Key Ideas and Terms

12-1 High-mass stars create heavy elements in their cores before violently blowing apart in supernova explosions, leaving behind remnants

• Unlike a moderately low-mass star, a high-mass star larger than 4 M_⊙ undergoes an extended sequence of thermonuclear reactions in its core and shells, including processes based on carbon, neon, oxygen, and silicon.
• Because thermonuclear reactions can take place simultaneously in several shells surrounding the core, energy is released at such a rapid rate that the star’s outer layers expand tremendously, resulting in a supergiant star, whose luminosity and radius are much larger than those of a giant.
• In the last stages of its life, a high-mass star has an iron-rich core surrounded by concentric shells hosting the various thermonuclear reactions.
• A star with an initial mass greater than 8 M_⊙ dies in a violent cataclysm in which its core collapses and most of its matter is ejected into space at high speeds, producing a core-collapse supernova.

12-2 Core-collapse supernovae can leave behind remnants, neutron stars, and pulsars

• The matter ejected from a core-collapse supernova, moving at supersonic speeds through interstellar gases and dust, glows as supernova remnants.
• A neutron star is a dense stellar corpse, supported by closely packed neutrons (neutron degeneracy pressure) remaining from a core-collapse supernova typically having a diameter of only about 12 miles, a mass less than 3 M_⊙, a magnetic field 10¹² times stronger than that of the Sun, and a rotation period of roughly 1 second.
• Intense beams of radiation emanate from regions of spinning neutron stars near the north and south magnetic poles of a neutron star observed as pulses of radio waves and called a pulsar, which slow their rotation as they lose energy with age.

12-3 Black holes are created in the death throes of the most massive of stars

• Surrounding a black hole, where the escape speed from the hole just equals the speed of light, is the event horizon.
• An object from which neither matter nor light (electromagnetic radiation) can escape is called a black hole.
• The distance from the center of a nonrotating black hole to its event horizon is called the Schwarzschild radius.
• The star’s entire mass is crushed to zero volume and hence infinite density—at a single point, known as the singularity, at its center.

12-4 Black holes cannot be seen directly

• The evidence for black holes comes from observed regions emanating X-rays where heated material glows as it accelerates toward a black hole, which can occur when a black hole shares a close binary system orbit with a star.
• Gas gravitationally collected at the center of most galaxies gives rise to a central supermassive black hole with a truly stupendous mass.

12-5 White dwarfs and pulsars in close binary systems can become novae, bursters, and supernovae

• Explosive hydrogen fusion may occur in the surface layer of a companion white dwarf in a close binary system, producing the sudden increase in luminosity that we call a nova.
• Supernovae that are caused by a core-collapse of a massive star are categorized as Type II supernovae.
• Type I supernova result from an accreting white dwarf in a close binary system, which becomes a supernova when carbon fusion ignites explosively throughout such a degenerate star.
• A graph of the brightness of a supernova versus time is known as a light curve showing that supernovae begin with a sudden rise in brightness that occurs in less than a day.
• Rapid and explosive fusion of helium may occur in the surface layer of a companion neutron star producing a sudden increase in X-rays, called an X-ray burster.