How to Find the Smallest Stars in the Universe

For astronomer Alexander von Boetticher, false positives in data are actually a good thing.

That’s how von Boetticher and his colleagues at the University of Cambridge discovered the smallest star ever measured. The star showed up in data from a British-led mission known as the Wide Angle Search for Planets, or WASP. WASP looks for potential exoplanets by looking for dimming in the brightness of stars, a signal that an object is passing in front and blocking the light. The mission has detected dozens of planets this way. Sometimes, the source of the dimming turns out to be something else entirely. “Every now and then, instead of finding an exoplanet orbiting some star, it actually finds a very small star that’s orbiting a star,” von Boetticher said.

The newly discovered star, known as EBLM J0555-57Ab, is located about 600 light-years away and is slightly bigger than Saturn. The star is likely as small as stars get; it has just enough mass to fuse hydrogen into helium in its core. The process is known as nuclear fusion, and it’s what makes stars stars. Any smaller, and EBLM J0555-57Ab would no longer be able to sustain this reaction. It would become a brown dwarf, a substellar object that emits mostly infrared, not visible, light. The discovery is described in a recent paper in the journal Astronomy & Astrophysics.

The newly discovered star is one half of a binary system, a common configuration in which two stars closely orbit each other. The nature of this system helps researchers make precise measurements of the smaller star, von Boetticher said. In a binary system, the smaller object exerts a gravitational pull that makes the larger object move back and forth ever so slightly. Scientists can observe this wobbling effect through telescopes and use it to determine the properties of the smaller object, like its mass and radius. These figures can help determine the nature of the object. “If it’s really heavy and it’s really small, it’s likely to be…

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