[jliptzin]

I have always wondered, how do astronomers identify a particular star? I assume there’s some kind of coordinate system, but how do they keep track of everything with star systems revolving around galaxies which itself are also moving and/or rotating as well as the earth moving and the universe always expanding?

[lacker]

Well in practice there are Python libraries to do it for you ;-)

For example Simbad:

https://astroquery.readthedocs.io/en/latest/simbad/simbad.ht...

Stars and various other objects get string ids like "HIP19550"

https://simbad.u-strasbg.fr/simbad/sim-id?protocol=html&Iden...

Then there are a bunch of different coordinate systems, you usually want to be doing lookups like "I have this particular telescope at this particular location and orientation, how should I configure it to point at this celestial object" and there is typically some astropy library function that will figure that stuff out for you.

In practice the stuff outside our solar system doesn't change its apparent motion much from our point of view. The further away an object, the less its apparent location changes. So these databases get updated every once in a while.

[jliptzin]

Makes sense, thanks! So I guess that means keeping track of stars in our own galaxy is probably harder than stars farther away?

[lacker]

Well, it isn't really that hard even for nearby stars. You need to be sure to get an updated version of the library like once per decade. I think once you're further away than 100 or so light years the star movement is no longer apparent, and our galaxy goes up to 100,000 light years away or so, so most of the stars in the Milky Way are still far enough away that there's no apparent movement. These numbers are a bit of a guess and I could be off.

[ceejayoz]

Spectrum, brightness, etc. all help, but mostly the fact that they don't move relative to other stars very fast on a human timeframe. Even the closest star only shows motion over a decade or so. https://hubblesite.org/contents/media/images/2013/22/3192-Im...