Wikipedia:Reference desk/Archives/Science/2010 December 1

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December 1

NGC identification in Veil Nebula

The Veil Nebula in Cygnus comprises several NGC/IC items: 6960 (the Witch's Broom), 6992/6995/IC1340 (Eastern Veil), 6974 (faint patch near north boundary) -- and the one I'm asking about: NGC 6979. This is frequently associated with Pickering's Triangle (see, e.g. Astronomy magazine, or indeed Google search for Pickering+triangle+"NGC+6979"), but the coords usually given for 6979 (and displayed in Uranometria) appear to be closer to 6974 -- and Pickering's Triangle was reputedly discovered only photographically, in the early 1900s. So the question is: is NGC 6979 really Pickering's Triangle, and is there a clear, definitive reference for the identity? -- Elphion (talk) 01:43, 1 December 2010 (UTC)

I should add, our own article on the Veil Nebula -- the one I've been working on -- is not yet "definitive" :-) -- Elphion (talk) 01:46, 1 December 2010 (UTC)

I think I've found a reasonable definitive source: the NGC/IC Project (under NGC 1979) makes it clear that although Herschel's original position was none too precise, it referred not to Pickering's Triangle but to another knot of nebulosity. -- Elphion (talk) 21:33, 1 December 2010 (UTC)

Although the error is trivial and obvious, do not be confused by Elphion above having inadvertantly mistyped "1979" intending 6979. Muphry strikes again! :-) 87.81.230.195 (talk) 14:18, 4 December 2010 (UTC)

Plastic-mirror

Can someone please tell if there's a material bendable and reflects (something like a plastic-mirror). If so, could you leave the name and the link in wikipedia (if exists).

Thank You. PD: sorry for my english, not my first language. —Preceding unsigned comment added by 190.218.125.158 (talk) 02:21, 1 December 2010 (UTC)

You can get stainless steel mirror sheets down to fractions of a millimeter that would be quite bendable. Something like this. Vespine (talk) 02:34, 1 December 2010 (UTC)

Also, aluminized mylar (we'll see if that comes up blue). --Trovatore (talk) 02:49, 1 December 2010 (UTC)

See also deformable mirror. --Mr.98 (talk) 03:21, 1 December 2010 (UTC)

Thank you, learned a lot. Is there anything solid that reflects exactly like a mirror? —Preceding unsigned comment added by 190.218.125.158 (talk) 04:14, 1 December 2010 (UTC)

I don't quite understand; if it's solid, and reflects like a mirror, then it is a mirror, right ? StuRat (talk) 05:25, 1 December 2010 (UTC)

The 3 mm stainless steel mirror sheets in the link above would be quite stiff, if that's what you mean by "solid". Or the 0.8 mm mirror sheets would be more bendable, but still much stiffer than aluminized mylar. Or were you looking for something like a curved mirror, which is bent but very stiff? Red Act (talk) 05:45, 1 December 2010 (UTC)

haha sorry :P . What i meant was something like silly putty —Preceding unsigned comment added by 190.218.125.158 (talk) 09:15, 1 December 2010 (UTC)

I don't think anything like putty could ever work as a mirror, because you need a even surface to make a recognizable reflection. A random blob would reflect light in all directions, and not make a clear image. StuRat (talk) 23:17, 1 December 2010 (UTC)

Silly Putty is actually a viscoelastic fluid, a type of non-Newtonian fluid, rather than being a (Hookean) solid. Specular fluids certainly exist; liquid mercury comes to mind, or even water at a grazing angle. And Silly Putty itself exhibits some specular reflection, in addition to diffuse reflection. But I'm having no luck trying to find a viscoelastic fluid that's highly specular in the visible spectrum. Red Act (talk) 00:50, 2 December 2010 (UTC)

What does the OP want to use it for? If they want to cast a mirrored shape, then an epoxy (or other casting resin) which contains a metal filler, can be rubbed with wire wool and buffed up to a mirror like shine. Resins and putties are available with a choice of several different fillers. Alternatively, coat the object with conductive paint, dry, then electroplate with chrome, etc. --Aspro (talk) 16:55, 2 December 2010 (UTC)

Laser distance meters

Are there any laser distance meters that are more-or-less equivalent to a Leica Disto (particularly in the sense of "having a range of at least 70 m"), but, rather than being hand-held units, come in the form of (essentially) a circuit board with a laser attached to it? That is to say, I'm looking for something programmable, intended for remote use, and long-range. --superioridad ^(discusión) 02:59, 1 December 2010 (UTC)

What does the OP want to use it for? Are you thinking of something along these lines. Laser scanner--Aspro (talk) 17:02, 2 December 2010 (UTC)

Those products are for imaging, i.e. constructing 3D models of objects. I want to use it to measure the distance between two objects that will be separated by several dozen meters, and measure how that distance changes (which will only be slightly, so the laser won't end up e.g. pointing in the wrong direction). --superioridad ^(discusión) 01:57, 3 December 2010 (UTC)

Sounds like a Michelson interferometer might be useful here. From the initial positions, you can get changes in distance measured down to a fraction of the wavelenth of your laser (limited by the detector's ability to sense changes in light intensity). It requires that the object's reflection always point back towards your device. Another way is based on time-of-flight, using a gated laser pulse or a waveform. That lets you measure absolute distances (rather than just changes), limited by resolution of your timer. Both are pretty common undergraduate-level lab projects, with various types of detectors, data aquisition/processing, etc. I'm sure there are USB or serial devices available for hobbiests and teaching-labs, but I don't see a premade one off-hand in my PASCO catalog. DMacks (talk) 21:40, 3 December 2010 (UTC)

Global Warming and Ice Age

In The Day After Tomorrow, global warming caused an Ice Age. Could this really possibly happen in real life? —Preceding unsigned comment added by 202.168.107.2 (talk) 03:11, 1 December 2010 (UTC)

Global warming is a misnomer; the preferred term is "climate change", since what happens as a result of increased greenhouse gases is more nuanced than simply "the earth gets hotter". What is much more likely to happen is actually greater swings in climatic changes rather than simply a gradual increase in temperatures; one could envision decades or centuries that were markedly hotter OR colder than the current "normal" temperatures. The problem faced with anthropogenic climate change is not that it is predicted to make everything uniformly warmer; if it were that simple then it would be easier to design a fix; the earth is not merely going to get warmer on a regimented schedule. What is predicted to happen is an increasing unpredictability in climate; so there is no way to prepare for rapid and large changes. One of the problems with popular media when dealing with these problems is that there is a misinterpretation of these predictions. What most mainstream scientists who work in the area agree on is "We know climate is going to change in unpredictable ways", which gets misinterpreted as "We can't predict how climate will change" which gets further assumed to mean "We can't predict anything". You may also want to check out Snowball Earth for a past event with similar implication. --Jayron32 03:55, 1 December 2010 (UTC)

Huh? For years and years we've been told about Global Warming, and now you say that global warming is not going to happen, but just "greater swings in climate changes", in other words the normal ups and downs of the weather. We've been had! 92.24.177.111 (talk) 12:08, 4 December 2010 (UTC)

The movie is based on a genuine possibility but is vastly exaggerated. The climate of Europe is much warmer than it ought to be given its latitude, mainly because the Gulf Stream carries large amounts of heat there from the tropics. The Gulf Stream is ultimately driven by the sinking of cold salty water in the Arctic and North Atlantic, as part of the thermohaline circulation. There is a significant possibility that global warming, by eliminating Arctic ice, could cause the sinking of cold water to stop, thereby stopping the Gulf Stream and greatly chilling the climate of Europe. Such an interruption is thought to have happened during a cold period called the Younger Dryas, over 10,000 years ago, as a result of a sudden massive influx of fresh water into the North Atlantic. Even if it did happen, though, there is no way it could be anywhere near as rapid and dramatic as shown in the movie (as I've heard; I haven't seen it). Furthermore, the main effects would be on Europe, with lesser effects on North America. Looie496 (talk) 04:05, 1 December 2010 (UTC)

For comparison, the abrupt climate change at the beginning of the Younger Dryas took place over the course of several years, as opposed to drastic climate change occurring over the course of several days in the movie. Red Act (talk) 06:23, 1 December 2010 (UTC)

If you're in Britain at the moment, it feels like it's happening right now. Ghmyrtle (talk) 07:43, 1 December 2010 (UTC)

Wow snow in winter. Who would have guessed? Googlemeister (talk) 16:27, 2 December 2010 (UTC)

Snow in late autumn in southern England: actually surprising. 86.161.108.241 (talk) 17:14, 2 December 2010 (UTC)

You mean this southern England? Googlemeister (talk) 19:43, 2 December 2010 (UTC)

• 
  The Frozen Thames, Unknown 1677.
• 
  Thames Frost Fair, Thomas Wyke 1683.
• 
  Frost Fair of 1814, Luke Clennell 1814.

Yes, but a bit before my time old chap ;-) [1] Alansplodge (talk) 23:46, 2 December 2010 (UTC)

Wait, you consider a return to the conditions of the Little Ice Age unsurprising? Isn't that exactly what people are considering a surprising and potentially worrying thing? 86.161.108.241 (talk) 14:54, 3 December 2010 (UTC)

Cats and chocolate

Is it true that cats can die if they eat chocolate? Or is it just a myth? The Utahraptor^Talk/_Contribs 03:52, 1 December 2010 (UTC)

Absoltely, 100% true. See Theobromine poisoning. You can get poisoned by chocolate too; its just that YOU have to eat a lot more than the cat. Whereas you, based on weight, would probably have to eat several pounds of chocolate to get sick, cats can have nasty effects from a single Hershey bar. Cats and dogs also metabolize chocolate differently, so on a per-pound basis chocolate is also much more poisonous to them than to us. --Jayron32 03:57, 1 December 2010 (UTC)

I think a single pound of chocolate would make you pretty sick. Might not be from the theobromine. Just the same, the danger to cats and dogs is sometimes overstated a bit, I think. If you look up the LD50's by weight, they're not what you would call casual exposure.

If I recall correctly, cats are a bit more sensitive than dogs, but tend not to be interested in chocolate, because they don't have taste receptors for sweet. I imagine they'd have to be really hungry. Dogs occasionally do get poisoned, but if the numbers in the article the last time I looked at it are accurate, they do have to eat quite a lot. --Trovatore (talk) 04:12, 1 December 2010 (UTC)

It's probably better, however, to avoid feeding chocolate to pets. The toxicity level for theobromine is going to vary widely from pet to pet, given especially that a Chihuahua would get much sicker than a great dane eating the same chocolate bar. It's one of those things where, year, maybe your dog or cat would be fine, but it's not worth the risk. Just because someone can sprint accross a busy freeway and not get hit doesn't mean it isn't contraindicated to do so. Likewise, just because it may be possible to feed your pet chocolate and, by dumb luck, not end up with any ill effects doesn't mean its a good idea to do so. --Jayron32 05:52, 1 December 2010 (UTC)

Oh, I wasn't even considering the possibility of actively feeding them chocolate. Why would you do that?

I just mean that if you happen to drop the lemon creme from your box of See's and your German Shepard gets to it before you do, it's probably not worth bothering the emergency line at the vet. --Trovatore (talk) 09:58, 1 December 2010 (UTC)

I just want to add to this, late to the party, that different types of chocolate have different levels of risk. If you dog gets a small amount of milk chocolate, no need to freak out — the theobromine levels are low. If they get dark chocolate, be really wary, especially if it is a small dog. If baker's chocolate, call the vet. Etc.

As a dog owner, I think the most dangerous situation is a dog accidentally getting into, say, a box of chocolates, which, depending on the dog, is not too impossible. I'm especially wary with presents at Christmastime because we have some relatives that will send us wrapped boxes of chocolate, and there's real danger that the dog would, in a moment of weakness, try to avail herself of these while we aren't home. Any package she takes an untoward interest in sniffing goes on top of the fridge until Christmas morning. ;-) --Mr.98 (talk) 13:21, 3 December 2010 (UTC)

That little black strap that depends from the backs of cars and scrapes along the road?

I'm seeing fewer of them these days (I'm in Australia) - but can anyone tell me what they're meant to be for, and what they're called?

Thanks Adambrowne666 (talk) 04:57, 1 December 2010 (UTC)

If they are at the sides, they could be curb feelers. I suspect that backup cameras are replacing them as the high-end option to let you know how you are doing backing up.

However, those really aren't meant to drag on the road. I can't think of anything that is, although theoretically they could use such a device to measure road friction and change shift points, etc., depending on how slippery the road is. I've not actually heard of this being done, though. One obvious problem would be that anything dragging on the road like that would quickly wear out. This could be handled by replacing the wearable portion, which just might be a cheap piece of plastic, but it would still be annoying to have to do this so often. Perhaps a system like a weed wacker could be used to automatically deploy the plastic string to the proper length from a large roll. The current traction-control system I am aware of detects slippage in the wheels and adjusts accordingly, but being able to prevent such slippage with proactive measures is certainly preferable to adjusting for it after it has happened. StuRat (talk) 05:10, 1 December 2010 (UTC)

Aren't they antistatic ground straps or something like that ? Not sure whether there is an article about them. Sean.hoyland - talk 05:16, 1 December 2010 (UTC)

List of auto parts has a link to ground strap but it redirects to a more general coverage of the topic. Sean.hoyland - talk 05:21, 1 December 2010 (UTC)

Cool idea StuRat, but no, those things are grounding (earthing) straps, to avoid static shock. [2][3][4] I personally have never gotten a shock from a car (at least not that I can remember), but it's possible that in dry, slightly dusty areas it could be a bigger problem. The last link I posted seems to indicate people are buying them as snake oil. They are obviously much more important in vehicles that are carrying lots of flammables, like gas trucks. Ariel. (talk) 06:56, 1 December 2010 (UTC)

Yes, the rubber in the neumatics isolates the car from the ground. I occasionally get static shocks when entering a car.

These straps have a very small side issue, they can make you the target of rays. When you are isolated, the charges in the ground won't reach you during a storm. But the strap connects you to the ground, and the charges in the ground can climb up your car and attract a ray, the same way as trees and housetops. --Enric Naval (talk) 07:52, 1 December 2010 (UTC)

I think you mean "lightning", not "rays" (or is this a national variant I'm unfamiliar with? "Lightning ray" seems to strike a chord...but given the many Manta rays and X-rays and gamma rays and orbital laser platform microwave rays, all of which can target you, it's probably better to be specific). I also think you are wrong. What keeps you safe in a car during a thunderstorm is not the isolation of the cars body from the ground (if it's raining, the water will conduct electricity, anyways), but the fact that the car is a Faraday's cage, and will conduct the electric charge from a lightning bolt into the ground on the outside of the vehicle. --Stephan Schulz (talk) 08:21, 1 December 2010 (UTC)

Yeah, I meant lightning, it's just my bad English. Yes, the vehicle will protect you... but make sure you don't touch any metal surface. Especially, don't touch the metal surface at two different places, since part of the current might decide to travel through your body instead of travelling through the metal. The car is not a perfect faraday cage since it's not a closed metal surface (there are openings in the cage, and part of the current could travel through the inside surface of the car chassis, depending on the shape and position of the holes).

I repeat that having a ground strap increases the chance of your car being hit by lighting. But I recognize that I don't know much the risk is increased. Maybe it's just increased by a tiny amount. I understand that car tires and not 100% rubber, they have conducting materials in their composition that make them resistors rather than insulators. A lighting has so much voltage that the little ground strap might not make much of a difference. Unfortunately, I can't find any definitive source on this topic. --Enric Naval (talk) 18:43, 1 December 2010 (UTC)

I don't think this is true. Lighting has enough potential that it can travel through several miles of air - the tiny bit of rubber in your tires makes no difference to it whatsoever. Adding a grounding strap doesn't change anything. Ariel. (talk) 20:08, 1 December 2010 (UTC)

Regardless of how far it has gone, most of it will still take the path of least resistance, or lightning rods would be useless. StuRat (talk) 23:14, 1 December 2010 (UTC)

They are frequently marketed, not as anti-shock devices, but as anti-motion-sickness devices. The science behind this seems a little fuzzy to me, but the idea is that the small build up of static electricity that naturally occurs in a car contributes to the sense of motion sickness in sensitive people. By releasing this static to the ground, people who are sensitive to motion sickness are supposed to feel more comfortable. While the ones for cars look like strips of plastic or cloth, larger vehicles sometimes use chains. My wife and daughter get terribly motion sick, so we tried them. There seemed to be a benefit, but it could well be chalked up to expectations of it working than anything "real". Matt Deres (talk) 14:52, 1 December 2010 (UTC)

Yes, I've seen them marketed as anti car-sickness devices. I think they work on the Tinkerbell principle. They certainly don't prevent static shocks when entering or alighting from cars in my experience. DuncanHill (talk) 15:29, 1 December 2010 (UTC)

Re: static buildup: In many years of driving, I only had one car which produced a big static buildup. It was a little compact car, and I learned to touch metal on the car as I stepped onto the ground to discharge it; otherwise if I touched it later there would be an easily felt spark. This also presented an explosion hazard when fueling it, if a spark had occurred. I expect the tires were of a composition which was less conductive than typically, and had not yet gained a coating of dirt. A piece of conductive plastic, perhaps carbon fiber filled, would easily drain off such static charge, or perhaps an anti-static conductive sprays could be applied to the tires to conduct from the metal wheel to the ground. The parallel combination of the conductive path of the four tires might not be as good an insulator as some people assume, since the rubber quickly gets covered with conductive grime, especially when damp, as in a typical lightning storm. Edison (talk) 17:03, 1 December 2010 (UTC)

Thanks heaps everyone - but what are they called? Adambrowne666 (talk) 22:16, 2 December 2010 (UTC) -- actually, from googling, it looks like they're just called antistatic straps - bit disappointing - more a description than a name...

bladder-caused sleep interruption

I just spent 90 minutes, equally divided between Wikipedia and a Medical Advice Encyclopedia, trying to determine HOW LONG IT TAKES for water to become urine stored in the bladder!

NO ANSWER FROM EITHER!

The information is simple and would be extremely useful to many of us who use your service to AVOID getting lost in the thickets of hyper-information (the disease many information providers suffer from) and hypochondria.

How long before hs (hour of sleep) should an otherwise healthy adult or child avoid ingesting liquids as a simple preventative to avoid an over-full bladder demanding attention!

This is extremely useful consumer advice, and SHOULD be simply and easily found in ANY discussion of physiology, bladdders, urine, or piss (pre-13th Century use). —Preceding unsigned comment added by 63.193.182.226 (talk) 08:24, 1 December 2010 (UTC)

Like so many simple questions this one does not have a simple answer that covers all eventualities. I have no source to point you at but my experience indicates that it can be somewhere between 30 and 45 minutes for ingested liquid to be excreted as urine in 'normal' circumstances. however there are a number of other factors that will affect this timing or the need to void urine. The age of the individual is a significant factor in determining the need for nocturnal voiding. More older people tend to have a reduced bladder capacity which is stimulated by smaller amounts of urine. A lot of food we eat contains water and this is likely to be extracted, processed and excreted over a longer time than my experience of 30 - 45 minutes. The activity of the kidneys is regulated to some degree by the individual's circadian rhythm which may reduce urine production during the night but start again at around dawn (or so). This widespread problem is not easily remedied and may be something that individuals have to accept as a consequence of ageing. And please, no-one come back to me on ageism - 'cos I am one of the aged! Richard Avery (talk) 08:51, 1 December 2010 (UTC)

63.193 you seem angry that the web is not up to your expectations. We are really sorry about that. I assure we will try to improve the web in order to avoid that you get angry again. --Lgriot (talk) 10:04, 1 December 2010 (UTC)

The IP may be referring to last night's episode of QI XL in the UK, in which it was claimed that if they needed to get up early, American Indians would drink a lot of water before they went to sleep, so that they would have to get up early to urinate.--Shantavira|^{feed me} 10:07, 1 December 2010 (UTC)

Beer is much more efficient than water for that - but then, you can't get a good pint in America, or a good cup of tea either, so I suppose it's fair enough to make do with water. DuncanHill (talk) 15:32, 1 December 2010 (UTC)

This is also handed down as Vietnam War lore: the person who was to take over the watch duties while a patrol was sacked out would drink a lot of water when he went to sleep, so he would wake up a couple of hours later. Also, anecdotally, parents of little bedwetters have been known to restrict the childrens' intake of fluids an hour before bedtime. Also anecdotally, an ageing male's bladder inevitably needs emptying in the middle of an 8 hour sleep period, even if there are no fluids in the hour before bedtime. Ageing females seem to do better on that count. Edison (talk) 16:53, 1 December 2010 (UTC)

Old men tend to have enlarged prostates, which limits bladder capacity. Also, drooling, sweating and respiration are other ways the body loses water at night, with the last two varying with temperature and humidity. StuRat (talk) 23:30, 1 December 2010 (UTC)

Part of the difficulty with this question is that water does not go straight from the stomach to the bladder. water is absorbed into the blood stream from the intestines (at a rate governed in part by the body's need for water - it's an osmotic action). once in the bloodstream it is governed by other hormonal and chemical regulatory processes. once it passes through the kidneys and begins accumulating in the bladder, awareness of it will vary from person to person. remember that the body is mostly water - unless you drink a lot of water you are not significantly changing the quantity of water in the system, and so the body will not feel any great haste to expel it. --Ludwigs2 16:37, 1 December 2010 (UTC)

The Reference Desks usually have a turnaround of several hours up to several days. Adjusting your expectations accordingly will reduce your frustration. --Sean 18:06, 1 December 2010 (UTC)

I think the 90 minutes on Wikipedia and the MAE was before coming to the reference desk. Red Act (talk) 20:30, 1 December 2010 (UTC)

There is such a thing as a Medical Advice Encyclopedia? It's the antimatter to the Wikipedia Reference Desk! Kill it with fire! Comet Tuttle (talk) 21:55, 1 December 2010 (UTC)

white poop

In an early episode of Life on Mars, Sam Tyler (shifted from 2006 to 1973) remarks, "White dog shit – that takes me back." Me too; I don't remember seeing it since 1965 (when I moved from Chicago to Pasadena). What changed? —Tamfang (talk) 09:47, 1 December 2010 (UTC)

No idea if it's the same reason but the dog poop in my yard turns white after it dries out. I noticed the difference once we switched our dogs over to a raw chicken diet. So, maybe commercial dog food became more prevalent around 1965? Dismas|^(talk) 10:01, 1 December 2010 (UTC)

(e/c) Dog food contains much less bone meal than it used to. That used to get excreted as calcium carbonate. There are multiple discussion posts all over the place about this, such as here. Ghmyrtle (talk) 10:02, 1 December 2010 (UTC)

I'm sure this has been asked on the RefDesks before - but I haven't been able to find the thread. Anyone else remember it? DuncanHill (talk) 15:38, 1 December 2010 (UTC)

I've only seen a lot of white poop after a snow has cleared. I assume it's either something being leached out or something to do with the poop being preserved longer than usual. At any rate, perhaps the Chicago->Pasadena move would account for the change in frequency? Ask your friends back home to take a white shit census! --Sean 16:08, 1 December 2010 (UTC)

My understanding is it's due to the reduction in tripe in dog diets.--DeKay01 (talk) 20:44, 1 December 2010 (UTC)

In The London Labour and the London Poor (a factual description of the poorest people in London in the 1840s), the lowliest occupation belongs to an old lady who is a "Dog Dung Collector" or "pure finder". She made her meagre living by picking up white dog droppings and selling them to the tanneries, who used them to treat leather somehow. She'd be out of a job now! Alansplodge (talk) 23:39, 2 December 2010 (UTC)

Bismuth reaction with acid

Does bismuth or its oxide react with any acid? Chemicalinterest (talk) 14:36, 1 December 2010 (UTC)

Yes. See the "Reactions with acids" section of this page. Red Act (talk)

I added the data from the page to the bismuth article. --Chemicalinterest (talk) 19:05, 1 December 2010 (UTC)

Drinking seawater

In the most typical case, it would kill you. But does that hold true for any ocean? The salinity of the oceans vary, so if it kills you in the dead sea, it doesn't mean that it also kills you in the Antarctic. Quest09 (talk) 17:38, 1 December 2010 (UTC)

We have an article about this. Drinking small amounts of seawater is not harmful, drinking large amounts can be. Exactly how much is "bad" will depend on the composition of the seawater and the person doing the drinking (including their hydration status, environment, level of exertion, any chronic health conditions, medications, etc). So, the direct answer to your question is almost certainly that any ocean's water can be harmful. A precise answer to the follow-on questions (how much of each would be) depends on many factors. -- Scray (talk) 18:06, 1 December 2010 (UTC)

The most interesting thing about that article is the revelation that simply eating fish will probably supply all or most of the water you need. Along with the apparent fact that consuming small amounts of seawater along with fish is minimally damaging. That's assuming the water itself is germ-free, which would be another story. ←Baseball Bugs ^{What's up, Doc?} carrots→ 16:25, 2 December 2010 (UTC)

(EC)Our Southern Ocean article, unhelpfully, does not provide information on its salinity. Ocean tells us that "Average oceanic salinity is around 35 parts per thousand (ppt) (3.5%), and nearly all seawater has a salinity in the range of 30 to 38 ppt" and elsewhere reminds us that there are localised variations, especially at river mouths and, presumably, ice melts. I think we can take it as a general rule that salinity will tend to be 3% or greater, and thus consumption is ill advised. --Tagishsimon (talk) 18:09, 1 December 2010 (UTC)

More than ill advised, I'll say it is lethal in any situation - Dead Sea or Southern Ocean. Drinking water has a upper limit of 1,000 ppt salinity. Much less than the less salty ocean. Mr.K. (talk) 18:17, 1 December 2010 (UTC)

Err, isn't 1,000 ppt salinity pure salt? Matt Deres (talk) 18:24, 1 December 2010 (UTC)

• Sorry, I meant 1,000 ppm instead of the typical 35,000 ppm that you normally find in seawater. Mr.K. (talk) 18:40, 1 December 2010 (UTC)

Depending on your definition of "Ocean". If you include the Dead Sea (which is unusual ;-), you should also include the Baltic Sea. The Baltic surface water is 4-5 times less salty than the average ocean water, and the northernmost reaches of the Gulf of Bothnia are basically fresh. --Stephan Schulz (talk) 18:34, 1 December 2010 (UTC)

This information shows it is all the more remarkable that one group of mammals, the marine mammals, live in or on the sea, have no access to fresh water and yet they survive and have done for millions of years. Does Wikipedia have some information about how these remarkable creatures satisfy their need for fresh water in an environment that consists entirely of salt water? Dolphin (t) 11:44, 2 December 2010 (UTC)

• You should know that better than us, Dolphin.Mr.K. (talk) 16:14, 2 December 2010 (UTC)

I was going to suggest salt glands, but it would appear from the article that marine mammals don't have these (there's no mention of them, anyway). Maybe evolution has made their kidney much more tolerant of salt than ours? --Kurt Shaped Box (talk) 11:59, 2 December 2010 (UTC)

The problem with drinking seawater is not that it will damage our kidneys. Your kidneys won't be able to excrete it and the concentration of sodium chloride in your blood will increase to a dangerous level. Sea mammals, on the other side, have kidneys adapted to their environment. See: "In making the change from terrestrial to aquatic living, cetaceans needed a way to accommodate for the higher salinity of their environment. Unlike human kidneys, which are just two singular renules (or balls), dolphins have two kidneys with multiple renules. These renules all function as separate kidneys which help filter out the higher amount of salt content they must deal with in their daily environment." From [5].Mr.K. (talk) 16:12, 2 December 2010 (UTC)

Our article on Sea Otters makes mention of the creatures large kidneys for the purpose of removing excess salt. Googlemeister (talk) 16:22, 2 December 2010 (UTC)

Not sure if the articles on marine mammals mention this or not, but marine mammals generally don't drink (to the point where frequent drinking is recognized as a symptom of illnesses including kidney damage.) They get their water from eating - fish, cephalopods, other mammals, rather than from drinking salt water directly. They still likely take in some amount of seawater just by being there. 173.11.110.109 (talk) 00:55, 3 December 2010 (UTC)

NASA astrobiology news

What astrobiology news from NASA await us tomorrow? --Mortense (talk) 19:02, 1 December 2010 (UTC)

We shall see tomorrow. Before that the reference desk is unlikely to provide you with solid information. For Internet denizens' speculation, you might enjoy a Slashdot discussion on the subject. 88.112.56.9 (talk) 19:06, 1 December 2010 (UTC)

Slashdot'ters are notoriously ignorant about astronomy subjects. --Mortense (talk) 19:16, 1 December 2010 (UTC)

How about the official-sounding MEDIA ADVISORY : M10-167 that was posted on NASA's homepage, announcing "NASA will hold a news conference at 2 p.m. EST on Thursday, Dec. 2, to discuss an astrobiology finding that will impact the search for evidence of extraterrestrial life." As has been pointed out, we will know tomorrow after they hold their conference. Nimur (talk) 19:21, 1 December 2010 (UTC)

Oddly, I'm pretty sure I do in fact know the answer, since my girlfriend accompanied Felisa Wolfe-Simon on her most recent trip to Mono Lake. Without giving anything much away, you can get an idea about the topic area by looking up Dr. Wolfe-Simon's prior work. Dragons flight (talk) 19:21, 1 December 2010 (UTC)

Feel free to give at least a little away. Where would these arsenic based life forms most likely be found outside earth? Beach drifter (talk) 20:12, 1 December 2010 (UTC)

I don't think that they'll have located anywhere in particular that it's more likely to find life. Nobody at present is really sure just how likely abiogenisis is and there's a school of thought that maintains that it's extremely unlikely. If they really have found a new organism that they can place right at the root of the phylogenetic tree it'll mean that life may have started twice, independently, here on Earth. That would make one of the inputs to the Drake equation slightly less of a guess. Blakk and ekka 22:15, 1 December 2010 (UTC)

It could be anything on a scale from interesting extremophile to second Abiogenesis. I saw Paul Davies talking about this a couple of times earlier this year and, if it's as promising as he seemed to think it was, it'll be very interesting indeed. 86.26.8.192 (talk) 21:58, 1 December 2010 (UTC)(edit Blakk and ekka 22:01, 1 December 2010 (UTC))

The news shall be: We have found such-and-such a chemical at such-and-such a location up in the outer space area above us in the sky. Comet Tuttle (talk) 21:54, 1 December 2010 (UTC)

Funny. I was asked to look at that astrobiology news and if we should aim for certain organometalics for the MOMA instrument on Exomars. The arsenic life forms (producing C-As bonds) I find them in the ocean, for example lobsters, the acumulate arsenobetaine in large quantities. The other group is mold they get ride of arsenic by converting it to trimethyarsin and then they let it go with the wind. So even here arsenic is a notable part of biochemistry.--Stone (talk) 22:14, 1 December 2010 (UTC)

Today some speculations came up that a arsenic based DNA was found. This would be one of the big stories every biologist would like to be involved.--Stone (talk) 16:54, 2 December 2010 (UTC)

Heres the News release. It is indeed a bacteria that can incorperate arsenic instead of phosphorus in itself. Unfortunately, it does not indicate a second abiogenisis; the bacteria in question, GFAJ-1, has merely evolved to tolerate and use arsenic (that's still pretty damn cool, though). Buddy431 (talk) 20:02, 2 December 2010 (UTC)

Synapse structure

What structurally maintains the proper synaptic cleft of the presynaptic and postsynaptic neurons of a synapse? If I simply lay two threads floating head to head in a liquid, unless there's something holding them in place, they're pretty much guaranteed to drift apart as soon as I move the system. 20.137.18.50 (talk) 19:13, 1 December 2010 (UTC)

There are cell adhesion molecules such as neuroligin that bind pre-synaptic and post-synaptic membranes; see Synaptogenesis#Signalling. Also, the entire synaptic cleft is encapsulated by glial cells; see Synaptogenesis#Central Nervous System Synapse Formation. Red Act (talk) 20:24, 1 December 2010 (UTC)

(ec)The article you linked gives a little info. "Both the presynaptic and postsynaptic sites contain extensive arrays of molecular machinery that link the two membranes together". The article specifically about chemical synapses (the usual kind discussed in schools, with chemical neurotransmitters, etc.) does not have any more info about this detail--would be great if someone could find it. The article about electrical synapses talks about the connexon channels and the diagrams suggest an interaction between the E1 and E2 domains of the connexin proteins. That last article has a section detailing what's currently known about how they link together. DMacks (talk) 20:28, 1 December 2010 (UTC)

Thanks Red Act! Could you add some notes to the synapse article about it? It's definitely important to the structure itself, not just the biological formation of them. DMacks (talk) 20:29, 1 December 2010 (UTC)

The chemical synapse article would be the place to add the information -- the synapse article is basically a disambig page, because the two types of synapses are so different structurally and functionally. Unfortunately this is an aspect of neuroscience I know very little about. Looie496 (talk) 21:00, 1 December 2010 (UTC)

There is a diagrammatic representation here on page 127. The chapter also goes into more detail.--Aspro (talk) 21:14, 1 December 2010 (UTC)

Christmas lights

My outdoor lights I have just hung says clearly not to connect more than three strands together or it will exceed their rating. Of course I have eight connected end to end on one side of the house and four on the other. Surely I am not the only one to go well over the limit stated on the box? Is it most likely the light stands would fail first or the outlet? Each plug on the strands has a fuse on it so would these go out first? Anyone have any solutions for wiring these lights without running extension cords from all over the place? Thanks. Beach drifter (talk) 20:07, 1 December 2010 (UTC)

If the lights are connected in parallel, then the expected failure mode is for a fuse or circuit breaker to blow. The secondary failure mode is for something somewhere to get very hot, so be careful. Looie496 (talk) 20:55, 1 December 2010 (UTC)

(ec)We would need more info, specifically the wire size, and the power consumed (watts) from each strand. Worst case scenario is a fire by overheating the wires that make up the strands. Hopefully the fuse would blow first, but it's not guaranteed. Can you link to the product if you don't know the specs? You can buy LED lights - they use a LOT less power. Ariel. (talk) 20:57, 1 December 2010 (UTC)

The product is rated at 61.2 watts/.51 amps, connected end to end for a maximum of 210 watts/1.75 amps. Beach drifter (talk) 21:01, 1 December 2010 (UTC)

If each plug is fused is the a UK 3 pin plug 240v. If so you'll likely have a 2 amp fuse and that is because the lights have 2 amp wire (why waist money on a heavier gauge just for little dinky lights). No, not a good idea. Hope you are also using a isolation transformer or an earth leakage trip to guard against finding electrocuted reindeer in the garden.--Aspro (talk) 21:29, 1 December 2010 (UTC)

Good point about the deer, we get them in the yard more frequently in the winter. Beach drifter (talk) 21:36, 1 December 2010 (UTC)

I don't really understand what you mean. 61.2 watts each, maximum of 4 does not equal 210 watts. Or do you mean each one is 210 watts? (So x 4 = 840 watts.) Also, what size are the wires? Ariel. (talk) 22:40, 1 December 2010 (UTC)

There are several different ways to answer this.

1. Good engineering practice -- which is sought for, and generally achieved, in the design of electric power systems and appliances -- is to design things so that they are inherently safe, not to depend on the user to keep them safe. As you suggest, you're certainly not the first person to have imagined that you can "probably get away with" stringing together eight strands of lights when the directions say there's a max of three.
2. With that said, though, Christmas lights were once a distinct exception to the above. A strand of incandescent lights consumes a significant amount of power. If they're strung in series (which is the common arrangement if you're, say, encircling your house), the total current drawn by all strands must pass through the plug and wires of the strand that's closest to the outlet. If you connect together arbitrarily many strands, sooner or later you can almost certainly get to the point where you've exceeded the current-carrying capacity of the wires of the first strand, but not exceeded the rating of the fuse or circuit breaker protecting the outlet you've got the whole mess plugged into.
3. With that said, though, the "Christmas light loophole" has, as near as I can tell, been closed. There are little fuses in the plugs, now, and these presumably protect the plugs and wiring from having too many series-connected strands tacked on downstream.
4. With that said, though, it's arguably good engineering practice to adopt a "belt and suspenders" approach, to try to be safe on your own, letting the protective devices like fuses and circuit breakers serve as a backup, rather than depending on them. So if the instructions say "no more than three", and if you really want to Follow The Rules, it would certainly be a Good Idea to connect no more than three. (But with that said, though, the no-more-than-three recommendation is sure to include a safety factor, whether or not it's enforced by a fuse. [But what's the safety factor? And is it a good idea to make use of it?])
5. With all of that said, though, the real question is, for multiple strands of Christmas lights constructed with wire of a certain gauge, all connected in series, plugged in to an outlet protected by a fuse or circuit breaker with a certain rating, how likely are you to actually start a fire? Let's say the wire is 18 gauge, and the circuit is protected at 20 amps. That 20 amp circuit is wired (if to code) using 12 gauge wire, which has twice the diameter and therefore four times the current-carrying capacity of 18 gauge. In other words, you can run 20 amps through 12 gauge wire all day long without it getting anything like too hot. By that token, you shouldn't run more than 5 amps through the 18 gauge wire. If you string together enough light strands to draw (say) 19 amps, you wouldn't blow the 20 amp circuit, but the 18 gauge wire would certainly get warm, if not hot. The question, then, is, would a nearly 4x overload cause the wire to heat up so much that the insulation would melt off or catch fire? And the answer is... I don't know. But these are definitely the kinds of numbers we're talking about, in that I can't imagine a string of Christmas lights wired with anything smaller than 18 gauge wire, and the outlet you plug it/them into is never (in the U.S., anyway) going to be fused at more than 20 amps. —Steve Summit (talk) 03:07, 2 December 2010 (UTC)

Dinosaur Combs and Wattles

Given than birds are descendants of dinosaurs, have there been anything found in the fossil record that tell us whether dinosaurs also had structures such as combs or wattles like their chicken or turkey descendants? Jeanpetr (talk) 20:50, 1 December 2010 (UTC)

Fossils only form under some very rare and special conditions. For flesh to fossilize it takes even rarer and more special conditions. While there have been billions of fossils found I believe there are only a tiny fraction have anything that can be recognized as flesh. However there are at least some scientists who believe dinosaurs may have had soft skin folds similar to combs and wattles. Vespine (talk) 21:50, 1 December 2010 (UTC)

Is this for reals?

http://www.youtube.com/watch?v=6QasA6NQXXQ&feature=related

Hoax? —Preceding unsigned comment added by 114.173.217.17 (talk) 21:04, 1 December 2010 (UTC)

No Hoax. Our Snakebite article even mentions this: A snake's detached head can immediately act by reflex and potentially bite. The induced bite can be just as severe as that of a live snake. Dead snakes are also incapable of regulating the venom they inject, so a bite from a dead snake can often contain large amounts of venom. I've heard that rattlesnakes are particularly notorious for this, there's several news stories to back this up. Vespine (talk) 02:50, 2 December 2010 (UTC)

About how long would it take for that reflex capability to dissipate? ←Baseball Bugs ^{What's up, Doc?} carrots→ 16:27, 2 December 2010 (UTC)

Circuits with LEDs

I'd like to play around with LEDs, and I'd like to understand a bit what I'm doing. In school, and at home, I did loads of circuits with little incandescent bulbs, and I know the basics of circuits with batteries, resistors, incandescent bulbs. I can calculate the voltages/currents/resistance of components in idealised parallel and series circuits. But we never did anything with diodes, apart from look at a graph of how voltage varied with current to get an idea of them being oneway (and make a circuitboard in Design and Technology without any explanation of the theory or how it worked, which wound me up). I have an idea that, apart from needed to hook them up with positive and negative the right way round, you have to be careful about the voltage across them, so you need to add a resistor of a particular value to get the right voltage.

Could someone recommend some easily-available write-up of relevant theory, ideally from a practical angle, to ease me into this area? If I get the voltage on an LED too high, will it blow up, fail early, or just not work? 86.161.108.241 (talk) 22:56, 1 December 2010 (UTC)

I have failed LEDs in all of the above methods. Connecting a LED directly to a 9v battery is a quick and easy way to blow them up. There are loads of tutorials online, just google LED totorial; here's a couple. Just because it's a chance to blow my own trumpet, here's my LED project I spent about 18 months on. Vespine (talk) 23:33, 1 December 2010 (UTC)

If LEDs are so sensitive to votages then why are they used in Opto-isolators? Hcobb (talk) 23:48, 1 December 2010 (UTC)

- because it is preferable to blow up a cheap LED than the expensive chips that it isolates from inappropriate voltages. Dbfirs 07:56, 2 December 2010 (UTC)

Thanks Vespine: it's hard to identify a good tutorial when you're new to something. I'd thought I'd need to carry out really careful iterative calculations with logs, based on what I was seeing in our articles, but that looks fairly easy. And your project fills me with confidence in your ability to judge the tutorial! Wow, that's cool. Looks like I'm buying components and solder tomorrow! 86.161.108.241 (talk) 23:54, 1 December 2010 (UTC)

(ec) I can direct you to some of the theory, though it might be a little short on the "practical angle" aspect. The electrical properties of semiconductors can be explained using band theory (note that semiconductors are crystals). (It may or may not be useful to read up a little on Maxwell-Boltzmann statistics, on how and why it breaks down when applied to fermions, and, subsequently, on Fermi statistics.) Once you understand semiconductors, you can move on to diodes, and then to LEDs. --superioridad ^(discusión) 00:00, 2 December 2010 (UTC)

My tip is just dive straight in, you'll learn heaps by just trying things out, you can get LEDs by the hundreds literally for a few dollars off ebay. Find a DC power pack and learn how to make a simple power supply using a 7805, or a variable one if you are adventurous (LM317). I'd also recommend a breadboard, it makes things a lot easier. The components you need are peanuts from places like futurlec.com , I don't usually shill but that place is really cheap and I've bought lots of stuff from them without any problems. Even if you visit a local shop it shouldn't be too expensive to get started. Vespine (talk) 00:10, 2 December 2010 (UTC)

Also might be worth mentioning, don't be tempted to start learning on the high power LEDs like CREE or Luxoen, they're a considerably different kettle of fish. Just start with a bunch of the cheapest LEDs you can find. Vespine (talk) 00:13, 2 December 2010 (UTC)

(edit conflict) We have a practical article: LED circuit. Basically: if you overdrive an LED, you'll cook it (when I did this, it just glowed quite brightly and rapidly dimmed into nothingness). You can overdrive it in either direction: backwards, it's a little sturdier, but also useless.

The math: P=IV, of which IV₀ is light (V₀ is approximately the minimum voltage to get light) and the rest heat, so $P_{h}=I(V-V_{0})$ is the heat your LED has to dissipate. When V increases beyond V₀, both of those factors increase rapidly (I increases exponentially in standard diode models). So the heating rate of your diode is very sensitive to voltage; there's a tiny range of voltages between "doesn't light" and "cooks". Hitting that target is really hard with just a voltage source (battery), so don't be tempted to say "Hey, these are 3V LED, says so on the box, so let me just put three in series on a 9V battery.". The basic trick is to add a resistor in series; then the voltage across the LED can't get high, because then the current would be large, but then the voltage drop across the resistor would exceed that of the source. See the graphical analysis: even if you move the bottom-right point (which represents the battery voltage) around, the intersection doesn't move much.

The article depicts this and shows how to calculate the appropriate resistance (always round up, of course, with real resistors). You can safely stack LEDs (even of different colors) in series with one resistor: you'll just be constrained by the lowest current rating, of course. It's not usually recommended to connect LEDs in parallel: it works, but tends to generate uneven lighting and shorter lifetimes. --Tardis (talk) 00:14, 2 December 2010 (UTC)