As normal stars go, our Sun is roughly in the middle of the size scale. The biggest are about 25 times the Sun’s diameter, the smallest about a tenth as wide. Still, it’s big. It’s ten times the diameter of Jupiter, the largest planet in the solar system, and over a hundred times wider than the Earth.
That lower end is pretty interesting, though. How small can a star get and still be a star? Well, astronomers recently found what looks to be the smallest actual star ever discovered. How small, you ask?
Roughly the size of Saturn. Yes, Saturn, the planet.
It’s right at the lower limit of what’s considered to be a true star. And while it may very well be the smallest star ever found, there’s just enough uncertainty in its size that we can’t be 100% sure. Still, no matter how you slice it, it’s a very teeny star.
It was found using WASP, the Wide Angle Search for Planets project. This is comprised of two sets of wide-angle camera arrays, one in the northern hemisphere and one in the southern, which take images about once per minute every clear night. They monitor a staggering 30 million stars in total, looking for variations in brightness that might indicate the presence of a planet. If the orbit of such an exoplanet is edge-on as seen from Earth, it blocks a tiny fraction of its host star’s light as it passes in front of it once per orbit (what we call a transit), and that dip can be detected.
One star observed by WASP, later to be called EBLM J0555-57, is about 630 light years from Earth. It was seen to exhibit transit-like dips in brightness, and so was flagged for follow-up. More observations taken by other telescopes revealed that it was actually a binary star, two stars orbiting each other.
The two stars have a wide orbit, with a distance between them of roughly 70 billion kilometers (well over ten times the distance from the Sun to Neptune), and so it takes centuries for them to orbit each other once. The brighter of the two, called EBLM J0555-57A (note the “A” at the end), is much like the Sun if a bit bigger and warmer, and the other, EBLM J0555-57B, is a bit smaller and a touch cooler.
But the plot thickened. The dips in light were coming from only one of the two stars, the brighter of the two (Star A), and whatever it was orbited with a period (a “year”) of 7.75 days. That means it’s closer to its star than Mercury is to the Sun. Because of that we can’t see it directly; the glare of the far brighter star overwhelms it. It revealed itself by blocking the light from its star. But it turns out there’s still more to this …
Normally, when you have a transiting exoplanet, all you can figure out is its size (by how much light it blocks from the star) and how long its year is. But, if the object is massive enough, as it orbits the star it tugs on the star with its gravity, and the star makes a little circle as well. I like to make the analogy of two kids, one heavier than the other, facing each other, clasping hands, and swinging around each other. The lighter kid makes a big circle, and the heavier kid makes a smaller circle.
This is critical. As the star moves around in a circle, its light is Doppler shifted; the color changes. It shifts toward the red end of the spectrum as it moves away from us in the circle, and toward the blue when it moves toward us. That can be measured with pretty good precision! Knowing the mass of the star from physical models, it’s then possible to figure out the mass of the smaller object.
And that’s when astronomers got a shock. The second object, called EBLM J0555-57Ab, has a mass of 0.081 times the Sun. That would be huge for a planet — 85 times the mass of Jupiter! — but very low for a star. In fact, it’s right at the limit for how low mass a star can be.
Stars are objects that are able to sustain the fusion of hydrogen into helium in their core. They need enough mass that the pressure in their core can squeeze the hydrogen atoms together hard enough to fuse them. There are other factors involved as well, including how rapidly the star spins, the abundance of heavier elements inside it like carbon and magnesium, and so on. For an object with the same physical composition as EBLM J0555-57Ab, that limit is about 83 times the mass of Jupiter.
This means that EBLM J0555-57Ab made the cut — at 85 Jupiter masses it’s a true star, if a very, very, very low-mass one. In fact, that’s why it has the name it does: EBLM stands for “eclipsing binary, low mass.” That term is reserved for stars, not a star orbited by a planet.
The star isn’t just low-mass, it’s tiny, too: By studying the eclipses, astronomers determined its diameter to be 0.084 times that of the Sun. That makes it smaller than Jupiter! It’s about 0.84 times the width of Jupiter, making it just about Saturn-sized, maybe a hair smaller.
It’s important to note that there is some uncertainty in both its mass and size; these observations don’t provide exact results. Because of this, we can’t be 100% sure it’s truly the smallest star ever found. Another teeny star, 2MASS J05233822-1403022, was found in 2013, and it also is smaller than Jupiter, about 0.86 times its width. That’s very similar to this new star! And it turns out the uncertainties in the measurements of both stars mean we can’t really know which one in reality is smaller. Be wary of news articles and such saying this is the smallest star. It might be, but we really don’t know.
But since it is a star, that means the system isn’t a binary! It’s what’s called a hierarchical triple, two stars orbiting each other like a binary system, and a third star orbiting just one of those stars. Cool.
And it’s not just an oddity. Extremely low-mass stars are very important to our understanding of how stars work! Because they are right at the lower limit for what a star can be, they test our models of just how stars can fuse hydrogen into helium. Because these stars are so faint we don’t see very many of them, so every one we find is a precious sample.
I’m fascinated by objects like these, ones that sit right on the border between two different kinds of things. In many cases the borders are fuzzy, and that’s true here as well. Very big planets slide into the brown dwarf range, and very massive brown dwarfs slide into the star range, but it’s not like you can draw a line to distinguish them. When we study these objects, we learn more not just about them, but about both classes of objects they kinda sorta fall in.
Objects in space display a huge diversity, and it’s by understanding that diversity that we better understand the Universe itself. I’m all for that.