APOD: Gemini Stars Pollux and Castor (2017 May 16)

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APOD: Gemini Stars Pollux and Castor (2017 May 16)

Postby APOD Robot » Tue May 16, 2017 4:07 am

Image Gemini Stars Pollux and Castor

Explanation: Who are the twins of Gemini? It terms of astronomical objects, the famous constellation is dominated by two bright stars: Pollux (left) and Castor (right). Pictured, the two stars stand out because they are so bright, so close together both in angle and brightness, but so different in color. Pollux, at 33 light years distant, is an evolved red giant star twice as massive as our Sun. Castor, at 51 light years distant, is a blue main sequence star about 2.7 times more massive that our Sun. Castor is known to have at least two stellar companions, while Pollux is now known to be circled by at least one massive planet. In terms of ancient Babylonian, Greek, and Roman mythology, Castor and Pollux represent twin brothers. Currently, the Earth's orbit is causing the Sun to appear to shift in front of the constellation of Gemini, with the result that, for much of humanity, Castor and Pollux will remain visible toward the west at sunset for only a few more weeks.

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Re: APOD: Gemini Stars Pollux and Castor (2017 May 16)

Postby Nitpicker » Tue May 16, 2017 6:40 am

APOD Robot wrote:Currently, the Earth's orbit is causing the Sun to appear to shift in front of the constellation of Gemini, with the result that, for much of humanity, Castor and Pollux will remain visible toward the west at sunset for only a few more weeks.


I hope no one is now worried that we'll never ever see Castor and Pollux again. They'll be back in easterly pre-dawn skies, in a few short months. :wink:

Twins with different fathers? Immaculate!

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Re: APOD: Gemini Stars Pollux and Castor (2017 May 16)

Postby Ann » Tue May 16, 2017 6:49 am

Castor and Pollux are so interesting because they are so typical and so modest as bright stars go.

Not that there is anything modest or common about bright stars in themselves. According to Planet Quest by Ken Croswell, page 79, 80% of all stars in the Milky Way are small red main sequence stars (also known as red dwarfs), 9% are larger but still small K-type main sequence stars, 5% are white dwarfs, 4% are G-type main sequence stars like the Sun, and 2% are all other stars.

Very many of those rare "all other stars" must be A-type main sequence stars and K-type giants. According to Sky Catalogue 2000.0, Volume 1, Stars to Magnitude 8.0, 2nd Edition, by Alan Hirschfeld, Roger W. Sinnott and François Ochsenbein, which lists ~45,000 stars down to magnitude 8.0, the most common type of star listed in the catalog are K-type stars, most of which are certainly evolved giants. The second most common type listed there are A-type stars, most of them certainly main sequence stars. Or to put it differently, A-type main sequence stars and K-type giants are rare in absolute terms, but they are bright enough and common enough to dominate the sky.

Sirius, Vega, Altair of the Summer Triangle, Mizar and Alcor, Alioth, Megrez, Phad and Merak of the Big Dipper, Kaus Australis and Ascella of the Teapot in Sagittarius, and many others, are A-type main sequence stars. Arcturus of Bootes, Dubhe of the Big Dipper, Aldebaran apparently in the Hyades, and Kaus Borealis, Kaus Meridionalis, and Nash of the Teapot of Sagittarius, and many others, are K-type evolved giants.

What is most interesting about Pollux, apart from its planet, is how small and pale this star actually is. Its Johnson B-V is 0.991 ± 0.005, which is paler and more non-red than the color of any of the evolved giants that I have listed above, apart from Nash. Pollux, on the other hand, is the faintest K-type giants of all of those I have listed, only 30.66 ± 0.17 times solar in visual light.

Jim Kaler wrote:
From its distance of 34 light years, we calculate a total luminosity (incuding infrared radiation) for Pollux 46 times that of the Sun, and coupled with its temperature, a diameter some 10 times solar, making it smaller than most of its cool giant brethren and only a quarter the dimension of Aldebaran. Direct measures of angular size, however, yield a somewhat smaller diameter 9 times that of the Sun.


The relatively small size of Pollux means that it definitely wouldn't "eat" the Earth if it was placed at the center of the solar system! If I remember correctly, the distance between the Earth and the Sun is ~100 times the diameter of the Sun.

Interestingly, the distance to Pollux is very close to 10 parsecs. One parsec is 3.26163344 light-years, and the distance to Pollux is 33.785 ± 0.094 light-years, or 10.365 ± 0.029 parsecs.

Wikipedia wrote:
An object's absolute magnitude is defined to be equal to the apparent magnitude that the object would have if were viewed from a distance of exactly 10 parsecs (32.6 light years), with no extinction (or dimming) of its light due to absorption by interstellar dust particles.


The fact that the distance to Pollux is very close to 10 parsecs, and the visual luminosity of Pollux is 30.66 ± 0.17 solar, means that its absolute visual magnitude is very close to +1, or +1.0835 ± 0.0061. So Pollux comes close to being a "standard star", even though it appears to be faint for its class. But perhaps it isn't? Perhaps most K-type giants are smaller than we think? The really big bright ones stand out more than the others in the sky, and Pollux stands out just because it it nearby. Pollux is the most nearby red giant in the sky.

And Castor? Oh, at 47 times solar it's a modest A-type star - make that that it's a modest multiple star made up of six components! A-type Castor A is a spectroscopic double, made up of one bright A-type component and a probably M-type second component, Castor B is another spectroscopic double, made up of one bright A-type component and a probably M-type second component, and Castor C is made up of two small possibly M-type components. The brightest A-type star of Castor is probably just a little bit brighter than Sirius, which is 23 times as bright as the Sun, and Castor A is definitely fainter than Vega.

The Castor system is indeed rather complicated, but in fact many stars more massive than the Sun belong to multiple star systems.

So both Pollux and Castor are modest stars with quirks, but in many ways they are typical and representative of the bright stars of the sky. For one thing, like most bright stars in the sky, Pollux, Castor A and Castor B are all more massive than the Sun, but they are not that much more massive. According to Jim Kaler, Pollux is about 1.8 times as massive as the Sun (or 1.8 M). Castor A, again according to Jim Kaler, is 2.4 M, and Castor B is 1.9 M. So yes, these stars are massive, but they are a far cry from the real, incredibly rare true behemoths and superheavyweights of the heavens.

Ann
Last edited by Ann on Tue May 16, 2017 4:32 pm, edited 4 times in total.
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Re: APOD: Gemini Stars Pollux and Castor (2017 May 16)

Postby Boomer12k » Tue May 16, 2017 7:06 am

And what Myths will they tell after YOUR passing?

Very nice image... two very different stars, and systems...

And yet, "Twins"... I wonder how many half-brothers are actually TWINS... :?
The answer seems to be....THREE...sets...

http://www.mirror.co.uk/news/technology-science/meet-billion-one-twins-who-150093
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Re: APOD: Gemini Stars Pollux and Castor (2017 May 16)

Postby De58te » Tue May 16, 2017 9:39 am

Nitpicker wrote:They'll be back in easterly pre-dawn skies, in a few short months.

That's one short month, June, and one long month, July. I don't know the exact day that Castor and Pollux will be seen rising in the east, but I guess it will be sometime around mid August.

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Re: APOD: Gemini Stars Pollux and Castor (2017 May 16)

Postby Nitpicker » Tue May 16, 2017 11:08 am

Oops... Duplicate
Last edited by Nitpicker on Tue May 16, 2017 11:18 am, edited 1 time in total.

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Re: APOD: Gemini Stars Pollux and Castor (2017 May 16)

Postby Nitpicker » Tue May 16, 2017 11:14 am

De58te wrote:
Nitpicker wrote:They'll be back in easterly pre-dawn skies, in a few short months.

That's one short month, June, and one long month, July. I don't know the exact day that Castor and Pollux will be seen rising in the east, but I guess it will be sometime around mid August.


Not to be out-nitpicked ... Short months by name. :P

Mid August seems right though.

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Re: APOD: Gemini Stars Pollux and Castor (2017 May 16)

Postby neufer » Tue May 16, 2017 1:17 pm

Boomer12k wrote:
"Twins"... I wonder how many half-brothers are actually TWINS... :?
The answer seems to be....THREE...sets...
https://en.wikipedia.org/wiki/Superfecundation wrote:
<<Superfecundation is the fertilization of two or more ova from the same cycle by sperm from separate acts of sexual intercourse. Heteropaternal superfecundation refers to the fertilization of two separate ova by two different fathers. While heteropaternal superfecundation is referred to as a form of atypical twinning, genetically, the twins are half siblings. Heteropaternal superfecundation is common in animals such as cats and dogs. Stray dogs can produce litters in which every puppy has a different sire. Though rare in humans, cases have been documented. In one study on humans, the frequency was 2.4% among dizygotic twins whose parents had been involved in paternity suits. Koen and Teun Stuart, Dutch boys who were the result of in vitro fertilization (IVF). In a mixup at the laboratory, equipment had been used twice, causing another man’s sperm to be mixed with the intended father's. In 1995, a young woman gave birth to diamniotic monochorionic twins, who were originally assumed to be monozygotic twins until a paternity suit led to a DNA test. This led to the discovery that the twins had different fathers. In 1982, twins who were born with two different skin colors were discovered to be conceived as a result of heteropaternal superfecundation.>>
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Re: APOD: Gemini Stars Pollux and Castor (2017 May 16)

Postby Chris Peterson » Tue May 16, 2017 1:46 pm

I'd like to offer a technical description of what we're seeing here, because it's something we often see in astronomical images.

There is a huge dynamic range in star brightness. In a stars-only image like this, just capturing the majority of stars in the field means that the bright stars- Castor and Pollux- will be massively overexposed. In an electronic image, "overexposed" means that a pixel (or pixels) has reached it's maximum possible value. At the sensor level, it is holding all the electrons it is capable of holding. That's called saturation, and it destroys color information. When all three color channels- red, green, blue- are saturated, the pixel appears white. That's what we see here. Castor and Pollux are saturated, and they appear white, well out into the zone of scattered and diffracted light around them. It is only far out in the scatter that the signal finally drops out of saturation and the true color appears, where the RGB ratios are meaningful.

We see this most often with deep-sky images. Extended objects are usually much dimmer than many stars, so exposing them well often results in overexposed stars, whose color we only see in their surrounding halos.

Something similar happens visually, which is why we don't see star colors very well (indeed, many people don't think stars have colors, except for a handful of extreme cases where they see color faintly). Blurring our eyes a bit can really help us see stellar color by spreading the light out onto more retinal cells. Still, stars mostly stimulate our rods, which overwhelm the response of our cones, resulting in the visual appearance of stars being very unsaturated. We need images like today's APOD to show saturated colors. (Whether we want to call those "real" or "true" is a matter of opinion; they certainly reveal important information about the stars, however.)
Chris

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Re: APOD: Gemini Stars Pollux and Castor (2017 May 16)

Postby Ann » Tue May 16, 2017 2:43 pm

Chris Peterson wrote: Extended objects are usually much dimmer than many stars, so exposing them well often results in overexposed stars, whose color we only see in their surrounding halos.

Something similar happens visually, which is why we don't see star colors very well (indeed, many people don't think stars have colors, except for a handful of extreme cases where they see color faintly). Blurring our eyes a bit can really help us see stellar color by spreading the light out onto more retinal cells. Still, stars mostly stimulate our rods, which overwhelm the response of our cones, resulting in the visual appearance of stars being very unsaturated. We need images like today's APOD to show saturated colors. (Whether we want to call those "real" or "true" is a matter of opinion; they certainly reveal important information about the stars, however.)


In my opinion, we have to take the color of the halos of stars into consideration when viewing stars through a telescope. Of course, we must compare blue and yellow stars to make sure that the halos are of different colors. On occasion, during certain misty conditions, even yellow stars can have bluish halos, and then the halo can't be used to ascertain the color of the star. Normally, though, the halo is where the color is - most definitely in the case of blue stars!

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Re: APOD: Gemini Stars Pollux and Castor (2017 May 16)

Postby MarkBour » Tue May 16, 2017 3:51 pm

As a person who comes to APOD to slowly learn basic astronomy, I found today's APOD to be a wonderful example. The image is striking and beautiful. The description was informative and wonderfully worded, as usual. Then Ann's article taught me a trove of additional information that I never would have guessed. I was formulating one of my usual questions about the image, but then I read Chris's note and that answered it (and then some) before I even asked!

I think that inhabitants of a planet of one of the stars in the Castor system would have really interesting night skies. I wonder if their night skies would be more interesting than ours. I think such a situation could produce a lot of astrologers, and it might be hard for a true astronomer, such as a Copernicus or Kepler, to formulate a real description of what was going on. Eventually, if they got as far as us in astronomical observation, the truth (I mean the actual orbital motions) would be unavoidable, of course (assuming they had minds similar to ours, etc).
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Re: APOD: Gemini Stars Pollux and Castor (2017 May 16)

Postby neufer » Tue May 16, 2017 5:04 pm

MarkBour wrote:
I think that inhabitants of a planet of one of the stars in the Castor system would have really interesting night skies. I wonder if their night skies would be more interesting than ours.

I was wondering if a Trojan planet in the Castor AB system might be habitable.

Unfortunately the equilateral triangle distances are ~100 AU such that a Trojan planet would only receive about 1/200 the radiation the Earth receives.

https://en.wikipedia.org/wiki/Trojan_(astronomy)
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Re: APOD: Gemini Stars Pollux and Castor (2017 May 16)

Postby MarkBour » Wed May 17, 2017 12:05 am

neufer wrote:I was wondering if a Trojan planet in the Castor AB system might be habitable.
Unfortunately the equilateral triangle distances are ~100 AU such that a Trojan planet would only receive about 1/200 the radiation the Earth receives.
https://en.wikipedia.org/wiki/Trojan_(astronomy)

So, you just need to find a binary pair that is much closer together ... maybe more like 4 AU ?
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Re: APOD: Gemini Stars Pollux and Castor (2017 May 16)

Postby neufer » Wed May 17, 2017 12:19 am

MarkBour wrote:
neufer wrote:
I was wondering if a Trojan planet in the Castor AB system might be habitable.
Unfortunately the equilateral triangle distances are ~100 AU such that a Trojan planet would only receive about 1/200 the radiation the Earth receives.
https://en.wikipedia.org/wiki/Trojan_(astronomy)

So, you just need to find a binary pair that is much closer together ... maybe more like 4 AU ?

    Maybe more like 7 AU.
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Re: APOD: Gemini Stars Pollux and Castor (2017 May 16)

Postby neufer » Wed May 17, 2017 5:11 pm

https://en.wikipedia.org/wiki/Bollocks wrote:
<<"Pollux" is a word of Middle English origin. The word is often used figuratively in colloquial British English and Hiberno-English as a noun to mean "nonsense", an expletive following a minor accident or misfortune, or an adjective to mean "poor quality" or "useless". Similarly, common phrases like "Pollux to this!" and "That's a load of old Pollux" generally indicate contempt for a certain task, subject or opinion. The word has a long and distinguished history, with the Oxford English Dictionary (OED) giving examples of its usage dating back to the 13th century. One of the early references is Wycliffe's Bible (1382), Leviticus xxii, 24: "Al beeste, that ... kitt and taken a wey the ballokes is, ye shulen not offre to the Lord..." (i.e. Castorated animals are not suitable as sacrifices).>>
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Re: APOD: Gemini Stars Pollux and Castor (2017 May 16)

Postby MarkBour » Thu May 18, 2017 8:39 pm

neufer wrote:
MarkBour wrote:
neufer wrote:
I was wondering if a Trojan planet in the Castor AB system might be habitable.
Unfortunately the equilateral triangle distances are ~100 AU such that a Trojan planet would only receive about 1/200 the radiation the Earth receives.
https://en.wikipedia.org/wiki/Trojan_(astronomy)

So, you just need to find a binary pair that is much closer together ... maybe more like 4 AU ?

    Maybe more like 7 AU.

I think you're considering that Castor A and B are much brighter than Sol, so you're getting a higher preferred distance. (And you're probably actually calculating it, whereas I'm just imagining two stars more like our sun and crudely guessing.) Incidentally, as both of those stars have red dwarf companions, I'm thinking that hanging on at a Lagrange point might get somewhat tricky, if their orbits are significant in size compared to the whole.
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