Telling the difference between stars and more distant objects

[Trido (Justin)]

When viewing astronomical photographs, I have always wondered how to tell the difference between the close milky way stars and more distant objects such as other galaxies. You can tell the brighter stars because of the optical refraction (Like star points), but beyond that, I just don't know how to tell if they are stars or far mroe distant. Is there a way to tell?

[citivolus (Ric)]

You could analyse spectra to check for red/blue shifting, which would give you a baseline distance to the object as well as its composition. Proper motion (by comparing two pictures taken at different times) can also be used.

I'm not certain that there is an easy way to determine if a given pin point object is a star or something else from a single RGB or LRGB type image, though, which I suspect is what you are asking.

[theodog (Jeff)]

Originally Posted by Trido

When viewing astronomical photographs, I have always wondered how to tell the difference between the close milky way stars and more distant objects such as other galaxies. You can tell the brighter stars because of the optical refraction (Like star points), but beyond that, I just don't know how to tell if they are stars or far mroe distant. Is there a way to tell?

Apart from the rough (very rough) guide that bright objects are often closer, you can't. All objects appear to be at the same infinite distance. This is the distance that all astronomical objects come into focus.

The simplest method to determine distance is to image the same area twice, seperated by up to 6 months to look for a position shift (parallax) due to the baseline of the earth's orbit. This is beyond small scopes except for the near solar system objects.

More distant objects require special spectral and Cephied techniques.

Research Cephieds, Hertzsprung-Russell diagrams and magnitude modulus. These should help your understanding.