APOD: Jupiter over 2 Hours and 30 Minutes (2024 Jan 19)

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APOD: Jupiter over 2 Hours and 30 Minutes (2024 Jan 19)

Post by APOD Robot » Fri Jan 19, 2024 5:05 am

Image Jupiter over 2 Hours and 30 Minutes

Explanation: Jupiter, our Solar System's ruling gas giant, is also the fastest spinning planet, rotating once in less than 10 hours. The gas giant doesn't rotate like a solid body though. A day on Jupiter is about 9 hours and 56 minutes long at the poles, decreasing to 9 hours and 50 minutes near the equator. The giant planet's fast rotation creates strong jet streams, separating its clouds into planet girdling bands of dark belts and bright zones. You can easily follow Jupiter's rapid rotation in this sharp sequence of images from the night of January 15, all taken with a camera and small telescope outside of Paris, France. Located just south of the equator, the giant planet's giant storm system, also known as the Great Red Spot, can be seen moving left to right with the planet's rotation. From lower left to upper right, the sequence spans about 2 hours and 30 minutes.

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Re: APOD: Jupiter over 2 Hours and 30 Minutes (2024 Jan 19)

Post by Ann » Fri Jan 19, 2024 7:10 am


Every day I keep hoping that a galaxy will be the APOD. In particular, I hope that a new galaxy picture will be the APOD. If I'm amazingly lucky, it will be a stunning RGB+Hα picture of a gorgeous spiral galaxy. These days, that feels about as realistic as pulling the Moon closer to the Earth, as Jim Carrey did in the movie Bruce Almighty.

Click to play embedded YouTube video.

But hey, for lack of galaxy images, I don't mind Jupiter. I like Jupiter. It is a handsome planet and very interesting. I read some place that most of the momentum of the Solar system is found in (or generated by?) Jupiter, because of the planet's high mass and fast rotation.

For me as the Color Commentator, I obviously have to talk about the Jovian hues, particularly about the Great Red Spot. It looks rather small, pale and orange in the APOD. Does anyone remember what it looked like when it was photographed by Pioneer in November 1973?


By the way, I can't keep this picture from you. It is an illustration by artist Rick Guidice, showing what Pioneer's journey to Jupiter would be like. Note the thick "ring system" that Pioneer would have to cross - that's the asteroid belt! :D


Perhaps NASA thought that crossing the asteroid belt would be like crossing the Hoth asteroid field in Star Wars? :D

Asteroid field of Star Wars.png

Anyway. Jupiter's Great Red Spot has shrunk quite dramatically since 1973, but in some pictures it still looks quite red. Wikipedia calls this a true-color image of Jupiter. I'm not sure about that:


Interestingly, according to the same Wikipedia page, the smallest size ever of the GRB was recorded by human instruments in 2014. I take that to mean that it has grown a little since then.

There's been a lot of talk recently about how NASA fooled us by making planet Neptune look "too blue". All right! I have long suspected as much, because I can't think of any natural processes that would make Neptune as cornflower-blue as it was seen to be in that first NASA/Voyager 2 picture of it. (Yes, but NASA also made Jupiter amazingly red in its first Voyager pictures.)


This is what Jupiter and Neptune really look like:


Let me leave off with a foldout of Jupiter's entire atmosphere. The picture is from 2015. Are the colors real and not enhanced? I guess not.


If you want to see Jupiter's clouds move, you'll have to click on this link to see a 7.5 MB animation!

Ann

Edit: I forgot! There are the Juno images of Jupiter, which are incredibly handsome. I don't always get the colors, though.

jpegPIA23605[1].jpg
Illustration by Prateek Sarpal with enhanced colors
based on an original image by JunoCam on Sept 12, 2019.
You do not have the required permissions to view the files attached to this post.
Last edited by Ann on Fri Jan 19, 2024 2:55 pm, edited 1 time in total.
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Re: APOD: Jupiter over 2 Hours and 30 Minutes (2024 Jan 19)

Post by Ironwood » Fri Jan 19, 2024 2:36 pm

Thanks Ann. Love the color commentary and miss it on your days off when APOD is a picture of a sunset or something. Keep up the good work!

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Re: APOD: Jupiter over 2 Hours and 30 Minutes (2024 Jan 19)

Post by johnnydeep » Fri Jan 19, 2024 6:57 pm

In her post above, Ann included this pic of Jupiter's atmosphere rolled out:


I must say that would make a very nice design for a rug!
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Re: APOD: Jupiter over 2 Hours and 30 Minutes (2024 Jan 19)

Post by johnnydeep » Fri Jan 19, 2024 7:01 pm

Also, in references to Ann's comment above about Jupiter containing most of the momentum of the Solar system (apart, perhaps from the Sun itself?), there could conceivably by three contributions from it: its mass, it's orbital velocity, and its axial rotational velocity. But does its fast rotation about its axis affect the total angular momentum of the Solar system at all, or is it irrelevant to that? I'd guess it would be irrelevant. Am I right?
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Re: APOD: Jupiter over 2 Hours and 30 Minutes (2024 Jan 19)

Post by Ann » Fri Jan 19, 2024 8:18 pm

johnnydeep wrote: Fri Jan 19, 2024 7:01 pm Also, in references to Ann's comment above about Jupiter containing most of the momentum of the Solar system (apart, perhaps from the Sun itself?), there could conceivably by three contributions from it: its mass, it's orbital velocity, and its axial rotational velocity. But does its fast rotation about its axis affect the total angular momentum of the Solar system at all, or is it irrelevant to that? I'd guess it would be irrelevant. Am I right?
Remember that Jupiter spins fast and the Sun spins slowly. I think it takes 8 hours for Jupiter to make a full turn around its axis, but for the Sun its takes 28 days. Not only that, but the slow-spinning Sun is also an extremely round object.

University of Hawaii/Astronomy Magazine wrote:

The Sun is nearly the roundest object ever measured. If scaled to the size of a beach ball, it would be so round that the difference between the widest and narrow diameters would be much less than the width of a human hair.

By contrast, fast-spinning Jupiter is definitely flattened. According to Wikipedia, the equatorial radius of Jupiter is 71,492 km (44,423 mi), while its polar radius is "only" 66,854 km (41,541 mi). That is a marked difference.

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Re: APOD: Jupiter over 2 Hours and 30 Minutes (2024 Jan 19)

Post by johnnydeep » Fri Jan 19, 2024 9:22 pm

Ann wrote: Fri Jan 19, 2024 8:18 pm
johnnydeep wrote: Fri Jan 19, 2024 7:01 pm Also, in references to Ann's comment above about Jupiter containing most of the momentum of the Solar system (apart, perhaps from the Sun itself?), there could conceivably by three contributions from it: its mass, it's orbital velocity, and its axial rotational velocity. But does its fast rotation about its axis affect the total angular momentum of the Solar system at all, or is it irrelevant to that? I'd guess it would be irrelevant. Am I right?
Remember that Jupiter spins fast and the Sun spins slowly. I think it takes 8 hours for Jupiter to make a full turn around its axis, but for the Sun its takes 28 days. Not only that, but the slow-spinning Sun is also an extremely round object.

University of Hawaii/Astronomy Magazine wrote:

The Sun is nearly the roundest object ever measured. If scaled to the size of a beach ball, it would be so round that the difference between the widest and narrow diameters would be much less than the width of a human hair.

By contrast, fast-spinning Jupiter is definitely flattened. According to Wikipedia, the equatorial radius of Jupiter is 71,492 km (44,423 mi), while its polar radius is "only" 66,854 km (41,541 mi). That is a marked difference.

Ann
Yes, Jupiter spins fast and is flattened, but I'm asking if that contributes AT ALL to the total angular momentum of the Solar system as a whole. I'd guess that the individual axial spins of the planets don't have ANY effect at all, only their masses and orbital velocities do.
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Avalon

Re: APOD: Jupiter over 2 Hours and 30 Minutes (2024 Jan 19)

Post by Avalon » Sat Jan 20, 2024 12:23 am

Do the "flattened" shapes of the planets have to do with rapid rotation and centrifugal force? Does the planetary material tend to be flung out further from the axis due to the high speed of rotation? In the furthest left image I noticed a couple of moons at the right edge of Jupiter which disappear over the Jovian limb in the subsequent images. Do Jupiter's moons orbit at a fast pace,too? What even starts a planet's rotation anyway?

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Re: APOD: Jupiter over 2 Hours and 30 Minutes (2024 Jan 19)

Post by Ann » Sat Jan 20, 2024 4:47 am

johnnydeep wrote: Fri Jan 19, 2024 9:22 pm
Yes, Jupiter spins fast and is flattened, but I'm asking if that contributes AT ALL to the total angular momentum of the Solar system as a whole. I'd guess that the individual axial spins of the planets don't have ANY effect at all, only their masses and orbital velocities do.
Okay, Johnny, it would seem I was wrong. On googling I found this:

Vedantu wrote:

Everything in our solar system exhibits rotational motion and that too in the same sense i.e., in the anti-clockwise sense. According to the solar nebula theory, the solar system is formed from a gaseous cloud which was flat and disc shaped, rotating in the anti-clockwise sense due to which all the components of the solar system i.e., the planets, sun and the moons also rotate in anticlockwise sense due to the conservation of angular momentum.
The conservation of angular momentum means that the angular momentum of a system remains constant even if the configuration of the system changes. The angular momentum of the solar system can be calculated by summing up the angular momentum of its components, due to the gigantic size of the sun, it contributes the most to the angular momentum of the solar system.

So you were right and I was wrong about the total angular momentum of the Solar system. However, it is true that the Sun is very round in shape and that it rotates very slowly. Not all stars are like that.

Wikipedia wrote:

As of 2015, Achernar was the least spherical star known in the Milky Way. It spins so rapidly that it has assumed the shape of an oblate spheroid with an equatorial diameter 35% greater than its polar diameter. The oblateness of Achernar is comparable to that of the dwarf planet Haumea, and the stars of Altair and Regulus. The polar axis is inclined about 60.6° to the line of sight from the Earth. Since it is actually a binary star, its highly distorted shape may cause non-negligible departures of the companion's orbital trajectory with respect to a Keplerian ellipse.
So how fast does Achernar spin?

Wikipedia wrote:

The brightness of Achernar varies very slightly, by a maximum of 0.06 magnitudes or about 6%. A period of 1.263 d is given in the General Catalogue of Variable Stars, but several periods have been identified between about 17 h and 35 h. The longest periods are very similar to the rotation period of the star, although the exact period appears to vary as the rotational velocity of its upper atmosphere changes.
So Achernar would seem to have a rotation period close to 35 hours. Since Achernar also contains 6 times as much mass as the Sun (according to Wikipedia), we must conclude that there is a lot - a lot!!! - more angular momentum in the Achernar system than in the Solar system.

And back to the Sun/Jupiter question, even though you were right that the Sun provides more angular momentum to the Solar system than Jupiter does, I'm still going to guess that Jupiter is also an important player in the Solar system angular momentum business.

And off topic, but I suspect that the Sun's slow rotation and generally mild behavior is one of many factors that has made life on the surface of Earth possible. 🌞

Ann
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Re: APOD: Jupiter over 2 Hours and 30 Minutes (2024 Jan 19)

Post by johnnydeep » Sat Jan 20, 2024 2:25 pm

Ann wrote: Sat Jan 20, 2024 4:47 am
johnnydeep wrote: Fri Jan 19, 2024 9:22 pm
Yes, Jupiter spins fast and is flattened, but I'm asking if that contributes AT ALL to the total angular momentum of the Solar system as a whole. I'd guess that the individual axial spins of the planets don't have ANY effect at all, only their masses and orbital velocities do.
Okay, Johnny, it would seem I was wrong. On googling I found this:

Vedantu wrote:

Everything in our solar system exhibits rotational motion and that too in the same sense i.e., in the anti-clockwise sense. According to the solar nebula theory, the solar system is formed from a gaseous cloud which was flat and disc shaped, rotating in the anti-clockwise sense due to which all the components of the solar system i.e., the planets, sun and the moons also rotate in anticlockwise sense due to the conservation of angular momentum.
The conservation of angular momentum means that the angular momentum of a system remains constant even if the configuration of the system changes. The angular momentum of the solar system can be calculated by summing up the angular momentum of its components, due to the gigantic size of the sun, it contributes the most to the angular momentum of the solar system.

So you were right and I was wrong about the total angular momentum of the Solar system. However, it is true that the Sun is very round in shape and that it rotates very slowly. Not all stars are like that.

Wikipedia wrote:

As of 2015, Achernar was the least spherical star known in the Milky Way. It spins so rapidly that it has assumed the shape of an oblate spheroid with an equatorial diameter 35% greater than its polar diameter. The oblateness of Achernar is comparable to that of the dwarf planet Haumea, and the stars of Altair and Regulus. The polar axis is inclined about 60.6° to the line of sight from the Earth. Since it is actually a binary star, its highly distorted shape may cause non-negligible departures of the companion's orbital trajectory with respect to a Keplerian ellipse.
So how fast does Achernar spin?

Wikipedia wrote:

The brightness of Achernar varies very slightly, by a maximum of 0.06 magnitudes or about 6%. A period of 1.263 d is given in the General Catalogue of Variable Stars, but several periods have been identified between about 17 h and 35 h. The longest periods are very similar to the rotation period of the star, although the exact period appears to vary as the rotational velocity of its upper atmosphere changes.
So Achernar would seem to have a rotation period close to 35 hours. Since Achernar also contains 6 times as much mass as the Sun (according to Wikipedia), we must conclude that there is a lot - a lot!!! - more angular momentum in the Achernar system than in the Solar system.

And back to the Sun/Jupiter question, even though you were right that the Sun provides more angular momentum to the Solar system than Jupiter does, I'm still going to guess that Jupiter is also an important player in the Solar system angular momentum business.

And off topic, but I suspect that the Sun's slow rotation and generally mild behavior is one of many factors that has made life on the surface of Earth possible. 🌞

Ann
I’m obviously not making myself clear. The question I had was whether the spins of planets contribute to the total angular momentum of a solar system or not. I had thought not, but now I think I'm wrong. Hypothetical case: our Sun with Jupiter as its only planet, masses, spins and orbital parameters identical to what they are now. Now, if Jupiter were to spin twice as fast, would the total angular momentum of the system change or not? I was saying no, but with this simple example if now seem like it would change. This is basic mechanics, I’m sure, but I last took physics 43 years ago.

Clearer case: binary star system with both stars spinning at rate A. If both stars spin at 2*A, then the total momentum would have to increase, since the only source of the momentum is that of the primordial disk they formed out of! No difference than in a system with just a star and no planets. If the star spins twice as fast, the total momentum of the primordial disk must have been double, and that must be conserved in the star that formed from it.
--
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Re: APOD: Jupiter over 2 Hours and 30 Minutes (2024 Jan 19)

Post by Ann » Sat Jan 20, 2024 6:36 pm

johnnydeep wrote: Sat Jan 20, 2024 2:25 pm
Ann wrote: Sat Jan 20, 2024 4:47 am
johnnydeep wrote: Fri Jan 19, 2024 9:22 pm
Yes, Jupiter spins fast and is flattened, but I'm asking if that contributes AT ALL to the total angular momentum of the Solar system as a whole. I'd guess that the individual axial spins of the planets don't have ANY effect at all, only their masses and orbital velocities do.
Okay, Johnny, it would seem I was wrong. On googling I found this:

Vedantu wrote:

Everything in our solar system exhibits rotational motion and that too in the same sense i.e., in the anti-clockwise sense. According to the solar nebula theory, the solar system is formed from a gaseous cloud which was flat and disc shaped, rotating in the anti-clockwise sense due to which all the components of the solar system i.e., the planets, sun and the moons also rotate in anticlockwise sense due to the conservation of angular momentum.
The conservation of angular momentum means that the angular momentum of a system remains constant even if the configuration of the system changes. The angular momentum of the solar system can be calculated by summing up the angular momentum of its components, due to the gigantic size of the sun, it contributes the most to the angular momentum of the solar system.

So you were right and I was wrong about the total angular momentum of the Solar system. However, it is true that the Sun is very round in shape and that it rotates very slowly. Not all stars are like that.

Wikipedia wrote:

As of 2015, Achernar was the least spherical star known in the Milky Way. It spins so rapidly that it has assumed the shape of an oblate spheroid with an equatorial diameter 35% greater than its polar diameter. The oblateness of Achernar is comparable to that of the dwarf planet Haumea, and the stars of Altair and Regulus. The polar axis is inclined about 60.6° to the line of sight from the Earth. Since it is actually a binary star, its highly distorted shape may cause non-negligible departures of the companion's orbital trajectory with respect to a Keplerian ellipse.
So how fast does Achernar spin?

Wikipedia wrote:

The brightness of Achernar varies very slightly, by a maximum of 0.06 magnitudes or about 6%. A period of 1.263 d is given in the General Catalogue of Variable Stars, but several periods have been identified between about 17 h and 35 h. The longest periods are very similar to the rotation period of the star, although the exact period appears to vary as the rotational velocity of its upper atmosphere changes.
So Achernar would seem to have a rotation period close to 35 hours. Since Achernar also contains 6 times as much mass as the Sun (according to Wikipedia), we must conclude that there is a lot - a lot!!! - more angular momentum in the Achernar system than in the Solar system.

And back to the Sun/Jupiter question, even though you were right that the Sun provides more angular momentum to the Solar system than Jupiter does, I'm still going to guess that Jupiter is also an important player in the Solar system angular momentum business.

And off topic, but I suspect that the Sun's slow rotation and generally mild behavior is one of many factors that has made life on the surface of Earth possible. 🌞

Ann
I’m obviously not making myself clear. The question I had was whether the spins of planets contribute to the total angular momentum of a solar system or not. I had thought not, but now I think I'm wrong. Hypothetical case: our Sun with Jupiter as its only planet, masses, spins and orbital parameters identical to what they are now. Now, if Jupiter were to spin twice as fast, would the total angular momentum of the system change or not? I was saying no, but with this simple example if now seem like it would change. This is basic mechanics, I’m sure, but I last took physics 43 years ago.

Clearer case: binary star system with both stars spinning at rate A. If both stars spin at 2*A, then the total momentum would have to increase, since the only source of the momentum is that of the primordial disk they formed out of! No difference than in a system with just a star and no planets. If the star spins twice as fast, the total momentum of the primordial disk must have been double, and that must be conserved in the star that formed from it.
I hope Chris will pop in and answer your questions, Johnny. You know I'm worthless at the kind of reasoning that you presented in your post.

But I'll say this much: I'm 99% per cent certain that the spin of stars (and planets) can change over time. I strongly believe that the Sun probably spun faster in its youth. And I know that the Earth spun faster in its early days. As for Venus, I believe that it has spun down so dramatically that it has actually reversed its spin. Perhaps Jupiter has spun up? Is that possible?

But seriously, isn't it possible for the total momentum of a star system to decrease over time? I guess Chris is the one that should tell us about that.

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Re: APOD: Jupiter over 2 Hours and 30 Minutes (2024 Jan 19)

Post by Chris Peterson » Sat Jan 20, 2024 6:49 pm

Ann wrote: Sat Jan 20, 2024 6:36 pm
johnnydeep wrote: Sat Jan 20, 2024 2:25 pm
Ann wrote: Sat Jan 20, 2024 4:47 am

Okay, Johnny, it would seem I was wrong. On googling I found this:





So you were right and I was wrong about the total angular momentum of the Solar system. However, it is true that the Sun is very round in shape and that it rotates very slowly. Not all stars are like that.

So how fast does Achernar spin?




So Achernar would seem to have a rotation period close to 35 hours. Since Achernar also contains 6 times as much mass as the Sun (according to Wikipedia), we must conclude that there is a lot - a lot!!! - more angular momentum in the Achernar system than in the Solar system.

And back to the Sun/Jupiter question, even though you were right that the Sun provides more angular momentum to the Solar system than Jupiter does, I'm still going to guess that Jupiter is also an important player in the Solar system angular momentum business.

And off topic, but I suspect that the Sun's slow rotation and generally mild behavior is one of many factors that has made life on the surface of Earth possible. 🌞

Ann
I’m obviously not making myself clear. The question I had was whether the spins of planets contribute to the total angular momentum of a solar system or not. I had thought not, but now I think I'm wrong. Hypothetical case: our Sun with Jupiter as its only planet, masses, spins and orbital parameters identical to what they are now. Now, if Jupiter were to spin twice as fast, would the total angular momentum of the system change or not? I was saying no, but with this simple example if now seem like it would change. This is basic mechanics, I’m sure, but I last took physics 43 years ago.

Clearer case: binary star system with both stars spinning at rate A. If both stars spin at 2*A, then the total momentum would have to increase, since the only source of the momentum is that of the primordial disk they formed out of! No difference than in a system with just a star and no planets. If the star spins twice as fast, the total momentum of the primordial disk must have been double, and that must be conserved in the star that formed from it.
I hope Chris will pop in and answer your questions, Johnny. You know I'm worthless at the kind of reasoning that you presented in your post.

But I'll say this much: I'm 99% per cent certain that the spin of stars (and planets) can change over time. I strongly believe that the Sun probably spun faster in its youth. And I know that the Earth spun faster in its early days. As for Venus, I believe that it has spun down so dramatically that it has actually reversed its spin. Perhaps Jupiter has spun up? Is that possible?

But seriously, isn't it possible for the total momentum of a star system to decrease over time? I guess Chris is the one that should tell us about that.

Ann
Angular momentum is a conserved property. The total angular momentum of a star system can only change if it transfers momentum to something outside the system, or receives momentum from outside the system. For example, a passing star could add or remove angular momentum from the system.
Chris

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Re: APOD: Jupiter over 2 Hours and 30 Minutes (2024 Jan 19)

Post by johnnydeep » Sat Jan 20, 2024 7:00 pm

Chris Peterson wrote: Sat Jan 20, 2024 6:49 pm
Ann wrote: Sat Jan 20, 2024 6:36 pm
johnnydeep wrote: Sat Jan 20, 2024 2:25 pm

I’m obviously not making myself clear. The question I had was whether the spins of planets contribute to the total angular momentum of a solar system or not. I had thought not, but now I think I'm wrong. Hypothetical case: our Sun with Jupiter as its only planet, masses, spins and orbital parameters identical to what they are now. Now, if Jupiter were to spin twice as fast, would the total angular momentum of the system change or not? I was saying no, but with this simple example if now seem like it would change. This is basic mechanics, I’m sure, but I last took physics 43 years ago.

Clearer case: binary star system with both stars spinning at rate A. If both stars spin at 2*A, then the total momentum would have to increase, since the only source of the momentum is that of the primordial disk they formed out of! No difference than in a system with just a star and no planets. If the star spins twice as fast, the total momentum of the primordial disk must have been double, and that must be conserved in the star that formed from it.
I hope Chris will pop in and answer your questions, Johnny. You know I'm worthless at the kind of reasoning that you presented in your post.

But I'll say this much: I'm 99% per cent certain that the spin of stars (and planets) can change over time. I strongly believe that the Sun probably spun faster in its youth. And I know that the Earth spun faster in its early days. As for Venus, I believe that it has spun down so dramatically that it has actually reversed its spin. Perhaps Jupiter has spun up? Is that possible?

But seriously, isn't it possible for the total momentum of a star system to decrease over time? I guess Chris is the one that should tell us about that.

Ann
Angular momentum is a conserved property. The total angular momentum of a star system can only change if it transfers momentum to something outside the system, or receives momentum from outside the system. For example, a passing star could add or remove angular momentum from the system.
And you didn't explicitly address it, but the individual angular momenta from the spins of the planets contribute to the total in the system, yes? Does it matter if some planets spin backward compared to the rest? That is, in a system with two identical planets, each spinning at the same rate but opposite to each other, does all their spin momentum cancel each other?
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Re: APOD: Jupiter over 2 Hours and 30 Minutes (2024 Jan 19)

Post by Chris Peterson » Sat Jan 20, 2024 7:04 pm

johnnydeep wrote: Sat Jan 20, 2024 7:00 pm
Chris Peterson wrote: Sat Jan 20, 2024 6:49 pm
Ann wrote: Sat Jan 20, 2024 6:36 pm

I hope Chris will pop in and answer your questions, Johnny. You know I'm worthless at the kind of reasoning that you presented in your post.

But I'll say this much: I'm 99% per cent certain that the spin of stars (and planets) can change over time. I strongly believe that the Sun probably spun faster in its youth. And I know that the Earth spun faster in its early days. As for Venus, I believe that it has spun down so dramatically that it has actually reversed its spin. Perhaps Jupiter has spun up? Is that possible?

But seriously, isn't it possible for the total momentum of a star system to decrease over time? I guess Chris is the one that should tell us about that.

Ann
Angular momentum is a conserved property. The total angular momentum of a star system can only change if it transfers momentum to something outside the system, or receives momentum from outside the system. For example, a passing star could add or remove angular momentum from the system.
And you didn't explicitly address it, but the individual angular momenta from the spins of the planets contribute to the total in the system, yes? Does it matter if some planets spin backward compared to the rest? That is, in a system with two identical planets, each spinning at the same rate but opposite to each other, does all their spin momentum cancel each other?
Yeah, it's just a big vector summation of every spinning/rotating mass in the system, from the central star down to molecules in oceans.
Chris

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Re: APOD: Jupiter over 2 Hours and 30 Minutes (2024 Jan 19)

Post by johnnydeep » Sat Jan 20, 2024 7:10 pm

Chris Peterson wrote: Sat Jan 20, 2024 7:04 pm
johnnydeep wrote: Sat Jan 20, 2024 7:00 pm
Chris Peterson wrote: Sat Jan 20, 2024 6:49 pm
Angular momentum is a conserved property. The total angular momentum of a star system can only change if it transfers momentum to something outside the system, or receives momentum from outside the system. For example, a passing star could add or remove angular momentum from the system.
And you didn't explicitly address it, but the individual angular momenta from the spins of the planets contribute to the total in the system, yes? Does it matter if some planets spin backward compared to the rest? That is, in a system with two identical planets, each spinning at the same rate but opposite to each other, does all their spin momentum cancel each other?
Yeah, it's just a big vector summation of every spinning/rotating mass in the system, from the central star down to molecules in oceans.
Ok - finally got it!
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Re: APOD: Jupiter over 2 Hours and 30 Minutes (2024 Jan 19)

Post by Ann » Sun Jan 21, 2024 4:36 am

Chris Peterson wrote: Sat Jan 20, 2024 6:49 pm
Angular momentum is a conserved property. The total angular momentum of a star system can only change if it transfers momentum to something outside the system, or receives momentum from outside the system. For example, a passing star could add or remove angular momentum from the system.
I realize that this is not something that you can know, Chris, but still I have to ask. The Sun spins very slowly. Is it likely that the original solar nebula spun that slowly, or is it more likely that the Sun has "spun down" over its life time?

Can we think of a reason for why Jupiter spins so fast and the Sun so slowly?

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Re: APOD: Jupiter over 2 Hours and 30 Minutes (2024 Jan 19)

Post by Chris Peterson » Sun Jan 21, 2024 2:32 pm

Ann wrote: Sun Jan 21, 2024 4:36 am
Chris Peterson wrote: Sat Jan 20, 2024 6:49 pm
Angular momentum is a conserved property. The total angular momentum of a star system can only change if it transfers momentum to something outside the system, or receives momentum from outside the system. For example, a passing star could add or remove angular momentum from the system.
I realize that this is not something that you can know, Chris, but still I have to ask. The Sun spins very slowly. Is it likely that the original solar nebula spun that slowly, or is it more likely that the Sun has "spun down" over its life time?

Can we think of a reason for why Jupiter spins so fast and the Sun so slowly?

Ann
Does the Sun spin "slowly"? In comparison to what? (In terms of surface velocity, the Sun is only 6 times slower than Jupiter.) And from a physical standpoint, the rotation of the Sun has a clear relationship to the rotation of the presolar nebula, but the spin of Jupiter does not. And, of course, the disk that condenses to a star, like the ultimate stellar system that forms, is not a rigid body with a constant angular velocity, but has material moving much more rapidly towards the center. And as it collapses, conservation of angular momentum should speed things up centrally, like an ice skater tucking in her arms.

I'd say the rotation speed of the Sun was determined by the nature of the presolar nebula collapse, and that it hasn't changed substantially over time. Where would that energy go? (Of course, every rotating body produces gravitational waves, resulting in slowing... but this is extremely tiny.)

And the planets? They would have initially rotated according the the turbulence driven local collapses of material in their original orbital positions, subsequently modified by the interactions that moved them around and set them in their current orbits.
Chris

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Re: APOD: Jupiter over 2 Hours and 30 Minutes (2024 Jan 19)

Post by johnnydeep » Sun Jan 21, 2024 2:49 pm

Chris Peterson wrote: Sun Jan 21, 2024 2:32 pm
Ann wrote: Sun Jan 21, 2024 4:36 am
Chris Peterson wrote: Sat Jan 20, 2024 6:49 pm
Angular momentum is a conserved property. The total angular momentum of a star system can only change if it transfers momentum to something outside the system, or receives momentum from outside the system. For example, a passing star could add or remove angular momentum from the system.
I realize that this is not something that you can know, Chris, but still I have to ask. The Sun spins very slowly. Is it likely that the original solar nebula spun that slowly, or is it more likely that the Sun has "spun down" over its life time?

Can we think of a reason for why Jupiter spins so fast and the Sun so slowly?

Ann
Does the Sun spin "slowly"? In comparison to what? (In terms of surface velocity, the Sun is only 6 times slower than Jupiter.) And from a physical standpoint, the rotation of the Sun has a clear relationship to the rotation of the presolar nebula, but the spin of Jupiter does not. And, of course, the disk that condenses to a star, like the ultimate stellar system that forms, is not a rigid body with a constant angular velocity, but has material moving much more rapidly towards the center. And as it collapses, conservation of angular momentum should speed things up centrally, like an ice skater tucking in her arms.

I'd say the rotation speed of the Sun was determined by the nature of the presolar nebula collapse, and that it hasn't changed substantially over time. Where would that energy go? (Of course, every rotating body produces gravitational waves, resulting in slowing... but this is extremely tiny.)

And the planets? They would have initially rotated according the the turbulence driven local collapses of material in their original orbital positions, subsequently modified by the interactions that moved them around and set them in their current orbits.
I'd add that just like the gradual migration of the Moon outward from the Earth results in a slowing of the Earth's rotational speed. So perhaps since Jupiter has migrated from the inner Solar system in its youth to the outer Solar system now, that that could have slowed the Sun's rotational speed a little? Or does the slowing Earth/outward migrating Moon only occur because of tidal friction and the fact that the Moon's rotational period is locked to its orbital period thereby keeping one side always facing Earth?
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Re: APOD: Jupiter over 2 Hours and 30 Minutes (2024 Jan 19)

Post by Chris Peterson » Sun Jan 21, 2024 2:54 pm

johnnydeep wrote: Sun Jan 21, 2024 2:49 pm
Chris Peterson wrote: Sun Jan 21, 2024 2:32 pm
Ann wrote: Sun Jan 21, 2024 4:36 am

I realize that this is not something that you can know, Chris, but still I have to ask. The Sun spins very slowly. Is it likely that the original solar nebula spun that slowly, or is it more likely that the Sun has "spun down" over its life time?

Can we think of a reason for why Jupiter spins so fast and the Sun so slowly?

Ann
Does the Sun spin "slowly"? In comparison to what? (In terms of surface velocity, the Sun is only 6 times slower than Jupiter.) And from a physical standpoint, the rotation of the Sun has a clear relationship to the rotation of the presolar nebula, but the spin of Jupiter does not. And, of course, the disk that condenses to a star, like the ultimate stellar system that forms, is not a rigid body with a constant angular velocity, but has material moving much more rapidly towards the center. And as it collapses, conservation of angular momentum should speed things up centrally, like an ice skater tucking in her arms.

I'd say the rotation speed of the Sun was determined by the nature of the presolar nebula collapse, and that it hasn't changed substantially over time. Where would that energy go? (Of course, every rotating body produces gravitational waves, resulting in slowing... but this is extremely tiny.)

And the planets? They would have initially rotated according the the turbulence driven local collapses of material in their original orbital positions, subsequently modified by the interactions that moved them around and set them in their current orbits.
I'd add that just like the gradual migration of the Moon outward from the Earth results in a slowing of the Earth's rotational speed. So perhaps since Jupiter has migrated from the inner Solar system in its youth to the outer Solar system now, that that could have slowed the Sun's rotational speed a little? Or does the slowing Earth/outward migrating Moon only occur because of tidal friction and the fact that the Moon's rotational period is locked to its orbital period thereby keeping one side always facing Earth?
All of the planetary orbit shifts since the beginning will have influenced the rotation speed of the Sun, either up or down. But the masses of these bodies are so small compared with the Sun, and their distances so great, that the changes must be very small.
Chris

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Re: APOD: Jupiter over 2 Hours and 30 Minutes (2024 Jan 19)

Post by johnnydeep » Sun Jan 21, 2024 3:02 pm

Chris Peterson wrote: Sun Jan 21, 2024 2:54 pm
johnnydeep wrote: Sun Jan 21, 2024 2:49 pm
Chris Peterson wrote: Sun Jan 21, 2024 2:32 pm
Does the Sun spin "slowly"? In comparison to what? (In terms of surface velocity, the Sun is only 6 times slower than Jupiter.) And from a physical standpoint, the rotation of the Sun has a clear relationship to the rotation of the presolar nebula, but the spin of Jupiter does not. And, of course, the disk that condenses to a star, like the ultimate stellar system that forms, is not a rigid body with a constant angular velocity, but has material moving much more rapidly towards the center. And as it collapses, conservation of angular momentum should speed things up centrally, like an ice skater tucking in her arms.

I'd say the rotation speed of the Sun was determined by the nature of the presolar nebula collapse, and that it hasn't changed substantially over time. Where would that energy go? (Of course, every rotating body produces gravitational waves, resulting in slowing... but this is extremely tiny.)

And the planets? They would have initially rotated according the the turbulence driven local collapses of material in their original orbital positions, subsequently modified by the interactions that moved them around and set them in their current orbits.
I'd add that just like the gradual migration of the Moon outward from the Earth results in a slowing of the Earth's rotational speed. So perhaps since Jupiter has migrated from the inner Solar system in its youth to the outer Solar system now, that that could have slowed the Sun's rotational speed a little? Or does the slowing Earth/outward migrating Moon only occur because of tidal friction and the fact that the Moon's rotational period is locked to its orbital period thereby keeping one side always facing Earth?
All of the planetary orbit shifts since the beginning will have influenced the rotation speed of the Sun, either up or down. But the masses of these bodies are so small compared with the Sun, and their distances so great, that the changes must be very small.
So, in a hypothetical two-body system, does their orbital period always gradually increase over time, and the rotational speeds of both bodies always slow as a consequence? Or does it only happen if there is tidal locking?
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Re: APOD: Jupiter over 2 Hours and 30 Minutes (2024 Jan 19)

Post by Chris Peterson » Sun Jan 21, 2024 3:07 pm

johnnydeep wrote: Sun Jan 21, 2024 3:02 pm
Chris Peterson wrote: Sun Jan 21, 2024 2:54 pm
johnnydeep wrote: Sun Jan 21, 2024 2:49 pm

I'd add that just like the gradual migration of the Moon outward from the Earth results in a slowing of the Earth's rotational speed. So perhaps since Jupiter has migrated from the inner Solar system in its youth to the outer Solar system now, that that could have slowed the Sun's rotational speed a little? Or does the slowing Earth/outward migrating Moon only occur because of tidal friction and the fact that the Moon's rotational period is locked to its orbital period thereby keeping one side always facing Earth?
All of the planetary orbit shifts since the beginning will have influenced the rotation speed of the Sun, either up or down. But the masses of these bodies are so small compared with the Sun, and their distances so great, that the changes must be very small.
So, in a hypothetical two-body system, does their orbital period always gradually increase over time, and the rotational speeds of both bodies always slow as a consequence? Or does it only happen if there is tidal locking?
That happens as they transition to a tidally locked state, because of a shift of rotational angular momentum to orbital angular momentum. Once they are tidally locked the orbits are stable forever (other than the previously mentioned loss of energy to gravitational radiation).
Chris

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Re: APOD: Jupiter over 2 Hours and 30 Minutes (2024 Jan 19)

Post by johnnydeep » Sun Jan 21, 2024 5:14 pm

Chris Peterson wrote: Sun Jan 21, 2024 3:07 pm
johnnydeep wrote: Sun Jan 21, 2024 3:02 pm
Chris Peterson wrote: Sun Jan 21, 2024 2:54 pm
All of the planetary orbit shifts since the beginning will have influenced the rotation speed of the Sun, either up or down. But the masses of these bodies are so small compared with the Sun, and their distances so great, that the changes must be very small.
So, in a hypothetical two-body system, does their orbital period always gradually increase over time, and the rotational speeds of both bodies always slow as a consequence? Or does it only happen if there is tidal locking?
That happens as they transition to a tidally locked state, because of a shift of rotational angular momentum to orbital angular momentum. Once they are tidally locked the orbits are stable forever (other than the previously mentioned loss of energy to gravitational radiation).
Thanks. I can see how being tidally locked would be the end state for solid bodies, but what about Stars and the gas giants in orbit around them?
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Re: APOD: Jupiter over 2 Hours and 30 Minutes (2024 Jan 19)

Post by Chris Peterson » Sun Jan 21, 2024 5:17 pm

johnnydeep wrote: Sun Jan 21, 2024 5:14 pm
Chris Peterson wrote: Sun Jan 21, 2024 3:07 pm
johnnydeep wrote: Sun Jan 21, 2024 3:02 pm

So, in a hypothetical two-body system, does their orbital period always gradually increase over time, and the rotational speeds of both bodies always slow as a consequence? Or does it only happen if there is tidal locking?
That happens as they transition to a tidally locked state, because of a shift of rotational angular momentum to orbital angular momentum. Once they are tidally locked the orbits are stable forever (other than the previously mentioned loss of energy to gravitational radiation).
Thanks. I can see how being tidally locked would be the end state for solid bodies, but what about Stars and the gas giants in orbit around them?
Well, now you're talking about non-rigid bodies and differential rotation. So energy will always be dissipated by tides, and over time the stars will get closer.
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Re: APOD: Jupiter over 2 Hours and 30 Minutes (2024 Jan 19)

Post by johnnydeep » Sun Jan 21, 2024 6:56 pm

Chris Peterson wrote: Sun Jan 21, 2024 5:17 pm
johnnydeep wrote: Sun Jan 21, 2024 5:14 pm
Chris Peterson wrote: Sun Jan 21, 2024 3:07 pm
That happens as they transition to a tidally locked state, because of a shift of rotational angular momentum to orbital angular momentum. Once they are tidally locked the orbits are stable forever (other than the previously mentioned loss of energy to gravitational radiation).
Thanks. I can see how being tidally locked would be the end state for solid bodies, but what about Stars and the gas giants in orbit around them?
Well, now you're talking about non-rigid bodies and differential rotation. So energy will always be dissipated by tides, and over time the stars will get closer.
Ah. "Sloshing" tides affecting non-rigid bodies (or those with non rigid parts) versus tides locking a side of a rigid body. And I guess there are oddball cases of egg shaped solid cores with surface oceans!
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Re: APOD: Jupiter over 2 Hours and 30 Minutes (2024 Jan 19)

Post by Ann » Sun Jan 21, 2024 7:29 pm

Chris Peterson wrote: Sun Jan 21, 2024 2:32 pm
Does the Sun spin "slowly"? In comparison to what? (In terms of surface velocity, the Sun is only 6 times slower than Jupiter.) And from a physical standpoint, the rotation of the Sun has a clear relationship to the rotation of the presolar nebula, but the spin of Jupiter does not. And, of course, the disk that condenses to a star, like the ultimate stellar system that forms, is not a rigid body with a constant angular velocity, but has material moving much more rapidly towards the center. And as it collapses, conservation of angular momentum should speed things up centrally, like an ice skater tucking in her arms.

I'd say the rotation speed of the Sun was determined by the nature of the presolar nebula collapse, and that it hasn't changed substantially over time. Where would that energy go? (Of course, every rotating body produces gravitational waves, resulting in slowing... but this is extremely tiny.)

And the planets? They would have initially rotated according the the turbulence driven local collapses of material in their original orbital positions, subsequently modified by the interactions that moved them around and set them in their current orbits.

Well... in comparison with those egg-shaped B and A-type stars like Achernar, Regulus and Altair? Unlike the Sun, B- and A-type stars are young.

And how about young M-type dwarfs that are highly magnetic and have tremendous outbursts? Perhaps they spin fast, too?
Astronomy & Astrophysics wrote:

Rotation is a property shared by most celestial bodies, including stars. Stars take birth in the core of molecular clouds from the infall of spinning matter driven by self-gravity. Rotation varies in time, it can be rapid or slow, but it persists all along stellar life. Ruled by the angular momentum conservation, rotation may lead to angular momentum, matter or/and energy transport between core and outer layers.
Don't know how to read that. Does it mean that different stars are born with different angular momentums that persist throughout their (main sequence?) life times, or does it mean that the rotation of one and the same star may change over time?

We plotted (Fig. 1) the distribution of apparent rotational velocities (V sin i) as a function of the spectral type. Two stellar populations spinning at different rates can be identified: stars cooler than F7 generally rotate at angular speeds lower than 50 km/s, while hotter stars are often rotating faster than 100 km/s. Rotation is indeed competing with other physical processes, and is impacted by the interaction between the magnetic field of cooler stars and their protostellar environment, or by the development of strong stellar winds at higher effective temperatures.

So I guess you could say that stars like the Sun spin slowly compared with stars hotter than spectral class F7.

And interestingly, just as the Sun appears to be almost perfectly round, it also seems to be less active and more "benign" than most other stars of its type.



Phys.org wrote:

By cosmic standards the sun is extraordinarily monotonous. This is the result of a study presented by researchers from the Max Planck Institute for Solar System Research in the upcoming issue of Science. For the first time, the scientists compared the sun with hundreds of other stars with similar rotation periods. Most displayed much stronger variations. This raises the question whether the sun has been going through an unusually quiet phase for several millennia.

Together with colleagues from the University of New South Wales in Australia and the School of Space Research in South Korea, the MPS researchers investigated, whether the Sun behaves "normally" in comparison to other stars. This may help to classify its current activity.

To this end, the researchers selected candidate stars that resemble the Sun in decisive properties. In addition to the surface temperature, the age, and the proportion of elements heavier than hydrogen and helium, the researchers looked above all at the rotation period. "The speed at which a star rotates around its own axis is a crucial variable," explains Prof. Dr. Sami Solanki, director at MPS and co-author of the new publication. A star's rotation contributes to the creation of its magnetic field in a dynamo process in its interior. "The magnetic field is the driving force responsible for all fluctuations in activity," says Solanki. The state of the magnetic field determines how often the Sun emits energetic radiation and hurls particles at high speeds into space in violent eruptions, how numerous dark sunspots and bright regions on its surface are—and thus also how brightly the Sun shines.
The exact analysis of the brightness variations of these stars from 2009 to 2013 reveals a clear picture. While between active and inactive phases solar irradiance fluctuated on average by just 0.07 percent, the other stars showed much larger variation. Their fluctuations were typically about five times as strong. "We were very surprised that most of the Sun-like stars are so much more active than the Sun," says Dr. Alexander Shapiro of MPS, who heads the research group "Connecting Solar and Stellar Variabilities."
Yes, but those G-type stars that vary so much in brightness compared with the Sun nevertheless display the same rotation period as the Sun. So apparently the Sun's rotation rate can't explain its mild behavior.

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