Degenerate matter

Degenerate matter is a special state of matter that happens when very small particles, called fermions, are squeezed very close together. This squeezing changes how the matter behaves because of a rule in physics called the Pauli exclusion principle. This rule says that two identical fermions cannot be in the same place at the same time with the same energy.

This kind of matter is very important in space for very dense stars like white dwarfs and neutron stars. Normal heat pressure is not enough to stop these stars from collapsing under their own gravity. Instead, the degeneracy pressure—created because the fermions cannot all be in the same low-energy state—helps keep these stars stable.

Even if the temperature were absolute zero, this degeneracy pressure would still exist. It depends only on how tightly packed the particles are. When we squeeze degenerate matter even more, the particles must move into higher energy states, creating a force that pushes back against the squeezing. This is why degenerate matter is so strong and can support huge stars against gravity.

Concept

Main article: Fermi-Dirac statistics

Quantum mechanics uses the word 'degenerate' to describe special states of matter at very low temperatures. When the temperature of a system of tiny particles called fermions gets very close to absolute zero, special rules called the Pauli exclusion principle and quantum confinement shape how these particles behave. This means only one particle can occupy each energy state, and as more particles are added, they must fill higher and higher energy levels, even when it’s very cold.

This creates a pressure called electron degeneracy pressure, which stops the material from being squeezed too tightly. This kind of matter is important in stars like white dwarfs and neutron stars, where normal heat pressure isn’t enough to stop them from collapsing under their own gravity. The way these particles spread out among different energy levels is described by the Fermi-Dirac distribution.

Degeneracy pressure

For regular gases, pressure depends on temperature. But for degenerate matter, pressure depends very little on temperature and stays strong even when things get very cold. This special pressure helps keep objects like white dwarfs and neutron stars from collapsing completely.

Degenerate matter has both normal pressure from heat and this special pressure. In very dense materials, the special pressure becomes much more important than the normal pressure. Even regular solids have a bit of this special pressure, but it’s usually not enough to matter much. Some special kinds of degenerate matter include things found in neutron stars and other extreme objects.

Degenerate gases

Degenerate gases are made of tiny particles called fermions, such as electrons, protons, and neutrons, instead of normal molecules. These gases follow a rule that no two fermions can share the same tiny space, called the Pauli exclusion principle. In degenerate gases, all these tiny spaces are filled up to a certain energy level, called the Fermi energy.

Stars like white dwarfs and neutron stars stay standing because of the pressure from degenerate gases, not just heat. In white dwarfs, the pressure comes from electrons being squeezed very close together. In neutron stars, it is neutrons that provide this pressure. When all the tiny spaces are filled, the gas becomes very hard to squeeze further, which helps keep these stars from collapsing under their own weight.

History

In 1914, a scientist named Walther Nernst noticed that gases lose some of their normal heat qualities at very cold temperatures. He called this "degeneration" and linked it to tiny, invisible forces called quantum effects. Later, scientists like Albert Einstein, Max Planck, and Erwin Schrödinger studied these cold gases and called the effect "gas degeneracy."

In 1927, two scientists, Enrico Fermi and Llewellyn Thomas, created a simple way to understand how tiny particles called electrons behave in metals. Soon after, Arnold Sommerfeld used special rules about these particles to explain why metals have certain heat qualities at low temperatures. He called this a "wholly degenerate gas."

Scientists began to understand that some very dense stars, called degenerate stars, are made of this special kind of matter. Arthur Eddington suggested that the star Sirius B was made of tightly packed particles. Ralph Fowler described white dwarfs—small, dense stars—as being made of particles that act differently at very cold temperatures. Later, Subrahmanyan Chandrasekhar helped create the main theory we use today to understand how these stars stay stable.