Wikipedia:Reference desk/Archives/Science/2019 August 21

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August 21

Atmospheres on objects at the heliopause

Should the near absence of solar wind there allow much smaller planetoids to maintain an atmosphere ? And containing lighter elements ? (At those temps, I'd expect a hydrogen/helium atmosphere, and even making it warm enough for those to be gases may require tidal heating from a moon.) SinisterLefty (talk) 04:35, 21 August 2019 (UTC)

It depends on the mass of the planetoid. Ruslik_Zero 19:52, 21 August 2019 (UTC)

Can white dwarf contain a neutron star at its center? Or vice versa

With the much high pressure that i presume occurs deeper inside a white dwarf, I wonder if matter like in a neutron star could form(much denser than white dwarf matter, held from further compression by neutron degeneracy pressure) at the central core, especially in higher mass white dwarfes. Vice versa, maybe I should ask instead if a neutron star could have an outer layer of white dwarf matter, so much less dense than neutron star material that it would be analogous to an atmosphere for a planet(although actually far denser than rock). ThanksRich (talk) 10:48, 21 August 2019 (UTC)

I doubt that the two can co-exist. The immense gravity would crush normal matter into neutrons, unless that normal matter was actually in orbit. So, I could imagine something like Saturn's rings, at least in the short term, but don't know if it could last (it might end up more like an accretion disk around a black hole). Perhaps if the distance is high enough, rings would be more stable. SinisterLefty (talk) 11:48, 21 August 2019 (UTC)

A white dwarf cannot "contain" a neutron star since stars cannot contain other stars. Their composition is quite different: white dwarfs have cores containing electron-degenerate matter, basically separated disordered electrons, neutrons and protons with a lot of kinetic energy, much like a gas, held apart by electron degeneracy pressure. Neutron stars, however, have cores composed of neutronium; pressures are so high that the protons and electrons combine to form neutrons, or at least that is one theory of their composition; they are held apart by neutron degeneracy pressure. So their compositions are very different, as they experience different amounts of gravitational pressure. --Jules (Mrjulesd) 12:24, 21 August 2019 (UTC)

I would like to add that we don't have clear ideas of the composition of these bodies, only theories. But there is no theory that a white dwarf can form a neutron star, but only a black dwarf. I would imagine that in the outer layers they could both contain similar matter. The atmosphere of a neutron star is only a few micrometers thick! --Jules (Mrjulesd) 12:38, 21 August 2019 (UTC)

• "since stars cannot contain other stars", this is likely to be incorrect as a generalised statement: Thorne–Żytkow object. Likely true in the specific case asked about here though. Fgf10 (talk) 08:46, 22 August 2019 (UTC)

• Duly noted. --Jules (Mrjulesd) 11:20, 22 August 2019 (UTC)

The outer part of a neutron star consists of an electron degenerate matter much like a white dwarf. But this layer is very thin (probably a few meters). So, in some sense any neutron star sits inside a white dwarf. Ruslik_Zero 20:03, 21 August 2019 (UTC)

There is a time when a white dwarf might have a neutron star in the middle, and that is when it is collapsing. If the star is heavy just below the Chandrasekhar limit, and spinning slows or more mass is added, it might exceed the limit and form a neutron star. This stage would only last for seconds tough and so much energy is released that it becomes a supernova. Graeme Bartlett (talk) 23:24, 21 August 2019 (UTC)

You may also be interested in Thorne–Żytkow object. Graeme Bartlett (talk) 04:20, 23 August 2019 (UTC)

One possible result of the merger of a white dwarf and a nuetron star is an R Coronae Borealis variable. SinisterLefty (talk) 12:12, 22 August 2019 (UTC)

No, R Coronae Borealis variable stars may be result of a merge of two white dwarfs only. Ruslik_Zero 20:13, 22 August 2019 (UTC)

Not according to the last sentence in Thorne–Żytkow object#Formation, which includes a reference. SinisterLefty (talk) 12:09, 23 August 2019 (UTC)

Mystery of the sliding fish

In a fish pond, we noticed a strange phenomenon: every fish that approached the lighting electricity cable - first 'tasted it', then immediately slid on its side and glided over it (the two images demonstrate it). There is enough space above the cable for the fish to travel freely. This happened with every single fish, and also in another location at the same pond. How can this behavior be explained? Gil_mo (talk) 19:23, 21 August 2019 (UTC)

• 
  sliding fish 1
• 
  sliding fish 2

Could it have something to do with the lateral line ? They could use that to sense the strange object more closely. Another possible explanation is that the wire puts out enough of a magnetic field to surpass the Earth's magnetic field, at that spot, which confuses the fish about what is up and down. I suggest turning off the electricity to see how the fish behavior changes, as that should kill the magnetic field. SinisterLefty (talk) 19:57, 21 August 2019 (UTC)

Great idea, I'll have to go there at daytime to observe. Stay tuned... Gil_mo (talk) 21:05, 21 August 2019 (UTC)

Fish appear to steer with magnets. Alansplodge (talk) 21:41, 21 August 2019 (UTC)

A few species of fishes are thought to use magnetic field as a clue for travel orientation, but even them are not using it to tell up from down. Only a couple of bacteria do that, AFAIK.

Gem fr (talk) 08:17, 22 August 2019 (UTC)

Somehow I doubt if all 33,600 known species of fish have been studied for this [1]. But it isn't actually necessary that they use the magnetic field to tell up from down. Let's say a fish is trying to swim west, and does so by positioning himself so his right side faces magnetic north. If some local magnetic field changes magnetic north to be below the fish, then one side would turn toward the bottom automatically. This is somewhat similar to how airplane pilots flying on instruments sometimes turn the plane sideways when those instruments give false readings. SinisterLefty (talk) 11:56, 22 August 2019 (UTC)

BTW, do the fish always swim by the wire with the same side up ? SinisterLefty (talk) 23:10, 21 August 2019 (UTC)

Will check. Gil_mo (talk) 04:44, 22 August 2019 (UTC)

more importantly:

1. does this happens only when the switch is on, and stop when the switch is off?
2. does this happens on AC just like DC?
3. is there a threshold (voltage or current) for this to happen?

Gem fr (talk) 08:29, 22 August 2019 (UTC)

To reply to this I'll have to have access to the pond main power, which I don't. All I can do is observe what happens once they shut it off. Gil_mo (talk) 11:48, 22 August 2019 (UTC)

I believe it to be a fairly common for fish to rub their sides on the bottom of a tank or on the sea floor to scrape off bacteria and parasites. I would suspect that if you experimented and put a non conducting wire of the same diameter and texture in the same place, the fish would continue to produce the same behaviour. Anton 81.131.40.58 (talk) 08:30, 22 August 2019 (UTC)

Ok, the fish behave the exact same even when electricity turned off. Gil_mo (talk) 19:13, 24 August 2019 (UTC)

good to know, although not really a surprise. I guess there is no light either, so it also is not the cause. Gem fr (talk) 19:21, 24 August 2019 (UTC)

Well, is there a marine biologist in the house? Gil_mo (talk) 20:45, 24 August 2019 (UTC)

Do they always swim by with the same side up ? SinisterLefty (talk) 04:16, 25 August 2019 (UTC)

I didn't check this per a single fish. But in general there's no rule for that. Gil_mo (talk) 06:42, 25 August 2019 (UTC)

Could be they suspect the thing could be a danger, a living thing in hiding, which would explain both they try to taste it before passing by, and keep an eye on it (literally) while they do. Gem fr (talk) 07:37, 27 August 2019 (UTC)

I guess they must be familiar with it, having done this routine for thousands of times before... I think it is time for a biologist to step in :) Gil_mo (talk) 21:06, 27 August 2019 (UTC)