APOD: Where Your Shadow Has Company (2016 Feb 20)

[APOD Robot]

Where Your Shadow Has Company

Explanation: Want to take a relaxing interstellar vacation? Consider visiting Kepler-16b, a world in a binary star system. In fact Kepler-16b is the first discovered circumbinary planet. It was detected in a wide 229 day orbit around a close pair of cool, low-mass stars some 200 light-years away. The parent stars eclipse one another in their orbits, observed as a dimming of starlight. But Kepler-16b itself was discovered by following the additional very slight dimming produced during its transits. Like sci-fi planet Tatooine of Star Wars fame, two suns would set over its horizon. Still, Kepler 16b is probably not a Tatooine-like terrestrial desert world. Instead, Kepler 16b is thought to be a cold, uninhabitable planet with about the mass of Saturn and a gaseous surface ... so plan to dress accordingly. Or, choose another Visions of the Future vacation destination.

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[Ann]

APOD Robot wrote:
It was detected in a wide 229 day orbit around a close pair of cool, low-mass stars

In today's APOD, one of the stars is white and the other is red. What are their spectral classes?

Cool illustration. I like the 1950s sci-fi feel.

Ann

[RichardSRussell]

You DO know that the illustration got the shadows reversed, right? The ground under the shadow pointing away from the red star is illuminated only by the white one, so it shouldn't be reddish.

[Ann]

Okay, I answered my own question.

https://en.wikipedia.org/wiki/Kepler-16 wrote:
Kepler-16 is a binary star system in the constellation of Cygnus[4] that was targeted by the Kepler spacecraft. Both stars are smaller than the Sun; the primary, Kepler-16A, is a K-type main-sequence star and the secondary, Kepler-16B, is an M-type red dwarf. They are separated by 0.22 AU, and complete an orbit around a common center of mass every 41 days.

The system is host to one known extrasolar planet in circumbinary orbit: the Saturn-sized Kepler-16b.

Ann

[Alhuru]

The shadow from the white star would be illuminated primarily by red light (ignoring reflections from the surface) so would appear red. The shadow from the red star would be (largely) white, but since the overall lighting is red/pink (which the eye would perceive as white-ish) the shadow from the red star would appear to the eye to have a blue/green hue.

[rstevenson]

You DO know that the illustration got the shadows reversed, right? The ground under the shadow pointing away from the red star is illuminated only by the white one, so it shouldn't be reddish.

The shadow from the white star would be illuminated primarily by red light (ignoring reflections from the surface) so would appear red. The shadow from the red star would be (largely) white, but since the overall lighting is red/pink (which the eye would perceive as white-ish) the shadow from the red star would appear to the eye to have a blue/green hue.

Whenever I look at my shadow here on Earth, it's just, well, dark. In fact, it's likely to look dark green if I'm on a lawn, or maybe dark gray if I'm standing in a parking lot -- despite the blueish cast it will actually have from the blue sky. So the shadows in this ilustration are quite accurately indicated, since the ground where the tourist/astronaut/scientist is standing appears to be reddish dirt.

Rob

[Randy Morris]

"Instead, Kepler 16b is thought to be a cold, uninhabitable planet with about the mass of Saturn and a gaseous surface ... so plan to dress accordingly. Or, choose another Visions of the Future vacation destination."

If Kepler 16b is a gas giant like Saturn, then it is not "cold" unless you are in the far reaches of the upper atmosphere, or unless it is very very old. Gas giants become very hot as you go down into their atmospheres. For instance, the temperature of Jupiter several thousand Km into its "atmosphere" is about the same temperature as the surface of the sun. The warmest places in our solar system, aside from the sun, are in the "gaseous surfaces" of our gas giants, Jupiter, Saturn, Neptune and Uranus. The largest volume of habitable zone by temperature in our solar system is a layer of Jupiter's atmosphere. However, the Jupiter zone has very little water, and visiting there would require a balloon of some kind, as has been proposed for habitats in the atmosphere of Venus. It is possible then that Kepler 16b could have a habitable zone at some depth in its atmosphere if it contained enough water, which may be unlikely.

[Chris Peterson]

You DO know that the illustration got the shadows reversed, right? The ground under the shadow pointing away from the red star is illuminated only by the white one, so it shouldn't be reddish.

The shadow from the white star would be illuminated primarily by red light (ignoring reflections from the surface) so would appear red. The shadow from the red star would be (largely) white, but since the overall lighting is red/pink (which the eye would perceive as white-ish) the shadow from the red star would appear to the eye to have a blue/green hue.

Whenever I look at my shadow here on Earth, it's just, well, dark. In fact, it's likely to look dark green if I'm on a lawn, or maybe dark gray if I'm standing in a parking lot -- despite the blueish cast it will actually have from the blue sky. So the shadows in this ilustration are quite accurately indicated, since the ground where the tourist/astronaut/scientist is standing appears to be reddish dirt.

It's also worth considering that shadows on Earth will often be noticeably colored if you are seeing them in a photo, as opposed to with your eye directly. Photos maintain more accurate color information.

[neufer]

You DO know that the illustration got the shadows reversed, right? The ground under the shadow pointing away from the red star is illuminated only by the white one, so it shouldn't be reddish.

The shadow from the white star would be illuminated primarily by red light (ignoring reflections from the surface) so would appear red. The shadow from the red star would be (largely) white, but since the overall lighting is red/pink (which the eye would perceive as white-ish) the shadow from the red star would appear to the eye to have a blue/green hue.

Whenever I look at my shadow here on Earth, it's just, well, dark. In fact, it's likely to look dark green if I'm on a lawn, or maybe dark gray if I'm standing in a parking lot -- despite the blueish cast it will actually have from the blue sky. So the shadows in this ilustration are quite accurately indicated, since the ground where the tourist/astronaut/scientist is standing appears to be reddish dirt.

[b][color=#0000FF]An artist's rendering of the Kepler-16 system Credit: NASA/JPL-Caltech[/color][/b]

---

And it basically had to be reddish dirt in order for the the secondary, Kepler-16B (an M-type red dwarf) to produce any sort of noticeable shadow at all vis-a-vis the shadow of the primary, Kepler-16A (a K-type main-sequence star).

The real problem with the illustration
is that the background red dwarf
is way too big

[RichardSRussell]

It should be possible to replicate these lighting conditions here on Earth in a darkened room with 2 bright lights, 1 with a red filter. I don't have enuf time to do it myself, but perhaps somebody else is curious enuf to give it a go and report back on their findings.

[neufer]

[url=http://www.iamsterdam.com/en/visiting/areas/amsterdam-neighbourhoods/centre/red-light-district]Amsterdam’s Red Light District[/url]

---

It should be possible to replicate these lighting conditions here on Earth in a darkened room with 2 bright lights, 1 with a red filter. I don't have enuf time to do it myself, but perhaps somebody else is curious enuf to give it a go and report back on their findings.

[madtom1999]

A quick blast on a ray tracer will show Mr Russel to be largely correct.

[geckzilla]

A quick blast on a ray tracer will show Mr Russel to be largely correct.

A quick blast on a ray tracer shows that arguing about one little detail being wrong in this illustration is completely silly. Both shadows take on the color of the sky, whatever that may be. The dwarf's shadow is negligible and barely noticeable. Richard Russell et al. are correct in one sense, but the illustration is so "wrong" that nitpicking this one detail over all the other "wrong" parts would seem to miss the forest for the leaf on one tree.

[neufer]

A quick blast on a ray tracer will show Mr Russel to be largely correct.

A quick blast on a ray tracer shows that arguing about one little detail being wrong in this illustration is completely silly. Both shadows take on the color of the sky, whatever that may be. The dwarf's shadow is negligible and barely noticeable. Richard Russell et al. are correct in one sense, but the illustration is so "wrong" that nitpicking this one detail over all the other "wrong" parts would seem to miss the forest for the leaf on one tree.

---

We appear to be on a Titan type moon (but with a much thinner atmosphere) orbiting around the Saturn gas giant Kepler 16b. The reddish-brown tholins on the surface dominate the shadow color.

<<Titan is the only moon in the solar system to have a thick atmosphere. Images from the Huygens probe show that the Titanean sky is a light tangerine color. However, an astronaut standing on the surface of Titan would see a hazy brownish/dark orange color. It seems likely that Saturn is permanently invisible behind orange smog, and even the Sun would only be a lighter patch in the haze, barely illuminating the surface of ice and methane lakes.>>

• . https://www.youtube.com/watch?v=rBHx0UwMHLc

[Boomer12k]

Science officer..."Immaterial, Captain...as at 2 times the speed of light, this ship's top speed, it would take 100 years...and you are not likely to survive. The odds are...."

Captain: "Don't quote me the odds....Helmsman, 2 times the speed of light..."

Helmsman: "Aye, Captain..."....with a sigh under his breath...another fruitless cruise....

:---[===] *

[Boomer12k]

A quick blast on a ray tracer will show Mr Russel to be largely correct.

A quick blast on a ray tracer shows that arguing about one little detail being wrong in this illustration is completely silly. Both shadows take on the color of the sky, whatever that may be. The dwarf's shadow is negligible and barely noticeable. Richard Russell et al. are correct in one sense, but the illustration is so "wrong" that nitpicking this one detail over all the other "wrong" parts would seem to miss the forest for the leaf on one tree.

Looks more like a darker color of the ground...not sky....at least to me...
But then my eyes might be screwed up....

:---[===] *

[geckzilla]

A quick blast on a ray tracer will show Mr Russel to be largely correct.

A quick blast on a ray tracer shows that arguing about one little detail being wrong in this illustration is completely silly. Both shadows take on the color of the sky, whatever that may be. The dwarf's shadow is negligible and barely noticeable. Richard Russell et al. are correct in one sense, but the illustration is so "wrong" that nitpicking this one detail over all the other "wrong" parts would seem to miss the forest for the leaf on one tree.

Looks more like a darker color of the ground...not sky....at least to me...
But then my eyes might be screwed up....

:---[===] *

The ground in my render is a plain gray plane. It has also taken on the color of the sky.

[Chris Peterson]

The ground in my render is a plain gray plane. It has also taken on the color of the sky.

Not to mention that if the stars were as close/large as shown in the poster, the shadows would be penumbral- just vaguely darker areas at most, not showing real edges.

[Ann]

A quick blast on a ray tracer will show Mr Russel to be largely correct.

A quick blast on a ray tracer shows that arguing about one little detail being wrong in this illustration is completely silly. Both shadows take on the color of the sky, whatever that may be. The dwarf's shadow is negligible and barely noticeable. Richard Russell et al. are correct in one sense, but the illustration is so "wrong" that nitpicking this one detail over all the other "wrong" parts would seem to miss the forest for the leaf on one tree.

Thanks for the illustration, Geck! That's really clarifying. There wouldn't be two obvious sets of shadows, just one, because the brightness difference would be too great for the smaller star to make much of an impact. I agree, too, that both stars might look whitish. Both would be too bright to stare at directly, of course. But also the color difference between the two stars wouldn't be overwhelming in the first place, and also the light from the whiter K star might actually reflect on the photosphere of the M star, thereby diluting its color.

So, Geck, imagine a planet orbiting Algol. There is an immediate problem here, because Algol A is a B8-type star, very much brighter than the Sun and, compared with our own star, terribly ultraviolet. Life could not exist on a planet orbiting Algol at one A.U. Let's forget about that little detail and pretend that a planet orbiting Algol at one A.U. would be a fine vacationing spot for future spacefaring humans.

What would these humans see? According to Jim Kaler, Algol consists of a primary B8-type star about 95 times brighter than the Sun in optical light, and a shrunken class K giant 4.5 times brighter than the Sun. That would make the bluish primary about 20 times brighter than the yellowish secondary. Also the stars are very close together, separated by only 0.05 A.U. Would Algol B be bright enough and sufficiently well separated from the primary to cast its own shadow on the ground of a planet orbiting at 1 A.U. from the stellar pair? In other words, would objects on the planet cast two obvious shadows on the ground?

Ann

[geckzilla]

The ground in my render is a plain gray plane. It has also taken on the color of the sky.

Not to mention that if the stars were as close/large as shown in the poster, the shadows would be penumbral- just vaguely darker areas at most, not showing real edges.

Indeed. If I play with it a bit I can get the left ball to be orange and look kinda like the illustration. This results in the orange from that sphere overtaking the sky color (the yellow) and now everything is a bit orangeish. The only place the sky color dominates is in its shadow, which now looks a bit greenish because of an optical illusion. Amusing.

[geckzilla]

So, Geck, imagine a planet orbiting Algol. There is an immediate problem here, because Algol A is a B8-type star, very much brighter than the Sun and, compared with our own star, terribly ultraviolet. Life could not exist on a planet orbiting Algol at one A.U. Let's forget about that little detail and pretend that a planet orbiting Algol at one A.U. would be a fine vacationing spot for future spacefaring humans.

What would these humans see? According to Jim Kaler, Algol consists of a primary B8-type star about 95 times brighter than the Sun in optical light, and a shrunken class K giant 4.5 times brighter than the Sun. That would make the bluish primary about 20 times brighter than the yellowish secondary. Also the stars are very close together, separated by only 0.05 A.U. Would Algol B be bright enough and sufficiently well separated from the primary to cast its own shadow on the ground of a planet orbiting at 1 A.U. from the stellar pair? In other words, would objects on the planet cast two obvious shadows on the ground?

Ann

If you can tell me the radius of each star then I can try to simulate it for you. I think you could probably guess better than I could if it has to be guessed. You could tell me in sun-radii if that is easiest for you. I pretty much just put in a temperature in kelvin and convert it to a blackbody for the color and have to guess at the intensity because Blender isn't exactly a physics sim, even though it kinda is.

[Ann]

So, Geck, imagine a planet orbiting Algol. There is an immediate problem here, because Algol A is a B8-type star, very much brighter than the Sun and, compared with our own star, terribly ultraviolet. Life could not exist on a planet orbiting Algol at one A.U. Let's forget about that little detail and pretend that a planet orbiting Algol at one A.U. would be a fine vacationing spot for future spacefaring humans.

What would these humans see? According to Jim Kaler, Algol consists of a primary B8-type star about 95 times brighter than the Sun in optical light, and a shrunken class K giant 4.5 times brighter than the Sun. That would make the bluish primary about 20 times brighter than the yellowish secondary. Also the stars are very close together, separated by only 0.05 A.U. Would Algol B be bright enough and sufficiently well separated from the primary to cast its own shadow on the ground of a planet orbiting at 1 A.U. from the stellar pair? In other words, would objects on the planet cast two obvious shadows on the ground?

Ann

If you can tell me the radius of each star then I can try to simulate it for you. I think you could probably guess better than I could if it has to be guessed. You could tell me in sun-radii if that is easiest for you. I pretty much just put in a temperature in kelvin and convert it to a blackbody for the color and have to guess at the intensity because Blender isn't exactly a physics sim, even though it kinda is.

Jim Kaler says the B star is 2.9 solar radii and the K star 3.5 solar radii. Also, he says that the temperature of the B star is 12,000 K, while the K star is 4,500 K.

Ann

[geckzilla]

Best I can do. The Algol system is so much brighter than our Sun that I can't make a comparison with a single image. To simplify things there is no sky.

[Ann]

Thanks a lot, Geck!

"Sort of two", but overlapping, shadows for the Algol planet, then.

What about Alpha Centauri?

Jim Kaler wrote:
The brighter is a yellow class G2 hydrogen-fusing dwarf star that, with a temperature of 5790-5850 Kelvin (a bit warmer than the solar temperature of 5780 K), appears almost identical to our G2 Sun, certainly an odd coincidence. The companion, over a magnitude fainter, is a cooler (5260-5320 Kelvin) class K1 dwarf, the two making an obvious color contrast. The pair orbit each other every 79.9 years. Though they average 23.2 Astronomical Units apart (21 percent farther than Uranus is from the Sun), the elliptical orbit sends them from a farthest distance of 35 AU to 11, just over Saturn's solar distance.
...
Direct measure of radius through interferometry gives a radius for Alpha-A of 1.21 solar and for Alpha-B of 0.85 solar

According to my software, Alpha Centauri A is 1.47 times as bright as the Sun, while Alpha Centauri B is 0.38 times as bright as the Sun.

Imagine a planet orbiting the brighter component at 1 A.U. Alternatively, imagine a planet orbiting the fainter component at 1 A.U.

Consider a time when both stars are as close to one another as they can get. Would a person standing on a planet orbiting at 1 A.U. of either star see two shadows of himself on the ground?

Ann

[geckzilla]

Yeah, and that's at maximum possible separation. Say if the planet somehow rotated in such a way that matched their orbital period and the only piece of land for a person to stand on made it so only one star was visible during sunset and sunrise, that person would eventually figure out there are two stars by looking at their shadow at noon, even though the fact would be obscured from direct observation by the glare.