Astrophysics for People in a Hurry

All atoms contain a central nucleus filled with protons and neutrons. The proton has an electric charge, and a neutron is basically a proton with no charge. Except for hydrogen, all elements contain neutrons with their number roughly the same as the number of protons. Atomic nuclei are surrounded by electrons, one for each proton; an electron has an electric charge that’s exactly opposite to that of the proton. Heavier atoms have more protons, neutrons, and electrons. The electrons orbit the nucleus in layers called shells; the outermost layer of electrons interacts with the electrons of other atoms’ outer shells to form groups of atoms called molecules.

When the universe began, it was made up mainly of simple protons, electrons, and photons of energy. Hydrogen, the most basic atom, contains one proton and one electron. A hydrogen atom without its electron is “ionized” and has a positive electric charge; in effect, a lone proton is simply an ionized hydrogen atom.

Except for lithium and some early helium, all larger atoms are created in the intense heat and pressure of the interiors of stars. That pressure forces nucleons to join together in a process called fusion. The first such atoms created by fusion are helium atoms, which contain two protons and two neutrons; helium released by a star will eventually collect two electrons that balance out the atom’s electric charge.

Iron is the last of the elements created during the life of a large star. Unlike the lighter atoms, its creation generates no energy, and the star, which depends on the heat of fusion to stay expanded, simply collapses in on itself. Its outer layers literally bounce off the deep interior and blast away, spreading out into space to mix with nearby gas clouds and become the next generation of stars and planets. This explosion is called a supernova, which means a giant new star; the name comes from the dying star’s sudden brightness, which is strong enough to be seen billions of light-years away.

The core of a supernova, crushed by the rebounding outer layers and its own collapsing density, creates all the elements heavier than iron in a few moments. Some of this material also blows off into space; the rest collapses into a “neutron star” that’s made entirely of nucleons with none of the usual wide spacing between atoms. It’s one of the densest materials in the universe: A teaspoon of it would weigh as much as a skyscraper.

Sometimes giant stars’ cores are so dense they collapse all the way down to a point called a black hole. The gravity is so enormous that within a certain distance of the point, called the event horizon, nothing can escape, not even photons of light. Many or most galaxies contain large black holes at their centers that are filled with millions of stellar masses. Any gas cloud or star system that wanders too near it will get sucked down into it, heating up and releasing high-energy photons in the process, some of which escape near the event horizon to become bright jets of energy we can detect.