some suggestion that our eyes have evolved to use the peak of our local solar black-body radiation spectrum, but I'm not sure about that. There are other advantages to the particular visual spectrum we use. Infrared is minimally scattered by the atmosphere, so shadows are poorly illuminated by long wavelengths. Ultraviolet is strongly scattered, and so the atmosphere is very hazy in short wavelengths. If you want to see into shadows and as far as the horizon, and you only have one set of eyes, and those eyes are limited by the simplicity of their optics to a single octave of EM radiation, then 400-700nm is a good compromise solution. For a plains-dwelling ape with an interest in foraging and a worry about great big predators in the distance, it's no surprise we use the wavelengths we do.

In addition, Alf Cen B is redder than Sol so would this colour difference be noticeable or would the retinas be saturated and the difference not noticed?

We'd see it as white. We see any bright continuous spectrum as white down to even red dwarf temperatures of 3000K ... that's the same temperature as the filament in an electric lightbulb, which is glaringly white to my eye. Alpha Cen B would still look white even when viewed from Alpha A, because it would still have a high apparent surface brightness - to see colour you'd need to move farther away.

Oh, and just for fun, what would be the combined light intensity from the A-B pair of the Alf Cen system from Proxima?

Rather than do the sums, I used Celestia to give me their magnitudes, and then combined them: comes out at -7.4, or a tad less than 100th of the full Moon.

Grant

[granthutchison]

So, something like this...

Nice . The hard-edged shadows are good. That would be a noticeable contrast between day and companion-illuminated "night" - the shadows would be very sharp. It occurs to me that the light would be similar to the spooky light that comes in the last few seconds before a total eclipse - I recall disturbingly sharp shadows as the solar disc was thinning down towards vanishing point.

Grant

[granthutchison]

Aha. Tracked down some information on contrast thresholds: for a static edge defined by contrast to be visible with colour vision, there has to be at least a 1% change in luminance. So to create double shadows on the planets of binary stars, the dimmer star must have a luminosity (in the planet's sky) of at least 1% of the brighter star. (If both stars show a disc then you'd presumably need to do a little better than that, since the shadow edges would be diffuse and therefore less noticeable.)

Handy rule of thumb, though, for world-builders: 100-fold difference in brightness, or a factor of 5 in apparent magnitude.

Just to tighten up my rough estimates above, by plugging in specifics from the Alpha Cen system:

Alpha Cen A
Total energy output ~1.7 Sun
Visual luminosity ~1.5 Sun
Habitable zone ~1.3 AU
Illumination in habitable zone ~0.9 Earth
Closest possible approach of habitable planet to B ~9.7AU
Illumination by B at closest approach ~0.005 Earth
Illumination ratio B:(A+B) ~0.0055

Alpha Cen B
Total energy output ~0.55x Sun
Visual luminosity ~0.45x Sun
Habitable zone ~0.74AU
Illumination in habitable zone ~0.8 Earth
Closest possible approach of habitable planet to A ~10.3AU
Illumination by A at closest approach ~0.014 Earth
Illumination ratio A:(A+B) ~0.017

So if the planets of Alpha Cen B orbited in roughly the same plane as its orbit around Alpha Cen A, and if the stars were at their closest approach to each other, and a habitable planet of B were passing through the section of its orbit closest to Alpha Cen A ... there would be a few months during which a faint second shadow would be visible in daytime. :)

Grant

[ajtribick]

Hmmm... on a planet around a K-star would there be a noticeable difference in colour of the general illumination, as opposed to the solar disc?

[granthutchison]

Hmmm... on a planet around a K-star would there be a noticeable difference in colour of the general illumination, as opposed to the solar disc?

Don't think so - our eyes and brains are very good at the trick of colour constancy - even in very different illuminants we judge the incoming light to be white and correctly sense the colours of the various objects illuminated. We don't have a problem, for instance, with indoor lighting at a colour temperature of 3000K - white paper still looks white to us.

However, if you had two equally bright stars of different temperatures in the sky at the same time, you'd notice a difference in the colour of the double shadows they cast: the bluer star would cast an orange shadow, while the cooler star would cast a blue shadow. You can demonstrate this to yourself by using daylight coming through a window and an incandescent lamp of some kind to cast simultaneous shadows of an object. There's no trick your brain can play to make both illuminants appear white when they're illuminating the same surface at the same time, and the shadows allow you to isolate the effects of one illuminant from the other and display them side by side. The effect is amazingly odd when you first see it, since your brain is telling you that the sunlight is white and the lamp filament is white, but between them they are visibly generating quite strongly coloured shadows.

Grant

[Empty86]

Aha. Tracked down some information on contrast thresholds: for a static edge defined by contrast to be visible with colour vision, there has to be at least a 1% change in luminance. So to create double shadows on the planets of binary stars, the dimmer star must have a luminosity (in the planet's sky) of at least 1% of the brighter star. (If both stars show a disc then you'd presumably need to do a little better than that, since the shadow edges would be diffuse and therefore less noticeable.)

Handy rule of thumb, though, for world-builders: 100-fold difference in brightness, or a factor of 5 in apparent magnitude.

Just to tighten up my rough estimates above, by plugging in specifics from the Alpha Cen system:

Alpha Cen A
Total energy output ~1.7 Sun
Visual luminosity ~1.5 Sun
Habitable zone ~1.3 AU
Illumination in habitable zone ~0.9 Earth
Closest possible approach of habitable planet to B ~9.7AU
Illumination by B at closest approach ~0.005 Earth
Illumination ratio B:(A+B) ~0.0055

Alpha Cen B
Total energy output ~0.55x Sun
Visual luminosity ~0.45x Sun
Habitable zone ~0.74AU
Illumination in habitable zone ~0.8 Earth
Closest possible approach of habitable planet to A ~10.3AU
Illumination by A at closest approach ~0.014 Earth
Illumination ratio A:(A+B) ~0.017

So if the planets of Alpha Cen B orbited in roughly the same plane as its orbit around Alpha Cen A, and if the stars were at their closest approach to each other, and a habitable planet of B were passing through the section of its orbit closest to Alpha Cen A ... there would be a few months during which a faint second shadow would be visible in daytime.

Grant

Is that a mistake in Celestia then? Because I dont see it get closer then 25AU

[Empty86]

oh yea.. of course, stars in celestia don't orbit each other

[Matt McIrvin]

In addition, Alf Cen B is redder than Sol so would this colour difference be noticeable or would the retinas be saturated and the difference not noticed?

We'd see it as white. We see any bright continuous spectrum as white down to even red dwarf temperatures of 3000K ... that's the same temperature as the filament in an electric lightbulb, which is glaringly white to my eye.

The more interesting question is whether there would be a noticeable cast to the resulting illumination during "half-night" as opposed to during the day. A lightbulb filament looks white to me, too, but the illumination from a regular incandescent has a noticeable yellow cast compared to the illumination from sunlight.

The descriptions of the appearance of "half-night" remind me of nothing so much as childhood memories of my father working in the open garage on warm summer nights by the light of a brilliant electric lantern.
Outside of the garage was blackness; inside, the harsh illumination of this intense yellow light source that was almost too bright to look at.