# Thread: Proper motion of stars

1. Member
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Oct 2018
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## Proper motion of stars

This question has been bugging me for a while. Stars orbiting around the galaxy are going to have a range of differing orbital characteristics relative to the galactic plane. We can then predict that from our perspective they will have apparent motion over very, very long time scales. This all seems obvious. But how is that calculated? Are parallax measurements or changes in luminosity that precise to determine how another star is moving relative to us?

2. Doppler effect tells us their velocity towards or away from us; measuring the change in their angular coordinates over time (which is the meaning of "proper motion") gives us transverse velocity when it's combined with parallax (for distance). And combining radial and transverse velocity gives us "peculiar velocity", which is the speed of a particular star relative to the Local Standard of Rest (which is the mean motion of star systems in the vicinity of the sun). Knowing the peculiar velocity lets us work out the galactic orbits of these stars.

Lots of estimates, though. The farther away a star is, the less angular displacement for a given velocity, so these estimates are only much good for relatively nearby stars, or stars that are moving very quickly. And then there's the mass distribution of the galaxy, and our distance from the centre of mass of the galaxy, which are still evolving estimates.

Grant Hutchison

This question has been bugging me for a while. Stars orbiting around the galaxy are going to have a range of differing orbital characteristics relative to the galactic plane. We can then predict that from our perspective they will have apparent motion over very, very long time scales. This all seems obvious. But how is that calculated? Are parallax measurements or changes in luminosity that precise to determine how another star is moving relative to us?
Are you asking more about the changes due to the acceleration during the bobbing action of stars across the galactic plane? Otherwise, Grant has nailed it nicely.

If you get the astrometry of stars (found on Wiki if you choose) then it's simple vector math to determine their future if you are limiting the time frame to thousands of years in most cases. Non-singular star systems can be far more complicate since their proper motions have acceleration components as they orbit one another.

4. Member
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Originally Posted by George
Are you asking more about the changes due to the acceleration during the bobbing action of stars across the galactic plane? Otherwise, Grant has nailed it nicely.

If you get the astrometry of stars (found on Wiki if you choose) then it's simple vector math to determine their future if you are limiting the time frame to thousands of years in most cases. Non-singular star systems can be far more complicate since their proper motions have acceleration components as they orbit one another.
Yep, I think Grant has a nice answer here for my brain even if it's a bit beyond my scope. I believe I was not taking into consideration just how precise observations can really be. I was googling around and came across a few pictures that showed just how far Barnard's star has moved relative to much more distant stars and it was very noticeable. But that's a fairly nearby star as well.

Yep, I think Grant has a nice answer here for my brain even if it's a bit beyond my scope. I believe I was not taking into consideration just how precise observations can really be. I was googling around and came across a few pictures that showed just how far Barnard's star has moved relative to much more distant stars and it was very noticeable. But that's a fairly nearby star as well.
Re: Barnard's, you may enjoy this thread here on the forum.