Zwicky

Fritz Zwicky was a Swiss scientist who had a notorious reputation for attacking people though his publications and had little patience with other points of view.

Zwicky was interested in what made stars explode into supernovae and where cosmic rays came from. Clues of these things would come with the discoveries on the internal structures of stars.

In the early 1900’s Ernest Rutherford in Cambridge, England searched for a way to get the laws of quantum mechanics square with the behavior of the atomic nucleus. He proposed a new particle called the neutron, which was indeed discovered in 1932 by a man he worked with named James Chadwick. One man that noticed was Fritz Zwicky.

By then Zwicky knew of two features of atoms that were a guide to understanding stars.

The first was the tremendous distribution of size inside atoms which are made of mostly empty space. What we call an “atom” is mostly a cloud of electrons, about 10^-8 cm in diameter. The nucleus is about 100,000 times smaller than the cloud.

Secondly was the balanced but strongly counteracting forces within atoms. Electrons in the cloud have to follow a rule called the Pauli Exclusion Principle, which means they are confined to orbitals that make them go at high speeds and unstable motion resulting in electron degeneracy.

To Zwicky, atoms resembled stars because the powerful forces of nuclei were just enough to prevent electron degeneracy pressure from making the atom fly apart.

Chadwick’s neutron was just what Zwicky needed for his theory. Zwicky was certain it was indeed possible for the core of a star with a density about a hundred grams per cubic centimeter to implode to the density of an atomic nucleus – about 10^14 grams per cubic centimeter.

The star’s circumference would be severely decreased at a very high speed under such a collapse.

Also, a tremendous amount of energy would be released. About 10% of the star’s mass would be converted to enough explosive energy to blow the star’s outer layers off.
Thus Zwicky introduced to the world the idea of the neutron star.

Today we know that when the thermal pressure inside a white dwarf finally gives out, the star collapses, crushing the core with so much pressure that it slams protons and electrons to make neutrons just a Fritz Zwicky predicted.

The collapse is so violent that neutrinos are blown into space, the temperature approaches 100 billion degrees Centigrade, and the outer layers are blown off. What is left is a core of neutrons that is so dense a teaspoon will weigh billions of tons. At the height of the explosion, it can be 5 billion times brighter than the sun.

A neutron star is like a spinning ice skater. With her arms outstretched, she spins at a moderate speed. But when she pulls her arms in, her angular momentum makes her spin extremely fast.

We know that neutron stars spin at extremely high speeds because the powerful magnetic fields they make produce radio waves. Since the waves do not come out the stars’ rotation axes we can detect them every time they sweep in the direction of the earth, much as we would see regular pulses of light from a rotating search light. This is why they are called pulsars. After J. Bell discovered them using an ingenious device made from chicken wire, some radio astronomers entertained the idea they could be radio messages from intelligent beings but scientists soon realized what was going on.

Supernovae that produce pulsars are very rare but visual sightings of them go back a long time. In 1054 Chinese astronomers observed an explosion in the Crab Nebula, in the constellation Taurus, about 600 light years away.

The pulsar that was left behind was observed in visible light in 1969 at the Steward Observatory in Arizona. We now know that it rotates at 30 times per second and it emits energy at all wavelengths of the electromagnetic spectrum.

So now we know that once a white dwarf approaches a maximum size, what we now call the Chandrasekhar limit, nothing can stop its collapse or implosion. Now that we know this, it seems easier to believe that a star’s gravity could be so powerful that light can not escape it and its local time will stop. But before we begin our discussion on black holes, we might ask do we have modern evidence that they exist at all.