Ask A Scientist

Why do stars explode when they get old?

Asked by: Kate Gell
School: Maine Endwell Middle School
Grade: 6
Teacher: Kevin Wagstaff
Hobbies/Interests: Softball & dance

Answer from George Catalano

Professor of Bioengineering; Director, BU Scholars Program

Research area: Turbulence, Fluid Mechanics, Aerodynamics, Environmental Ethics, and Modeling Ecosystems, Predator-prey Modeling, Restoration of Wolves
PhD school: University of Virginia, Aerospace Engineering, 1977
Interests/hobbies: All things Italian, Creative Arts, Model trains & cars
Family: Wife, Karen, is a registered yoga teacher at Yoga for Everybody at the Orthopedic Associates; lives with 2 Alaskan Malamutes, four more in our hearts

Traditionally, stars have been characterized as immutable or unchanging points of light, but in the last one hundred years scientists have redefined these seemingly timeless objects. Today stars are seen to have a finite life span just as you and I do. They are born, reach maturity, then old age and eventually come to an end. In fact the death of stars has led to something quite remarkable in this universe so let’s see if we can detect what that actually might be! Let’s begin then by taking a look at each of the individual stages of a star’s life.

Scientists believe that the universe was created about 15 billion years ago with the "big bang," a cosmic explosion that resulted in an expanding cloud of hydrogen and helium. The entire universe consisted of only these two elements. For example, imagine for a moment a world in which there are two and only two kinds of plants or animals. That would be a far different world then the world in which we live. As the gases cooled, the mutual gravitational attractions of gas molecules led to the growth of the first generation stars. Eventually enough of the helium and hydrogen were present so that the internal pressure finally became large enough to start the process of nuclear fusion, in which the nuclei of hydrogen and helium merge to form heavier elements. This was accompanied by the release of energy, which made the star begin to shine. For a time then, the star reached a place in which there was a balance—it remained at approximately the same size and same brilliance in the night time sky.

Eventually, though, time catches up to the stars as it does with us. As the hydrogen and helium begin to run out in the center core of the star, the core contracts and the outer layers expand, cool, and become less bright. Stars die in different ways according to their original size. In some cases it may collapse to what astronomers refer to as a white dwarf star which looks very much like a planet in the night sky. A larger star undergoes catastrophic core collapse, exploding violently and rapidly. Scientists refer to this as a supernova. It is the supernova that creates other elements in the universe which has eventually led to the creation of you and me. So Joni Mitchell was right. We are stardust after all.  

Last Updated: 3/1/17