Wikipedia:Reference desk/Archives/Science/2013 April 3

= April 3 =

Finding a star with a specific set of properties.
Hi there, I'm having a little trouble searching for something. Any star is fine, not a particular one, but I have a specific set of criteria; it should have a spectral class similar to the sun (G2V), should be pretty far away (far enough that it's not part of the exoplanet search YET, so I'm thinking 100-200 light years?), and preferably a bit older than the sun, thought they probably won't know that much about the star if it's that far out. Does anyone understand enough about the catalogues to be able to find something like that? All I could find was a list of G2V stars from the Hipparcos catalog, which is a start, but I can't tell how far away they are -> http://web.njit.edu/~gary/322/G2V_stars.html. Thanks! 96.49.66.5 (talk) 01:12, 3 April 2013 (UTC)
 * I suppose the solar twins listed here are mostly pretty good... http://en.wikipedia.org/wiki/Solar_twin 96.49.66.5 (talk) 01:14, 3 April 2013 (UTC)


 * For your "not part of the exoplanet search", criterion, you have a bit of a problem. 100 to 200 light years isn't nearly enough distance to prevent exoplanet searchers from looking at it.  Extragalactic planet says that planets have been discovered orbiting stars in other galaxies.  I would also call out HIP 13044 which is known to have at least one planet (which we have an article about: HIP 13044 b) and is 2,300 light years away.  So any star that can be imaged from Earth could (in principle) be checked for planets.
 * That said, those planets were found either by extraordinary means (gravitational lensing) that doesn't let you image just any old star - or (as in the case of HIP 13044 b) they happened to examine it closely because the star is very special and the search for planets orbiting it is more than just idle curiosity. So probably - if you pick a suitable star that's a good distance away and otherwise highly unremarkable - then it'll be a while before anyone bothers to check it for planets.
 * Now that we've established that planets are orbiting almost every star - and that earth-like planets are fairly common, it's doubtful that astronomers will bother checking every G-class star they can find - it's just pointless to do that now that so many exoplanets have been found.
 * SteveBaker (talk) 14:21, 3 April 2013 (UTC)


 * Pretty much any star you choose, no matter how close you look, has not been part of an exoplanet search. Pretty much any star that has been part of an exoplanet search could have undetected planets around them.  The most common search method, looking for transits, only works if the orbital plane is highly aligned with our line of sight.  A randomly situated alien astronomer looking at our Sun, for example, only has a 0.2% probability of seeing an Earth transit.  If you don't see a transit, the next best option is to try Doppler spectroscopy.  Current human technology cannot detect an Earth-like planet around a Sun-like star with 300-day periods for even the closest star.  Alpha Centauri B, part of the nearest star system, was found to have an Earth-sized planet only 5 months ago, and this planet had a 0.04 AU orbit.  If this same star had an Earth-sized habitable planet around it with plants and animals, we'd have no way of knowing about it.  --140.180.250.241 (talk) 15:51, 3 April 2013 (UTC)

Rain and shoelaces
Why do shoelaces come untied more often during rainy weather? 24.23.196.85 (talk) 01:18, 3 April 2013 (UTC)


 * It depends on what they're made of. Many synthetic fibers can become slippery when wet, meaning that the water acts to lubricate the knot by reducing the coefficient of friction. This applies to things like nylon, dacron, and polypropylene which are a common lace material in athletic shoes. This slipperiness makes the knot easier to come undone through normal vibrations and perturbations in the course of walking. However, natural fibers, such as cotton, hemp, or even leather strips, tend to absorb moisture and make knots tighter, so that they hold even better when wet than when dry. These materials are more common in work boots, hiking wear, and some dress/casual shoes. For further reading, I suggest Ian's shoelace site as a great resource. SemanticMantis (talk) 01:42, 3 April 2013 (UTC)
 * Specifically, this "secure knot" is closely related to surgeon's knots and fisherman's knots, and will hold quite well no matter what the cordage or conditions.  SemanticMantis (talk) 01:55, 3 April 2013 (UTC)
 * Thanks for the info! As an aside, isn't Semantic's secure knot essentially the same as a square knot, used by sailors when reefing a sail? 24.23.196.85 (talk) 03:37, 3 April 2013 (UTC)
 * No problem, knots are fun :) And no, it is similar, but a little different to a square knot. Ian's "secure knot" has two twists on the last part, a square knot has only one. That extra twist makes all the difference, especially one fishing line or wet shoelaces. See also the picture at surgeon's knot, which is sort of the reverse of the secure knot (ignoring the bighted working ends on the latter). The secure knot starts with one twist and ends with two opposed, while the surgeon's knot starts with two twists and ends with one opposed. Actually, the they are the same knot if you consider it with four free ends, but for shoelaces, we usually consider two fixed ends and two free ends. SemanticMantis (talk) 04:13, 3 April 2013 (UTC)
 * Thanks once again! 24.23.196.85 (talk) 04:24, 3 April 2013 (UTC)
 * regardless of material, I think the extra weight of the long ends as we walk in the rain and more often run to get out of it pull the knot looser.68.36.148.100 (talk) 02:11, 5 April 2013 (UTC)

Check in time
How long before the scheduled departure time do you have to check in for a flight from Baikonur to LEO? What's the baggage allowance? 202.155.85.18 (talk) 07:07, 3 April 2013 (UTC)


 * You will need to ask the airline company. The information is probably on their website.--Shantavira|feed me 07:19, 3 April 2013 (UTC)


 * Do space tourists use airline companies? Technically, they won't be traveling just through air. OsmanRF34 (talk) 21:24, 3 April 2013 (UTC)
 * English words aren't necessarily logical. Astronauts don't fly to stars. And very few teamsters drive teams of horses nowadays. ←Baseball Bugs What's up, Doc? carrots→ 07:03, 4 April 2013 (UTC)


 * It's hard to tell, but the mission photos suggest that the luggage allowance may depend on position and experience. Pavel Vinogradov, as Soyuz TMA-08M commander with two previous spaceflights, was allowed to bring one moustache, whereas it looks like the other 2 chaps were required to leave theirs at the Baikonur Cosmodrome. Oddly the moustache article doesn't have any information about the first moustache in space.  Sean.hoyland  - talk 08:23, 3 April 2013 (UTC)

Economy 7 Fridge/freezer
Is there a reason that might prevent manufacturers producing a refrigerator and/or freezer that benefits the owner on an 'Economy 7' electricity plan? A freezer could run the compressor more during the seven hours of reduced-cost electricity and less during the 17 hours of increased-cost electricity. Could it run constantly during those seven hours and then not at all (or relatively briefly) during the 17? — Preceding unsigned comment added by 89.241.233.102 (talk) 21:35, 3 April 2013 (UTC)


 * Yes, but you would find the temperature varies more than you might like. Thicker insulation would help, but opening the door to access food somewhat defeats the point.  I certainly wouldn't store anything that could rot in such a fridge, as the cost of the thrown-out food would be more than the electricity savings, but perhaps it would be OK for things like soda, where getting a bit warmer from time to time is OK.  If this is what you intend it for, perhaps you could set an appliance timer to only give it power for 7 hours a day.  I'd also set the temperature in the fridge a bit lower, but not so low that things will freeze when it does have power.  You'd also want to disable any defrost cycle, in a freezer/refrigerator, since the 17 hours a day without power will be more than enough of a defrost.


 * Now, if we want to talk about completely redesigning the fridge, you could also use a chilled water system to store "coolth" (my own word) generated at night for use during the day. (Although fresh water doesn't get cold enough for this, without freezing, so perhaps salt-water or another fluid would need to be used.)  Such a system would add weight and bulk, for the storage tank, however.  You could also somewhat simulate this effect, by filling the fridge (or, preferable, the freezer portion of a combo unit) with items of high thermal capacity, like beer.  (Now who could object to this requirement ?)


 * If you have two refrigerators, you might try this with just one, and leave the other as is, for foods which can decompose easily. StuRat (talk) 22:07, 3 April 2013 (UTC)


 * Yeah, so temperature variation is a bigger problem for the fridge than for the freezer. Let's focus on the freezer. What if the compressor runs during the seven hours and brings the temperature down to, geez, how should I know, -32 C and then over the course of the day, including access, it increases to -20 C (I'm not even suggesting that the device is forbidden from activating during the 17-hour period; of course it should switch on to prevent the temperature exceeding a set-point - the point is that it lowers the temperature to a lower limit during the seven-hour period) 89.241.233.102 (talk) 22:27, 3 April 2013 (UTC)


 * Here in Australia we have the Esky. It uses no electricity, and keeps stuff cold for days. (Yes, I know such things exist elsewhere, under other names.) It's really just a matter of how you use the device. HiLo48 (talk) 22:31, 3 April 2013 (UTC)


 * It looks like that's just an insulated cooler. However, I think you'll find that it doesn't keep the temperature constant, especially if it's frequently opened and closed. StuRat (talk) 22:34, 3 April 2013 (UTC)


 * You CAN have a constant temperature in an Esky. What we do in Australia sometimes, where we might drive long distances to visit friends or cousins etc in remote areas, is put the food they can't get locally in an Esky (large box made of thick polysterene foam)), packed in regular ice, which all food shops and alcohol suppliers can supply, with the regular ice just covering the food. Then. we put a judged quantity of ice made from salt water on top.  Adding salt to water lowers the freezing point.  During the long drive, and overnight if necessary, the salt water ice steadily melts.  Actually, its more a case of stiff mush turning into runny mush.  While the salt water ice is melting, it holds the temperature constant at the melting point.  The regular ice around the food serves merely to conduct the "coolth" to the food, and add a bit of thermal capacity.  Not long ago me and my lady were invited to a cousin's 8-year old daughter's birthday party, 800 km and one overnight sleep away.  Because they live in a remote are, she had never seen an icecream birthday cake, in fact she had only ever seen home made icecream.  We bought a large icecream birthday cake and had the supplier write her name in coloured cream on it.  The supplier told us the cake should be kept between -16 and -22 C for optimum eating pleasure.  I made up some tubs salt water ice in our home freezer, progressively stiring in salt until it was stiff frozen mush at -22 C.  When the cake was checked after being in the Esky for over 24 hours, the salt water ice had almost fully melted, the normal ice was still completely frozen solid, and the icecream cake was in perfect condition.  Wickwack 124.182.6.147 (talk) 00:34, 4 April 2013 (UTC)


 * Nice. I have a portable cooler which plugs into the cigarette lighter.  This really doesn't provide enough cooling to cool anything off, but it will pretty much keep it at the starting temperature.  Ice packs can help out, of course. StuRat (talk) 00:40, 4 April 2013 (UTC)


 * Japanese portable fridges with oscillating mass - type compressors (e.g, "Engel" brand) run on 12 VDC and easily maintain -16 C, but they flatten the car battery if left on while you eat or sleep, and they have a high failure rate if left in a parked car during Australian hot weather. However you can also run them on house current, so you can bring them inside the motel room or whatever if a sleep or meal is needed.  Some people install a second battery in the car with diode isolation, so at least they know they can start the car engine next morning. Wickwack 124.178.45.240 (talk) 01:20, 4 April 2013 (UTC)


 * Yes, I have to admit to having drained the battery dead by leaving it plugged in. StuRat (talk) 02:04, 4 April 2013 (UTC)


 * That would work, but the problem is that the lower you make the lowest temperature, the more expensive the special equipment, and the more coolth you lose to the environment, since heat transfer is proportional to the differential, also increasing the operations cost. StuRat (talk) 22:36, 3 April 2013 (UTC)


 * My, that reminds me! I briefly saw a refrigerator in Fiji some time back which required no electricity (which wasn't available much of the time) and seemed to make no noise and I was told ran on natural gas (though there was certainly no distribution network nor any large visible tank).  Seemed like a transplant from a future version of the Cosmos, but I have no idea how many of these details were scrambled in the telling (probably by me) Wnt (talk) 00:20, 4 April 2013 (UTC)


 * Yes, you can make one that burns fuel directly to produce coolth, counter-intuitive as that may seem. See absorption refrigerator.  As for the silent part, I have an ammonia cycle fridge (a type of absorption refrigerator) right next to me here (I keep it by the PC for cold beverages) that does that, although it does use electricity (inefficiently) and I'd have to evacuate the house if the ammonia ever leaked. StuRat (talk) 00:29, 4 April 2013 (UTC)


 * Gas operated friges are very commonly in use in caravans and mobile homes(="trailer home" and "winebago" in USA terminology). Since they run off bottled gas, they a portable, but they are very inefficient thermodynamically compared to compressor type friges.  You can also get 12 VDC operated peltier effect beer coolers, but they are even more inefficient.  Wickwack 124.182.6.147 (talk) 00:42, 4 April 2013 (UTC)


 * At least in my part of the US, the term "RV" (for "recreational vehicle") is used for the thing you drive and live in on vacation, and the term "mobile home" is used for the manufactured home towed to a site (where you pay rapidly increasing lot-rental fees and await the next tornado to put you out of your misery). StuRat (talk) 00:48, 4 April 2013 (UTC)


 * Ahah! I had a feeling I had the American terminology not quite right. I thought an "RV" was a 4-wheel drive vehicle.  Reminds me of a CEO we had, who came over from the USA.  He knew how to run big companies right enough, but he was hopeless at across-the-table negotiation, because he could not understand Australian English! Or Australian manners.  In Australia, the portable dwellings that are trucked to site, and trucked somewhere else when required are known as "dongas"  (because during cyclones, their sheetmetals walls make loud "Dong" noises when rocks, rubbish, and bits of other people's homes bounce off the sides).  You can get them with heavy chains to chain them down to concrete blocks so you don't get blown away in cyclones.  Wickwack 124.178.45.240 (talk) 01:27, 4 April 2013 (UTC)


 * A 4-wheel drive vehicle that's fully enclosed is called an SUV, like the Chevrolet Suburban. An F5 tornado will tear apart any mobile home known to man.  Even those in real houses had best hide out in the basement (and pray).  StuRat (talk) 02:03, 4 April 2013 (UTC)


 * We get F2's routinely in many parts of Australia. An F3 would be quite unusual and as bad as it gets.  We usually get 3 to 4 F1's per year in the city I live in - the biggest impact of which is cars wrecked by trees falling on them, and fires started by electricity infrastructure.  I was amused by the descriptions given in the Wikipedia article on the Fujita scale - F1 is descibed as "moderate damge" to homes, roofs peeled off etc.  If that actually typically happened here (as distinct from just the odd home out of thousands), builders would get sued.  To get a building permit from local government, homes and all buildings larger than 3 x 3 x 3 m must be engineered to withstand winds according to the cyclone rating of the area. Wickwack 124.178.51.92 (talk) 02:36, 4 April 2013 (UTC)


 * I suspect a language problem here. You're probably thinking of a "roof" as in timber.  That was the original meaning here, too, but it seems to now be used to just refer to the roof shingles.  I believe an F1 only peels off the shingles, not the framing.  StuRat (talk) 03:07, 4 April 2013 (UTC)