This New Map of Nearby Stars May Solve a Cosmic Mystery

Scientific American

2 days ago

I don't think it's overly poetic to say that stars are how we know the universe.
When we look out at the night sky, stars are overwhelmingly what we see—thousands of them, ranging from Sirius, the brightest in the night sky, to ones so dim they are known by mere catalog designations instead of names.
And for every star we can see by eye in our sky, telescopes can see millions more. Astronomers study them to understand the shape, size, structure, history, and fate of our galaxy and use them to gauge the distances and behavior of other galaxies. Even when studying exoplanets, we need to understand their host stars to make sense of these alien worlds. The heavier elements making up our planet and even our body were forged in stars long ago, and our own sun is a star, of course—so in a very real sense, to study stars is to study ourselves.
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Yet for all that, some basic questions about stars remain unanswered. While we have a pretty decent grasp of how individual stars are born, there are still gaps in our knowledge of their statistics en masse.
One of my favorite unanswered questions concerns the most fundamental properties of stars as a group, which is how different kinds are born out of a gas cloud. Say you have a giant gas cloud that is busily manufacturing stars. As a percentage, how many of them will be like the sun? How many will be feeble red dwarfs, how many will be massive blue beasts, and how many will be so low in mass that they will straddle the line between a true star and a planet? The mass of a star—how much matter is in it—determines most of its properties, including its temperature, color, brightness, evolution and even its destiny, so this is something astronomers are very keen to understand. Understanding stars' distribution in our galaxy tells us about the galaxy itself, much like poring over the contents of a construction kit offers insights about the finished product and the way it's assembled.
The 'initial mass function' is the technical term for this unresolved question of stellar demographics. And in theory, it's not too hard to answer: just observe a bunch of stars, determine their masses and then figuratively put them into the proper box.
In practice, though, it's very difficult indeed. Massive stars are bright and easy to spot. We can't, however, see small, dim ones if they are very far away—which means our cosmic census of low-mass stars is largely limited to whatever happens to be relatively near our solar system. Still, given time, these stars mix pretty well throughout the galaxy, so we can assume such fainter stellar next-door neighbors are representative of the galaxy at large.
Still, to give you an idea of just how hard this undertaking is, after millennia of astronomical observations, the first nearly complete tally of all stars out to a distance of about 65 light-years only appeared—in 2024! Published by a huge team of astronomers, the paper, which was published in the Astrophysical Journal, is a sprawling description of the Herculean effort involved.
Performing the survey in the first place required using several space- and ground-based astronomical observatories. The European Space Agency's Milky Way–mapping Gaia mission was chief among them, and it pinned down key data for most of the bright stars within the requisite 65 light-years. For lower-mass stars, which shine brightest in infrared, Gaia's observations were augmented by data from NASA's Wide-field Infrared Survey Explorer and the Spitzer Space Telescope.
Such stellar dim bulbs are critical for determining the lowest-mass end of the initial mass function. Brown dwarfs, which are objects of intermediate mass between planets and stars, give off essentially no visible light and can only be detected in the infrared. The first brown dwarfs were only found in the 1990s. And in general, these objects are so dim that even nearby ones can elude detection. Luhman 16 is a binary brown dwarf system just 6.5 light-years from the sun— the third closest system to us —yet was only discovered in 2013.
After carefully teasing out the details of objects confirmed to be within the 65 light-year region, the study team found a total of approximately 3,000 stars and 600 brown dwarfs. Right away this is a remarkable finding. I'm used to thinking on much larger spatial scales, so finding 3,600 objects within a mere 65 light-years of the sun is more than I'd have guessed. Mind you, our Milky Way galaxy is a flat disk 120,000 light-across, which is roomy enough for hundreds of billions of stars and other celestial citizenry.
The astronomers were careful to note that their survey wasn't complete at the low-mass end, either. Brown dwarfs cooler than about 325 degrees Celsius are so faint that our current technology can't directly see them beyond about 50 light-years. Also, some brighter brown dwarfs may still be hiding in crowded parts of the sky, such as the star-rich disk of the galaxy. There could also be smaller binary companions to some stars that have gone undiscovered.
What that means is that some brown dwarfs have likely been undercounted, which is actually pretty problematic for trying to determine the full range of object masses spawned in galactic gas clouds. Think of it this way: if you smash a rock with a hammer, you're likely to get one or two big pieces, a dozen or two midsize ones, hundreds of small chunks and thousands or tens of thousands of tiny grains. If you don't count all the grains, you can't really know how the size of the debris is distributed.
Still, this new, best-yet census of our interstellar neighborhood does extend our understanding of the initial mass function. Before, it wasn't clear if objects had a mass cutoff at the lower end. We know that gas clouds in stellar nurseries have to form clumps that collapse under their own gravity and that these clumps become stars. Is there a limit to how small a clump can be to collapse? Possibly, but until brown dwarfs were discovered and counted, we weren't sure they could even form like stars do. What the census finds is that the number of objects formed generally increases as mass decreases, as expected, much like the debris distribution from a hammer-struck rock. But the census does reveal some quirks: the tally of objects flattens out a bit as mass descends from the stellar regime to brown dwarfs but then starts rising again at lower, more planetary-scale masses. Does it flatten out again at some lower mass, such as around a few times that of Jupiter? That's for future telescopes to determine.
Still, this survey is a big step forward. Extrapolating it to the Milky Way and other galaxies will help us understand how galaxies behave—and how they change their behavior over time, churning out different mixes of stars as they age. The confidence we have in our scientific knowledge depends on each link in the chain, so the better we determine the initial mass function, the better we will comprehend the universe.