Star Formation

Imagine a cloud, drifting along through the interstellar medium. The cloud is unthinkably cold, in excess of -400°F (-240°C). It is not very dense, but it is so large that it renders the stars behind it either invisible or as dim, red points of light. It is made mostly of hydrogen, and has had its present shape and size for thousands of years.

Then, one year, something happens. A hundred parsecs away (about 190 trillion miles), a star explodes. It is a supernova, the violent end to a massive star's life. An expanding, devastating blast races outward, forming a shock wave. It sweeps everything before it, clearing the space through which it passes of the interstellar medium. And eventually, it encounters the cloud.

The shock wave slams into the cloud. The cold gas and dust is violently compressed by the shock, and as the particles are squeezed together, their mutual gravitational attraction grows. So tightly are they now packed that they begin to coalesce under their own gravity. The shock has transformed the cloud: many parts are still thin and diffuse, but now there are multitudes of condensing blobs of gas. They did not contract by themselves before, but now they have been given the necessary impetus.

When a blob of gas condenses, energy is released, and one of the beautiful theorems of physics shows us that half the energy goes into heating the gas. So as the blobs in the disrupted cloud condense, they get progressively hotter. Eventually they begin to glow a dull red, much as an electric burner on your stove begins to glow when it becomes sufficiently hot.

This process of contraction cannot continue indefinitely. As the temperature in a contracting blob of gas becomes higher, the gas exerts a pressure that counteracts the inward force of gravity. At this point, perhaps millions of years after the shock wave slammed into the dark cloud, the contraction stops. If the blob of gas has become hot enough at its center to begin thermonuclear fusion of hydrogen into helium, it will remain in this stable configuration for millions or billions of years. It has become a star.

Nature is filled with symmetries, and this is one of the most enchanting. The death of one star triggers the birth of new stars. And what of the rest of the dead star, the expanding blast of gas and dust that encountered no interstellar clouds? Eventually it comes to a halt, cooling and fading into darkness, where it becomes part of the interstellar medium. Perhaps, millions of years in the future, a shock wave will plow into it.