Chadwick A. Tolman

Transcription

The ancients believed the stars of the night sky were eternal and unchanging. Today we know this is not true. Stars are born, live their lives, and then die. The way a star dies depends largely on its mass. A low mass star ends as a white dwarf. A high mass star becomes a black hole. But in between, a star becomes a neutron star. Stars spend their lives fusing matter together. This process begins with the simplest of atoms: hydrogen. Fusing hydrogen nuclei gives you helium and releases some energy. It’s this energy which causes the stars to shine. If the star is big enough, then it continues to evolve by fusing matter together to make HEAVIER elements: helium, carbon, neon, oxygen… But at some point the star runs out of steam. Fusion stops, stellar evolution comes to an end, and the star dies. Smaller stars end their lives as a WHITE DWARF, a glowing ball of white hot matter which slowly cools down over billions of years. Although fusion has stopped for white dwarfs, they still shine because of their astronomically high temperature. This is the death that awaits our sun. For the really big stars, the end of fusion enables gravity to do some real damage. Unconstrained by fusion, the gravity of the star breaks down particles and squeezes everything together as tightly as nature will allow. The result is a BLACK HOLE. The gravity of a black hole is so strong that anything that gets close enough is sucked inside - including light. The danger zone is called the Schwarzschild radius. In between white dwarfs and black holes are NEUTRON STARS. These stars are made primarily of neutrons which are NEUTRAL particles. Ernest Rutherford predicted the existence of neutrons in 1920, and a dozen years later, they were observed by James Chadwick. You can find neutrons in the nucleus of most atoms. They can also be created in a process called “electron capture.” With enough force, a proton and electron combine to form a neutron and a neutrino. Neutrinos are super fast and elusive, so they just fly off. But the neutron stays behind. This is the key to understanding how neutron stars are made. Imagine you have a dying star about 50% more massive than our sun. The star’s gravity is strong enough to squeeze the electrons and protons together to form neutrons and neutrinos. The neutrinos dart off into space leaving behind a big sphere of neutrons. Gravity continues to squeeze the neutrons together, but eventually hits a wall - the Pauli Exclusion Principle. This says roughly that two particles cannot occupy the same place at the same time. You now have a neutron star! Let’s quantify the transition from white dwarf to neutron star to black hole. Suppose we have a dead star, and an imaginary dial that lets us change its mass. We’ll set the dial to 1 solar mass - the mass of our sun. This produces a white dwarf, a spinning sphere of white hot matter about the size of the Earth. As we increase the mass by turning the dial, gravity gets stronger, the white dwarf gets smaller, and it spins more quickly. Once we turn the dial to 1.39 solar masses, gravity is strong enough to combine electrons and protons to make neutrons and neutrinos. This value on the dial is called the Chandrasekhar limit. The dead star is now a neutron star. It shrinks down to a sphere with a radius of about 10 kilometers, and the spinning can be as fast as hundreds of times per second. If we move the dial further, gravity eventually becomes strong enough to break down the neutrons, and the neutron star collapses into a black hole. This point on the dial is called the Tolman–Oppenheimer–Volkoff limit and while its exact value is not known, it ranges from 1.5 to 3.0 solar masses. If you were to look at the ingredients of a neutron star, it wouldn’t be 100% neutrons. The number one ingredient is definitely neutrons, but there are still some protons and electrons in there, too. Because the rapidly spinning neutron star contains these charged particles, there will be a massive magnetic field. Just like on Earth, the magnetic field doesn’t have to line up with the axis of rotation. Like a stellar lighthouse, the magnetic field sweeps across the sky emitting regular bursts of electromagnetic radiation. Because of this pulsing signal, neutron stars are sometimes called pulsars. Neutron stars, like the neutron, were predicted to exist before they were observed. Almost as soon as the neutron was detected, astronomers Walter Baade and Fritz Zwicky predicted that a supernova could produce neutron stars. And in 1967, a pulsating neutron star was first observed. In the decades since many more have been discovered. The universe is a pretty big place, and so is that subscribe button. I’m not going to tell you to click it, because I’m certain you’ll do the right thing…… The right thing is to click the button.