# Thread: Minimum peculiar motion

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## Minimum peculiar motion

What is the star with slowest peculiar motion, where all components are known, including lower bound?

2. What do you mean by "peculiar motion" and "lower bound"?

3. Originally Posted by Hornblower
What do you mean by "peculiar motion" and "lower bound"?
Probably Wikipedia: Peculiar velocity

In physical cosmology, the term peculiar velocity refers to the components of a receding galaxy's velocity that cannot be explained by Hubble's law.
Wikipedia: Comoving distance

The velocity of an observer relative to the local comoving frame is called the peculiar velocity of the observer.

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I thought that "peculiar motion" is the motion of a star or any other heavenly body with respect to Sun - unlike "proper motion", which is movement along the heavenly sphere, "peculiar motion" is three-dimensional.

5. Order of Kilopi
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I believe chornedsnorkack is referring to the original usage of "peculiar motion", which is the velocity of a star relative to the Local Standard of Rest. It's derived from the observed values of proper motion, distance and radial velocity, corrected for the sun's (inferred) state of motion.

Edit: I see chornedsnorkack replied while I was typing. Chornedsnorkack, peculiar motion is not usually given relative to the sun, but relative to the Local Standard of Rest, which moves around the galaxy at the local circular orbital velocity. For this reason, the sun has a non-zero peculiar motion.

Grant Hutchison

6. Originally Posted by chornedsnorkack
I thought that "peculiar motion" is the motion of a star or any other heavenly body with respect to Sun - unlike "proper motion", which is movement along the heavenly sphere, "peculiar motion" is three-dimensional.
How about ScienceWorld: Peculiar motion (Peculiar velocity):

The random motion of a galaxy with respect to the Hubble recessional velocity.
There's this old BAUT topic Peculiar Motion, where the denizens tried to come up with a better term.

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Where is the Sun currently located with respect to the spiral arms of Milky Way?

And the original question can then be restated as: which stars have been noted for having very slow speed with respect to the Sun?

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Originally Posted by chornedsnorkack
Where is the Sun currently located with respect to the spiral arms of Milky Way?
We're near the inner edge of what used to be called the Orion Arm, which now seems to be called (in the current two-armed model of the galaxy) the Orion Spur.
The distance to the inner edge of the Orion Spur is seven or eight hundred light years, if you take the shortest route: just the other side of the Coalsack Nebula.

Grant Hutchison

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Originally Posted by grant hutchison
We're near the inner edge of what used to be called the Orion Arm, which now seems to be called (in the current two-armed model of the galaxy) the Orion Spur.
The distance to the inner edge of the Orion Spur is seven or eight hundred light years, if you take the shortest route: just the other side of the Coalsack Nebula.

Grant Hutchison
What is the angle between spiral arms and direction to the centre?

Taking the direct line between Sun and galactic centre, what is the distance from Sun to the midline of Orion Arm, outward of the Milky Way? And what is the distance to the midline of the Sagittarius Arm, to the inward of the Sun?

10. Originally Posted by chornedsnorkack
What is the angle between spiral arms and direction to the centre?
You are here: see images at NASA Spitzer Space Telescope: Our Milky Way Gets a Makeover (Annotated) (From topic Two of the Milky Way's Spiral Arms Go Missing, which links to other older images.)

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