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# 13.1 Star Birth and Death

Stars are forming even today within a few hundred parsecs of the Sun and in more distant regions of space. The starry sky is not a static scene but the site of continual births of new stars out of interstellar dust and gas. Many stars and star systems are less than a few million years old — much less than 1% of the age of our galaxy. Some have become visible since humanity evolved.

Star formation begins with proto stars, which are clouds of dust and gas that begin to contract due to their own gravity. They collapse fairly rapidly to stellar dimensions and become pre-main-sequence star-like objects. The process of star formation depends strongly on magnetic fields and rotation in the collapsing gas cloud. There has been much theoretical progress in the last two decades on understanding the basic features of star formation. Many of these features have been confirmed by observation, especially by infrared instruments, which detect the radiation from low temperature dust in nebulae around the newly formed stars. Once the main sequence is reached, energy is generated in the star as hydrogen converts into helium by means of the proton-proton cycle for smaller stars and the carbon cycle for larger stars. All stars in the universe shine by energy released from the fusion of light elements into heavier elements.

The fate of all stars is governed by the irresistible force of gravity. At the low-mass end, stars are dim, red, and slow to evolve. The coolest-sequence stars have not yet evolved off the main sequence in the entire age of the universe. Every star more massive than the Sun goes through a phase of mass loss, either involving a stellar wind or a more violent explosion. Stars below 1-2 solar masses end their lives quietly, as cooling white dwarf embers. Massive stars are rare, and they evolve quickly toward a spectacular demise. Supernovae are responsible for the production of neutron stars and probably black holes, and they recycle rare and important heavy elements into the universe.

Stellar old age leads to two basic phenomena: high-energy nuclear reactions, as ever more massive elements interact and fuse, and inexorable contraction, as energy sources are eventually exhausted. During the first stages of old age, as stars evolve off the main sequence, the large release of energy causes expansion of stars' outer atmospheres, producing giants and super giants. As heavier elements go through quick reaction sequences, various kinds of instability may produce variable stars and slow mass loss. After energy generation declines to a rate too low to resist contraction, low-mass stars contract to a dense state known as a white dwarf, with final mass less than 1.4 solar masses. The less common massive stars, which start out with as much as 8 solar masses or more, undergo supernova explosions and blow off much of their initial material. The entire course of stellar evolution involves the release of a small fraction of the mass-energy of a star, in the form of radiant energy.

If the remnant cores of stars end up between 1.4 solar masses and about 2.5 to 3 solar masses, they form dense, rapidly rotating neutron stars known as pulsars. If the remnant cores have more than about 3 solar masses, they may form black holes. Although virtually no radiation escapes from these strange objects, they may be detectable by orbital motions of their companion stars and by high-energy radiation from material falling into them. The search for the forms of dead and dying stars has yielded some of the most fascinating objects now being studied both by physicists and by astronomers.

Consider our cosmic origins. The carbon atoms on which life depends were produced in the cores of previous generations of stars. If you wear any jewelry, look at the gold or silver and consider the incredible journey those atoms have taken. All the iron and aluminum used to build our modern civilization, and all the precious metals that we treasure and use for adornment, were forged in distant stars long ago.