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13.2 Understanding Star Birth and Death

We are trapped in both time and space as we try to understand the nature of stars. We cannot travel and probe the physical conditions inside stars of different types. For the most part, we must study the clues that are contained in starlight. That starlight comes from a cool outer layer of the star, so it only indirect evidence of the real action that takes place in the fusion core. Neither can we observe the evolution of stars. A human life is like a blink of the eye in the life span of any star — the night sky presents us with a frozen moment in long cycles of star birth and death.
 

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Three rings of glowing gas around supernova 1987a. Click here for original source URL.

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Star forming region NGC 2264, also known as the Cone Nebula. Click here for original source URL.

Understanding the birth and death of stars is a particular challenge. Star birth is shrouded behind dense clouds of gas and dust; it is difficult to penetrate this material with our telescopes. The collapse and violent death of a massive star occupies no more than a millionth of its main sequence lifetime. Therefore, we would have to observe a million stars at various stages of their evolution to expect to find one case of star death occuring now. In general, statistically large samples are needed to recognize and understand fleeting stages of stellar evolution. Sometimes we get lucky. The star that died in a nearby galaxy and became momentarily visible to the naked eye, SN 1987A, is one such great opportunity.
 

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Hertzsprung-Russell diagram showing color and size of stars. Click here for original source URL.

Despite the lack of direct experimentation possible for stars, the study of stellar birth and death is still firmly grounded in the scientific method. Careful spectroscopy allows astronomers to measure the surface temperature and luminosity of a star. This in turn allows them to estimate the star's size. The star can then be plotted on an H-R diagram, which tells us its evolutionary state. (An H-R diagram is a plot of surface temperature against luminosity. When astronomers talk about stars "moving across" the H-R diagram or "leaving" the main sequence, it is shorthand for describing how the physical properties of a star change with time.) They can compare this data to stellar models and deduce the quantity that specifies the past and future of any star: its mass. Astronomers then deduce more information by studying populations of stars and check out theories against the few direct or nearby observations of birth and death.
 

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Hodge 301, an area of multiple generations of stars in the Large Magellanic Cloud. Click here for original source URL.

Star birth and star death illustrate universal laws of physics. The laws of gravity and radiation that we observe on Earth also apply across space and time. The life story of any star is a battle between two competing physical forces: gravity and the pressure caused by nuclear reactions in the core. Sometimes pressure wins and material is shot off into space — gently in the case of a giant star, and viol

ently in the case of a supernova. More often gravity locks material forever in a collapsed core: white dwarfs, neutron stars, and black holes. The entire life story of a star is determined by its mass. Thus astronomers can observe a nearby star and confidently predict its fate, even though nobody will be here to witness it!