# Thread: Fast moving stars and parallax

1. ## Fast moving stars and parallax

A recent thread here on this board, Cool Image: New Nearby Star Discovered made me wonder: How do astronomers know if a star that has moved much on their photographic plates is a close star, or just a star that moves really fast through space?

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Stuff like this always interests me

3. Well, if it's moved in exactly a year (i.e., the Earth is in the same place as before), then it has to be a proper motion. If it moves in six months and then moves back, it's parallax.

4. different stars, even of the same color (surface temperature), have different spectra. So, even though a red dwarf and a red giant may be the same "color", we can still tell the difference between a close dwarf and a distant giant.

5. Usually, it's a combination of high speed and proximity; many nearby stars have a relatively high proper motion. However, the parallax is key to determining true velocity; if it's been shown to be distant but still has a high proper motion, then it's moving very quickly.

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## Re: Fast moving stars and parallax

Originally Posted by Padawan
A recent thread here on this board, Cool Image: New Nearby Star Discovered made me wonder: How do astronomers know if a star that has moved much on their photographic plates is a close star, or just a star that moves really fast through space?
To know which it is, astronomers have to measure the star's distance. The high proper motion alerts them to the possibility that the star is nearby.

A star's proper motion is 0.688 (V/D), where V is the tangential velocity--the velocity across our line of sight--in kilometers per second, D is the star's distance in light-years, and the proper motion is expressed in arcseconds per year. If Crimson is permitted to say that 2/3 = 0.688, then this equation simplifies to the more memorable:

Proper motion = (2/3)(V/D).

So a star can have a large proper motion if its tangential velocity V is large or its distance D is small.

As excellent examples, consider Alpha Centauri and Mu Cassiopeiae. Both stars have nearly the same large proper motion. But Mu Cassiopeiae, located 25 light-years from Earth, is nearly six times farther than Alpha Centauri. Thus, its tangential velocity is nearly six times greater than Alpha Centauri's. Alpha Centauri has an ordinary velocity, so its large proper motion is due to its proximity; but Mu Cassiopeiae's large proper motion is due to its high tangential velocity.

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