Wikipedia:Reference desk/Archives/Science/2007 June 15

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June 15

What's the name of this plant?

Recently I came across the following plant (picture), but I cannot determine which plant it is exactly. It was found in the South of the Netherlands (Limburg). The four petals made me think it would belong to the Brassicaceae, but I'm not sure. I've looked a bit around at Dutch sites [1] and [2], but I'm not able to find a matching plant.

Some more information: It's a small plant, I think, the leaves you see in the background (for size) are, probably, red deadnettle. Nearby a Cardamine hirsuta (Bitter cress) was growing too.

It's the only picture I have, but if someone would be willing to give a shot, I'd be very happy. Berteun 11:33, 15 June 2007 (UTC)

I've no idea, but you might try asking at Wikipedia talk:WikiProject Plants. Sorry if that is redundant. — RJH (talk) 22:44, 15 June 2007 (UTC)

pyelonephritis

after treating this dissease and after it dissapears, will the kidney dammage that it has already caused be permanent? or can the kidneys 'self-regenerate' somehow?

The Article Pyelonephritis mentions that Chronic_(medicine) infections can result in fibrosis and scarring. I would interpret this as being permanent damage, but non chronic cases could heal fully. I am not a Doctor. This is not Medical advice, just a referance to an article. Blah Blah Blah. Czmtzc 16:58, 15 June 2007 (UTC)

D lol. thank u

It depends how severe. Most cases will not cause significant deterioration of kidney function (BUN and creatinine, or creatinine clearance), nor will the kidneys show visible scarring on CT scanning.

In fact, losing one kidney usually does not impact kidney function (people can donate a kidney for transplant or have a kidney removed to treat cancer); the remaining kidney undergoes something called "compensatory hypertrophy" to make up for the absent kidney.

Hope this helps.

Boat rudder

I know this sounds like a trick question or a joke but seriously why does a boat have a rudder in the back instead of in the front while the wheels that trun on a car or bicycle are in front? Ugly bag of water 15:29, 15 June 2007 (UTC)

Well, one good reason (I'm sure there are others) is that the back of the boat (usually) stays in the water, even in high waves, while the front will frequently come out of the water, which would render the rudder useless. --TotoBaggins 20:18, 15 June 2007 (UTC)

A rudder at the front would be impossible to control as the current would pull it all the way to one side, while at the back the current pulls it straight and work must be done to steer away from a straight course. Bendž|Ť 21:28, 15 June 2007 (UTC)

Side note: some wheeled vehicles (e.g. forklifts, and many hand carts) steer from the rear. —Steve Summit (talk) 13:55, 16 June 2007 (UTC)

Thanks Steve. That helps me to visualize the difference a little better but not completely - the act of parallel parking also helps but still not quit there. It does give me an idea that might help the other gender get the boat down the ramp to the water - mount an auxillary trailer hitch up front. Ugly bag of water 19:27, 16 June 2007 (UTC)

In boats and planes it's a matter of stability. If you had a rudder at the front and the boat is going in a straight line when you turned it just a TINY bit to one side, the flow of water would push against the rudder and make it want to flap all the way over to one side. So the smallest turn of the wheel would result in the boat wanting to turn at it's maximum turn rate and the boat would need constant attention to keep it going the way you wanted. In it's conventional position at the back, the flow of water acts to stabilise the rudder - so that once you are moving straight or in a particular curve, the flow of water tries to keep the rudder pointed in that direction - so you have built-in automatic stability. This is also true for airplanes - but not for cars. In the case of a car, the wheels don't have enough air pressure on them to matter and they can be kept stabilised by giving them a small amount of 'toe in' where the front wheels are angled slightly inwards at the front - so the lack of a stability problem makes it possible to put the wheels at either end and still be stable. Cars benefit greatly from front wheel steering because the back end of the car naturally follows very closely the path of the front - so the driver only has to aim the front end and doesn't have to worry too much about where the back end is heading. If you had a rear-wheel steered car, you'd have to concern youself with how the back end was swinging around as you turned. You can easily understand this problem if you imagine your rear-wheel steered car was parallel parked on the right side of the street close to the sidewalk. As you steered to the left to pull out into traffic, the rear wheel steering would cause the back end of the car to go to the right - when it would immediately hit the curb. On a front wheel drive car, you can steer to the left and the front end of the car leaves the curb cleanly and the back end just kinda follows along. So rear-wheel steered vehicles are only used in very specialised situations. There are also four-wheel steered cars and trucks out there - and those have to have some very fancy mechanics to ensure that you can pull out from a parallel parking situation without scraping the back tires. SteveBaker 00:16, 17 June 2007 (UTC)

A rudder at the back can direct the flow of water created by the propeller and not just the flow of water created by the forward motion of the boat.

Atlant 00:19, 17 June 2007 (UTC)

That's true - that makes the rudder more effective - but it's not the reason for the rudder being at the back. After all, if you really wanted the rudder at the front, you could put the propeller at the front (just like most airplanes). Also consider that most propeller driven single-engined aircraft have the engine at the front and the rudder at the back - and there are plenty of twin-engined aircraft with just a single rudder - so it's not necessary for a rudder to be directly in the flow from the engine. I'm sure that stability is the primary reason. SteveBaker 15:00, 17 June 2007 (UTC)

Putting the rudder at the front wouldn't mean you had to put it backwards... that would be a very unstable rudder indeed. I guess the reason you put the rudder at the back of the boat is that you want to place it as far as possible from the mass centre, that way a certain force at the rudder results in the greatest torque possible - which means a relativly light rudder could steer a relativly large boat. Any practical boat* has its centre of mass somewhere between its middle and bow, therefore the farthest point is at the stern. /SvNH 22:32, 17 June 2007 (UTC)

*) this does not hold for some ferrys, for instance, designed to travel "both ways", such ferrys have propellers that can be turned in both ends

Altimeters

Since the pressure altimeter works on the same concept as the barometer, in practice are changes in weather accounted for when trying to measure altitude? Thanks -BEN

Yes. Barometric-pressure altimeters have a way to set the local atmospheric pressure at a known altitude precisely for this reason. See the external URLs linked from the altimeter article for more (and more-specific) information. DMacks 19:27, 15 June 2007 (UTC)

Note that for aircraft flying more than a certain minimum altitude above ground, the distinction between true altitude and the so-called pressure altitude is ignored. Instead, flight levels are assigned, where a nominal flight level is equated with a specific pressure, which in turn roughly corresponds to a particular real distance above sea level. See the linked articles for more information. TenOfAllTrades(talk) 19:33, 15 June 2007 (UTC)

Because of that weather changes, the altimeter setting is NOT the same as the barometric pressure. As for flight levels, the minimum flight level assigned depends on the local altimeter setting. Flight levels are always referenced to an altimeter setting of 29.92 inHg which means that the actual altitiude above sea level will vary with local weather changes but it prevents airplanes flying from one are to another using the wrong local setting. --Tbeatty 04:47, 16 June 2007 (UTC)

Removal of SIO2 from Iron Ore

Quiktruk 16:45, 15 June 2007 (UTC)

Can SIO2 be reduces in iron ore through the introduction of HF acid? Or will it destroy the iron as well?

Is there a chemical process that will remove the SIo2?

Are you talking about SiO₂, or Silicon dioxide? You might be able to get more information from the Hydrofluoric acid article. -- JSBillings 17:06, 15 June 2007 (UTC)

Processes that produce iron from iron ore have to be very cheap, otherwise some other company will undercut the price and the inefficient iron producer will lose their business. HF acid is too expensive to be used for a low cost commodity like iron. In your case the iron ore would be converted to iron fluoride, which will be less convenient to convert into iron than the iron ore. All thoese fluorine containing gasses that you produce will have to be disposed of properly, as SiF₄ is fairly noxious, and all the CF₄ you make would contribute to a green house effect! Instead you can use the differences in density or different prospensity to stick to bubbles to separate these minerals. GB 22:08, 15 June 2007 (UTC)

Kinetic Energy of Light

I was wondering...
The equation for kinetic energy is

E=½mv2

If a photon has zero rest mass, then it's kinetic energy would also be zero. If this is so, how is a solar sail supposed to work?69.205.185.158 17:40, 15 June 2007 (UTC)

See radiation pressure. Now I don't know why that doesn't square with the above equation but my total guess is that you are using a classical Newtonian equation for kinetic energy when dealing with things like photons. But that is just a guess as to where the issue lies. --24.147.86.187 18:01, 15 June 2007 (UTC)

The equation you've given is only correct for non-relativistic bodies. The full expression (coming from Einstein's theory of relativity) is given in our article on kinetic energy. Using a rest mass for a photon of zero, one finds that the kinetic energy of a photon is given by

E_k = p·c.

Its momentum (p) is related to its wavelength (λ) and equal to h/λ. TenOfAllTrades(talk) 18:08, 15 June 2007 (UTC)

I know about radiation pressure, I guess what I really meant when I asked the question was this...
Electomagnetic radiation...

1. Has no rest mass.
2. Is not affected in either direction or magnitude when passing through the most powerful electric or magnetic fields mankind can produce.

Despite this, it always produces an acceleration on any matter it encounters.
How is this possible? 69.205.185.158 18:21, 15 June 2007 (UTC)

As others have said, the equation you are using "is only correct for non-relativistic bodies". Essentially, it's an approximation that neglects relativistic effects...those effects aren't important for things like a tennis ball, but when mass becomes zero, it's all that there is, so it cannot be neglected. DMacks 19:21, 15 June 2007 (UTC)

So if approximately one kilowatt of solar energy per square meter hits a solar sail, what is an upper bound for the pressure exerted? It can't be much because proposed sails are huge to achieve a modest acceleration. Edison 20:13, 15 June 2007 (UTC)

I'm not sure I reason correctly here, but the photon article says that the energy $E$ and momentum $p$ of a photon flying through empty space are related by $E=cp$, where $c$ is the speed of the photon (i.e., the speed of light). Probably, the sail will reflect the photons back (not absorb them) so that they will contribute with twice their momentum: $E=2cp$ If the sail during a time interval $T$ gets hit by energy $E$, the power is $E/T=2cp/T$, and $p/T$ is the force on the sail. Divide by the area $A$ of the sail to get $E/(TA)=2cp/(TA)$. In your example, $E/(TA)=1\,\mathrm {kW/m} ^{2}$ and the pressure you are looking for is $2p/(TA)=6.671\times 10^{-6}\,\mathrm {Pa}$. Could this be correct? (In reality, there is plasma in the solar system, not really empty space, but I'm not sure it will affect this very much.) —Bromskloss 20:53, 15 June 2007 (UTC)

Correct result, but confusion in the algebra. Momentum and energy of photon are related by $E=cp$. The change in momentum for total reflection is $2p=2E/c$. The force $F$ is change in momentum divided by time $T$, $F=2p/T$ (not $p/T$) . The pressure is $P=F/A=2p/(TA)=2E/(cTA)$. The irradiance (power per unit area) is $I=E/(TA)$ so pressure $P=2I/c$. This last result is the simplest relation to remember. --Prophys 12:26, 16 June 2007 (UTC)

Ah, I see the point of confusion. I implicitly redefined $p$ from meaning "the momentum of the photon" to "the momentum gained by the sail". I should probably have used different symbols instead. —Bromskloss 12:59, 16 June 2007 (UTC)

Getting back to why it works, it's true that static electric and magnetic fields don't affect e/m radiation propagation, but time-varying fields can. In the classical explanation, the incident e/m wave excites corresponding oscillations of the same frequency in the electrons of the material (think of a shiny metal for argument's sake), which then reradiate e/m waves because they are charges and they are accelerating. The amplitudes and phases of all the oscillating charges work out such that in the forwards direction they generate a total field which exactly cancels out the incident field by destructive interference, and in the backwards direction they produce a propagating e/m field of the same strength as the original. The force comes about because of the motion of the electrons of the conductor in the magnetic field of the incident wave: the Lorentz force.

In the quantum mechanical picture, the explanation is really much simpler, as long as you can accept that the photon has zero rest mass, but finite momentum. --Prophys 12:26, 16 June 2007 (UTC)

The article Solar sail is short of references, and also has no information of the force which would be exerted on a proposed 250,000 square meter solar sail. Radiation pressure says "the radiation of the Sun at the Earth has an energy flux density of 1370 W/m2, so the radiation pressure is 4.6 µPa (absorbed)" and twice that reflected, so the 1 meter sail would experience a force of 9.2 *10^-6 Newton. A realistic proposed sail of 500 meters square or 250,000 sq meters would experience a force of 2.3 Newton. So if a hypothetical solar sail ship weighed 1000 kilograms, it could add 5962 meters/second to its velocity every month. (assuming my math is right). Edison 21:02, 16 June 2007 (UTC)

geology

why is sulphur associated with vulcanism ?

Sulfur is among the stuff spewed out by Volcanos. Friday (talk) 19:39, 15 June 2007 (UTC)

Sulfur dioxide is third after (odorless) water vapor and CO₂ among gases farted out by most volcanoes, so its strong and distinctive smell dominates your experience of the air near a volcano. --TotoBaggins 20:26, 15 June 2007 (UTC)

The resson is due to the heat of the volcano, and the lack of oxygen undergound. In the air the sulphur dioxide is converted to sulphuric acid and sulphates. But when these are reduced they form sulphites that are easily destroyed by heat, being converted to sulphur dioxide. With even more reduction you will get sulphur and hydrogen sulphide, both of which are gasses at molten rock temperatures and atmospheric pressure. By the way its the hydrogen sulphide that has the rotten egg smell. The gasses can escape from the magma and find their ways to the surface through cracks in the rocks. You may also get hydrogen chloride in the gas. GB 22:00, 15 June 2007 (UTC)

Se also vulcanisation

exploding cars

I have been watching reruns of Walker: Texas Ranger on the Hallmark Channel, and I noticed that many times, someone shoots a car or a barrel with a pistol and the car or barrel explodes violently. From what I know of combustion and ballistics, this seems to be impossible for several reasons:

1. A pistol cannot possibly be expected to shoot a slug that is much larger than a .45 caliber, or the recoil would break the user's wrist. Such a small slug would create a very small hole that would not admit enough oxygen to start a fire.
2. The slugs in almost all bullets are made of lead or copper, neither of which spark on impact.
3. A single spark cannot possibly carry enough energy to raise gasoline to it's ignition temperature.
4. The slowest bullets travel at more than 1000 fps, so if you had an enourmous gasoline tank, say, 3 feet across, the slug would actually be in contact with the fuel for about ¹/₁₅₀₀ of a second, too little time to foster combustion.

Is my reasoning sound? 69.205.185.158 18:54, 15 June 2007 (UTC)

Things explode on TV way more than in real life. However, as for your reasons.. bullet weight is a factor in recoil, not diameter. Also, a spark can most certainly ignite gasoline vapor (this happens accidentally sometimes). Slow handgun bullets quite frequently go less than 1000fps, but I don't think this matters. Friday (talk) 19:05, 15 June 2007 (UTC)

As for exposure to oxygen, it usually takes more than one shot (even on TV). After the first hole makes the gasoline leak, the second bullet might strike and create a spark that ignites the gas in open air, wich in turn heats and explodes the rest of the gas tank.  Adam2288  T  C  19:17, 15 June 2007 (UTC)

MythBusters handled this one fairly thoroughly, with an entire firing squad using a variety of different caliber weapons were unable to cause an explosion. A quick perusal of the episode list pages shows it was proven false in Car Capers, and again in Exploding Gas Tank (though tracer ammunition was shown to work). GeeJo ^(t)⁄_(c) • 19:43, 15 June 2007 (UTC)

If I recall, MythBusters did this with cars that weren't moving. A car with its engine running might have responded differently. Even if only because the tailpipe would be hot. 69.95.50.15 15:51, 18 June 2007 (UTC)

One disagreement with the questioner: if there is a mixture of gasoline vapor and oxygen within the explosive range, it would only take a very tiny spark indeed to initiate an explosion, since the reaction is so exothermic. It should not be a question of raising the temperature of the liquid gasoline, which would certainly take a lot of energy. I would expect a very tiny amount of energy would suffice to ignite the gasoline vapor if there was the right mixture with air. But there is both an upper explosive limit and a lower explosive limit with most combustible vapors. Edison 20:08, 15 June 2007 (UTC)

Here's the movie physics page on it. --Wirbelwindヴィルヴェルヴィント (talk) 23:34, 15 June 2007 (UTC)

If I wanted to blow up a car with a gun, I'd use an M79. --Carnildo 00:35, 17 June 2007 (UTC)

Yes, rescue professionals are even getting concerned about this whole matter. I remember reading about a statement by some German association of firefighters reminding people that cars only explode in movies. It seems that first aiders are so afraid that they way too often refrain from helping people inside cars after an accident. And a personal story: When I was a kid we came across a car with burning engine compartment at an autobahn (slowly burning! nowhere near exploding). The driver was standing outside, waiting for the fire truck. My parents happened to have small fire extinguisher in their car, and this turned out to be fully sufficient to safely extinguish the fire. The firefighters, arriving little later, had nothing to do. Simon A. 07:53, 16 June 2007 (UTC)

I'm glad that gas tanks don't readily explode, or I would be extra crispy right now. I once was in a car accident where my car ran over a street sign which tore the gas tank open. Fortunately, the gasoline didn't ignite. It was in an extremely dense patch of fog (hence the accident), which might have helped to dampen the spirits of any sparks. The fire department was apparently worried enough that they hosed down the entire area, however. So, while exploding gas tanks are possible, they are extremely rare. I've often thought that gas tanks should all be made of plastic, both to prevent sparks and to prevent failure from rust. The usual enemy of plastic, UV light, doesn't seem to be an issue for gas tanks (assuming your car has "the shiny side up and the rubber side down" more often than not). 19:21, 18 June 2007 (UTC)

Genetics question

This is all hypothetical...multipart question here
a) If everyone in the world married someone from a different race and had kids, would we be able to completely blur the line between different races so that there's just a complete "spectrum" with no clear line between races?
b) If so, and we continue to reproduce with people that look different from us, would we eventually merge that spectrum in to one race?
c) And if we did that, then each went to one country and stayed there, and reproduced only with people in that country, would we split back into different looking races? Would they look like the current (or indiginous) people of that country? 209.53.180.75 19:11, 15 June 2007 (UTC)

• A: Thats already the case, to some extent, in many parts of the world. Take Latin Americans for example. They come in every shape and color from Asian-looking to blonde white to brunette to brown to black and everything in between.
• C: I imagine they will seperate because of evolution. They may not look like the current or even indiginous (afterall even Native Americans and other indiginous populations are settlers) because the current populations may still carry traits they picked up elsewhere.  Adam2288  T  C  19:13, 15 June 2007 (UTC)

The current mainstream scientific theory of the origin of modern humans is that they came out of Africa, probably looking like one race, then after migrating to the ends of the earth, eventually became all the present races outside Africa, from Arabs to Swedes, Japanese to Navajo, by virtue of the survival value of certain traits helping people to survive and reproduce with different skull shapes, eye shapes, skin pigmentation, and body shapes (endomorph, mesomorph, ectomorph). So, yes, they might well split into different races again. But the original migrating humans went through population bottlenecks, wherein only a small number survined the harsh conditions, In modern society, and with an assumption that there were many millions in each country, some of the traits found in different populations might have less survival value. In Northern countries, Vitamin D supplements might decrease the need for fair skin. In tropical areas, SPF45 sunblock might decrease the need for dark skin. In the arctic north, central heating and Eddie Bauer winterwear might decrease the need to develop mesomorphic physiques like Eskimos, while in equatorial countries air conditioning might decrease the survival of thin, tall physiques.The re-emergence of similar physical characteristics found in different countries might not happen, or might tend to emerge more slowly, unless the original conditions of climate, population,availability of food, technology, disease, wildlife and plant life were reproduced. Edison 20:04, 15 June 2007 (UTC)

The notion of 'races' of humans is highly dubious (many would simply say wrong), as the "clear line between races" that you suggest does not in fact exist. That essentially is the response to (a) and (b). But if we entertain the idea of races for the sake of discussion for (c), then your question is very hard to answer, as Edison suggests. It may depend on exactly how well you isolated the populations, and how small the populations were that you started with, and were working with in each country. Are you shutting down all communications, modern technology, etc? Technology, medicine, etc, at least in the developed world, has largely negated the effect of evolution on humans as any selective pressures from the environment are very minor, if they are there at all; the main pressures these days would come from diseases. Take away the technologies that sustain us, and you'll reintroduce selective pressures. So it's a very hypothetical question. --jjron 14:53, 16 June 2007 (UTC)

The notion of "races" may be questionable, but anyone could tell a group of Swedes from a group of Japanese, from a group of Australian aborigines, etc. There are differences in skin color and bone structure of the face that are not self chosen ethnicity labels. This is despite the fact that there is far more variation within a national/racial group (said to be 85%)than between them (said to be 5%). Apparently because "race" calls up bad memories of racists, eugenics, obsolete theories of "subspecies" and Nazi theories of racial superiority, the term, however useful and well understood, is politically incorrect and we should say "population" or "cline" instead. The article Race is apparently a hotbed of controversy and editing by persons with various viewpoints, but still an important and interesting article.Edison 18:22, 16 June 2007 (UTC)

I'm surprised that no one has yet mentioned that locally distinctive traits may result from random drift rather than from selective advantage. —Tamfang 01:49, 17 June 2007 (UTC)

I think it would be interesting to do principal component analysis on the codistribution of alleles, and see how much of our genetic diversity can be described with a small number of dimensions. —Tamfang 01:49, 17 June 2007 (UTC)

We humans, I believe, must be inherently racist or races wouldn't exist today. Ozone 08:44, 17 June 2007 (UTC)

Whilst I don't entirely disagree about 'inherent racism' - I do strongly disagree with your 'proof'. People with different racial characteristics clearly evolved in response to local conditions - dark skins closer to the equator - lighter skins nearer the poles being the most obvious variation. Before we had routine long distance travel (maybe 200 years ago) it was almost impossible for people of one geographical area to mingle completely with people from another area - hence it was very unlikely that there would be significant amounts of interbreeding between people with these different characteristics. So we don't have to hypothesise an innate racism to explain that racial characteristics are still present in the population today - there has simply not been enough time - or enough travel between regions for this obvious diversity to average out.

As for whether racist tendancies are inherent - I'd say that they probably are, although we mostly don't like to admit it and our superior brains allow us to avoid it if we wish. It is known that all animals choose mates based on subtle (or not so subtle) genetic factors. From an evolutionary perspective, all life evolves to perpetuate it's genes into future generations. If your genes are to survive, it makes sense to mate with someone with a similar genetic mix to your own. So if you have a particular gene, then it will get passed on to your children with 100% certainty if your mate also has the same gene - so if you mate with someone from a different race then the odds of those race-related genes being present in your children and subsequent generations is smaller). Of course we are also programmed to avoid inbreeding - so this tendancy is limited. SteveBaker 14:49, 17 June 2007 (UTC)

Galaxy size

Why are galaxies relatively flat, i.e. the distance from one end of the spiral to the other is far more than the "thickness" of the spiral? Wouldn't you expect stars to be distributed spherically? The solar system is also flat (in terms of the planets' orbits), which I assume is for similar reasons on a smaller scale. But why? zafiroblue05 | Talk 22:53, 15 June 2007 (UTC)

Not all galaxies are spiral in shape, have a look at galaxy article. The three major forms are ellipticals, spirals, and irregulars. The flat spiral ones are a result of the galaxies rotating. Likewise with the solar system; the solar system formed when a rotating star exploded, and the heavier elements went onto form the planets (I think that's the reason why). Basically, a lot of things in the universe rotates. - Akamad 00:47, 16 June 2007 (UTC)

There is some information to be found at Galaxy formation. I don't see these as fully explaining it though, but I know little about the topic. Someguy1221 01:39, 16 June 2007 (UTC)

Just a conjecture by an amateur, but maybe this is somehow related to the conservation of angular momentum. When we have a central force, what makes angular momentum remain constant, a particle's position equations simplify greatly, because we know its trajectory will be flat. As we all know, gravity is a central force, therefore angular momentum remains constant. However, this is just a conjecture, I can't say precisely why these things happen. --Taraborn 08:01, 16 June 2007 (UTC)

Yes, conservation of angular momentum is likely to be part of the explanation. It also explains why accretion discs form around black holes and neutron stars; the shape of the solar system; and why water spirals around the plug hole when you empty your bath. However, galaxy formation is made more complicated by the effects of interstellar magnetic fields, shock waves from supernovae, and interactions between colliding galaxies. Gandalf61 11:58, 16 June 2007 (UTC)

Take a weight and tie it onto the end of a length os string - now spin it around your head as fast as you can. Notice that the path taken by the string is a flat circle (well, more or less - gravity makes it droop a bit - but in zero-g it would be a dead flat disk). You can't (for example) make it spin around such that the weight is far above your hand so that the string traces out a narrow cone. Well, it's pretty much the same with the stars in our galaxy. The central bulge of stars (which is much closer to being spherical) exerts a gravitational pull on each star just like the string pulls on the weight in our little experiment. So each star wants to orbit in a flat circle with the center of the circle being at the center of the galaxy. OK - but why are all of those circular orbits lined up in a more-or-less flat plane? Well, the stars are also attracting each other so if (say) two stars were orbiting the central core with orbits that were at an angle of (say) 2 degrees to each other, whenever the two stars came close - they'd attract each other - so the two orbits would gradually move to be in the same plane. Taken over all of the orbits of all of the stars, everything comes out flat...more or less. The spiral 'arms' come about the same kind of way - a bunch of stars on one side of the galaxy want to get together because of their mutual gravity - but the ball of stars that would naturally form is smooshed out and stretched into a curve because the stars further out to the edge are moving at a different speed to the ones close in to the galactic core. SteveBaker 23:34, 16 June 2007 (UTC)

If the galaxies were made of spinning disks at different angles, wouldn't this increase the chance of stars hitting each other, and would each disk interfere with the movement of the others :) HS7 14:05, 16 June 2007 (UTC)

Space is BIG - out here at the outer rim - our nearest star is four light years away - the odds of orbits causing things to collide would be very remote indeed...that's not the reason. SteveBaker 23:35, 16 June 2007 (UTC)