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An astronomer is studying a star that appears to be in a galaxy. How does the astronomer know the star is actually in the galaxy and not just on the same line of sight as the galaxy? I'm guessing parallax won't work as galactic distances are too great.

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  • $\begingroup$ Can individual stars in another galaxy be resolved? Or is this question about special cases like supernovae? $\endgroup$
    – mmesser314
    Mar 4, 2015 at 14:14

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By estimating the distance is the most obvious method, but you are correct, the parallax will be too small to measure. If we can tell what type of star it is (by measuring its spectrum, or using its colour(s)), then we know roughly how intrinsically luminous the star is. The actual brightness then tells us how far away it is.

Fortunately, the diameter of galaxies is small compared with their separation, so stars in other galaxies, even the nearby Small and Large Magellanic clouds, are much further away than stars in our galaxy in the same line of sight.

Another thing that can be done in the case of the nearby dwarf galaxies is that you can separate them kinematically. That is, they may have distinctive line of sight velocities. But again, this does need a spectrum.

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  • $\begingroup$ Mind you, this sort of confusion is a big problem in the study of nearby dwarf galaxies, like the LMC and SMC that you mention. It's usually possible to figure out where a star belongs with a decent spectrum, but with just photometry, even in a few bands, it can be very difficult for some types of stars. And spectra are fairly expensive in terms of observing time/resources. $\endgroup$
    – Kyle Oman
    Mar 4, 2015 at 17:53
  • $\begingroup$ How do you use a spectrum to estimate distance? $\endgroup$
    – seldon
    Mar 4, 2015 at 20:35
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    $\begingroup$ @mattecapu By estimating a spectral type and then using the absolute magnitude for a star of that spectral type and its apparent magnitude to estimate the distance. $\endgroup$
    – ProfRob
    Mar 4, 2015 at 20:51

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