Wikipedia:Reference desk/Archives/Science/2010 September 21

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September 21

Turtle Star

Is there a star known as the Turtle Star? DuncanHill (talk) 02:29, 21 September 2010 (UTC)

Well, there's this one. WikiDao ☯ (talk) 02:37, 21 September 2010 (UTC)

There may well be, but Chrome can't connect to tattootribes.com DuncanHill (talk) 02:42, 21 September 2010 (UTC)

• Seems to be a reference to Mayan folklore, see this. This fits with the context in which I heard it, "Just by the shoulder of Orion, Bright Satellite outshone the Turtle Star". DuncanHill (talk) 03:05, 21 September 2010 (UTC)

I daresay that's the right answer. But just for interest, I searched on "Turtle Star" in Wikipedia and was intrigued to find out that there is an asteroid named Turtlestar, which was named after something called the Turtle Star Observatory (named in English even though it's in Germany). Putting the observatory's German web page into Google Language Tools for a translation, I was led to a page at the Minor Planet Center. If you go to this list and click on "Turtlestar", it explains:

(12053) Turtlestar = 1997 PK2

The Turtle Star Observatory, located in Mülheim/Ruhr, Germany, contributed follow-up observations for this object. This private observatory was built in 1995 by A. Boeker, K. Kleemann-Boeker, A. Martin and M. Tator and is named for a neighbor's turtle, which observed the construction with apparent great interest.

So apparently it's the Turtle Star-Observatory, not the Turtle-Star Observatory. --Anonymous, 17:42 UTC, September 21, 2010.

What is AGTP?

While watching some of my old DVDs, I came across this dialogue in Red Planet:

"I'm a geneticist. I write code, okay? "A", "G", "T", "P", in different combinations. Hacking the human genome, okay? I choose what, I choose where, and either your kidneys work or your grow a sixth finger. "

What does "A", "G", "T", "P" mean? -- 24.251.101.130 (talk) 06:28, 21 September 2010 (UTC)

Sure it's not A/T/G/C? Those are the standard DNA bases that encode the genome. DMacks (talk) 06:32, 21 September 2010 (UTC)

Nope, the actor definitely says "P". Thanks for the link to the correct information! -- 24.251.101.130 (talk) 06:39, 21 September 2010 (UTC)

The script, on page 59, clearly states A, G, T, P. However, the line is slightly different. It is: "I'm a geneticist. I write code. Like a hacker. Four elements, A-G-T-P, in different combinations. Hacking the human genome. I choose what, I choose where, and your kidneys work, or you grow a sixth finger." He refers to them as "four elements". I believe that it is safe to assume that the screenwriters simply got it wrong. It is A, G, T, C. They accidentally typed P and the actor recited what was on the page. It isn't the first time a movie has gotten some bit of science incorrect. -- kainaw™ 12:08, 21 September 2010 (UTC)

Is that the movie with the floating candies in a double helix, written in by someone who didn't know the difference between a base pair and a chromosome? The question of why Hollywood feels the need to show such gratuitous contempt for hard-scifi fans and refuse all access to anyone knowledgeable would be an interesting question for the Humanities desk. Wnt (talk) 15:18, 21 September 2010 (UTC)

No. That's Mission to Mars with Jerry O'Connell being the one who builds the DNA of his idea of a perfect woman out of M&Ms. Both movies were released in the same year, so it's not surprising that there would be some confusion between the two. Dismas|^(talk) 01:03, 22 September 2010 (UTC)

Physical exertion/weird taste

I have a question regarding the human body. Whenever I physically exert myself (mostly by continuous running), I eventually end up with a taste in my mouth similar to blood. I have talked to a few different people and they claim to have experienced this, including relatives of mine. What causes this taste to arise under near-exhaustion? ^{Here's the only} _{new, improved} Finalius! 11:33, 21 September 2010 (UTC)

As the major taste of blood is salt, I guess that the taste you experience after exercise is salt too. This paper says that after anaerobic exercise (near exhaustion) lactate, sodium, protein and RNAase concentrations increase in saliva. Smartse (talk) 14:23, 21 September 2010 (UTC)

Power supply for electricmotor

Can dc motor work on a.c. power supply?? —Preceding unsigned comment added by 115.248.134.49 (talk) 12:18, 21 September 2010 (UTC)

No. Dolphin (t) 12:22, 21 September 2010 (UTC)

Well, it probably would if you used a Rectifier to convert the AC to DC. Might also require a transformer. SemanticMantis (talk) 12:27, 21 September 2010 (UTC)

See Electric motor#Comparison of motor types. A Electric motor#universal will work on either. Something called a "DC motor" would appear, by definition, unsuited to AC operation. Edison (talk) 15:38, 21 September 2010 (UTC)

Has anyone tried hooking up a dc motor to an ac power supply? John Riemann Soong (talk) 20:26, 21 September 2010 (UTC)

My baby brother tried it and he says that it just vibrates. --Chemicalinterest (talk) 21:28, 21 September 2010 (UTC)

LAGRANGIAN AND HAMILTONIAN'S CONCEPT FOR GETTING EQUATION OF MOTION

I NEED SOME BASIC EXAMPLES LIKE GETTING EQUATION OF MOTION OF A FALLING BODY, SIMPLE PENDULUM BY LAGRANGIAN AND HAMILTONIAN METHODS. FURTHER, PLEASE PROVIDE ME ALL NECESSARY BASIC CONCEPTS OF THESE METHOD, AND WORKING RULE TOO.Ajay3HIMACHAL (talk) 14:29, 21 September 2010 (UTC)

PLEASE DON'T SHOUT - turn off your caps lock. --Cameron Scott (talk) 14:33, 21 September 2010 (UTC)

Did you try using a search engine such as Bing or Google? --Chemicalinterest (talk) 14:47, 21 September 2010 (UTC)

Nuclear power cost

Is it true that nuclear fission reactors are very expsnive to run? I thought they were quite cheap. What is the cost per megawatt from an "average" nuclear power plant and an average coal power plant? How does this compare to dependable renewables like hydro?--92.251.154.40 (talk) 14:42, 21 September 2010 (UTC)

We have an article on economics of new nuclear power plants that answers most of your questions. Gandalf61 (talk) 14:54, 21 September 2010 (UTC)

As well as an article on cost of electricity by source. But the figures vary and are hotly debated. Hydro power at its best is dirt cheap, but there isn't that much to go around.--Rallette (talk) 15:01, 21 September 2010 (UTC)

An important factor in their low expense is that nuclear operators don't pay insurance for damages over a set amount (that amount varies by country). Unlike a coal plant, a bad enough accident at a nuclear plant could make hundreds of square miles uninhabitable. If nuclear operators had to buy private insurance for that risk their rates would have to go way up to cover the premiums. See Price–Anderson Nuclear Industries Indemnity Act for the US version of this government largesse. --Sean 15:54, 21 September 2010 (UTC)

Not sure if you could technically call it largesse since the government has not really needed to pay for a huge nuclear accident... Googlemeister (talk) 16:36, 21 September 2010 (UTC)

Assumption of risk is an economic incentive, so I would definitely say it's largesse. Comet Tuttle (talk) 17:31, 21 September 2010 (UTC)

• I agree with Gandalf that you should check out those articles. The short answer is that nuclear power has high construction costs but cheap fuel. There's a similar dynamic in wind power, but you're dealing with smaller units, and don't have the nuclear industry's unique insurance problem. --M@rēino 19:46, 21 September 2010 (UTC)

It's worth pointing out that "construction costs" are not just physical construction, but regulatory, legal, and security related expenses. --Mr.98 (talk) 22:25, 21 September 2010 (UTC)

It's also worth pointing out that disposition and management of radioactive waste is an open problem - I don't think there is a single final repository operational on the planet, and hence estimating cost for disposal is essentially arbitrary. --Stephan Schulz (talk) 16:21, 23 September 2010 (UTC)

DNA

If you took the DNA out of a single cell, from a human or other large mammal, and stretched it out, how long would it be? --The High Fin Sperm Whale 16:25, 21 September 2010 (UTC)

DNA is an inherently 'stretchy' molecule, and it is much shorter if you keep it partially rolled up on its histone scaffold, but this page lists five textbook or encyclopedia sources reporting lengths between 1.5 and 3.0 meters. TenOfAllTrades(talk) 16:29, 21 September 2010 (UTC)

Each base pair "rung" on a DNA ladder (as it is conventionally shown as an extended double helix) is 0.33 nm tall (see DNA). The human genome is just over 3 billion base pairs, so the multiplication makes that out to be just under 1 meter. That's the haploid genome, though, so a diploid cell will have about 2 meters of DNA. Note, however, that it isn't one continuous ladder, but rather split into 46 separate pieces (the chromosomes). The website www.genomesize.com lists various genome sizes (in gigabases). Sperm whales aren't listed, but elephants are, at a (haploid) genome size of 4.03 Gbp, or 2.66 m of DNA per cell. (Elephant shrews, on the other hand, are at 5.98 Gbp, or 4 m.) -- 140.142.20.229 (talk) 18:48, 21 September 2010 (UTC)

gold purity

If you had a gold sample, what is the highest amount of impurity that would be undetectable by current testing methods? Googlemeister (talk) 16:38, 21 September 2010 (UTC)

Define "current testing methods". Accelerator mass spectrometry has demonstrated a routine detection ability of about 1 part in 10¹⁶, and in certain research applications is been made many orders of magnitude more sensitive than that (e.g. 1 in 10²⁴). However, AMS is crazy expensive, and hence would not be a normal test anyone would do on gold. So, I think the answer is going to be determined mostly by the choice of test(s) you are going to consider. Dragons flight (talk) 17:32, 21 September 2010 (UTC)

I mean by methods that might be used on an industrial scale, such as a refiner. Googlemeister (talk) 18:08, 21 September 2010 (UTC)

It will depend somewhat on which impurity you are worried about, but standard chemical analysis can easily detect parts per million levels, whereas you would need to take lots of expensive precautions to go lower than parts per billion levels. The main problem, especially for a refinery which does many such analyses, is impurities in the chemicals you use for the analysis and any contamination of the equipment. Gold bullion is 99.999%, that is a maximum of 10 ppm impurities, so that gives you some idea of the scale refiners are working to. Physchim62 (talk) 18:41, 21 September 2010 (UTC)

ghost light

After staring at a bright light for a few seconds then turning off the light, one notices, behind closed eyelids, a bright spot of light that disappears after 15-30 seconds. What causes this ghost light? WSC —Preceding unsigned comment added by 66.27.180.64 (talk) 16:44, 21 September 2010 (UTC)

At afterimage it says:

Afterimages come in two forms, negative (inverted) and positive (retaining original color). The process behind positive afterimages is unknown, though thought to be related to neural adaptation... Negative afterimages are caused when the eye's photoreceptors, primarily those known as cone cells, adapt from the overstimulation and lose sensitivity.

--Anonymous, 17:10 UTC, September 21, 2010.

It's due to neural adaptation. See also Optical illusion for many other interesting effects like that. WikiDao ☯ (talk) 17:52, 21 September 2010 (UTC)

Direct vision/averted vision

According to John Bortle, M33 should be visible in direct vision if the sky is dark enough, see here. Bortle class 1 or 2 is dark enough for this, while it is visible with averted vision at class 4 or better.

However, this is disputed here. Where I live the skies are not dark enough to be able to see M33 with the naked eye at all, but I have experienced the same thing about averted/direct vision. When you look at a faint object that is clearly visible and you don't consciously use averted vision, you may still see it because of a minimal unconscious use of averted vision, while you have the feeling that you are seeing it in direct vision. If you force yourself to stare at the object directly, it becomes much fainter. But I have to say that looking at an object in that way is not natural. If you do the opposite and use averted vison consciously, the object becomes far more apparant than if you just look at it normally.

If my eyes are fully dark adapted then precisely in the direction of view, there seems to be something like a weak haze. This is effect is not noticable if I'm just a few minutes outside in the dark. Only after 20 to 30 minutes does this more narrow difference between 100% direct vision and not 100% direct vision become apparant. So, perhaps one should distinguish betwee different levels of averted vision? Count Iblis (talk) 18:19, 21 September 2010 (UTC)

I'm not too sure what your question is. Do you want an answer in terms of rods and cones and neurons...? WikiDao ☯ (talk) 21:54, 21 September 2010 (UTC)

Yes, if an explanation can be given. So, the question is about being fully dark adapted. Then it seems that precisely in the view of direction your eyes are lot less sensitive to light, compared to looking just a little away from the object, but not so much away that you are engaging your averted vision. Count Iblis (talk) 00:36, 23 September 2010 (UTC)

Okay, well, dark-adapted vision explains how rod cells are the cells in the retina most involved in night vision. Then, the fovea article says:

The human fovea has a diameter of about 1.0 mm with a high concentration of cone photoreceptors. The center of the fovea is the foveola - about 0.2 mm in diameter - where only cone photoreceptors are present and there are virtually no rods.

The fovea is what you use to see "precisely in the view of direction your eyes", and you cannot see well with it in the dark because it has relatively few rod cells in proportion to cone cells compared to the rest of the retina. The foveola would then be even more precise in the same way, which may account for the third perception you are asking about, when you look "just a little away". That could also be due to slight involuntary movements of the eye called saccades, which may bring the image of the object very briefly (and imperceptibly) just enough right outside the fovea to activate some rod cells. WikiDao ☯ (talk) 02:10, 23 September 2010 (UTC)

Thanks for explaining! Count Iblis (talk) 14:34, 23 September 2010 (UTC)

Liquid helium production

How is liquid helium cooled before it is liquefied? I looked at helium and liquid helium but only saw that it is cooled to a low temperature than liquefied through expansion. How is it cooled to that low temperature? Thanks, --Chemicalinterest (talk) 18:35, 21 September 2010 (UTC)

Liquid hydrogen. It's the only readily available coolent that can get the helium down to 40–45 K (the Joule–Thomson inversion temperature). The hydrogen itself has to be cooled by liquid nitrogen before it can be liquified. Physchim62 (talk) 18:45, 21 September 2010 (UTC)

The history of attaining successively colder temperatures (and liquefying successively lower boiling gases: nitrogen, then oxygen, then hydrogen, and finally helium) is a fascinating one, and our articles really don't seem to do it justice. Heike Kamerlingh Onnes was the first person to liquefy helium, but he was in competition with another scientist (whose name eludes me at the moment) to be the first. I read a fascinating book on the subject a few years ago. It might have been this one [1], but I don't really recall.

As for the specific process Onnes used, he liquefied different gases and then used them to cool the next gas to be liquefied. If I recall correctly, he made liquid air(essentially liquid nitrogen) and used it to cool hydrogen, which was liquefied to cool helium, which was then liquefied by the aforementioned Joule–Thomson effect. Buddy431 (talk) 19:18, 21 September 2010 (UTC)

Edit: yeah, it's definitely the Absolute Zero book that I linked. It's pretty good for the non-expert, if you're interested in this sort of thing. It looks like you can read most of it online at Amazon, which sort of defeats the purpose of selling it... Buddy431 (talk) 19:23, 21 September 2010 (UTC)

Even more to add: it looks like PBS made a two hour NOVA episode about the subject (based on Shachtman's book that I linked). I haven't seen it, but they usually do a pretty good job with science subjects. It looks like I'm going to have to go to the library and see if they have that episode. Buddy431 (talk) 19:27, 21 September 2010 (UTC)

Here's the website for the NOVA episode. Buddy431 (talk) 19:28, 21 September 2010 (UTC)

From what I understand, in order to create liquid helium, you would compress standard helium, remove the heat created by the compression (while maintaining pressure) and then expand the helium. If your original compression ratio was sufficient, then you should be able to remove enough heat (using the above liquid hydrogen or liquid nitrogen) that when you expand the helium you will get some condensate. Googlemeister (talk) 19:47, 21 September 2010 (UTC)

Maybe some info can be placed in the articles by some of the more knowledgeable people about the process for liquefying helium. I am better at writing simple articles. --Chemicalinterest (talk) 19:53, 21 September 2010 (UTC)

I am a big documentary fan, I watch lots and lots of documentaries, and "Absolute Zero" is still in my fav docos of all time. I can not recommend it highly enough. It's a topic almost everyone thinks there's "nothing" to, the subject is so personally familiar to everyone, you can't not take it for granted. But the story of how people first tried to explain "cold" and incrementally discovered how to control it is actually incredibly fascinating. Vespine (talk) 23:19, 21 September 2010 (UTC)

As far as liquid helium production is concerned, I think this is involved Hampson–Linde cycle . Vespine (talk) 23:47, 21 September 2010 (UTC)

It's a Hampson–Linde cycle once you've cooled the gas down to 40 K. Above about 45 K, the Hampson–Linde cycle won't work, because the Joule–Thomson coefficient is negative: that means that helium warms up as it expands is it starts at more than about 45 K (the exact temperature depends on the pressure). This is why you need liquid hydrogen to cool the helium down before you can run the Hampson–Linde cycle. Physchim62 (talk) 00:50, 22 September 2010 (UTC)

Atmospheric tides of the moon

Tidal forces.

What makes the lunar tides effect on the atmosphere strongest at the 4 o'clocks & weakest at the 10s? I would think it would vary with the moon's phase & its rise and set times. —Preceding unsigned comment added by Woodelf68 (talk • contribs) 19:19, 21 September 2010 (UTC)

Can you point to a source discussing this? Our article on atmospheric tides doesn't mention it. — Lomn 19:48, 21 September 2010 (UTC)

Well, our atmospheric tides article does claim that atmospheric tides are "are generated by the motion of the Earth's oceans (caused by the Moon) and to a lesser extent the effect of the Moon's gravitational attraction on the atmosphere." So maybe the the diagram on the right, from our Tide article, helps some? I'm not entirely sure what you are asking, Woodelf68. WikiDao ☯ (talk) 20:53, 21 September 2010 (UTC)

It sounds like you are actually describing solar atmospheric tides, which are sun-synchronous (so at the same time each day) and have a 6-hour component of oscillation. Lunar atmospheric tides are generated by the same mechanism that produces ocean tides, so they should have the same (or perhaps orthogonal) timing as ocean tides. Franamax (talk) 18:02, 22 September 2010 (UTC)

Richard_Lindzen#Atmospheric_tides also William M. Connolley (talk) 18:22, 22 September 2010 (UTC)

this

why cant i find articles about this event http://www.documentingreality.com/forum/f166/police-shoot-couple-execution-style-15488/ —Preceding unsigned comment added by Kj650 (talk • contribs) 20:46, 21 September 2010 (UTC)

I think that's a bad link. I couldn't see the video there. If you give a better link or explain your question better, we might be able to help you find more information about your topic of interest. WikiDao ☯ (talk) 21:15, 21 September 2010 (UTC)

the link works —Preceding unsigned comment added by Kj650 (talk • contribs) 21:19, 21 September 2010 (UTC)

The link only works if you're a member of that forum. And since it's invite-only it's not very useful. APL (talk) 21:45, 21 September 2010 (UTC)

try this http://www.youtube.com/watch?v=7K3j2UfwDTo —Preceding unsigned comment added by Kj650 (talk • contribs) 00:58, 22 September 2010 (UTC)

Looks like it originally came from the History Channel. I couldn't quite catch the names of the deceased. But as explained in the video, police snipers shot to wound and disable; the wife then shot and killed her husband and then herself. Happens all the time; why would you expect to find an article about that one incident? WikiDao ☯ (talk) 01:09, 22 September 2010 (UTC)

It does not happen all the time, and, as I demonstrated below, something like this is quite likely to get written up in at least the state newspapers. Buddy431 (talk) 01:46, 22 September 2010 (UTC)

a news article i would expect —Preceding unsigned comment added by Kj650 (talk • contribs) 01:41, 22 September 2010 (UTC)

Here are a couple articles I found: [2], [3], using Google News Archive. Buddy431 (talk) 01:45, 22 September 2010 (UTC)

Okay, good work getting their names. That got me to our article Keith Haigler, which describes the incident in the video. WikiDao ☯ (talk) 01:51, 22 September 2010 (UTC)