Why Hot Stars Spin Faster express
The Gaia space telescope has measured how fast more than a hundred thousand stars spin. It turns out that a star's rotation speed depends on its temperature. For stars cooler than 6500 degrees, the surface moves at 10–20 kilometers per second. But once a star gets hotter, the speed jumps to 100 kilometers per second.
The key lies in the magnetic field. For cool stars like our Sun, the outer layers are constantly churning like boiling porridge. This churning generates a strong magnetic field that brakes the star's rotation, like an invisible anchor. Hot stars have calm outer layers, no magnetic brake, so they spin fast.
But over time, a hot star ages and puffs up. Its outer layers start to churn, a magnetic field appears, and the rotation slows down. The astronomers confirmed this by dividing stars into age groups. Cool stars slow down steadily throughout their lives, while hot stars only slow down after they age and begin to expand.
Original abstract
as written by the authors arXiv:2601.20008
We used more than $$10^5$$ Gaia DR3 line broadening vbroad measurements to examine stellar rotation as a function of stellar temperature, mass and age. The large sample clearly displays the Kraft break at $$\sim 6{,}500$$\,K, or mass of $$\sim1.3\,M_{\odot}$$-while vbroad are small, on the order of $$10$$-$$20$$ km/s, for stars cooler than the Kraft break, they sharply rise above the break, reaching up $$\sim100$$ km/s with temperature of $$7{,}000$$ K. To follow the stellar rotation as a function of age, we consider vbroad as a function of scaled age-stellar age divided by the relevant Terminal Age Main Sequence (MS), for four narrow mass bins. We find that stellar rotation deceleration is slow during the MS phase and fast afterwards for stars hotter than the break, whereas deceleration rate is relatively high and does not vary much for the cool stars. Our findings are consistent with the theory that stellar rotation slowing is due to magnetic breaking, emanating from magnetic fields that are anchored to the stellar convective envelopes. Therefore, deceleration is high in cool stars, but in hot stars only after they leave the MS and develop convective outer layers
A Sun-like star spun several times faster in its youth than it does now. Magnetic braking over billions of years has slowed its rotation.