Key Concepts in Stellar Evolution and Astronomy
Properties of Stars and Matter
Opacity of Gas: The resistance of matter to the passage of radiation, which depends on the temperature.
Stellar Model: A table of numbers representing the conditions in various layers within a star.
Nuclear Forces: The two forces of nature that only affect the particles in the nuclei of atoms.
Nuclear Fission: Reactions that break the nuclei of atoms into fragments.
Nuclear Fusion: Reactions that join nuclei to form more massive nuclei.
Coulomb Barrier: The electrostatic force of repulsion between bodies of like charge, commonly applied to atomic particles.
Energy Production in Stars
Proton-Proton Chain: A series of three nuclear reactions that builds a helium atom by adding together protons. This is the main energy source in the Sun.
Deuterium: An isotope of hydrogen in which the nucleus contains a proton and a neutron.
Neutrino: A neutral, nearly massless atomic particle that travels at or nearly at the speed of light.
Carbon-Nitrogen-Oxygen Cycle: A series of nuclear reactions that use carbon as a catalyst to combine four hydrogen nuclei to make one helium nucleus plus energy, effective in stars more massive than the Sun.
Stellar Formation and Evolution
Brown Dwarf: A stellar object with such low mass that it cannot raise its central temperature high enough to sustain hydrogen fusion.
Zero-Age Main Sequence: The location in the H-R diagram where stars first reach stability as hydrogen-burning stars.
Interstellar Medium: The low-density gas and dust distributed between the stars.
Interstellar Dust: Microscopic solid grains in the interstellar medium.
Nebula: A relatively dense cloud of interstellar gas and dust.
Interstellar Reddening: The process in which dust scatters blue light out of starlight and makes the stars look redder.
Molecular Cloud: A dense interstellar gas cloud in which atoms are able to link together to form molecules such as H2 and CO.
Shock Wave: A sudden change in pressure that travels as an intense sound wave.
Star Cluster: A group of stars that formed together and orbit a common center of mass.
Stellar Association: A group of stars that formed together but are not gravitationally bound to one another.
Protostar: A collapsing cloud of gas and dust destined to become a star.
Bok Globule: Small, dark cloud only about 1 light-year in diameter that contains 10 to 1000 solar masses of gas and dust, thought to be related to star formation.
Bipolar Flow: Jets flowing in opposite directions away from a central object.
Herbig-Haro Object: A small nebula that varies irregularly in brightness, evidently associated with star formation.
T Tauri Star: A young star surrounded by gas and dust, understood to be contracting toward the main sequence.
Stellar Events and Remnants
Nova: A sudden and temporary brightening of a star, making it appear as a new star in the sky, evidently caused by an explosion of nuclear fuel on the surface of a white dwarf.
Supernova: A new star in the sky that is up to 50,000 times more luminous than a normal nova and longer-lasting, evidently the result of an explosion of a star.
Giant Star: Large, cool, highly luminous star in the upper right of the H-R diagram, typically 10 to 100 times the diameter of the Sun.
Supergiant Star: Exceptionally luminous star whose diameter is 100 to 1000 times that of the Sun.
Horizontal Branch: The location in the H-R diagram of giant stars fusing helium.
Planetary Nebula: An expanding shell of gas ejected from a medium-mass star during the latter stages of its evolution.
Open Cluster: A collection of 10 to 1000 stars in a region about 25 parsecs in diameter.
Degenerate Matter: Extremely high-density matter in which pressure does not depend on temperature.
Compact Object: One of the three final states of stellar evolution, which generates no nuclear energy and is much smaller and denser than a normal star.
Chandrasekhar Limit: The maximum mass of a white dwarf, about 1.4 solar masses. A white dwarf of greater mass cannot support itself and will collapse.
Roche Lobe: The volume of space a star controls gravitationally within a binary system.
Angular Momentum: A measure of the tendency of a rotating body to continue rotating. Mathematically, the product of mass, velocity, and radius.