I remember going to a lecture as an undergraduate wherein my professor compared what astronomers do to a hypothetical alien crew on a fast-moving ship that can only take one photo of the Earth as it passes by. We can assume they have a special camera that can see through buildings, but otherwise they just get one photo of, say, a major city, and from that they have to try and learn as much as they can about the human race. How hard would it be to discover that our species has two genders? Or that both of them are required to propagate the species, but only one gives birth? Would it be possible to figure out the whole human life cycle? To discover what disease is? To distinguish between genetics and culture (nature and nurture)? Just having one picture is limiting, but with careful study you can learn more than you think.

One of the most fundamental plots in astronomy.

The professor was drawing an analogy with the Hertzsprung-Russell diagram in particular, which we’ve talked about before on our hangouts: to make it, you record the color and the luminosity of all the stars you can and plot them up against each other, one point per star. The stars group together in interesting ways in particular areas of the diagram, and it turns out that from this diagram alone you can recover an enormous amount about the life cycles of a population of stars (for example, in a star cluster, a neighborhood of our own galaxy, or a nearby galaxy). You can learn even more if you couple the diagram with spectra of stars from different parts of it. Studying stellar populations has helped us understand the fundamentals of what kinds of stars exist, how they are born and die, and how many stars of any given mass are likely to develop in a galaxy in relation to stars of different masses. That last thing is called the Initial Mass Function (IMF for short). Essentially it says that, when stars form in groups, more low-mass stars form than high-mass stars. Put that together with what we know about how much brighter high-mass stars shine and how much faster they die than low-mass stars, and you can start to understand how whole populations of stars in galaxies form and evolve.

And we can apply our studies of nearby galaxies and groups of stars to galaxies we observe much farther away. It’s a good thing, too, since most galaxies are far enough away that our current telescopes can’t resolve individual stars. We just get the sum of the light from all the stars. That combined light is sometimes made up of multiple populations of stars that formed in groups at different times and now all live together in a particular galaxy. Taking that single picture combining the light from billions (often hundreds of billions) of stars and using it to learn about the stars’ masses, ages and histories is an important process, and there are several ways to do it: one way combines models of stellar populations made by forming and evolving many stars in a computer simulation. This is sometimes called Stellar Population Synthesis, or SPS.

On today’s live Hangout, we once again let your questions guide us as we talked about IMFs and what they have to do with SPS and measuring the stellar masses of galaxies. The work that laid the foundations for today’s study of galaxy stellar populations was done in large part by women (Bill mentioned Beatrice Tinsley, for example), which is fitting since today is International Women’s Day. We talked about that too, and about diversity in general in astronomy. Just as you can learn a lot from even one snapshot of a galaxy, you can do a lot with just a bit of mindfulness about being an ally for diversity (Kyle noted on Twitter that World Day for Cultural Diversity is May 21), be it equality for women or for any other minority groups in science, or indeed any field.

Here’s the audio-only version of the Hangout: click to listen to mp3 version.

And the video:

We’ll post about our next hangout soon; in the meantime, keep those questions coming!