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SYFY WIRE Bad Astronomy

The fastest spinning star in the galaxy

By Phil Plait
Artwork depicting a star rotating so rapidly it’s flattened, or oblate, and flinging material away from its equator. Credit: NASA, ESA, and G. Bacon (STScI)

An astronomer has found the fastest spinning star known in our galaxy… and it's a doozy. It's rotating at the dizzying speed of at least 540 kilometers per second. In fact, if it were spinning much faster, it would tear itself apart!

The star is called LAMOST J040643.69+542347.8, but let's call it J0406 for short. It's located about 30,000 light years from us, toward the outer edge of the galaxy. LAMOST is a sky survey being done in China that looks at wide areas of the sky and takes low-resolution spectra of astronomical objects. That means it breaks the light up into thousands of individual slices of color, which in turn can tell us a lot about the object's properties: How fast it's moving toward or away from us, how hot it is, what it's made of, and — crucially in this case — how rapidly it spins.

In this case, the astronomer was looking for a special kind of star when they noticed one star had an odd spectrum, with unusual features in it associated with rapid spin. Careful measurement indicated that the star has the phenomenal rotation speed of 540 kilometers per second.

That's fast. The Sun's rotation at its equator is 2 kilometers per second, so this star is spinning at a speed 270 times faster than the Sun!

The star LAMOST J040643.69+542347.8 (arrow), the fastest rotating star known in the Milky Way. Credit: DSS2 / Aladin

I should note that a couple of stars in a nearby galaxy have been found to rotate at more than 600 kilometers per second, but J0406 is the fastest spinner known in the Milky Way, our galaxy. [Note (added 16:00 UTC 08 July 2020): My colleague Scott Manley points out that neutron stars spin faster as well. I wasn't counting them because I was thinking of "normal" stars, ones like the Sun that are still fusing elements in their cores; neutron stars are the cores of massive stars that have exploded. They can spin close to the speed of light at their equators! So I should be clear: J0406 is the fastest spinning star like the Sun in the galaxy.]

J0406 is what we call an O star, one of the most massive and hot stars there is. Its mass and size aren't known but a decent (though very rough) estimate is that it's 20 times the Sun's mass and 10 times the size. Given that, at that speed it would take just under a day to spin once. The Sun takes nearly a month! So this star is whizzing around.

An actual image of the bright star Altair, spinning so rapidly it’s oblate or flattened. Credit: Ming Zhao, University of Michigan

In fact, it's spinning so rapidly that the centrifugal force at the equator is very strong. Playing with the numbers (and again acknowledging I'm guessing at the size and mass), the force outward due to spin is a substantial fraction of the force inward due to gravity. That means the star must be significantly flattened, much narrower through the poles than across its equator; what we call oblate. Our Sun is nearly a perfect sphere, but this star must look like a beach ball someone has sat on.

A handful of rapidly rotating stars are close and big enough to image using special techniques, and they can be seen to be visibly flattened, as well as darker at their equators, an effect called gravitational darkening. Credit: Monnier et al via CHARA

There's subtle evidence of this in the spectrum too, indicating that the gravity at the poles is higher than at the equator. This also causes an effect called gravitational darkening: The higher gravity at the poles compresses the gas there, heating it up and making it shine more brightly. If you were close to this star, what a sight it would be! Egg-shaped, and darker around the middle than at the poles. Weird.

Incidentally, this star probably can't spin much faster than this. If it did, the force outward at the equator would be stronger than gravity, and material there would fly off. It would quite literally spin itself apart.

Artwork depicting a star rotating so rapidly it’s flattened, or oblate, and flinging material away from its equator. Credit: NASA, ESA, and G. Bacon (STScI)

The question is, how did this star get to spin so rapidly? The most likely answer is that it has or had a binary companion, another star orbiting it very closely. In that case tides might spin up the two stars, as the orbital energy of the companion is transferred to the star. If the stars are very close, matter from one can get transferred to the other, and as it spirals in it would speed up the star's rotation as well.

However, despite looking, no evidence for a companion star was found. This part is cool though: More massive stars age more rapidly than less massive ones. The more massive star in a close binary would expand into a red supergiant and dump a lot of matter on the second star, then explode as a supernova. When that happens the second star would've been flung away like a rock from a sling, sending it moving through space at high speed. As it happens, the spectrum of J0406 reveals it is indeed moving very rapidly through the galaxy, just as you'd expect if it once had a massive companion that blew up.

So all the pieces fit. It's funny: By eye it's just another star lost among thousands of others in a picture of the sky. But by spectrum, well, it's a very different story. That's true for every star; they all have their story to tell. In this case, though, it's a story of long-dead exploded companion, a cannonball screaming through the galaxy, and a spin so strong the star is hugely flattened and weirdly darkened at the equator.

The Universe is such a strange place. That's one reason it's so much fun to explore.