Wikipedia:Reference desk/Archives/Science/2019 September 12

= September 12 =

Expected value of Earth’s distance from:
(Statistically, probabalisticallhat is the expected value of the distance of Earth from the closest 1: stellar black hole; 2: neutron star; 3: white dwarf? thanks Rich (talk) 09:04, 12 September 2019 (UTC)
 * Well, the articles we have already give the closest detected such objects. (1) Sagittarius A* is 26000 light-years away, (2) Neutron_star says RX J1856.5−3754 is about 400 light-years away, and (3) White_dwarf says Sirius B, 8.7 light-years away. Tigraan Click here to contact me 10:16, 12 September 2019 (UTC)


 * They asked about the closest stellar mass black hole: A0620-00 at 3000 light years. Ruslik_ Zero 10:21, 12 September 2019 (UTC)
 * Who reads questions before answering anyway? Tigraan Click here to contact me 10:40, 12 September 2019 (UTC)


 * Since they said "Expected value", not actual distance, they might be asking what the distance should be, on average, based on the density of such objects in the galaxy. SinisterLefty (talk) 11:00, 12 September 2019 (UTC)
 * I did mean statistically/probalistically.Rich (talk) 16:09, 12 September 2019 (UTC)
 * For white dwarfs there is no difference as all nearby white dwarfs are known. For other two type of objects it is tricky. For instance at least 100 million neutron stars should exist in Milky Way, which translates to the minimum distance of at least 15 pc. However it depends on the spatial distribution of them (disk or spheroid). The statistics for black holes is even more fuzzy. Ruslik_ Zero  16:25, 12 September 2019 (UTC)
 * Why can’t there be other white dwarfs, maybe small ones, closer than the ones that are known?Rich (talk) 23:47, 12 September 2019 (UTC)
 * Because White dwarf stars have a minimum possible size which, combined with their currently possible range of temperatures, means they have a minimum brightness that we would be able to detect at any distances closer than those of the closest already known. Of course, eventually they will radiatively cool sufficiently to become dim or invisible in the range of visible light (see Black dwarf), but this takes markedly longer than the current age of the universe, so none can yet have cooled to this degree. {The poster formerly known as 87.81.230.195} 90.202.210.107 (talk) 11:42, 13 September 2019 (UTC)
 * If you have refernces for these statements, I would be grateful.Rich (talk) 21:46, 13 September 2019 (UTC)
 * I mean, it’s not sufficient logically to guess/deduce plausible reasons for Ruslik’s(admittedly true, since he’s an expert in this area)assertions.Rich (talk) 23:37, 13 September 2019 (UTC)
 * Rich, I'm not sure from your indentation whether you were addressing me or not, but anyway . ..
 * in Star it's mentioned that white dwarfs form from stars with a minimum mass of 0.5 M☉ – less mass than that and the star will not become a white dwarf.
 * List of smallest stars gives the smallest known white dwarf, likely to be close to the lower size limit, as GRW +70 8247 which is about 43 light years away. If this star, discovered in the 19th century and identified as a white dwarf in 1934, were as close as even the nearest known white dwarf (Sirius B, linked in Tigraan's initial reply) it would appear about 5 times brighter than it does, and would therefore be very easy to detect, as would any similar or slightly smaller white dwarf even closer. The only way it would have to "hide" from direct observation is if it were in very close orbit around a larger companion (which is what delayed Sirius B's discovery), but in any stellar systems closer than Sirius (of which there are six by my count), we could detect it indirectly by observations of Fraunhofer lines if not by proper motion observations. {The poster formerly known as 87.81.230.195} 90.202.210.107 (talk) 16:33, 16 September 2019 (UTC)


 * Wow that is intensely good answer, thank you Rich (talk) 04:20, 18 September 2019 (UTC)
 * You can read this for the neutron star birth rates and then calculate the average distance based on the known Milky Way's disk dimensions. Ruslik_ Zero 16:06, 14 September 2019 (UTC)
 * Thanks i’ll read through it, although i won’t have the skills or time to do the calculation myself.Rich (talk) 04:56, 18 September 2019 (UTC)

Are tapirs odd or even-toed ungulates??
The classification classifies them as odd-toed ungulates. However, within the article it says that they have 4 (an even number) of toes on their front feet and 3 (an odd number) on their hind feet. This means they're in fact a mixture. Can odd-toed ungulates really be partially even-toed?? Georgia guy (talk) 15:59, 12 September 2019 (UTC)
 * The classification of ungulates by toe number is superseded by the genetic relationship between animals. Counting genetic difference is a better test of relatedness than counting toes is.  Tapirs are of the order Perissodactyla, the same as equines and rhinos, because their genetic history branches off from other animals at the point that puts them in that grouping.  That they have evolved 4 toes on their front legs is not relevant otherwise.  Yes, "perissodactyla" means roughly "odd digits", but (I suspect) the taxon got its name long before tapirs were included in it.  -- Jayron 32 16:09, 12 September 2019 (UTC)
 * Also, if you really want to split hairs, toes are the digits on the back legs. The front leg digits are fingers.  -- Jayron 32 16:11, 12 September 2019 (UTC)
 * Perhaps the more "scientific" word would be digit, insofar as a Latin word sounds more officious. Nimur (talk) 23:53, 12 September 2019 (UTC)


 * "Odd" and "even" mean one and two (i.e. hoof vs. cloven hoof).  It breaks down above that. It's a useful distinction, but only between the simple groups where it does work. (Otherwise though, tapirs behave as for the odd-toed ungulates.) Andy Dingley (talk) 16:29, 12 September 2019 (UTC)
 * Yeah somewhat confused by the question at a basic level. While this is not something I know anything about, both Odd-toed ungulate and Even-toed ungulate suggest he distinction between odd and even has nothing to do with the number of toes being odd and even per se, but whether they are weight bearing on one or two toes. Others may be present but aren't weight bearing. Of course the other points people have made still stand, but I don't quite understand why the meaning of odd-toed ungulate and even-toed ungulate would make you think that having 3 or 4 toes relates to being either odd or even-toed. Nil Einne (talk) 10:29, 14 September 2019 (UTC)
 * It appears that the answer to the original question is "Yes." ←Baseball Bugs What's up, Doc? carrots→ 16:34, 14 September 2019 (UTC)