HOW COME? Night sky is full of spinning stars

Do stars spin like planets? asks a reader.We don't feel the Earth turning, even though it's spinning at nearly 1,000 mph at the equator. But evidence for its rotation is in the sky every day: As the Earth turns from west to east, the sun appears to rise, climb the sky, and then set again.

And we don't see the sun, our home star, spinning. But we know it is, thanks to sunspots. By the 1600s, using the first real telescopes, astronomers discovered that the same dark blotches disappeared and then eventually came back into view, showing that the sun was turning.

Meanwhile, all of the planets and moons of our solar system are rotating at their own "daily" rates: 24 hours on Earth, a brief 10.7 hours on Saturn, 1,408 hours on Mercury, and about 708 hours on our own moon.

So it's not surprising that what's happening here is happening everywhere we look in the universe. The night sky, which looks so sedate, is actually full of spinning stars, with their own whirling planets and moons.

The sun is a ball of glowing gases (mainly hydrogen and helium), rather than solid, like the rocky Earth. The result?

Different parts of the sun rotate on their own, at different speeds.

Gases near the sun's equator take more than 24 Earth-days to rotate once around; gases near the sun's poles take more than 34 days. (Planets Jupiter and Saturn, the "gas giants," also rotate in slices, gases at the equator turning faster than those at the poles.)

Other stars have their own average rotation rates, depending on age and type. An ordinary star may spin from about 7 to about 130 miles a second.

The most rapidly rotating stars are pulsars (short for "pulsating star"). Pulsars are a kind of neutron star, the collapsed remnant of a star that exploded in a supernova. Instead of atoms like hydrogen, neutron stars are made mainly of naked neutrons, particles that are normally part of an atom's nucleus. Our sun is about 865,000 miles across, but a neutron star is only about 12 miles across -- with 1.4 to two times the sun's mass. (A scant teaspoonful of neutron star would weigh more than a billion tons on Earth.)

Like the beacon in a lighthouse, a pulsar emits a beam of radiation each time it swings around. A pulsar may rotate hundreds of time a second, sending out a flash of visible light, X-rays, radio waves, or gamma rays each time it does.

The most rapidly spinning pulsar discovered spins 716 times a second. At its equator, the pulsar's rotational speed is nearly 43,500 miles a second, or about 24 percent of the speed of light (186,000 miles a second).

CORRECTION: A previous version of this story gave an incorrect figure for the Earth's rate of spin.