Wikipedia:Reference desk/Archives/Science/2019 April 28

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April 28

Photo of black hole in M87

The photo of the black hole in M87 - can someone give an indication of the angle that subtends, in everyday terms? Something like "the size of a dime 1 mile away". Bubba73 ^{You talkin' to me?} 00:24, 28 April 2019 (UTC)

If you mean the black hole itself, it's much too small to see without super duper telescopes. According to the article, the black hole's Schwarzchild radius is 5.9e-4 parsecs so the diameter is 2x that, about 1.2e-3 parsecs. M87's distance from Earth is around 16e6 parsecs so the angle subtended by the black hole would be around 7.5e-11 radians. A US dime is about 18mm so the equivalent distance would be 18e-3/7.5e-11 = 2.4e8 meters, about 2/3 of the distance to the Moon. 173.228.123.207 (talk) 01:27, 28 April 2019 (UTC)

Thanks! Bubba73 ^{You talkin' to me?} 01:40, 28 April 2019 (UTC)

The black hole that visible in the image is not the Schwarzschild sphere. It is the photon sphere, which has radius of 3/2 of that of Schwarzschild sphere. Ruslik_Zero 20:40, 28 April 2019 (UTC)

Yes, I knew about that. Bubba73 ^{You talkin' to me?} 23:02, 29 April 2019 (UTC)

I didn't know about it! Thanks Ruslik0. 173.228.123.207 (talk) 07:32, 30 April 2019 (UTC)

"Still, it’s tiny from a vantage point on Earth, less than 50 microarcseconds wide in the sky, which makes it about as hard to see as a donut placed on the moon."[1] Also, it is technically not a "photo" as that implies a camera attached to an optical telescope. This is an image generated from radio telescope signals. --mikeu ^talk 14:57, 5 May 2019 (UTC)

Camouflage and chromatophores

When an animal (such as chameleon or squid) rapidly does crypsis, does it mean it has color vision to identify the background? Also, is it known how exactly color matching through chromatophores for blending into background occurs in such cases? 212.180.235.46 (talk) 07:55, 28 April 2019 (UTC)

Quick side note: as our article at chameleon indicates, for the most part, chameleons change colour for the purposes of signalling more so than camouflage. Matt Deres (talk) 18:57, 29 April 2019 (UTC)

There are two different camouflage cases:

1. An animal which must change color/pattern to match a wide range of backgrounds. In this case, then yes, they would need good color vision to be able to determine the colors and patterns they need to match. These are likely to have chromatophores.

2. An animal which only needs to match a small, fixed set of colors and patterns. For example, a fish that sometimes rests on the sand at the bottom of the sea might have a sand-colored back, while maybe having a light colored belly for when it swims near the surface, to make it blend in with the sky when viewed from below. Such an animal wouldn't need colored vision, at least not for camo purposes.

As for how it's done, it must require lots of nerve cells to map the observed colors and patterns to a map of open and closed chromophores, and then deliver those signals to the skin throughout the body. Presumably none of this requires conscious thought, other than perhaps the initial thought "blend in", say when they spot a predator nearby. You might compare with our digestive system, which makes all sorts of "decisions" about how to digest or vomit up food, without any conscious thought from us. It's releasing enzymes, performing contractions to break up food and move it along, and opening and closing valves, all by itself. SinisterLefty (talk) 19:28, 29 April 2019 (UTC)

Heat and Sound Conduction Through Air

Heat is conducted through air very slowly, on the order of one foot per second. Sound is conducted through air very rapidly, on the order of one thousand feet per second. Why the big difference? Both phenomena depend on the movement of air molecules. Indexguy (talk) 09:23, 28 April 2019 (UTC)

Not necessarily. Consider the initial velocity of the heat source. ←Baseball Bugs ^{What's up, Doc?} carrots→ 10:42, 28 April 2019 (UTC)

The phase velocity and group velocity of a wave (such as a sound wave), have little relation to the particle velocity of the medium (air) due to the passage of the wave though it. The particle velocity is typically much smaller than the wave velocity (see speed of electricity for another example). catslash (talk) 18:04, 28 April 2019 (UTC)

On further reflection, the speed of thermal conduction is likewise a different thing from the thermal velocity, so that was a poor answer. catslash (talk) 18:14, 28 April 2019 (UTC)

Conduction (diffusion) is not the main mode of heat transfer in air. Your reply was quite good in pinpointing the error in the "sound = motion of molecules" view. Tigraan^{Click here to contact me} 11:00, 29 April 2019 (UTC)

You both are quite wrong. The sound speed in a gas is directly proportional to the average speed of molecules. In fact these two parameters are not that different from each other

$c_{s}={\sqrt {\frac {\gamma }{3}}}{\sqrt {\overline {v^{2}}}}$,

the speed of sound being slightly smaller than the average speed of molecules. Ruslik_Zero 20:24, 30 April 2019 (UTC)

The sound speed in a gas is directly proportional to the average speed of molecules. Yes, but not because the molecules move along the pressure wave (which was probably the assumption behind OP's question and needed to be dispelled). Tigraan^{Click here to contact me} 09:29, 2 May 2019 (UTC)

• Both phenomena depend on the movement of air molecules is a misleading way of thinking about it.

The dominant form of heat transfer in air (for common conditions) is convection, where large chunks of air carry the heat they contain when they move around. How fast heat transfer happens (which is rarely quantifiable by a speed) is roughly linked to how well individual molecules move in the room, which is relatively slow: even though the thermal velocity of a single air molecule is a few hundreds of m/s, it keeps bouncing back and forth by collisions with other molecules, so its net motion in any given direction is "quite slow".

Sound, on the other hand, depend on the propagation of a pressure wave. As Catslash wrote, that does not mean the molecules need to travel along the wave. Tigraan^{Click here to contact me} 11:00, 29 April 2019 (UTC)

An even faster method of heat transfer is thermal radiation, which can occur at the speed of light. It becomes more of a factor with large temperature differences between objects, and with a transparent medium (or a vacuum) between the objects, so not a smoke-filled room during a fire, for example. SinisterLefty (talk) 19:38, 29 April 2019 (UTC)

For an example of how slow thermal conduction would be in air, consider Styrofoam, which is designed to prevent convection and limit thermal radiation, leaving only conduction as the major mode of heat transfer. And it's so slow it makes a good insulator. SinisterLefty (talk) 20:52, 30 April 2019 (UTC)

Helium hydride ions

If the ion consists of a hydrogen atom bonded to a helium atom with a missing electron, than shouldn't it just be a lithium cation since it has three protons in the nucleus? --Puzzledvegetable (talk) 14:40, 28 April 2019 (UTC)

No because sharing an electron cloud is not the same thing as sharing a nucleus which would require nuclear fusion, not just a chemical bond. Sagittarian Milky Way (talk) 15:00, 28 April 2019 (UTC)

I did a little research, and found what my error was. I incorrectly assumed that an ion must be a charged atom. I didn't realise it can be a charged molecule as well.--Puzzledvegetable (talk) 16:46, 28 April 2019 (UTC)

Bumblebee species classification

This bumblebee was found today in Tel Aviv. Many of these are nesting (?) now in dry bamboo shoots in the garden. Help is needed to classify the species. Etan J. Tal^(talk) 17:19, 28 April 2019 (UTC)

• Looks like a carpenter bee to me. Abductive (reasoning) 03:45, 29 April 2019 (UTC)

• Probably Xylocopa sulcatipes or Xylocopa pubescens. Abductive (reasoning) 03:56, 29 April 2019 (UTC)

Thank you! Etan J. Tal^(talk) 06:26, 27 May 2019 (UTC)