Stars have an average lifetime of 5 billion years, so we obviously cannot study one from birth to death.

We don’t need to. We don’t study people from birth to death, either. We study lots of different people of different ages and then build a progression of characteristics of various ages. Astronomers have done just that by studying thousands upon thousands of stars.

Stars are born from clouds of gas that gravity compresses until the density and temperature become great enough for nuclear fusion to begin.

A helium nucleus weighs less than the sum of the weights of its hydrogen components. Fusing hydrogen into helium releases energy equivalent to the difference in mass. The same phenomenon is true for carbon, nitrogen and oxygen. For example, the carbon nucleus is slightly lighter than three helium nuclei, which can fuse to create it.

This “mass defect” grows smaller as the number of nucleons increases in the nucleus. The mass defect reaches zero in iron, so for elements with more nucleons than iron (a nucleus with 26 protons and 29 neutrons) fusion requires an input of energy and cannot proceed spontaneously.

In young stars hydrogen fusion predominates, releasing energy and creating helium, which accumulates in the core. As hydrogen at the core is exhausted, the star contracts, raising the temperature high enough for helium fusion to begin, which creates carbon that builds up in the core.

Different things happen depending on the mass of the star. Stars below eight solar masses never reach carbon-burning temperature. They gradually expand to become a red giant the size of the orbit of Mars as hydrogen outside the helium core begins fusion. When the hydrogen is gone, the star shrinks to become a white dwarf.

Massive stars go through repeated stages of contraction and become layered like onions. They undergo stages where fusion in the core stops and the core collapses until the pressure and temperature are sufficient to begin the next stage of fusion. An outermost layer of hydrogen gas surrounds a layer of hydrogen fusing into helium, which surrounds a layer of helium fusing into carbon, which surrounds layers that fuse to progressively heavier elements until it produces an inert core of iron.

If the mass of the inert core exceeds about 1.4 solar masses, the repulsive forces of the core nucleons can no longer support the crushing force of gravity. Then a cataclysmic implosion takes place. Within seconds the outer core collapses at one-quarter the speed of light, and the inner core reaches temperatures up to 100 billion degrees.

The implosion crushes protons and electrons in the core into neutrons. The shock wave bounces off the incompressible neutron core and expands outward, blowing away the outer layers. The exploding supernova provides energy to fuse some of the nuclei into small quantities of heavier elements such as lead, gold and uranium.

This debris mixes with debris from other supernovae to form the gas clouds from which stars and solar systems are made. Earth and everything on it, including the carbon in our bodies and the iron in our blood, is made from this star stuff.

Richard Brill is a retired professor of science at Honolulu Community College. His column runs on the first and third Fridays of the month. Email questions and comments to brill@hawaii.edu.