Explanation: Closest to the Sun on March 1, and closest to planet Earth on April 23, this Comet ATLAS (C/2020 R4) shows a faint greenish coma and short tail in this pretty, telescopic field of view. Captured at its position on May 5, the comet was within the boundaries of northern constellation Canes Venatici (the Hunting Dogs), and near the line-of-sight to intriguing background galaxies popularly known as the Whale and the Hockey Stick. Cetacean in appearance but Milky Way sized, NGC 4631 is a spiral galaxy seen edge-on at the top right, some 25 million light-years away. NGC 4656/7 sports the bent-stick shape of interacting galaxies below and left of NGC 4631. In fact, the distortions and mingling trails of gas detected at other wavelengths suggest the cosmic Whale and Hockey Stick have had close encounters with each other in their distant past. Outbound and only about 7 light-minutes from Earth this Comet ATLAS should revisit the inner solar system in just under 1,000 years.
Re: APOD: The Comet, the Whale, and the Hockey... (2021 May 13)
Posted: Thu May 13, 2021 6:20 am
by Ann
The Comet, the Whale, and the Hockey Stick.
Image Credit & Copyright: Grand Mesa Observatory, Terry Hancock / Tom Masterson
NGC 4631 (left) and NGC 4656 (right). Photo: Mark Hanson.
Click on the image to see a larger size.
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!
Note that NGC 4631 has a bright (but dust-reddened) yellow center. It has a blue disk peppered with pink emission nebulas (click on the image at right to see them). Note the soft sometimes bluish but often beige light that seems to envelop the entire galaxy. That is the thick disk of NGC 4631, which may have been puffed up due to interactions between the large galaxy and its small companion galaxy, NGC 4627.
Note how different NGC 4656 is. There is just a hint of yellow at its center. There are no visible pink emission nebulas. There is no thick disk enveloping the galaxy. In short, NGC 4656 is "all blue"! I must love it, right? No, I think it is rather boring. Galaxies should be both blue, yellow and pink, in my opinion - or at least blue and pink!
GALEX, the now-defunct space telescope that photographed galaxies in ultraviolet light at two wavelengths, far ultraviolet (mapped as blue) and near ultraviolet (mapped as yellow), picked NGC 4656 as "the most far-ultraviolet" galaxy on a poster of ultraviolet atlas of nearby galaxies! Can you see NGC 4656 at top left in the image at right? NGC 4631 is the third galaxy from the left in the second row from the top.
Anyway, NGC 4656 is a mystery to me. How did it get that way? It looks as if NGC 4656 started out as a puny little galaxy with a puny little barely yellow center, and then all of a sudden, BANG! It somehow received a tremendous helping of gas. Maybe it had to to the with "blade" of the hockey stick, which may be an interacting galaxy. In any case, NGC 4656 started forming bright blue stars all over itself, used up of all its gas, dispersed its nebulas, blew away all of its dust, and - well, this all-blue ghost of a galaxy is all that remains! We talk about elliptical galaxies as red and dead, but NGC 4656 appears to be blue and dead!
But there is a lot of "life" going on around NGC 4631, which is one of my favorite galaxies! And I've got to admit that even if I spoke kind of harshly of NGC 4656, the probably interacting pair of NGC 4631 and NGC 4656 makes for a splendid sight at least in photographs!
Ann
Re: APOD: The Comet, the Whale, and the Hockey... (2021 May 13)
Posted: Thu May 13, 2021 6:58 am
by VictorBorun
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.
Re: APOD: The Comet, the Whale, and the Hockey... (2021 May 13)
Posted: Thu May 13, 2021 10:18 am
by 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.
NGC 2146, whose twisted shape suggests that the galaxy is a merger product.
Photo: Ken Crawford.
Any question that has anything to do with figures and numbers is a bad question to ask me about.
But certainly a collision between two galaxies is going to give the two galaxies, or the single collisional product, weird shapes!
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.
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
Re: APOD: The Comet, the Whale, and the Hockey... (2021 May 13)
Posted: Thu May 13, 2021 10:59 am
by orin stepanek
Comet ATLAS; the Whale; and the Hockey Stick!
Reminds me of a similar APOD from the past! Must have been just
the Whale and the Hockey Stick! Beautiful!
Re: APOD: The Comet, the Whale, and the Hockey... (2021 May 13)
Posted: Thu May 13, 2021 11:47 am
by XgeoX
Hi Ann,
Nice work!
The first thing I thought when I saw Elliptical galaxy NGC 3923 is that it is a magnificent example of gravitational lensing with almost perfect Einstein rings. But on closer look You are right.
Eric
Re: APOD: The Comet, the Whale, and the Hockey... (2021 May 13)
Posted: Thu May 13, 2021 7:20 pm
by Aeroflake123
Good timing for a pic of the Whale and the Hockey Stick. It's been a while, but in 1997 the Whalers took their hockey sticks from Hartford to Carolina and took the ice as the Hurricanes. And next week they will begin their quest for their second Stanley Cup. That's the trophy for the best (known) team in our galaxy.
Re: APOD: The Comet, the Whale, and the Hockey... (2021 May 13)
Posted: Thu May 13, 2021 8:59 pm
by SpookyAstro
Ann, that's a TON of awesome information thanks for sharing!
Glad you all like the image
Tom
Re: APOD: The Comet, the Whale, and the Hockey... (2021 May 13)
Posted: Fri May 14, 2021 12:29 am
by WWW
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
Definitely one of those astronomy sites that I haven't seen yet....
Re: APOD: The Comet, the Whale, and the Hockey... (2021 May 13)
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.
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
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
Re: APOD: The Comet, the Whale, and the Hockey... (2021 May 13)
Posted: Fri May 14, 2021 5:07 am
by Ann
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
Galactic disks are not rigid at all, but very fluffy. We are not physically "bound" to any other star in our galactic neighborhood than our own Sun, are we? We don't "feel" the gravity of Alpha Centauri or Sirius, do we? A tremendous force from outside, like the enormous gravity of an interacting galaxy, can most certainly mess up a galaxy's disk and scatter stars every which way.
An example of what a post-merger product can look like is galaxy NGC 6240 at left. More interesting, in my opinion, is the Antennae galaxy, NGC 4038 and NGC 4039. We can still see their disks. But what is going to happen to these disks when their massive yellow centers smack-bang into each other? It does not seem unlikely that these two galaxies may turn into a single elliptical galaxy.
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
Re: APOD: The Comet, the Whale, and the Hockey... (2021 May 13)
Posted: Fri May 14, 2021 1:08 pm
by Chris Peterson
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. So when galaxies collide, we need to understand what is going on by considering the gravitational impact separately on billions of individual bodies.
Re: APOD: The Comet, the Whale, and the Hockey... (2021 May 13)
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.
Re: APOD: The Comet, the Whale, and the Hockey... (2021 May 13)
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.
Quite.
Re: APOD: The Comet, the Whale, and the Hockey... (2021 May 13)
Posted: Sat May 15, 2021 7:33 am
by VictorBorun
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?
Re: APOD: The Comet, the Whale, and the Hockey... (2021 May 13)
Posted: Sat May 15, 2021 1:13 pm
by Chris Peterson
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?
The disc isn't a "thing". Nor is Saturn's ring system.
Re: APOD: The Comet, the Whale, and the Hockey... (2021 May 13)
Posted: Sun May 16, 2021 6:29 am
by VictorBorun
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.
Re: APOD: The Comet, the Whale, and the Hockey... (2021 May 13)
Posted: Sun May 16, 2021 1:55 pm
by Chris Peterson
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.
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.)
Re: APOD: The Comet, the Whale, and the Hockey... (2021 May 13)
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.)
Re: APOD: The Comet, the Whale, and the Hockey... (2021 May 13)
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.)
It is important not to conflate "fluid" and "liquid".
Re: APOD: The Comet, the Whale, and the Hockey... (2021 May 13)
Posted: Sun May 16, 2021 6:35 pm
by neufer
Chris Peterson wrote: ↑Sun May 16, 2021 3:07 pm
It is important not to conflate "fluid" and "liquid".
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.
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.
The Comet, the Whale, and the Hockey Stick
We talk about elliptical galaxies as red and dead, but NGC 4656 appears to be blue and dead! 
Must have been just 
Beautiful!
