Wikipedia:Reference desk/Archives/Science/2013 August 15

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August 15

Binary star systems and their planets

In sci-fi shows, planets with multiple suns are often shown. Are the distances between binary systems really small enough to allow a planet to be significantly affected by both stars, to the point where they can be seriously seen to have "two suns" by an observer on the surface? 2.223.230.86 (talk) 01:32, 15 August 2013 (UTC)

For some binary systems, yes. See Alpha_Centauri_B#Theoretical_planets for some discussion, as well as an artist's rendition of what such a sky would look like. However, in this case the planet would be in the ballpark of 10 times closer to one star than the other to be in a habitable zone, so you would never see two huge suns in the sky. Someguy1221 (talk) 01:39, 15 August 2013 (UTC)

The OP didn't specify that it has to be a habitable planet. Also, sometimes one star is more than 10X the diameter of another. I wonder if a planet can exist at the barycenter between the two stars, or at Lagrange points between them. StuRat (talk) 07:43, 15 August 2013 (UTC)

Planets have been discovered orbiting close binary stars at a larger distance, like at Kepler-47.

A planet placed (with 0 velocity) at the barycenter the planet would start falling towards the heavier star, and wouldn't have a stable orbit; and the Lagrange point L1 isn't the place for a stable orbit either. There are stable orbit around the Lagrange points L4 and L5 if one star is a lot more massive than the other (like the Trojan asteroids of Jupiter), and also around both of them (horseshoe orbit). Icek (talk) 08:36, 15 August 2013 (UTC)

One star has to be at least about 25 times heavier than the other in order for some of the Trojan orbits to be stable. Other stable orbit possibilities are, as Someguy1221 pointed out, when the planet is much closer to one of the stars, or, as Icek pointed out, when the stars are much closer to each other than to the planet. Dauto (talk) 15:08, 15 August 2013 (UTC)

See circumbinary. Wnt (talk) 15:42, 15 August 2013 (UTC)

Thank you. 2.223.230.86 (talk) 00:31, 16 August 2013 (UTC)

nipples

Why do men have nipples — Preceding unsigned comment added by 67.197.49.21 (talk) 04:46, 15 August 2013 (UTC)

See Nipple#In_male_mammals. Someguy1221 (talk) 04:49, 15 August 2013 (UTC)

Men don't have nipples in South Carolina. It's against their religion. ←Baseball Bugs ^{What's up, Doc?} carrots→ 05:07, 15 August 2013 (UTC)

But opossums have a baker's dozen, so it evens out. Sadly, it is still far too few overall nipples, if you're a baby opossum. InedibleHulk (talk) 05:32, 15 August 2013 (UTC)

Basically, if there is no evolutionary advantage to not having them, it is more efficient to use the same DNA for males and females. For men to have nipples doesn't cause us any problems whatever in daily life - so there is no natural selection pressure that would eventually eliminate them. Evolution tends to put as little information as possible on the sex chromosomes for reasons of efficiency - so men and women tend to be identical in ways that don't matter to the perpetuation of our species. Even the organs that we do have that are visibly different are often relatively simple modifications of the same underlying design. Another example of that would be facial hair. Men typically grow full beards and mustaches - women still have hair in those places - it's just fine and short so it's (usually) barely noticeable. SteveBaker (talk) 12:04, 15 August 2013 (UTC)

It's a little more complex than that - there actually is a phenomenon of male lactation, and however insignificant it may be at this point in history, who knows how often natural selection calls on it on a geologic time scale? And, they're unarguably pretty, i.e. there's sexual selection, which maintains many a useless structure. Wnt (talk) 15:32, 15 August 2013 (UTC)

There is also male breast cancer, despite the "pinkness" the disease has in pop culture. Though rare (about 1% of breast cancer cases), it can strike even as "manly" a name as Rod Roddy. InedibleHulk (talk) 17:41, 15 August 2013 (UTC)

Butterfly Wings

I understand that a caterpillar turns into a butterfly after time- so here is my question: If I cut the wings of a butterfly, does it 'revert' back into a caterpillar? Why is it that when a butterflies wings are cut off, it flaps around and dies, instead of living it's old life as a caterpillar? Thank you. --Howdalikya eggsdone (talk) 15:05, 15 August 2013 (UTC)

It's programmed to behave like a butterfly, also it can't eat anymore. Count Iblis (talk) 15:11, 15 August 2013 (UTC)

Every part of the butterfly is made anew from imaginal discs - pupation is more than just popping out a pair of wings. The legs are different from those of the caterpillar and the prolegs it used to help hold on are just gone, and the mouthparts are also changed for new foods. Wnt (talk) 15:28, 15 August 2013 (UTC)

I'm not an expert, but the way I understand it, the caterpillar can be viewed roughly like a highly evolved "egg". The butterfly actually hatches out of it. Vespine (talk) 22:54, 15 August 2013 (UTC)

No, that's not really accurate. Maybe the transition from caterpillar to adult has a certain amount in common with the stage at which mammalian embryos develop a stratum granulosum (the start of the thickened stratified epidermis) based on the arguable homology of T3 and ecdysone. But understand that adult tissues in many species begin with simple diploid 2n nuclei and develop into more "industrialized" cells with 4n, 8n, etc. nuclei that are big, highly functional, but not meant to replicate. For us, this transition is usually quite gradual and continuous, like liver cells that have a gradual gradient within the liver acinus. Many of our fastest replicating epithelial tissues never get the chance to go polyploid. But in insects there's a huge distinction between the fast-replicating imaginal disc, which is dividing for future use, and the polyploid and nondividing (and indeed rigid) external tissues, which are fully differentiated and finished growing. Still, it is an exaggeration of our more steady cell turnover, I would say. Wnt (talk) 03:59, 16 August 2013 (UTC)

I have no idea what the following was about. Wnt (talk) 03:59, 16 August 2013 (UTC) :

Please read the two questions together before answering. CambridgeBayWeather (talk) 01:16, 16 August 2013 (UTC)
The following discussion has been closed. Please do not modify it.

Is the daytime aurora visible in Northern Norway?

Or isn't it dark enough at Noon in Winter and you would need to go farther North to Svalbard to see it? Count Iblis (talk) 15:07, 15 August 2013 (UTC)

If you are north of the Polar Circle, it is quite possible to see the aurora at 'noon' - since the sun is below the horizon. I myself saw it around lunchtime in Bodø one January about a decade back, when I was working there for a short while.

The further north you go the more days of no sun there is, logically that gives you a better chance of seeing it. WegianWarrior (talk) 17:13, 15 August 2013 (UTC)

Yes, but then given how extremely expensive Norway is, I'm not going to stay there for long (I paid 60 Euros for just a simple meal with potatoes vegetables and meat :( ). I was worrying about about the typical daylight aurora not being so bright as the evening one, combined with the fact that the Sun at Noon time not very far below the Horizon in Northern Norway, so you'll have some twilight that may drown out the aurora unless it is unusually bright. In Tromso the Sun will be only about 3 degrees below the Horizon around 21 December, while in Svalbard it will be 12 degrees below the Horizon.

But then I could go to Svalbard in Winter anyway to get to see a totally dark night sky for the first time in my life... Count Iblis (talk) 23:39, 15 August 2013 (UTC)

Electric rocket propulsion: propellants and ground based use

I read the Ion thruster & Electrically powered spacecraft propulsion articles, and I am not sure what prevents the use of air as a propellant. Why can’t one be used in a vehicle that runs on the ground? I know that chemical rockets & jets are not used in a medium sized ground vehicles like cars because of low efficiency and massive fuel consumption. But that shouldn't stop us from using an arcjet with air as a propellant, right? Diwakark86 (talk) 16:34, 15 August 2013 (UTC)

Those sorts of thrusters have a high propellant efficiency, but very low thrust. The majority of the examples listed in ion thruster have power outputs that don't compare favorably to a gasoline engine at all. Luckily, on the ground, we don't have to worry about propellant efficiency - instead of ejecting propellant to gain momentum we can just push off the ground with wheels. This is a very efficient way to transfer energy and it doesn't require propellant to be consumed. Also, the arcjet article mentions that the 100kW jet has a 10mile/sec exhaust - I don't think you want cars driving around with that coming out the back. The ion thruster article also explains that propellants are chosen for ionization properties and low erosion, you can't just replace xenon with air and have it work the same way. Jessica Ryan (talk) 16:53, 15 August 2013 (UTC)

For an example of an ion thruster that uses air as the propellant, see Ionocraft. --Carnildo (talk) 00:56, 16 August 2013 (UTC)

Chrysler Turbine Cars might be of interest here. While the exhaust wasn't nearly as fast as that from an ion thruster, it's heat was still problematic, as was the noise. That technology was used in the M1 Abrams main battle tank, though, where heat and noise are less problematic. StuRat (talk) 07:02, 16 August 2013 (UTC)

That Chrysler is a really interesting car - I never realized that any manufacturer developed a gas-turbine car all the way to being basically production-ready! Old helicopter turbines show up in boats every so often, and I think I read about Jay Leno driving a motorcycle that someone managed to fit one into... However, I read the question as using the rocket effect to accelerate - gaining forward momentum by putting an equal amount of momentum into the propellant, which is ejected. A gas turbine car still moves by pushing wheels against the ground. I suppose you could duct an ion thruster through a turbine and use that to get power to the ground, but I have a feeling it is a lot more efficient (and simpler/cheaper) to use the electricity to run an electric motor instead. Jessica Ryan (talk) 11:46, 16 August 2013 (UTC)

Thanks for the response. Looking back at the article there is one more thing I didn’t understand: why is the thrust for an electricity/ion based jet low? Is there a design constraint that prevents us from building a more powerful thruster, like material that handles high plasma temperatures or is it just that it is not as efficient as a magnet pushing a current carrying wire in a motor? Also is there any method available to compare the efficiency of pushing the ground for thrust as opposed to pushing air?Diwakark86 (talk) 17:13, 19 August 2013 (UTC)