Movement of stars

[saac]

22 minutes ago, ollypenrice said:

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

That is wholly correct. the red shift of the object has different origins, metrical expansion of space being one, gravitational red shift being another and doppler a third. On a really physics pedandarty note - my understanding is that all objects are "born" into this universe in a state of motion - therefore no need for any initial acceleration to happen to get things up to speed as such. And again, if the object were to be accelerating then this would be shown as a distinctive signature in the successive redshift measurements. So yes we would see the redshift signature and we would be able to discern if the object is accelerating when we take another measurement.

I go back to the OP's original question though where he/she asks which direction are the stars moving relative to the Earth.  My understanding is that if we only known the red shift taken along a particular line of sight then we only know the direction of movement along that line of sight. We can say the object is moving away from us along that axis. But the star has another component of motion, as you have said, its proper motion. The proper motion is derived from the stars tangential velocity. So to have a more complete understanding of the stars direction we need to measure the tangential velocity as well - it's a vector resolution problem. 

To be honest I think we are all pushing against an open door here    It would be good to hear back from the OP with their thoughts! 

Jim 

Edited April 20 by saac

[Gfamily]

13 hours ago, saac said:

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    

Is that detecting doppler shift? I'd have thought that reduced reflection time is more likely to be used to determine when adaptive cruise control kicks in - i.e., it's proximity rather than delta speed that triggers it.

Edited April 20 by Gfamily

[saac]

12 hours ago, Gfamily said:

Is that detecting doppler shift? I'd have thought that reduced reflection time is more likely to be used to determine when adaptive cruise control kicks in - i.e., it's proximity rather than delta speed that triggers it.

With adaptive cruise control we need both a distance measurement and an indication of acceleration or deceleration in other words a speed measurement as well.  The distance measurement is given by the time of return of the radar but to determine the velocity it uses doppler. So what does this mean; once you set the trigger distance (safe distance), a car which pulls in front of you but is accelerating away will not unnecessarily trigger your brakes. While the car has pulled theoretically into your trigger distance (safe zone) the doppler shift tells the system that the car is actually accelerating away from you so no need to brake. Non doppler radar cruise control (before active cruise control) would simply take a distance measurement and would have activated your brakes. This very clever technology is one reason why car prices have gone silly over a very short period of time. Most of the acc radar units are mounted just behind the front grill behind a radar transparent window - sometimes incorporated into the car badge.  A simple low speed front end bump say in a car park can now result in a very expensive repair as the radar units are expensive to replace. 

Jim

Edited April 21 by saac

[dph1nm]

Velocity is a relative quantity, it has no absolute value - so to answer the original question you have to ask the question moving relative to what? These days it can be popular to use the cosmic microwave background, especially in cosmology, but is that useful for stars in our galaxy? I think one might consider motion relative to the centre of the galaxy to be more useful.

NigelM

[andrew s]

On 20/04/2024 at 11:07, ollypenrice said:

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.

All velocities are relative. You only have a velocity with respect to something else. Change the "something else" and you (may) change the velocity.  Being earth bound we tend to only think of a singular velocity relative to the ground.

Regards Andrew 

[Albir phil]

On 18/04/2024 at 23:31, 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.

Like as been said it depends on your perspective, imagine you are standing out in space with no stars, you know that your feet are down and your head is up but the space around you has no up or down left or right

[Xilman]

If one star is a white dwarf, it could be moving towards, away from, or be stationary relative to the Earth.

A strong gravitational field also creates a red shift which may be larger than the Doppler-induced one.

 

Edited April 23 by Xilman
Clarify

[saac]

4 hours ago, Albir phil said:

Like as been said it depends on your perspective, imagine you are standing out in space with no stars, you know that your feet are down and your head is up but the space around you has no up or down left or right

I'm not so sure that matters though. The measurement of red shift is a relative velocity measurement - it already ecompases the relative information by default. If the light is redshift then we know the object is moving away from us. If you want a direction (polar,  cartesian or otherwise) then you would need to specify a reference axis. You would also need to determine proper motion of the star as well. 

Jim

[saac]

Just now, Xilman said:

If both stars are white dwarfs, either could be moving towards, away from or stationary relative to the Earth.

A strong gravitational field also creates a red shift which may be larger than the Doppler-induced one.

 

If orbiting a black hole -  one week they could be coming towards us, the next moving away  

Jim 

[Earl]

Are they moving or is the space expanding in which they are, its a bot of both IIRC the greater the distance the more the spacial effect.

 

[vlaiv]

5 minutes ago, Xilman said:

If one star is a white dwarf, it could be moving towards, away from, or be stationary relative to the Earth.

A strong gravitational field also creates a red shift which may be larger than the Doppler-induced one.

 

I always wondered how large this effect is - particularly on galactic scales.

For example - when computing Hubble's law - do we have to take into account relative difference between galaxy masses? Origin galaxy and MW?

When light leaves origin galaxy it will be red shifted, but then when it "falls into" MW it will be blue shifted - difference between those two will be some percent of total red shift - but how large is the effect?

[andrew s]

1 hour ago, vlaiv said:

I always wondered how large this effect is - particularly on galactic scales.

For example - when computing Hubble's law - do we have to take into account relative difference between galaxy masses? Origin galaxy and MW?

When light leaves origin galaxy it will be red shifted, but then when it "falls into" MW it will be blue shifted - difference between those two will be some percent of total red shift - but how large is the effect?

See here the sun's gravitational [removed word] is approximately equivalent to 633 m/s receeding. Regards Andrew 

Removed word was shift !

Edited April 23 by andrew s