If looking up at the sky sometimes makes you dizzy, consider the state of LAMOST J040643.69+542347.8, a star spinning at 540 kms−1 (1,200,000 mph), at least 250 times as fast as the Sun. It's true that pulsars can turn much faster than that, but they are thousands of times smaller than stars like this one, still fusing elements in their cores.
Simply holding a record may not seem like a big deal, but J040643 (for short) has really pushed this one, turning at a rate more than 100 km/s faster than any previous fusing star in our Milky Way Galaxy, a little like running a three-minute mile. To be fair, even faster stars have been found before, but they are in the Large Magellanic Cloud, rather than our own galaxy, and they do things differently there. There is a limit to how fast an ordinary star can spin without tearing itself apart and these objects are getting close. Neutron stars, including pulsars, can go much faster because their extreme density creates a magnetic field that can hold them together.
Dr Guang-Wei Li of China's National Astronomical Observatories has reported J040643's astounding rotational speed in The Astrophysical Journal. The rate of spin is measured by observing the Doppler shift of lines in its light spectrum as the material that made it turns toward and away from us.
Li notes J040643 is an O-type star, the most massive and hottest category, with a mass around 20 times that of the Sun. As well as turning very fast, J040643 is moving very rapidly through the galaxy. This sort of linear speed, which leads it to be categorized as a “run-away star”, is usually a result of having once been part of a binary system and getting kicked out by its partner, probably in a supernova explosion. It's likely this ex-companion is also responsible for the spin, its gravity having induced tides that wound it up.
We lack the telescopes to get a good view of J040643's shape, but we know at these speeds it must be quite flattened at the poles – an exaggerated version of the squashing the Earth and Jupiter experience.
The speed with which J040643 turns is not just a curiosity of nature, it should induce mixing between the core and the outer layers in a way most stars don't experience, extending the stellar lifespan and ensuring all the hydrogen is consumed.
A previous example of a very fast spinning star was more mysterious, having a planet orbiting at an improbable distance and having lost its dust strangely early. Whether these oddities have anything to do with its rapid rate of spin, we don't yet know.
[H/T: Bad Astronomy]