Starship Asterisk*

Chris Peterson wrote: ↑Sun May 16, 2021 3:07 pm
It is important not to conflate "fluid" and "liquid".

https://www.etymonline.com/search?q=fluid wrote:

affluent

---

fluid (adj.) early 15c., "liquid, capable of flowing," from Old French fluide (14c.) and directly from Latin fluidus "fluid, flowing, moist," from fluere "to flow" (see fluent). Figurative use, of non-material things, "not fixed or rigid," from 1640s.

fluent (adj.) 1580s, "flowing freely" (of water), also, of speakers, "able and nimble in the use of words," from Latin fluentem (nominative fluens) "lax, relaxed," figuratively "flowing, fluent," present participle of fluere "to flow, stream, run, melt,"

affluent (adj.) early-15c., "abounding in, copious" (of God's grace); mid-15c. "flowing to" (of liquids), from Old French afluent (14c.) or directly from Latin affluentem (nominative affluens) "abounding, rich, copious," literally "flowing toward," present participle of affluere "flow toward," from assimilated form of ad "to" (see ad-) + fluere "to flow" (see fluent). The especial sense of "abounding in wealth or possessions" is from 1753.

https://www.etymonline.com/search?q=liquid wrote:

liquidate

---

liquid (adj.) late 14c., "flowing, capable of flowing; neither solid nor gaseous," from Old French liquide "liquid, running" (13c.), from Latin liquidus "fluid, liquid, moist," figuratively "flowing, continuing," also of sounds and voices, from liquere "be fluid," related to liqui "to melt, flow," from PIE *wleik- "to flow, run."

liquidate (v.) 1570s, of accounts, "to reduce to order, to set out clearly", from Late Latin or Medieval Latin liquidatus, past participle of liquidare "to melt, make liquid, make clear, clarify," from Latin liquidus "fluid, liquid, moist". Sense of "clear away" (a debt) first recorded 1755. The meaning "wipe out, kill" is from 1924, possibly from Russian likvidirovat, ultimately from the Latin word.

neufer wrote: ↑Sun May 16, 2021 3:02 pm

Click to play embedded YouTube video.

Chris Peterson wrote: ↑Sun May 16, 2021 1:55 pm

VictorBorun wrote: ↑Sun May 16, 2021 6:29 am
Is a Champagne glass rigid? For a 1000 years it can be: it can hold its shape against some external force. For a trillion years it's rather liquid: it would become a glass puddle.

Rigidness is well defined. Fluids are not rigid. No fluid material can hold its shape against an external force, it can merely alter its shape more slowly. A rigid body does not do that. (FWIW, glass is not fluid, and will not change its shape over time; that is an urban myth.)

Chris Peterson wrote: ↑Sun May 16, 2021 1:55 pm

VictorBorun wrote: ↑Sun May 16, 2021 6:29 am
Is a Champagne glass rigid? For a 1000 years it can be: it can hold its shape against some external force. For a trillion years it's rather liquid: it would become a glass puddle.

Rigidness is well defined. Fluids are not rigid. No fluid material can hold its shape against an external force, it can merely alter its shape more slowly. A rigid body does not do that. (FWIW, glass is not fluid, and will not change its shape over time; that is an urban myth.)

VictorBorun wrote: ↑Sun May 16, 2021 6:29 am Every rigidness implies a context.

Is a Champagne glass rigid?
For a 1000 years it can be: it can hold its shape against some external force.
For a trillion years it's rather liquid: it would become a glass puddle.
For alpha particles it's rather thin media: they would mostly go through.

Saturn's ring system is rigid in the sense that it holds against tidal forces from asteroids, satellites, Sun.
The poor thing is raining on Saturn and may drain in a 100 million years though.
And it is an easy 100 meters walk for a tourist in a space suit crossing through all the snow from one surface to the other.

VictorBorun wrote: ↑Sat May 15, 2021 7:33 am I thought we may call a thing somewhat rigid if it can hold its shape against some external force.
Like a gas disk of a disk galaxy.
Its viscousity punishes any non-circular or not-in-the-main-plane orbits of any part of gas; therefore the stars and stellar clusters tend to form within the disk and stay within the disk for a while.
The rotation of the disk blocks the sinking into the core bulge.
The rotation of far parts is lagging behind but that level of friction is tolerated. (Even Saturn's snowy ring obeying Keppler's law T~R^(3/2) is not disrupted, and for a galaxy with a constant orbital velocity, T~R, thanks to dark halo, that source of turmoil is even weaker.)

So the disk can survive as long as it rotates, can't it?

neufer wrote: ↑Fri May 14, 2021 3:15 pm

Chris Peterson wrote: ↑Fri May 14, 2021 1:08 pm

VictorBorun wrote: ↑Fri May 14, 2021 4:20 am
So a pair of disk galaxies can realistically arrest each other's rotation and like a pair of fighting circular saws lose the rigidness of their disks :)

Galaxies are not rigid bodies, so in the strictest sense, dynamically, they don't rotate. Rather, the stars and gas that make them up are, individually, rotating within the gravitational field that all contribute to.

Stars and gas are not rigid bodies, so in the strictest sense, dynamically, they don't rotate. Rather, the stars and gas that make up galaxies are, individually, orbiting within the gravitational field to which all contribute.

Chris Peterson wrote: ↑Fri May 14, 2021 1:08 pm

VictorBorun wrote: ↑Fri May 14, 2021 4:20 am
So a pair of disk galaxies can realistically arrest each other's rotation and like a pair of fighting circular saws lose the rigidness of their disks

Galaxies are not rigid bodies, so in the strictest sense, dynamically, they don't rotate. Rather, the stars and gas that make them up are, individually, rotating within the gravitational field that all contribute to.

VictorBorun wrote: ↑Fri May 14, 2021 4:20 am So a pair of disk galaxies can realistically arrest each other's rotation and like a pair of fighting circular saws lose the rigidness of their disks :)

VictorBorun wrote: ↑Fri May 14, 2021 4:20 am
So a pair of disk galaxies can realistically arrest each other's rotation and like a pair of fighting circular saws lose the rigidness of their disks

COSMOS - The SAO Encyclopedia of Astronomy wrote:

Unlike spiral galaxies, elliptical galaxies are not supported by rotation. The orbits of the constituent stars are random and often very elongated, leading to a shape for the galaxy determined by the speed of the stars in each direction. Faster moving stars can travel further before they are turned back by gravity, resulting in the creation of the long axis of the elliptical galaxy in the direction these stars are moving.

Ann wrote: ↑Thu May 13, 2021 10:18 am

NGC 2146 Ken Crawford.png

NGC 2146, whose twisted shape suggests that the galaxy is a merger product.
Photo: Ken Crawford.

Check out the picture of galaxy NGC 2146. Its weird shape is certainly the product of a galactic interaction, and maybe even a collision. You may want to check out the image caption at Anne's Astronomy News:

The most likely explanation {for the galaxy's shape} is that the strong tidal interaction of a companion galaxy is gravitationally perturbing NGC 2146 and distorting the orbits of many of its stars (possibly NGC 2146a, which is a smaller, nearby barred spiral galaxy, or to a collision with another galaxy which has merged with the larger galaxy, and is therefore no longer visible as a separate object). It is probable that we are currently witnessing the end stages of a process which started about 80 million years ago.

So it may have taken 80 million years for NGC 2146 to change from a normal-looking galaxy into its current shape.

Elliptical galaxy NGC 3923 with shells. Photo: Rolf Wahl Olsen.

---

Elliptical galaxy NGC 1316 with dust bunnies. Image: NASA, ESA, and The Hubble Heritage Team (STScI/AURA).

---

But a galaxy that has been strongly disturbed will eventually "relax" and look more symmetrical. It is certainly possible that the galactic merger product will become an elliptical galaxy, perhaps surrounded by some "fuzz". Elliptical galaxy NGC 3923 is surrounded by shells, which must be products of mergers of some kind. An elliptical galaxy may also contain conspicuous dust bunnies which may also be the remnants of a collision, as in NGC 1316.
Ann

Ann wrote: ↑Thu May 13, 2021 6:20 am
Got to show you my favorite picture of NGC 4631 and NGC 4656. It is the picture at right by Mark Hanson, and it has been picked as the "Space Porn of the Day Picture". Do click on the image to see a larger size of it!

Ann

VictorBorun wrote: ↑Thu May 13, 2021 6:58 am I still wonder how do the speed of a galaxy collision compares to the rotation speed in a disk galaxy.
Can a galaxy stop rotating and get weirdly shaped?
All in 100 million years before the blue stars die out.

The most likely explanation {for the galaxy's shape} is that the strong tidal interaction of a companion galaxy is gravitationally perturbing NGC 2146 and distorting the orbits of many of its stars (possibly NGC 2146a, which is a smaller, nearby barred spiral galaxy, or to a collision with another galaxy which has merged with the larger galaxy, and is therefore no longer visible as a separate object). It is probable that we are currently witnessing the end stages of a process which started about 80 million years ago.