Foundations of Modern Cosmology

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Chapter 5: The Lives of the Stars

Chapter Summary

This is chapter summarizes some important aspects of stars and stellar physics. Stars play several important roles in cosmology. Most obviously, stars make up the majority of the luminous matter in the universe. Many cosmological questions are related to the lives of the stars. How many stars are there? What are their masses? How much of the mass of the universe is made up of stars, including those too dim to see? How are stars born? How do they die?

Stars are born in huge clouds of interstellar gas and dust. In the hearts of these molecular clouds, gravitational forces overwhelm regions of cold gas, drawing these cold clumps into dense cores. From such cores stars will form. An example of a region of active star formation is the Orion Nebula here seen in an HST photo. The Orion nebula is visible to the naked eye as the fuzzy patch in the sword of Orion. Here is a recent HST image of an Evaporating Gaseous Globule where stars are forming in great pillars of molecular hydrogen.

We argue, using some basic physical considerations, that hydrogen burning stars (so-called main sequence stars) increase in luminosity approximately as their mass to the third power. For a mathematical demonstration of this see the Mass-Luminosity Relation. Since a star's lifetime will be affected mainly by its mass divided by its luminosity, it follows that the more massive stars die first. Thus we can use the main sequence on the Herzsprung-Russell (HR) diagram to determine the ages of the oldest star clusters. The most ancient star clusters are the globular clusters, dense balls of stars that are found surrounding galaxies such as the Milky Way. M3, pictured below, is an example of a globular cluster. Determining the ages of stars in such clusters provides a lower limit to the age of the universe. Currently the best data and calculations imply that the oldest clusters are 12 to 15 billion years old.

M3, a globular cluster

Astronomers know that there is a great deal of mass in the universe that is not in ordinary stars. This unseen material is often referred to as the missing mass or dark matter. One of the possible candidates for this dark matter is small star-like objects that have too little mass for nuclear fusion to occur in their cores. These objects are called brown dwarfs. Recently the HST took the first picture of a brown dwarf, orbiting a cool red star known as Gliese 229.

Brown Dwarf orbiting Gliese229

Though the lives of stars are important for cosmology, perhaps even more interesting is their deaths. Stars cannot live forever, since they emit energy and nothing has an infinite source of energy. Once the star has run out of useable hydrogen fuel for the nuclear furnace at its core, it must find another source of energy or die. The next most easily fused element is helium. When the star begins to burn helium at its core, its outer layers are no longer heated as effectively and cool, causing the star's size to swell drastically, creating a red giant . But stars begin with only about a quarter as much helium as hydrogen, and an even smaller fraction of the helium can be fused because of practical limitations; thus the helium-burning red giant phase of the star's existence is relatively brief. When the useable helium is gone, the star has even fewer options. Modest stars such as the Sun, and stars up to about six times its mass, simply fade away. They have too little mass to generate a sufficiently high temperature at their cores to fuse any elements heavier than helium, so they cease nuclear fusion. The bloated outer layers are expelled, forming a planetary nebula (so named because of a vague resemblance in color and shape to a gas giant planet, though a planetary nebula is several orders of magnitude larger than any planet). Without the extreme gas pressure generated by the heat of nuclear fusion, the remnant core collapses until the electrons of its atoms can no longer be squeezed any closer. This phenomenon, called electron degeneracy, is a consequence of the Pauli exclusion principle of quantum mechanics discussed in Chapter 4. The core, now a white dwarf , continues to shine feebly as light diffuses through it, cooling over billions of years until finally it leaves behind a dead, compact, black dwarf.

Larger stars have more spectacular ends. The most massive stars are able to ignite heavier elements, moving from helium to carbon to continue to shine. The sequence continues until iron is reached, but iron refuses to fuse unless energy is supplied to it; at this point, no more energy production is possible. If the star was able to shed enough mass during its giant phase, it might fade away as a white dwarf like its smaller brethren. If not, it collapses catastrophically, blowing its outer layers into space in a supernova , one of the most brilliant displays of cosmic fireworks. The energy released in a supernova is so great that iron is induced to fuse, starting a chain of reactions that produces the heavy metals. The core left behind is too massive even for electron degeneracy to support it; instead the electrons and protons are squeezed together into neutrons, and the core becomes a great ball of neutrons, packed as densely as possible, called a neutron star . Neutron stars are visible only when they beam radiation as they rotate, in which case we detect them as pulsars . In addition to accounting for some of the most bizarre phenomena in the universe, neutron stars are another of the faint stellar remnants that must be considered when we add up the total mass in stars in the universe. Supernovae themselves also play an important role in cosmology because their brilliance allows them to be seen for great distances. A knowledge of their intrinsic luminsosity would thus allow luminosity distances to be derived for remote galaxies.

Points to Ponder
Potential Pitfalls
Questions & Answers

Questions and Answers related to Chapter 5.

Web World

Researchers using the Lick Observatory in California have discovered planets around normal stars. These jupiter-sized planets were discovered indirectly by measurements of Doppler shifts in the stars around which they orbit. Furtehr information can be found from the Other Worlds, Distant Suns website, the Planets around normal stars site, and the Extra Solar Planets Encyclopedia. NASA has a new program to investigate the origins of planets and life in the universe. Visit the NASA Origins website for further information.

Learn more about stars at the NASA Star Page. You can also learn more about Binary Star Systems, or take a Star Journey at National Geographic. Finally check out the explosive deaths of stars at the NASA Supernova Page.

Looking for pretty pictures of an astronomical nature? Check out the NASA Photo Gallery

To learn more about spectra, galaxies, observations and the like, go to the Whispers from the Cosmos Map available from NCSA. This gives the background information on the Berkeley, Illinois, Maryland Array, a large radio telescope array.

If you would like to know more about the space satellite that has obtained parallax measurements for tens of thousands of nearby stars, check out the Hipparcos home page.