Thank you for understanding. Have a great day!

[phinds]

If we imagine in some inertial reference frame there are two objects in relative motion,
are there inertial reference frames in which the two objects are not in relative motion?

Any frame in which one of the objects is not in motion is going to be a direct translation to one of the original frames so by definition the other object will have to be in motion in that frame, so no.

 

[bahamagreen]

That's what I thought... so with regard to the two objects can you say, "There is motion" without qualifying it as some kind of relative motion?
What would you call this intrinsic motion that can't be removed by switching frames and does not depend on absolute rest with which to be relative?

 

[phinds]

That's what I thought... so with regard to the two objects can you say, "There is motion" without qualifying it as some kind of relative motion?
What would you call this intrinsic motion that can't be removed by switching frames and does not depend on absolute rest with which to be relative?

There IS NO "intrinsic motion". All motion is relative and it is always incorrect to state velocity without stating what that velocity is relative to. You call any motion just "motion" but you have to say what it is relative to.

If you have object A and in its rest frame there is an object B that is moving at 100mph, there are an infinite number of other inertial frames in which object B has a corresponding infinite number of velocities.

 

[bahamagreen]

There IS NO "intrinsic motion". All motion is relative and it is always incorrect to state velocity without stating what that velocity is relative to. You call any motion just "motion" but you have to say what it is relative to.

If you have object A and in its rest frame there is an object B that is moving at 100mph, there are an infinite number of other inertial frames in which object B has a corresponding infinite number of velocities.

May we go one step at a time? How do you logically disagree with this:

If there are no IFRs in which the two objects aren't in motion,
then one can conclude, "There is motion"
without stipulating either an absolute or relative reference.

 

[phinds]

May we go one step at a time? How do you logically disagree with this:

If there are no IFRs in which the two objects aren't in motion,
then one can conclude, "There is motion"
without stipulating either an absolute or relative reference.

Well, you can never legitimately say that there is motion of ONE BODY without saying what it is in motion relative to. Yes, you can say that two objects are in motion relative to each other but I don't see that that gets you anything. If you want to talk about one of the bodies you STILL have to say what it is in motion relative to, you cannot simple say "it is in motion".

 

[Dale]

May we go one step at a time? How do you logically disagree with this:

If there are no IFRs in which the two objects aren't in motion,
then one can conclude, "There is motion"
without stipulating either an absolute or relative reference.

You can probably say that they are moving relative to each other. That can be said without specifying any specific inertial frame because it is true in all inertial frames. However, technically it may not be true since you could construct non inertial frames where they are both at rest (assuming they don’t collide). But failing to qualify it is unlikely to cause confusion.

 

[lomidrevo]

I wonder how accurately one can identify halo stars from spectroscopy only.

I've just recently read about the kinematics of the Milky Way, and what I understood is that info about metallicity can be combined with observed peculiar motion of the stars (relative to the local standard of rest LSR). In particular, stars with large value of velocity component toward the galactic pole (perpendicular to the disk) are mainly to be identified as old, metal-poor stars, like red dwarfs. These stars are considered as being memebers of the stellar halo. So as I understand it, spectroscopy alone is not sufficient.

 

[alantheastronomer]

info about metallicity can be combined with observed peculiar motion of the stars

Yes, the two methods are used in concert, reinforcing each other's findings. However, low metallicity alone is the very definition of population II halo stars. The high observed relative velocities are an artifact of their being halo stars, not the prime indicator. Pop. I stars might have a peculiar velocity mimicking a halo star; it's metallicity can then be used to rule it out. The textbook "Galactic Astronomy" by Mihalas and Binney describes these methods. The relative motion that the OP hadn't realized he was referring to, was with respect to the center of the galaxy.

 

[Tom.G]

Just to confound matters.

If you are on an object in free space and you suddenly experience an increase (or decrease) in apparent gravity due to acceleration of the object, would it be appropriate to conclude that the motion of the object has changed?

 

[lomidrevo]

However, low metallicity alone is the very definition of population II halo stars. The high observed relative velocities are an artifact of their being halo stars, not the prime indicator. Pop. I stars might have a peculiar velocity mimicking a halo star; it's metallicity can then be used to rule it out.

But, vice versa, isn't the case that metal-poor, population II stars can be also part of the galactic thick disk or bulge? If yes, you would need the peculiar velocity measurements anyway to filter them out when identifying the actual halo stars. I must fully agree with your first sentence :)

Yes, the two methods are used in concert, reinforcing each other's findings.

...
Maybe dividing the stars only into two categories (population I and II) is somehow artificial in this context, in reality there is no strict border and the metallicity is changing continually. The ranges of star's metallicity might be somehow overlapping in various galactic structures.

 

[lomidrevo]

Just to confound matters.

If you are on an object in free space and you suddenly experience an increase (or decrease) in apparent gravity due to acceleration of the object, would it be appropriate to conclude that the motion of the object has changed?

I am not sure I understand the situation you described. What do you mean by apparent gravity? You and the object would experience the same change of the gravitational acceleration. If you were at rest relative to the object before the change, you would be at rest to the object after the change of the gravitational field.

 

[phinds]

Just to confound matters.

If you are on an object in free space and you suddenly experience an increase (or decrease) in apparent gravity due to acceleration of the object, would it be appropriate to conclude that the motion of the object has changed?

Yes, because for an increase in acceleration to be due to an actual increase in gravity, that would require magic to suddenly increases the mass of the object you are on so if you feel acceleration you know you are changing velocity.

 

[alantheastronomer]

But, vice versa, isn't the case that metal-poor, population II stars can be also part of the galactic thick disk or bulge? If yes, you would need the peculiar velocity measurements anyway to filter them out when identifying the actual halo stars. I must fully agree with your first sentence :)...
Maybe dividing the stars only into two categories (population I and II) is somehow artificial in this context, in reality there is no strict border and the metallicity is changing continually. The ranges of star's metallicity might be somehow overlapping in various galactic structures.

Yes, you're absolutely right; you need kinematics to separate the bulge stars from the halo. I'm glad you brought up the thick disk; it's an area of controversy that persists to this day! The thick disk is an observed density enhancement out to roughly 1000kpc (while the thin disk extends only 400kpc). There are two major theories for the origin of the thick disk; 1) As the early galaxy settled down from a spherical distribution, a second generation of star formation occurred in a thick disk just before finally becoming the thin spiral disk.2) Stars from the thin disk are being kinematically "slingshot" by Giant Molecular Clouds (GMC's) out of the plane of the Milky Way(like a satellite being given a gravity assist by a giant planet) "puffing up" into a thick disk.
Recently, abundance measurements seem to favor the former. So you're also right in that the thick disk should kind of be looked at as a sort of pop. 1+1/2...

The pop. II stars, including halo, bulge, and globular clusters, were all formed in one fell swoop, roughly all at the same time and so are considered one population. Their metallicities don't change. The thin disk on the other hand is comprised of a significant amount of gas, and so star formation is continually ongoing. As heavier elements are continuously being produced and mixed into the gas, the metallicity of the disk stars is continuously changing; as you mention.

 

[rootone]

Please read my statements very carefuly. I SAID THAT we don't look for relative speed, we looking for its true speed. relative speed is just comparison of two movement speed of objects. WE measure object speed in SPACE. You can transform movement speed to termodynamical energy, and it going to gave its own units. if you cannot feel space, its your own psychological problem

I think where you are going wrong is in having an idea that there some absolute frame of reference.
There isn't.

 

[phinds]

I think where you are going wrong is in having an idea that there some absolute frame of reference.
There isn't.

The OP CLEARLY didn't want to hear that (have you followed this thread? We said that to him over and over). He has left the building. See post #21

 

[lomidrevo]

The pop. II stars, including halo, bulge, and globular clusters, were all formed in one fell swoop, roughly all at the same time and so are considered one population. Their metallicities don't change. The thin disk on the other hand is comprised of a significant amount of gas, and so star formation is continually ongoing. As heavier elements are continuously being produced and mixed into the gas, the metallicity of the disk stars is continuously changing; as you mention.

Yes, that makes sense to me. However, below is a snapshot of a table from a textbook I am reading, Carroll&Ostlie. The metallicity is expressed here as iron-to-hydrogen ratio of the observed star compared to the Sun's value [Fe/H]:
$$
[Fe/H] = \log_{10} \left [ \frac{(N_{Fe}/N_H)_{star}}{(N_{Fe}/N_H)_{sun}} \right ]
$$
Halo: from -5.4 to -0.5
Bulge: from -2.0 to +0.5
(Thick disk: from -2.2 to -0.5)

That is what I had in my mind, when mentioned changing and overlapping of metallicities in my previous post. The declared range for Halo stars is quite wide, isn't it? Indeed, I am not sure how accurate or up-to-date are the data in the table, when browsing I found slightly different values from different sources, eg. this one.

But what is also interesting, the authors mention in the textbook, that correlation between age and [Fe/H] may not be 100% reliable. They suggest that some astronomers rather use [O/H] or [O/Fe] ratios. Probably it is not easy to define stellar populations with such a variety of parameters :)

If you talk about Population II stars, do you mean stars with [Fe/H] very close to -5?

 

[Tom.G]

(warning! Some Devil's advocacy here.)

I am not sure I understand the situation you described. What do you mean by apparent gravity? You and the object would experience the same change of the gravitational acceleration. If you were at rest relative to the object before the change, you would be at rest to the object after the change of the gravitational field.

Yes, because for an increase in acceleration to be due to an actual increase in gravity, that would require magic to suddenly increases the mass of the object you are on so if you feel acceleration you know you are changing velocity. (emphasis added)

I was referring to the "true speed" phrase. As an example if you are on an asteroid in free space with no other objects, and experience a change in force holding you to the asteroid. Knowing that a change in gravity is not a viable assumption, a likely conclusion would be that there was an outgassing acting as a jet to change the asteroid course. With no other objects available for reference, then there is no 'speed' (velocity) either before or after the course change, yet something changed as shown by the delta acceleration that was felt.

Can somebody define what changed if it was not 'speed'?

 

[lomidrevo]

(warning! Some Devil's advocacy here.)I was referring to the "true speed" phrase. As an example if you are on an asteroid in free space with no other objects, and experience a change in force holding you to the asteroid. Knowing that a change in gravity is not a viable assumption, a likely conclusion would be that there was an outgassing acting as a jet to change the asteroid course. With no other objects available for reference, then there is no 'speed' (velocity) either before or after the course change, yet something changed as shown by the delta acceleration that was felt.

Can somebody define what changed if it was not 'speed'?

Yes, there would be some change of speed. Initially, the 3 reference frames of you, the asteroid and the gasses were all at rest, relative to each other. Assuming, that you and the asteroid remain in relative rest, after releasing the gasses, you could measure the relative velocity between your frame of reference and the one of released gasses.

Btw. the outgassing itself (more typical for comets), would require some source of additional thermal energy, so the space couldn't be empty anyway -> additional frame of reference to compare with, maybe? [emoji846]

 

[alantheastronomer]

The declared range [of metallicities] for Halo stars is quite wide, isn't it? That is what I had in mind, when mentioning changing and overlapping of metallicities...

Yes that's basically because they're all from the same population. Unlike terrestrial physics, because we're limited by our observations and can't tailor our experiments to what we're trying to determine, some areas of astronomy are unfortunately imprecise. Measured iron abundances vary widely from globular cluster to globular cluster, and even within the same cluster. These variations are not errors of measurement, they're what's found in nature. They show that all these structures are all of the same population, and they're only differentiated by their kinematics. It also shows that the thick disk was formed concurrently and not formed by larger abundance disk stars kicked up out of the galactic plane.

The authors mention...that correlation between age and [Fe/H] may not be 100% reliable. They suggest that [we] rather use [O/H] or [O/Fe] ratios.

Yes this was some time ago and they made a good suggestion. Iron alone is just produced in supernovae, while oxygen is formed (along with carbon and nitrogen) in lower mass stars that die as planetary nebulae and white dwarfs. Using all three abundance markers together then gives a more accurate picture of abundance evolution. Ages are still mostly determined by finding a population's turn off point on the main sequence, but this can still vary by as much as a billion years!

 

[Tom.G]

On star metallicities, see: https://phys.org/news/2018-08-stars-rebirth-home-galaxy.html

 

[fresh_42]

Thread closed.

As we apparently discuss a complete different topic now, it is time to end this one. If you wish to go on discussing metallicity please create a new thread with that topic.