Wikipedia:Reference desk/Archives/Science/2010 January 5

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January 5

sky colors over a blue stars

This question tends to trick everyone, everyone is having a tough time deciding if the star is blue and the planet have earthlike atmosphere, would the sky still be blue or it would have color of something else. If star is any other color, then the sky is blue that easy if it have earthlike atmosphere , if no atmosphere the sky is black.--69.226.34.161 (talk) 01:36, 5 January 2010 (UTC)

Is there a specific question? -- kainaw™ 01:39, 5 January 2010 (UTC)

I meant if earth was orbiting a blue star what color will the sky be, because the blue star has blue light, people would wonder if a blue star will emit blue light, the sky wouldn't be black at this case.--69.226.34.161 (talk) 02:55, 5 January 2010 (UTC)

Of course a blue star emits blue light, that's what makes it blue... Despite your rather confused description of the problem, I think I understand. You want to know what colour the sky would be for an Earth-like atmosphere of a planet orbiting a blue star. That's simple enough - it would be blue. The colour of the sky is due to Rayleigh scattering. The particles in the air scatter blue light more than they scatter light of longer wavelengths, and it is that scattered light that we see when we look up but not towards the sun. The light from a blue star will be scattered the same way. Blue stars still emit over a wide range of wavelengths, just with the peak nearer the blue end of the spectrum (actually, probably in the UV part of spectrum). More interesting would be the sky of a planet orbiting a red star - red stars emit very little blue light (not none, though), so the sky would look very dark (but still blue - or, at least, bluer than the star itself looks). This is all assuming human eyes - life that evolved on those planets would have eyes that see in the range where their star emits most of its light (just as we do), so people on the blue star planet would actually see the sky as UV (which they would be able to see) and people on the red star planet would probably see the sky as green (since they wouldn't be able to see blue). --Tango (talk) 03:23, 5 January 2010 (UTC)

It's a bad idea to speculate on how life around some other star might evolve. Sure, humans have evolved vision that's a good 'fit' for the brightest part of our sun's spectrum - but plenty of other animals see in the infra-red (Owls and some kinds of snake, for example) while others have evolved to see in the ultra-violet (bees, for example). So we can't conclude that evolution would necessarily drive all life to seeing colors that match that of the peak output of their local star. We truly don't know how (and cannot meaningfully speculate) how these hypothetical aliens might see their sky. SteveBaker (talk) 17:40, 5 January 2010 (UTC)

"It's a bad idea to speculate on how life around some other star might evolve." No, it is not. Speculating about what could be is the driving force of science. Of course, it has to be accompanied with calculation, experimentation and observation (the later two being very difficult is this special case). What has gotten into you? I know you from previous years of posting and providing knowledge and help here but lately you tend to answer questions by something equivalent to "It is not, so it cannot be. Full stop." I am worried. 93.132.156.195 (talk) 17:25, 6 January 2010 (UTC)

As I understand it, animals that see into UV or IR only see very near UV or IR. An O-type main sequence star (which is described as being blue) has a minimum surface temperature of 30000K, which corresponds to a peak wavelength of 100nm - that is Extreme ultraviolet, a long way from the near UV that bees can see. (The upper end of O-type is 52000K, or 56nm peak.) I think it is fair to say that all animals on Earth (with the exception of blind cave dwellers, I suppose) are adapted to where the Sun emits most of its light, the differences are fairly minor. --Tango (talk) 21:09, 5 January 2010 (UTC)

You should also keep in mind that the atmosphere matters. An atmosphere with a large oxygen content, like ours, is fully opaque below about 280 nm. It doesn't matter what is emitted if the life forms can never see it. Dragons flight (talk) 21:23, 5 January 2010 (UTC)

That is an excellent point. --Tango (talk) 21:54, 5 January 2010 (UTC)

Bees see UV out to about 300nm - so that makes perfect sense. They can see all the way to the top of the UV spectrum - as transmitted by the atmosphere. SteveBaker (talk) 00:02, 6 January 2010 (UTC)

Anyone want to add information on bee eyesight to bee? Seems like a useful detail. Dragons flight (talk) 21:28, 5 January 2010 (UTC)

I did a little googling, but didn't find any useful reliable sources, just a few unreliable sources that were enough for me to make an educated guess from. I wouldn't be happy including educated guesses in an article. If someone can find better sources, it would be good to include it in the article. We also have Colour_vision#In_other_animals, that could do with expanding (there is more than one paragraph to say on the subject - it could probably have its own article). --Tango (talk) 21:54, 5 January 2010 (UTC)

It's described in detail in Chapter 8 of Richard Dawkins' Climbing Mount Improbable - which includes a pair of photos of a flower, one in normal light and the other in UV - showing how the flower has an ultraviolet 'target' on it, showing the bees the way to the nectar at the center. His bibliography references: "The Honey Bee" by Gould J.L and Gould C.G, "Bumblebee Economics" by Heinrich, B. Online, you can view Dawkin's lecture to the Royal Institution (you don't get much more prestigious than that!) entitled "The UltraViolet Garden" here. "The Natural History of Pollination" by Michael Proctor, Peter Yeo has a diagram at the top of page 131 that shows that Bee vision uses three color receptors - one in yellow, one in blue and the other in UV - going out to 300nm wavelengths. Also "The Pollination of Flowers" by Proctor and Yeo, Collins, 1973, ISBN 0 00 213178 1 has a section about this - and there is more online stuff here Hopefully, that's more than enough reference for such a well-known fact about bee vision. SteveBaker (talk) 23:51, 5 January 2010 (UTC)

UV stands for ultraviolet, looking at this article it actually means black light. Spectrum drawn shown UV end as more or less black.--69.226.34.161 (talk) 04:10, 5 January 2010 (UTC)

"Black light" is actually a casual non-scientific term used in limited applications for what is more formally called Ultraviolet, not the opposite as you seem to suggest. It's shown as black because we humans cannot see it directly (just as we cannot see Infrared) but many other creatures (for example, Bees) can, so it is not "black" to them and they presumably perceive the coloration of the sky and the rest of the world differently to us. 87.81.230.195 (talk) 15:11, 5 January 2010 (UTC)

It's interesting to note that humans actually can see UV, if you remove the cornea, which normally blocks it for health and safety reasons. We can also see x-rays, to a degree, fuzzily, dangerously, in a dark room, but I believe that's through a different mechanism to our normal vision. --Sean 16:31, 5 January 2010 (UTC)

Of course UV isn't a single color/frequency. It's a band of frequencies. People who have had their cornea(s) removed can see a little way into the 'near' ultraviolet - but not as far as (for example) bees can. My mother had hers removed during cataract surgery - it's important to note that you really don't see any 'new colors' - you still only have red, green and blue sensors - so UV light looks just like a very normal blueish violet color. What is interesting is that flowers that are pollinated by bees have evolved to present certain patterns in UV light that are not there in normal human vision. My mother (being an avid gardener) has noticed odd (and very feint) blue/violet stripes and splotches on some kinds of flowers that she couldn't see before - so there is not doubt that she's seeing some of what a bee sees. SteveBaker (talk) 17:40, 5 January 2010 (UTC)

Since this thread has wandered into speculation about superhuman forms of vision we might revisit the archaic idea that the eye sees by projecting instead of receiving rays. That means that what a colour would look like is defined before one sees it. (This is not nonsense, it is how ray tracing in CGI works. Superman's X-ray spectacles must work this way too.) X-rays have a 4-decade frequency range (3E16 to 3E19 Hz) but AFAIK all medical X-ray photographs are monochrome. What wondrous false-colour images they might show through us! I have read that a British local council once decreed a prohibition against use of X-ray spectacles. Cuddlyable3 (talk) 00:02, 6 January 2010 (UTC)

Would we get interesting false colour images? That would require different types of body tissue/bones to have different variations in their opacity to different frequencies of x-ray. Is that the case? --Tango (talk) 00:20, 6 January 2010 (UTC)

I suspect that is the case. Different target (anode) materials have different characteristic spectral emission and absorption frequencies. Charles Barkla received the 1917 Nobel Prize in Physics for his discovery of the characteristic X-ray line spectra of different elements. Furthermore any diffraction effects, such as from crystal structures, are wavelength dependent. One certainly gets differing X-ray medical images with and without injected radiopaque contrast masterial. Cuddlyable3 (talk) 01:55, 6 January 2010 (UTC)

I don't see what injecting a contrast material has to do with it - that increases the opacity at the one wavelength you are using. I have no idea what it does at other wavelengths. --Tango (talk) 02:30, 6 January 2010 (UTC)

In general, I suspect it wouldn't be very interesting. At low energies there is a lot variation in x-ray cross-sections for different elements (it's the operating principle for energy dispersive X-ray spectroscopy), but at the high energies typically needed for penetrating x-ray scans there isn't much variation. Specifically, the high energy x-rays pretty much just scatter/attenuate in relation to the electron densities they encounter. Electron rich elements (calcium in bones, iodine or barium in x-ray contrasts) are more visible because they have more electrons, but they aren't really fundamentally different colors they way one can distinguish them in light. As one may recall, colors come from exciting electronic transitions that are typically a few eV. Once you start firing penetrating x-rays at 40-50 keV there are no more discrete transitions to excite, since everything is ionizing. Dragons flight (talk) 04:03, 6 January 2010 (UTC)

french spiderman

have they ever commited him? here in the us if they feel u did crime but will get a light settence they commite u instread becausse they can keep u there for a long time. —Preceding unsigned comment added by 67.246.254.35 (talk) 02:17, 5 January 2010 (UTC)

Alain Robert mentions many arrests but no involuntary commitment. I'm not sure your US description is fair but the reference desk is not for such debates. PrimeHunter (talk) 03:22, 5 January 2010 (UTC)

Who is "they"? Psychiatrists commit people, not the justice system. --Tango (talk) 03:25, 5 January 2010 (UTC)

In the U.S., involuntary commitment is a legal procedure, not a medical one. But I would like to see evidence that "they" commit people they think are going to get light sentences. 75.41.110.200 (talk) 05:24, 5 January 2010 (UTC)

The Pacific Gull's huge beak...

As far as I am aware, the Pacific Gull of Australia has the largest beak of any gull (it's almost puffinesque) and looking from the front, the gull appears to have a rather wide and muscular jaw. Now, I recall someone mentioning on here (may not have been on this desk, per se) that this bird has a very powerful bite, as might be expected from any large gull. What I'm curious about here is whether the Pacific Gull has a more powerful bite than other gull species of a similar size (say the Great Black-backed Gull, for example) - can it, say crack bivalves with its jaws that other gulls might have to drop from a height onto rocks/concrete to open? Can it tear flesh more efficiently? Can it carry larger amounts of food in its bill? Or is the size of the thing mainly for 'show' (I'm thinking of how a toucan beaks, whilst enormous and impressive are actually much less powerful than they look like they should be)? Any ideas? I've never personally encountered this gull, nor am I likely to at any time in the foreseeable future. --Kurt Shaped Box (talk) 04:02, 5 January 2010 (UTC)

A larger, thicker beak may protect it from fracture when subjected to greater forces, but strength would derive from larger or more efficient muscles of mastication -- I am unaware, though, of the avian set-up of such muscles. As a frequent science-question-related editor, I'm sure you're aware of this, but for the record, it should be mentioned overtly in the text of the discussion on this point :) DRosenbach ^{(Talk | Contribs)} 14:21, 5 January 2010 (UTC)

Tangentially related - why is there a Journal of Hippocampus but not a Journal of Gull Science? With such specificity in scientific publication, I would think that there should be an authoritative reference periodical for Kurt Shaped Box's various gull-related questions. What is the authoritative gull science reference work? ...is it Wikipedia/the Science Reference Desk?Nimur (talk) 15:07, 5 January 2010 (UTC)

Well, ever since we deleted RD/Seagulls.... — Lomn 16:54, 5 January 2010 (UTC)

If nothing else, you've provoked me to create Hippocampus (journal) -- which I should have done long ago, since my dissertation paper is published there. Looie496 (talk) 17:03, 6 January 2010 (UTC)

Which is more important in the grand scheme of things, would you say - gulls or hippocampi? --Kurt Shaped Box (talk) 22:20, 6 January 2010 (UTC)

Yeah, I'm aware of that DR - though I probably didn't make it clear enough. From looking at pictures of the Pacific Gull, it would appear that it is in fact wider in the face/head than other species of gull, which might be indicative of larger jaw muscles - but it's quite difficult to determine with birds, considering how they can puff up and deflate at will (I wonder what doing that feels like? Goosebumps - but consciously controlled?), significantly changing their apparent size and volume. --Kurt Shaped Box (talk) 02:22, 6 January 2010 (UTC)

Hi! Does anybody know the approximate biting force of a gull? I need it for a science project.

Power Curtain

What is a power curtain. I think they have them as a feature on vehicles.174.3.123.13 (talk) 16:11, 5 January 2010 (UTC)

Interesting...I could not find a single google hit in the top 20 or so that discusses automotive power curtains. The only reference to 'curtain' is in curtain-style air bags, but I hope that comes powered as a standard. Wouldn't be much use if you had to crank it open like rolling a window. DRosenbach ^{(Talk | Contribs)} 17:17, 5 January 2010 (UTC)

I saw several auto-related references in foreign (i.e. not US/UK) classified ads. As such, I expect it's a rough translation or perhaps idiomatic. I don't see overlap between "power curtain" and "power windows", so my best guess is that the two are equivalent. Alternately, it might refer to the partition between driver and passengers on a limo. — Lomn 19:04, 5 January 2010 (UTC)

I believe it's a power-operated side window curtain like the one in this photo. You get them on MPVs in hot countries, but not on small cars. --Heron (talk) 10:18, 6 January 2010 (UTC)

It may be a powered window curtain (or shade) as seen on many luxury vehicles, located on the rear deck to shade the back seat from sun. They extend and retract via motor. From the Google results, it sounds like they are quite failure prone (being electro-mechanical in nature). --Jmeden2000 (talk) 16:44, 6 January 2010 (UTC)

Impreza

What is standard-edition trim?174.3.123.13 (talk) 16:15, 5 January 2010 (UTC)

Note that the WRX STI you've linked is far from what many would consider "standard-edition"; however, you can find the specs of that model (and all others available) at Subaru's website. — Lomn 16:51, 5 January 2010 (UTC)

It says in lede, doesn't it (or is it a "newer" version)?174.3.123.13 (talk) 17:32, 5 January 2010 (UTC)

I just googled it, I didn't click on the article, so it may just be a cached version.174.3.123.13 (talk) 17:33, 5 January 2010 (UTC)

I suppose it's "standard" in that it's a mass-produced model, but the STI is the most expensive trim available (out of about 7, by my count) for the Impreza -- thus, it's not what I think most people would consider a "standard" Impreza. — Lomn 19:01, 5 January 2010 (UTC)

From nothing to electricity

A question purely out of curiosity:

In order to generate current, most power plants rely on some form of heat-generation which drives a steam engine which drives a dynamo which generates the current.

In order to make a dynamo, you need a magnet.

In order to magnetize materials, you need another magnet or a current (to produce an electromagnet which can magnetize the material).

So how did people get from naturally-occurring and presumably rather weak magnets to today's strong magnets? Is it possible to produce a magnet which is stronger than the magnet used to magnetize it (or stronger than the magnet in the dynamo used to produce the current used to magnetize it)? (obviousely the answer is yes, but how was it done? Did it involve large transformer circuits to get a higher current or something?) —Preceding unsigned comment added by 83.134.170.251 (talk) 18:53, 5 January 2010 (UTC)

Spontaneous magnetization. Dauto (talk) 19:08, 5 January 2010 (UTC)

Electric current and more windings. Or do you mean before electromagnets? Percussion —Preceding unsigned comment added by 75.41.110.200 (talk) 19:40, 5 January 2010 (UTC)

There are Lodestones, which are natural magnetic rocks. But actually magnets are usually created by electric currents, not by other magnets. The original magnets (except for Lodestones) were also electromagnets. To get the electricity they used a chemical battery a Voltaic pile, not a dynamo. Also you don't need a strong magnet necessarily to make a lot of electricity - you can also use a larger magnet, or spin it faster. Which will give you enough electricity to make any strength magnet you like. Ariel. (talk) 21:51, 5 January 2010 (UTC)

Just out of interest, big power station generators tend not to contain large permanent magnets. See Excitation (magnetic). Tonywalton ^Talk 23:16, 5 January 2010 (UTC)

Lots of turns of insulated copper wire around an iron horseshoe or other form creates a very strong magnet when a small electric current flows through it, far stronger than any natural magnet. These date back to the 1820's, and the work of William Sturgeon and Joseph Henry. There is no paradox such as the OP described, where only a strong magnet could create another strong magnet. Such an electromagnet can be used to magnetize steel or some alloys of nickel, cobalt and aluminum, or Neodymium to create amazingly strong permanent magnets. The current to power the early strong magnets came from zinc and copper plates dipped in dilute acid. Unmagnetized iron, weighing 59.5 pounds, became a strong electromagnet, capable of lifting 2063 pounds [1], just from the passage of a small electric current through thousands of turns of insulated wire around it. From the work of Michael Faraday, a coil moving past the gap of such an electromagnet has a voltage induced in it, giving the basis of the dynamo. One dynamo can furnish the power for its own coils, with the small amount of residual magnetism left in the iron being sufficient to start up the process when the dynamo starts spinning. By the 1890's the electromagnets used in generators were about 30 times more powerful than Henry's magnet of the 1820's. Electromagnets or field coils in generators today can be made even stronger. The strength of any natural or permanent magnets was of no importance in the development of these powerful electromagnets, nor are powerful permanent magnets needed to make the strong neodymium magnets sold today. Edison (talk) 23:10, 5 January 2010 (UTC)

For example, O +ve, and A1 -ve are rare types.

hi

Human blood group systems#Rare blood types:

A blood type is classified as rare when more than 200 donors have to be screened to find one compatible donor with blood of that type. In the "ABO" system, all blood belongs to one of four major group: A, B, AB, or O. But there are more than two hundred minor blood groups that can complicate blood transfusions. These are known as rare blood types. About one person in 1,000 will inherit a rare blood type. Whereas common blood types are expressed in a letter or two, with maybe a plus or a minus, a fewer number of people express their blood type in an extensive series of letters in addition to their 'ABO' type designation. For example, O +ve, and A1 -ve are rare types.

o+ve is Rare Blood type??????? —Preceding unsigned comment added by 212.77.204.150 (talk) 19:49, 5 January 2010 (UTC)

Formatted the original post for greater clarity. I agree that O +ve seems unlikely to be classified as "rare". Dragons flight (talk) 20:30, 5 January 2010 (UTC)

This wouldn't have anything to do with genetic linkage, would it? John Riemann Soong (talk) 20:46, 5 January 2010 (UTC)

The article was recently changed - I've reverted it. O+ is not a rare blood type, about 30-40% of people have it, depending on country (Blood_type#ABO_and_Rh_distribution_by_country). --Tango (talk) 21:17, 5 January 2010 (UTC)

Why are some metals more conductive than others?

I understand why metals are good conductors (namely a decentralized 'sea' of electrons caused by metallic bonding) but why is Silver, for example, more conductive than Gold? I'm guessing that it's at least partially related to the atoms electron shell structure given that copper, gold and silver are all in the same group but exactly why are some metals more conductive than others? I have't been able to find the answer anywhere I look, any help is much appreciated. —Preceding unsigned comment added by 95.150.243.104 (talk) 21:26, 5 January 2010 (UTC)

The "sea of electrons" model is an idealized version of what happens. In reality, all elements will exist on a continuum between a perfect, ideal "sea of electrons" model, and a perfectly "localized" model, where each atom retains its own electrons. So, every metal features some "localization" character along with the "sea of electrons" character; the better conductors have more "sea of electrons" character. Does that help? --Jayron32 21:57, 5 January 2010 (UTC)

These articles should talk about this topic, but some are better, or more complete than others: Electrical conduction, Classical and quantum conductivity#Classical conductivity, Drude model, Free electron model. (Actually most of those articles are pretty bad, but maybe the terms mentioned will help your find your answer in other places.) Ariel. (talk) 21:59, 5 January 2010 (UTC)

Thank you both very much, that's very helpful! —Preceding unsigned comment added by 95.150.243.104 (talk) 22:48, 5 January 2010 (UTC)