Wikipedia:Reference desk/Archives/Science/2018 September 26

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September 26

Neutron star cooldown time

According to the article on neutron stars the majority of the examples in our galaxy are already cold (or at least cold enough to not be detectable), but according to the article on white dwarfs, that type of star is not expected to cool down for at least trillions of years. Why do neutron stars cool down so much faster than white dwarfs? 182.0.151.47 (talk) 10:07, 26 September 2018 (UTC)

Maybe because while a white dwarf has radius on the order of 3000 6000 kilometres, a neutron star has a radius on the order of 10 kilometres? 194.174.73.80 (talk) 10:39, 26 September 2018 (UTC) Marco Pagliero Berlin

If anything, shouldn't a larger surface area lead to faster cooling, as you can get a greater heat flux through it? 139.194.67.236 (talk) 10:42, 26 September 2018 (UTC)

Of course not. A drop of molten steel gets cold faster than a ton of molten steel. Can you imagine why? 194.174.73.80 (talk) 11:51, 26 September 2018 (UTC) Marco Pagliero Berlin

Could it have something to do with the ratio between volume and surface area not being linear? 216.59.42.36 (talk) 18:05, 26 September 2018 (UTC)

The reason a droplet of steel cools faster than a ton of steel is because the surface area to mass ratio is greater in the droplet than in the ton due to square cube law. That will only work for comparisons of objects of similar densities. The neutron star (with a mass of about 1.5 solar masses) has far greater thermal energy to fit through its tiny surface area, compared to a white dwarf with its surface area several orders of magnitude greater, but a smaller mass of around 1 solar mass. So, the comparison of the radii doesn't answer the question. It just makes it more puzzling. 114.124.239.108 (talk) 23:29, 26 September 2018 (UTC)

You are right, I didn't think of this and IP139, I beg your pardon. So it can be that heat conduction in a neutron star is much faster and the whole star cools quickly, or in the opposite, conduction is very slow, so the surface cools while the core stays very hot. 194.174.73.80 (talk) 15:07, 27 September 2018 (UTC) Marco Pagliero Berlin

PS Neither nor: neutrino emission from the core cools the neutron star very quickly: https://academic.oup.com/mnras/article/324/3/725/1025405

"After a period of intense accretion the neutron star surface cools on a time scale of years." Source: Thermal and transport properties of neutron star matter.[1]

--Guy Macon (talk) 04:07, 27 September 2018 (UTC)

Thermal conductivity is roughly proportional to density. A neutron star has ~1×10⁸ times the density of a white dwarf, which contributes greatly to its ability to cool down much faster than a white dwarf. Dragons flight (talk) 19:44, 27 September 2018 (UTC)

Wouldn't the conductive heat loss be zero? It seems like the heat loss would be 100% radiative. BTW, do we know what neutronium looks like? I keep imagining shiny silver, but now that I think about it I am pretty sure that I got that from Larry Niven's books. Is a neutron star a black body radiator? --Guy Macon (talk) 21:41, 27 September 2018 (UTC)

From the surface it's presumably (almost) 100% radiative, but the heat has to get to the surface. Radiation isn't going to move far inside a neutron star. --Trovatore (talk) 21:43, 27 September 2018 (UTC)

Ah, but of course neutrinos can, as in Marco's link above. I guess that's "radiative", more or less? --Trovatore (talk) 23:41, 27 September 2018 (UTC)

Manhole cover in space?

Assuming that the shaft steel plate cap of the Pascal-B nuclear test actually survived its launch (it was never found), where would it be now? Is that fast enough to leave the solar system? Be ahead of Voyager? It would be ironic if the first contact an alien race had with humans was to be hit by a giant manhole cover travelling at 150,000 mph. SpinningSpark 17:21, 26 September 2018 (UTC)

So close to a nuclear explosion everything simply gets evaporated in some nanoseconds by the heat. The physical expansion is rather slow in comparison. Little Boy exploded 580 metres (1,900 ft) above the city of Hiroshima and caused temperatures of 6000 C° in a cone beneath it on the ground. The nuclear core is so hot that it starts rising up by heating the air around it so viciously that the resulting Convection airflow takes the core with it up. That is how the typical nuclear mushroom forms. --Kharon (talk) 18:15, 26 September 2018 (UTC)

• No, that's rubbish (as usual for your comments). Andy Dingley (talk) 18:17, 26 September 2018 (UTC)
• No, that's rubbish (as usual for your comments) II. --Doroletho (talk) 10:24, 27 September 2018 (UTC)

• It was definitely faster than Earth escape, definitely slower than Solar escape. However no-one knows how it would survive passing through the atmosphere. If it stayed in one piece, it's probably now in a solar orbit. If it broke in two, it probably broke further and burned up on ascent. Andy Dingley (talk) 18:16, 26 September 2018 (UTC)

Well thanks but no thanks for the flowers Andy. Even a 500 ton Tungsten (wolfram) plate would evaporate faster that the physical blast could reach it. Nuclear cores heat up to 100,000,000 C° in their chain reaction. If you put anything close enough to a chain reaction to theoretically push it to 66 km/s, it wont get pushed but turned to its Plasma (physics) state. --Kharon (talk) 18:32, 26 September 2018 (UTC)

The first frame of the high-speed film taken at the time has the thing airborne (that's how they got the lower bound on the speed) so that kind of makes you, well ... wrong. SpinningSpark 18:41, 26 September 2018 (UTC)

I doubt that but since there is no physical evidence (1 frame is no prove), you believe what you like. All i know is that 100,000,000 C° evaporates everything close to it faster than a 1957 highspeed camera can record. --Kharon (talk) 19:04, 26 September 2018 (UTC)

Wrong. Not even close to being correct. Read the reference that you yourself cited: "Two pulses of thermal radiation emerge from the fireball. The first pulse, which lasts about a tenth of a second, consists of radiation in the ultraviolet region. The second pulse which may last for several seconds, carries about 99 percent of the total thermal radiation energy."

In 1957 high speed cameras were commercially available that took 600 pictures per second, and the fastest cameras took 4.5 million pictures per second.[2]

Last time I checked 1/600 of a second is less than either 1/10 of a second or several seconds.

The force that blew the lid off, on the other hand arrived at the speed of sound. Nobody who understands the physics thinks that the nuke vaporized the lid. The question is whether it was traveling fast enough for atmospheric friction to vaporize it before it exited the atmosphere. If the answer is no, then a manhole cover beat Sputnik into space. --Guy Macon (talk) 20:32, 26 September 2018 (UTC)

Even if vaporization of the object does have time to proceed significantly before the shock wave reaches it, that will kickstart the acceleration of the object due to ablation, so no matter which way you slice it the object starts traveling very fast. 114.124.239.237 (talk) 23:48, 26 September 2018 (UTC)

• If you're that interested, you can read my work (and many others, originally Peter Hagelstein) in the '80s on X-ray lasers and the production of them via irradiation of thin metal foils by either lasers or nuclear devices. See the underground Excalibur tests. And these were thin foils, not manhole covers. So please don't give me this crap about 500 ton tungsten manhole covers turning instantly to plasma without moving. Andy Dingley (talk) 11:34, 27 September 2018 (UTC)

The Escape velocity from Earth's gravity is 11.186 km/s and from the Sun's gravity 617.5 km/s. Designer Dr. Robert Brownlee estimated that the explosion accelerated the plate to six times the former, which would not allow its remains to escape the Solar System. The Voyager space probes had escaped from the Sun's gravity by 1980 after flying past Jupiter, see [3]. DroneB (talk) 19:05, 26 September 2018 (UTC)

Sorry, this is plainly wrong. The velocity necessary to leave the Solar System from the Earth' surface is minimum 16.6 km/s though it depends on direction. This is so called total escape velocity. You need to learn some basic physics before making such claims. Ruslik_Zero 19:42, 26 September 2018 (UTC)

With the initial velocity of 66 km/s the final velocity upon leaving Solar System will be 85 km/s if the initial velocity was in the direction of the Earth's orbital motion. If it was launched in the direction opposite to the Earth's orbital motion, it will now orbit Sun in the retrograde direction somewhere between Earth and Mars. The real situation is more messy, of course. Ruslik_Zero 19:58, 26 September 2018 (UTC)

I am no nuclear physicists so i may have overlooked something in my "vaporizing"-answer. Luckily we have good articles to cite from like Effects of nuclear explosions:

(cite) "Energy from a nuclear explosive is initially released in several forms of penetrating radiation. When there is a surrounding material such as air, rock, or water, this radiation interacts with and rapidly heats it to an equilibrium temperature (i.e. so that the matter is at the same temperature as the atomic bomb's matter). This causes vaporization of surrounding material resulting in its rapid expansion." (cite end). So the "blast" is clearly just a secondary phenomenon of the vaporization, or rapid change to a plasma state, given the just 100 MILLION DEGREES Celsius or Kelvin(273.15° difference doesnt matter anymore at such numbers). The only chance that this "Manhole cover" had is when it was far enough away and shielded from the radiation. I concluded for an impulse from 0 to 55Km/s it must have been very close. --Kharon (talk) 02:45, 27 September 2018 (UTC)

With on object like a manhole cover, the radiation is first incident on the top layer of object, heating it directly and very, very quickly. This causes that layer to immediately vaporize and expand as a cloud of hot gas (a process known as ablation), pushing the remaining non-vaporized portion of the cover in the direction away from the blast. With the top layer of the manhole cover removed, the second layer is now irradiated and vaporized, which accelerates the bulk of the manhole cover further. As the cover moves further from the blast, the degree of radiation it is subject to is reduced very rapidly (due to the inverse square law). This allows the object to be accelerated to a very high speed without the bulk of its matter being heated to vaporization. 202.155.85.18 (talk) 03:27, 27 September 2018 (UTC)

• There are too many factors involved to sum this up in a simple answer. First of all, when talking about heat, we're talking about energy. When talking about vaporization, we're talking about energy transfer. How that energy transfers all depends on the method of transfer and the material it transfers to (ie: absorption and dissipation). The initial release of energy comes in the form of a blast from the detonation charges, followed by an intense radiation (light) wave that outruns the supersonic shockwave. At enough intensities, this light exerts more pressure than heat, and this is especially true for reflective objects like metal. (There are actually laser meters that measuer power by the pressure the beams exerts on the mirror.) In close enough proximity, this light blast will arrive at roughly the same time as the concussion, and, since the metal does not absorb radiation well, nor the thermal heat fast enough, it is conceivable it would absorb most of that energy as kinetic and become airborne.

At mach 2 you can expect to be heated to around 200 degrees F. At mach 5 you're up to about 1325 degrees, hitting 4000 by mach 8. Temperatures of the nose of the Apollo capsules reached up to 19,800 degrees (the temp of a blue star). While it is extremely plausible such a thing could be blasted far from the explosion, that it would survive traveling through the atmosphere for any distance before ablating to death or exploding itself like that meteor over Russia a few years ago, is highly unlikely. (Or that it would retain it's energy long enough, as the atmosphere will dissipate it quickly for such a small object. Remember, skin pressure goes up dramatically once you go supersonic, because the air just can't get out of the way fast enough. Zaereth (talk) 03:25, 27 September 2018 (UTC)

You can clearly see at 1:00 on this video: https://www.youtube.com/watch?v=KQp1ox-SdRIt=60 that the expanding blast wave hits nearby objects first (you can see it expanding at the speed of sound) and that the high temperatures come a fraction of a second later (you can see this when the fireball becomes much brighter). Thus a manhole cover that is blown into space by the blast wave is long gone before the high temperatures you keep SHOUTING ABOUT IN ALL CAPS AS IF WHAT MATTERS IS HOW HOT AND NOT HOW SOON reach the place where the manhole cover used to be. And of course you assumed that "the speed of a 1957 highspeed camera" was much lower than the actual figure of 4,500,000 FRAMES PER SECOND.

The distance between the bomb and the cover was 500 feet.

The camera was recording 1 frame per millisecond.

The blast wave hit the manhole cover 506 milliseconds after the bomb went off. Note that being at the bottom of a shaft magnifies the blast wave compared to a free-air detonation, but does little to concentrate the thermal effects, most of which end up vaporizing a big hole 500 feet down.

The welds held for some short amount of time, allowing the pressure to increase, then the manhole cover was launched at a calculated velocity of 180 feet per millisecond.

By the time the thermal maximum happened, roughly 1000 milliseconds later, the manhole cover is calculated to have been roughly 35 miles up and still climbing, having passed through the stratosphere and into the mesosphere.

The scientist who did that calculation concluded that it was going too fast to burn up before reaching outer space: "I was in the business and did my own missile launches. I realized that that piece of iron didn’t have time to burn".[4] This is easily confirmed by examining nickle-iron meteorites that are considerably smaller and faster than the manhole cover, and arrive on the ground icy cold with zero evidence of surface melting.

All of which is just a long-winded way of saying "Kharon is wrong". Zaereth, on the other hand, is likely to be right. It would be very interesting to see the calculations for a gun attempting to fire a metal projectile into space. We have an article on this: Space gun. --Guy Macon (talk) 04:15, 27 September 2018 (UTC)

Not sure if you're talking to me, but you make a good point. You also have to take into account that air pressure and speed all play a role. Meteors that are traveling fast enough may explode before ever reaching the ground, such as the one in Tunguska, or those that hit Jupiter a few years ago. Those that are small enough may ablate completely. Ablation transfers little to almost no heat to the main body, which is why it is used for cooling space vehicles. These are typically 80,000 mph +, and not every meteor is traveling that fast relative to the Earth. Then there is the physical stress of accelerating that at that rate, which I know from experience can lead to an object completely spalling before traveling more than a few yards. It seems entirely plausible either way, but, like I said, too many factors involved to sum it up so simply. I do agree that vaporization from heat of the blast is the least likely. Zaereth (talk) 04:13, 27 September 2018 (UTC)

For whatever it is worth, my physicist's intuition is that the scenario described would not be capable of reaching orbit, too much atmospheric drag relative to the mass and speed of the projectile. However, when I tried to do a rough estimation of the drag forces involved, my calculation made it seem like a much closer thing than I had expected. For 900 kg disk 1.2 m across and 60 km/s initial velocity, I estimated that it would reach an apex of more than 35 km in altitude, which is rather quite a lot. In my rough calculation, the disk slows from 60 km/s to only 2 km/s by 2 seconds after launch, but has already hit an altitude of 10 km by that point. Though the cover would likely be somewhat ablated by the initial explosion and the subsequent hypersonic drag, if the initial acceleration of the launch doesn't tear the cover apart, I would guess that the cover would remain mostly intact until it returned to the Earth. Dragons flight (talk) 14:14, 27 September 2018 (UTC)

My educated guess is that if the steel was brittle it would break up, but if it was ductile it would stay intact, possibly bent or even folded. My uneducated guess is that is went tens of kilometers up and then came down somewhere, but don't ask me to prove it. --Guy Macon (talk) 15:53, 27 September 2018 (UTC)

My physicist's intuition leads me to a slightly different result. Since 66 km/s vastly exceeds the speed of sound in the air, the air in front of the disk will be simply compressed and accelerated. This process will look like an inelastic collision between the disk of 900 kg with the air column above it with about 11000 kg mass. The momentum conservation means that the final speed will be around 5.5 km/s with about 88% of the kinetic energy dissipating into heat. This is about 17 170 MJ per kg. The naive calculation using 600 J/kg/K thermal heat capacity of the air leads to about 30,000 300,000 K temperature. In reality (taking into account the increase of thermal capacity with temperature and ionization) it will be probably lower - around 10,000 50,000 K. Ruslik_Zero 21:05, 27 September 2018 (UTC)

That sounds about right to me. Do you have any guesses as to why space guns aren't rejected at the planning stage for the same reasons? --Guy Macon (talk) 21:47, 27 September 2018 (UTC)

I actually made an error: it should be around 170 MJ per kg. And all guns that I heard of involved velocities less than 4 km/s, which means that my assumption about inelastic collision may not be right. Ruslik_Zero 18:00, 28 September 2018 (UTC)

1) Project Orion found that coating the pusher plate with oil resulted in almost no ablation so I wonder if they could have gotten that (much smaller) plate to survive with a little such preparation. 2) the space gun design that I read about a while back used a pointy projectile with an ablative coating. Energy and momentum transfer was modelled as elastic collisions between air molecules and the oblique projectile surface. 3) Streak cameras and rotating mirror cameras (High-speed_photography#Rotating_mirror) even in the 1950s could film at millions of fps so it's unfortunate that they didn't set up something like that. 173.228.123.166 (talk) 04:36, 29 September 2018 (UTC)

Fascinating story. I followed back two refs from the article to the blog (well, pre-blog) that everything is based on: [5] Note of course that the calculation he cites was either manual or damn near, so it is possible now to do far more elegant numerical simulations that might shed some light on the topic. Alas, I found nothing in ArXiv about "Pascal B". Anybody good at computer science and wanna get published? Wnt (talk) 23:41, 29 September 2018 (UTC)

Plant identification help

Unidentified plant

The flower

The leaves

Could anyone please help identify this plant, growing in a Sussex garden in September? The stalk is a distinctive red, matching the colour of the flowers. Shallow, chalky soil, full sun, facing south. This specimen is about 2'6" tall. DuncanHill (talk) 18:34, 26 September 2018 (UTC)

This plant is Leycesteria formosa, known commonly in the UK as Himalayan honeysuckle or Pheasant berry. Richard Avery (talk) 07:10, 27 September 2018 (UTC)

Splendid, thank you. DuncanHill (talk) 14:29, 27 September 2018 (UTC)

Dutasteride and erectile dysfunction

Please consult your doctor or pharmacist -- we don't answer this kind of question here

After how many days of daily Dutasteride 0.5 mg administration will the erectile dysfunction become irreversible? — Preceding unsigned comment added by 2401:FA00:C:702:DC27:9C1A:E16F:22E3 (talk) 21:21, 26 September 2018 (UTC) Didn't you ask that question already? ←Baseball Bugs What's up, Doc? carrots→ 22:38, 26 September 2018 (UTC) This is a different one and the previous question was not answered in a satisfactory manner. — Preceding unsigned comment added by 2401:FA00:C:702:DC27:9C1A:E16F:22E3 (talk) 23:02, 26 September 2018 (UTC) When are you going to see your doctor about this? ←Baseball Bugs What's up, Doc? carrots→ 00:23, 27 September 2018 (UTC)