I am new to Astronomy, I got interested lately thanks to James Webb Space Telescope. I have some questions. In this zoomable picture of Andromeda galaxy, how many of our stars (Milky Way stars) in this picture? Are Milky Way stars those objects with spikes?
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$\begingroup$ BTW (a side note to your question): The spikes you see are known as diffraction spikes; they occur because light diffracts off of and around various objects on the telescope that are in the way - like support beams. $\endgroup$– Filip MilovanovićCommented Jul 11, 2022 at 23:36
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TL;DR - Yes, most of the objects with spikes are probably inside the Milky Way, the only exception could be some of the smallest blue objects with spikes.
Great question! This is a question that many, Many astronomers have asked before you. Each patch of the sky has objects at many many distances, and it is rarely immediately obvious how far away any individual object is. For example, a friend and collaborator of mine once found that an object in andromeda was actually a much more distant supermassive blackhole! That being said there are a few ways that you can distinguish between foreground (inside the Milky Way) and background (outside of the Milky Way) objects.
Visual brightness
Because brightness drops off as $\frac{1}{r^2}$ (this is an inverse square law) the same object with the same intrinsic brightness placed inside the Milky Way at, say, 1,000 parsecs would be over 600,000 times brighter than that same object in the Andromeda galaxy. Because this image wanted to look at the much fainter stars in the Andromeda galaxy they had to overexpose the brighter foreground stars, which is why the closest objects tend to look like large circles with spikes.
The same thing can happen for the very brightest stars in Andromeda, however. The vast majority of stars are very small and red. But, the most massive stars that form, like $\eta$ Carina, can be millions of times brighter than our sun.
So by looking at the brightest stars, with the spikes coming out of them, you are looking at either 1. foreground Milky Way stars and 2. the most massive, most luminous stars in the Andromeda galaxy. It is safe to say, though, that nearly all of the bright, red stars are foreground objects since, in general, blue stars are brighter than red stars.
Parallax
For a more definitive answer, you could use parallax to try to measure how far away each star is. The Gaia spacecraft has spent years measuring parallax for huge numbers of Milky Way stars, and probably has measured parallax angles for every star in this image aside from those in the Andromeda galaxy, which is too distant for this method to work.
Spectroscopic Identification
That being said, I think most astronomers would agree that only a reliable spectral classification could truly confirm how far away any individual star is. That is because a spectrum can tell an astronomer how luminous an object is. If you know a star's luminosity, and how bright it appears to be, you can use the inverse square law to determine how far away that star is.
Hopefully this helps! :-)
