Movement of stars

[Space Traveller]

Hi All,

this is something that puzzles me about stars and their movement in relationship to Earth, consider this.

Star A ------ Earth ----- Star B

The light spectrum from both stars is shifted towards the red end of the spectrum so my question is, in which direction are these stars moving?

Dave R

[AstroKeith]

They are both travelling away from the Earth

[Carbon Brush]

You asked this question in January. It was answered.
Are we missing something?

[Space Traveller]

Sorry all, I've been very sick the last few months or so in hospital undergoing open heart surgery and forgot where I posted this question.

Yes, they are both moving away from earth but in which direction - is Star A moving to the left and Sta B to the right.

If that is correct, how do we know.

[cajen2]

20 minutes ago, Space Traveller said:

Sorry all, I've been very sick the last few months or so in hospital undergoing open heart surgery and forgot where I posted this question.

Yes, they are both moving away from earth but in which direction - is Star A moving to the left and Sta B to the right.

If that is correct, how do we know.

Because they're both red shifted - Doppler effect shows they're both moving away from us.

[Space Traveller]

Sure they are moving away from us but in which direction.

Based on the basic diagram is Star A moving to the right and Star B moving to the left.

Consider this, Star B is moving to the right at a speed of 6 units.  The earth is moving to the right at a speed of 4 units and Star A is also moving to the right at a speed of 2 units, thus the stars show a Red-Shift.

However when we observe the stars, we are studying them from what I would describe as 'Thumb-Tack Earth', and there is nothing we can do about it.

[Stu]

15 minutes ago, Space Traveller said:

Sure they are moving away from us but in which direction.

Based on the basic diagram is Star A moving to the right and Star B moving to the left.

Consider this, Star B is moving to the right at a speed of 6 units.  The earth is moving to the right at a speed of 4 units and Star A is also moving to the right at a speed of 2 units, thus the stars show a Red-Shift.

However when we observe the stars, we are studying them from what I would describe as 'Thumb-Tack Earth', and there is nothing we can do about it.

Everything has to be based on a frame of reference I think, as there are no fixed points in space. From Earth, the would be moving away (simplistically left and right in your diagram), but from another frame of reference they could all be moving right at different speeds, both could be correct.

I’m sure someone far more intelligent will be along to help soon, where is @vlaiv when you need him 😉

[Stu]

Anyway, you are a space traveller, shouldn’t you know this stuff already 😉😉🤣🤣

[vlaiv]

19 minutes ago, Space Traveller said:

Sure they are moving away from us but in which direction.

Based on the basic diagram is Star A moving to the right and Star B moving to the left.

Consider this, Star B is moving to the right at a speed of 6 units.  The earth is moving to the right at a speed of 4 units and Star A is also moving to the right at a speed of 2 units, thus the stars show a Red-Shift.

However when we observe the stars, we are studying them from what I would describe as 'Thumb-Tack Earth', and there is nothing we can do about it.

Depends on where you put observer.

If you stand on earth - then earth is moving with 0 units of speed relative to you and thus star A must move to the left and star B must move to the right.

For some external observer, standing near star C - it can indeed appear that all three are moving to the left or to the right or some other combination - because motion is relative to the observer.

[Stu]

2 minutes ago, vlaiv said:

Depends on where you put observer.

If you stand on earth - then earth is moving with 0 units of speed relative to you and thus star A must move to the left and star B must move to the right.

For some external observer, standing near star C - it can indeed appear that all three are moving to the left or to the right or some other combination - because motion is relative to the observer.

Did I pass the test then Vlad?? 🤞🤞

[vlaiv]

3 minutes ago, Stu said:

Did I pass the test then Vlad?? 🤞🤞

Only if you do the analogy with moving train, three people on the train - one seated, one exiting car on one side and other exiting car on the other side versus bystander watching the train go by

Who will see what?

[ollypenrice]

 

I'm always nervous around Vlad (despite his being such a nice chap! ) but here goes...

There are two kinds of redshift.  Though they are mathematically equivalent, their origins differ - because there are two kinds of 'movement.'

1) The Doppler redshift. This is created when the star or the observer (or both) are accelerated by some force. This kind of acceleration must have a direction and we are familiar with 'movement' in this sense.  In this case we would talk of the star's proper motion. (It's own motion.)

2) The cosmological redshift. This applies on cosmological scales, outside gravitationaly bound systems, and does not arise from an object's being accelerated.  Although the galaxies on this scale are moving away from each other, each one considers itself to be more or less at rest (ignoring the fact that they are all whirling around to some extent in curved spacetime.) In this unfamiliar situation each galaxy experiences no movement of itself but sees all the others as moving away from it as the space expands between them.  Describing the way in which they move apart from each other requires a metric for the expansion of spacetime. Any one galaxy will find it easy enough to describe all the others as moving away from it along radiating lines like shrapnel flying from an explosion. That description, however, will not work for more than one galaxy because each one will see itself as being at the centre of an explosion - and they can't all be right!

Olly

 

Edited April 19 by ollypenrice

[vlaiv]

27 minutes ago, ollypenrice said:

1) The Doppler redshift. This is created when the star or the observer (or both) are accelerated by some force. This kind of acceleration must have a direction and we are familiar with 'movement' in this sense.  In this case we would talk of the star's proper motion. (It's own motion.)

No need for acceleration - just uniform motion is enough to produce Doppler shift.

On the other hand, red shift (or blue shift for that matter) - can be due to acceleration - but we might not need force for that either - curvature of space time is enough to produce the effect. Light emitted from vicinity of large mass will be red shifted to observer that is far away, while light produced by observer floating in intergalactic space and observed near massive body (or even inside the galaxy) will be blue shifted due to it "falling" into gravitational potential well of mass concentration.

This is closely related to time dilation effects by the way (think of laser producing exact wavelength of light and those oscillations being slowed down due to time dilation and thus producing longer wavelengths - red shift).

[ollypenrice]

1 hour ago, vlaiv said:

No need for acceleration - just uniform motion is enough to produce Doppler shift.

 

But where did this uniform motion originate?

Olly

[vlaiv]

22 minutes ago, ollypenrice said:

But where did this uniform motion originate?

Olly

It does not matter.

Consider this case:

We have a source of waves moving on a line from our right to left like in image above. At first - it is approaching - so there is component of motion that is in our direction. Source is blue shifted. At the mid point - source is still moving at constant speed - but component of motion in our direction is zero - so we receive normal frequency. After it passes this point - it starts moving away from us - now there is component of motion in our direction that is away from us - source is red shifted.

No acceleration takes place in above case - but we have 3 different scenarios happening: from blue shift thru no shift to red shift. This shows that Doppler effect in its essence is not related to acceleration (that once must have happened for object to be moving with respect to us) - but rather to component of its relative speed in our direction.

This is regular red shift - and what we observe when we look at for example rotational curves of galaxies (stars moving from/away depending if they are on one side or other side of the galaxy). It also happens when ambulance passes by - we first hear high pitched siren, then regular, then low pitched siren ...

There are other two sources of red shift - gravitational (or the one related to acceleration / curvature of space time) and cosmological red shift - which happens due to expansion of space. Last one is "equivalent" to regular red shift - as if galaxy is really moving away from us at certain speed. There are only a few indicators that the space is actually expanding - like fact that we can have different places recede faster than the speed of light - which would not be possible if galaxies were moving thru the space instead of space expanding.

Edited April 19 by vlaiv

[saac]

Doppler redshift is a measure of the stars motion due to velocity not acceleration. and it is measured along the axial line of sight. However stars also have a tangential velocity from which they derive "proper motion"  To determine the  combined motion of the star one would need both properties. Typically a measure of the star's proper motion would be taken by position measurement taken a few years apart.  So to answer the OPs question if both stars show red shift then all we can conclude is that their axial line of sight motion is away from us. To determine their true motion relative to Earth would also need information on their tangential velocity. 

Jim 

Edited April 19 by saac

[ollypenrice]

40 minutes ago, vlaiv said:

It does not matter.

Consider this case:

We have a source of waves moving on a line from our right to left like in image above. At first - it is approaching - so there is component of motion that is in our direction. Source is blue shifted. At the mid point - source is still moving at constant speed - but component of motion in our direction is zero - so we receive normal frequency. After it passes this point - it starts moving away from us - now there is component of motion in our direction that is away from us - source is red shifted.

No acceleration takes place in above case - but we have 3 different scenarios happening: from blue shift thru no shift to red shift. This shows that Doppler effect in its essence is not related to acceleration (that once must have happened for object to be moving with respect to us) - but rather to component of its relative speed in our direction.

This is regular red shift - and what we observe when we look at for example rotational curves of galaxies (stars moving from/away depending if they are on one side or other side of the galaxy). It also happens when ambulance passes by - we first hear high pitched siren, then regular, then low pitched siren ...

There are other two sources of red shift - gravitational (or the one related to acceleration / curvature of space time) and cosmological red shift - which happens due to expansion of space. Last one is "equivalent" to regular red shift - as if galaxy is really moving away from us at certain speed. There are only a few indicators that the space is actually expanding - like fact that we can have different places recede faster than the speed of light - which would not be possible if galaxies were moving thru the space instead of space expanding.

I agree that redshift/blueshift don't need acceleration (as Jim also points out) but a body's proper motion does, surely, need to originate with an acceleration? What I'm getting at is that the OP's question is about movement since he asks in which direction the stars are moving. Is there not a fundamental difference between a body 'moving' because of the expansion of space and 'moving' because it has, at some point, been accelerated?  If not, why do cosmologists distinguish between Doppler and Cosmological redshift?

Olly

[vlaiv]

17 minutes ago, ollypenrice said:

I agree that redshift/blueshift don't need acceleration (as Jim also points out) but a body's proper motion does, surely, need to originate with an acceleration? What I'm getting at is that the OP's question is about movement since he asks in which direction the stars are moving. Is there not a fundamental difference between a body 'moving' because of the expansion of space and 'moving' because it has, at some point, been accelerated?  If not, why do cosmologists distinguish between Doppler and Cosmological redshift?

Olly

I'm not sure that OP is concerned about origin of motion, but rather the fact that there is no preferred direction.

If you look at this post:

7 hours ago, Space Traveller said:

Consider this, Star B is moving to the right at a speed of 6 units.  The earth is moving to the right at a speed of 4 units and Star A is also moving to the right at a speed of 2 units, thus the stars show a Red-Shift.

However when we observe the stars, we are studying them from what I would describe as 'Thumb-Tack Earth', and there is nothing we can do about it.

I believe it shows what is the heart of the question. We tend to study stars from earth and study their motion relative to us.

Someone else might see different motion - after all motion is relative, and their Doppler shift would be in agreement with what they observe - as movement is relative.

[saac]

1 hour ago, ollypenrice said:

I agree that redshift/blueshift don't need acceleration (as Jim also points out) but a body's proper motion does, surely, need to originate with an acceleration? What I'm getting at is that the OP's question is about movement since he asks in which direction the stars are moving. Is there not a fundamental difference between a body 'moving' because of the expansion of space and 'moving' because it has, at some point, been accelerated?  If not, why do cosmologists distinguish between Doppler and Cosmological redshift?

Olly

In truth stars will have an acceleration component to their motion for a number of reasons; for example the rotational motion around their galaxy's centre of mass. That change in direction by definition means the velocity is changing (even if there was a constant speed), in turn by definition acceleration. This is what we measure when we detect a star's radial doppler shift as it rotates in its orbit around the centre of mass.  So at some point we will detect a red shift and at others a blue shift, and inbetween a changing red and blue shift.

A star that is not gravitationally bound to its galaxy, so called rogue or intergalactic stars, will travel through space at a constant speed unless they come into the influence of say a black hole or other concentration of mass. Here the star will similarly accelerate until it either collides or is caught in orbit. But a star that is accelerating like that, say along our axial line of sight, would present with an ever changing red shift/blue shift. 

Just as you said red shift can also be caused by the cosmological expansion of space (recessional velocity) independent of the star's relative motion to us due to its motion through space.  I'm not sure how they differentiate between the two, most likely the red shift due to expansion is a known component and rather like a background count rate of radiation it can be separated from the "motion" red shift - I guessing here.   We need an input from Andrew  S here. 

 

ps - just an after thought.  From what I understand cosmological expansion does not impart motion onto bodies in space - rather it is the space between these objects that is expanding -  the motion of stars remain dominated by gravity. Any motion that a star has then comes from either conservation of the momentum from the gas cloud from which it formed or from a gravitational attraction.  As far as I'm aware cosmological expansion does not impart motion onto the galaxies/stars.

 

Jim 

Edited April 19 by saac

[ollypenrice]

8 hours ago, saac said:

 

ps - just an after thought.  From what I understand cosmological expansion does not impart motion onto bodies in space - rather it is the space between these objects that is expanding -  the motion of stars remain dominated by gravity. Any motion that a star has then comes from either conservation of the momentum from the gas cloud from which it formed or from a gravitational attraction.  As far as I'm aware cosmological expansion does not impart motion onto the galaxies/stars.

 

Jim 

This was my point. The galaxies do move apart from each other without the need for proper motion.

It seems to me that there are two ways of looking at the 'motion' imparted by the Hubble flow. We can take the point of view of an observer on a single galaxy and see radial recession on the part of the others, or we can conceive of galaxies within a universal expansion. We cannot observe this conception because an observer, of course, is pinned to a single location.

Olly

Edited April 20 by ollypenrice

[FunkyKoval35]

12 hours ago, saac said:

In truth stars will have an acceleration component to their motion for a number of reasons; for example the rotational motion around their galaxy's centre of mass. That change in direction by definition means the velocity is changing (even if there was a constant speed), in turn by definition acceleration. This is what we measure when we detect a star's radial doppler shift as it rotates in its orbit around the centre of mass.  So at some point we will detect a red shift and at others a blue shift, and inbetween a changing red and blue shift.

It doesn't matter whether the mass is accelerating or not. The Doppler Effect is a function of velocity, not acceleration. When you measure a spectral shift, you do it at a specific point in time at which the mass has a specific velocity - even if it is accelerating.

[saac]

4 minutes ago, FunkyKoval35 said:

It doesn't matter whether the mass is accelerating or not. The Doppler Effect is a function of velocity, not acceleration. When you measure a spectral shift, you do it at a specific point in time at which the mass has a specific velocity - even if it is accelerating.

If the object was accelerating through then as we take and then retake the red shift measurement we would see it progressively change reflecting the change in velocity. 

Jim 

Edited April 20 by saac

[FunkyKoval35]

10 minutes ago, saac said:

If the object was accelerating through then as we take the red shift measurement we would see it progressively change reflecting the change in velocity. 

Yes, but still the Doppler Effect is a function of velocity. You will see progressive change in velocity but you have to do at least two mesurments of Doppler Effect. For each of them you will get diffrent velocities.

Edited April 20 by FunkyKoval35

[ollypenrice]

7 minutes ago, FunkyKoval35 said:

It doesn't matter whether the mass is accelerating or not. The Doppler Effect is a function of velocity, not acceleration. When you measure a spectral shift, you do it at a specific point in time at which the mass has a specific velocity - even if it is accelerating.

I realize that the Doppler redshift is a product of velocity, not acceleration, but my point is that, to have a velocity, an object must at some point have been accelerated.

The cosmological redshift, however, is not introduced by velocity but by the expansion of the space through which the light is travelling.  While following the same formulae, these redshifts are not of the same origin.

Olly

[saac]

Just now, FunkyKoval35 said:

Yes, but still the Doppler Effect is a function of velocity. You will see progressive change in velocity but you have to do at least two mesurments of Doppler Effect. For each of them you will get diffrent velicities.

Correct it is, although not exclusively as has been commented on.  As such, any red shift measurement taken along the line of sight on an accelerating object will reflect that changing velocity (hence change in red shift). It is a well practiced technique - here on Earth it comes in handy in my car to stop me bumping into the car in front when I have cruise control on  

Jim