I believe there is a difference between the mass calculated using the orbital speed and the stellar mass of the galaxy. So how is the stellar mass calculated?

Thank you :)

  • $\begingroup$ Are you asking about how the mass of individual stars is determined, or how the total mass of stars in a galaxy is determined? Both are reasonable and interesting, but they are different questions. $\endgroup$ Commented May 17, 2019 at 9:12
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    $\begingroup$ Thank you, I want to know the total mass of stars in the galaxy (I changed the question, I hope it's now more clear) $\endgroup$
    – Vloer
    Commented May 17, 2019 at 9:23
  • $\begingroup$ I'm afraid it still doesn't make sense - the title is good, but the body of the question seems to ask whether "stellar mass" is the same as "mass" $\endgroup$
    – Rory Alsop
    Commented May 17, 2019 at 12:56
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    $\begingroup$ It is perfectly clear. Stellar mass is the mass attributable to stars. $\endgroup$
    – ProfRob
    Commented May 17, 2019 at 21:23

1 Answer 1


In short, you count stars. Of course you can't count or even see all the stars, so you have to count certain types of (bright) stars and then assume that the ratio of these to all the stars in their neighbourhood is similar to the ratio in a local part of the Galaxy where we can do a more complete census.

Once we have an idea of a number for the stars in any region, we can use an assumed mass distribution for the stars to calculate the total stellar mass.

There are details and nuances (e.g. the form of the stellar mass distribution depends on the age of the population), but that's the basic process in our own Galaxy.

In other galaxies, where you don't resolve individual stars, you work on the basis of mass to luminosity ratios for typical populations to convert a luminosity to a stellar mass.


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