Never neutral about neutrons
When a big star runs short of fuel -- when its hydrogen has fused into helium, and eventually slightly heavier elements like carbon and oxygen -- it eventually develops a mass of iron, which cannot fuse under stellar conditions.

With the expansive force of fusion energy suddenly absent, the star's gravitational force has nothing to balance it, and quicker than you can say "galloping gravity," the star collapses, sending off a shower of energy and shrinking into an object called a neutron star. The star is now about the size of Milwaukee, but it still contains most of the original mass.

It's dense.

Neutron stars were dreamed up by astronomers Walter Baade and Fritz Zwicky in 1934. They were discovered in 1967 by the British astronomers Jocelyn Bell and Antony Hewish, who noticed a periodic pulsing in a radio signal. Due to this signal, the object began to be called "pulsars." We now think that pulsars are rapidly spinning neutron stars.

Over the years, scientists have come to appreciate that neutron stars are true oddballs. At their center, they are 100 trillion times as dense as water. Their magnetic fields are one billion times as intense as the sun's. And their gravitation is - well, it's not quite as intense as a black hole. But trust us: Even on asteroids, Arnold Schwarzenegger would need a cane to walk upright on one of these critters!

The gravitational pressure in the little stars is so intense that electrons and protons fuse to become neutrons, which are packed together at incredible density. Many neutron stars spin a dozen or more times per second, emitting jets of X-rays. One spins so rapidly that its surface moves at one-seventh the speed of light!

And while you wouldn't want to get too close to a neutron star, the Chandra observatory should give us an arms'-length view of them, along with some other strange celestial objects recently rescued from sci-fi status.

Our bibliography oozes gravitational forces.

©1999, University of Wisconsin, Board of Regents.