Anyone who’s ever tried to make a perfect snowball has some inkling how difficult it is to create a sphere. Even nature has a hard time of it. But a star discovered by NASA’s Kepler Mission, dubbed Kepler 11145123, comes pretty darn close. Rounder than the sun, the Earth, the moon and all the rest, it’s the most spherical natural object we know of in the entire universe.
At first blush, you might think I’m kidding. The sun looks like a perfect circle when viewed through a safe filter. And everything from globes to satellite images of the Earth show what appears to our eyes as a sphere. But as in many things, humans can only see with so much finesse. Stars and planets rotate, and the centrifugal force causes all of them to bulge at their equators. Instead of perfect globes, they’re oblate spheroids, a ten-dollar word for flattened spheres.
Our Sun rotates with a period of 27 days, causing it to bulge 12.4 miles (20 km) more around its middle than from top to bottom. For the Earth this difference is even greater: 26 miles or 42 km. When spread across a globe some 8,000 miles (12,875 km) across, that 26 mile difference is much too small for the eye to perceive when viewing the whole Earth. Jupiter is one of the oblatest of the oblate with an equatorial diameter 5,763 miles (9,275 km) larger than polar, a difference that’s very apparent in even a small telescope.
Prof. Laurent Gizon of the Max Planck Institute in Germany and his colleagues selected a slowly rotating star named Kepler 11145123. This hot and luminous star is more than twice the size of the Sun and rotates three times more slowly than the Sun. Naturally, the slower a star rotates, the less flattened it becomes. Some stars like Altair in Aquila the Eagle (in the Summer Triangle) rotate so quickly — 10 hours in this case — they look more like footballs than globes.
Gizon selected the star because it periodically expands and contracts like your lungs do when breathing in fresh air and exhaling carbon dioxide. The periodic expansions and contractions of the star were detected as fluctuations in the star’s brightness.
NASA’s Kepler mission kept tabs on the star for more than four years. Its instruments were so sensitive they could detect different pulsations depending on the latitude (distance north or south of the equator) they examined. Think of a plucked string, which can vibrate at many different frequencies or modes. The technique of determining what goes on beneath the surface of stars using their pulsations is called asteroseismology.
Comparing the frequencies of low-latitude regions vs. those of high latitudes, they were able to show that the difference in the star’s radius — the distance from center to edge — is only 1.86 miles (3 km) with a precision of 0.6 miles (1 km). Wow, now that’s a sphere! Well, almost. Euclid would still furrow his brow at this sliver of imperfection, I suppose.
Still, Kepler 11145123 is the roundest natural thing known. One more surprise: the star is even less oblate than implied by its rotation rate, the reason the authors propose that a magnetic field at low latitudes could “sphere up” the star even more. What I enjoy most about the new finding is how much it helps me appreciate the imperfect in this world, since most everything is.