The total stellar mass of a galaxy cannot be measured directly. Astronomers can infer it from the galaxy spectrum by stellar population synthesis. However, since high-mass stars dominate the spectrum but contribute little to the total stellar mass, the inferred total stellar mass is sensitive to initial mass function (IMF) which is usually assumed a priori. One can also measure the total stellar mass by kinematics, but it is the total mass of stars, dark matter and gas enclosed that is measured in this way, and we still need some assumptions (say, on the dark matter profile) to obtain the total STELLAR mass. In a word, it's very difficult to measure the total stellar mass of a galaxy precisely.

So I wonder why people are still interested in this parameter. What can it tell us about the formation and evolution of galaxies?

It seems that what can be measured relatively more accurately is the properties of high-mass stars which are also the main sources of stellar feedback and metal enrichment. Why not just focus on these stars? What makes low-mass stars (or to be more concrete, their mass) important to the galaxy as a whole?

up vote 5 down vote accepted

Actually, the light of most galaxies is dominated by red giants, which are predominantly not very massive stars (more massive than the Sun sure, but less than a few solar masses). These are much more common than short-lived high mass stars, which in any case don't exist in many galaxies (e.g. old ellipticals).

In galaxies that not gas-rich - again most ellipticals, the stars form most of the baryonic mass and most of the stellar mass is in low-mass stars. Even in galaxies where there is lots of gas (the dust is a negligible mass component), there are ways of measuring this too. Estimating the total mass of baryonic versus dark matter is essential in understanding the formation and evolution of galaxies, since they behave differently.

Stars less massive than about 0.9 solar masses have had insufficient time to evolve. The baryonic mass in these stars is locked away and cannot be recycled into other stars. Knowing the fraction of mass in this situation is important in understanding the chemical evolution of galaxies.

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