Key Ideas and Terms

11-1 Stars form from the gravitational collapse of immense clouds of interstellar gas and dust

• Star lifetimes exceed human life spans, so they must be studied by looking at numerous stars at different stages in their life cycles.
• Gas and dust, which make up the interstellar medium, are clumped into clouds called nebulae.
• Emission nebulae are glowing, ionized clouds of gas, powered by ultraviolet light that they absorb from nearby hot stars.
• Dark nebulae are so dense that they are opaque. They appear as dark blots against a background of distant stars.
• Reflection nebulae are produced when starlight is reflected from dust grains in the interstellar medium, producing a characteristic bluish glow.
• Star formation begins in dense, cold clouds, where gravitational attraction causes a clump of material to condense into a protostar.
• T Tauri stars are protostars with emission lines as well as absorption lines in their spectra and whose luminosity can change irregularly as they eject material into space.
• The dusty material being added to the protostar is known as a circumstellar accretion disk, and the surrounding materials that form planets are called protoplanetary disks, or proplyds.
• Supernova remnants are the remains of large, exploded stars that have run out of usable fuel.

11-2 Most stars shine throughout their lives by converting hydrogen into helium through nuclear fusion

• Nuclear fusion occurs when smaller atoms combine together to make heavier atoms.
• Stars spend the majority of their life cycles consuming hydrogen and forging heavier elements, releasing energy in the process.
• Stars often form in clusters from the same large interstellar cloud, with all stars having a similar chronological age and initial chemical composition.
• A star cluster’s age is equal to the age of the main-sequence stars at the upper end of the remaining main sequence. As a cluster ages, the main sequence is “eaten away” from the upper left as stars of progressively smaller mass evolve into red giants.

11-3 Careful observations of star clusters provide insight into how a star’s mass influences how stars change over time

• Population I stars formed from remains of other stars and are metal-rich.
• Population II stars are first-generation stars formed in the very early galaxy and are metal-poor.
• The most important characteristic determining a star’s main-sequence lifetime is its mass.

11-4 Stars slowly become red giants

• The central core of a star is the location of highest pressure and temperature and where core fusion occurs.
• Spherical shells of material surround a star’s core and shell fusion can occur there if hot enough.
• Stars join the main-sequence group when they begin hydrogen fusion in their cores.
• Stars leave the main-sequence group and become brighter and cooler giant stars when their core hydrogen is depleted.
• The triple alpha process combines helium atoms to form carbon and release energy.
• In a more massive red giant, fusion of helium begins gradually; in a less massive red giant, it begins suddenly, in a process called the helium flash.
• Degenerate-electron pressure supports a helium-rich core with atoms packed as closely as negatively charged, electric-repulsing electrons will allow.
• Stars in a second red-giant phase are asymptotic giant branch stars, or AGB stars, and their evolutionary tracks approach the red-giant branch from the left on an H-R diagram.

11-5 Low-mass stars pulsate and eject planetary nebulae, leaving behind a white dwarf at the end of their life cycles

• Helium shell flashes in an old, moderately low-mass star produce thermal pulses during which more than half the star’s mass may be ejected into space, exposing the hot, dense, carbon-oxygen core of the star, called a white dwarf.
• The maximum mass of a white dwarf is given by the Chandrasekhar Limit of 1.4 M_⊙.
• Ultraviolet light from the exposed core ionizes and excites the ejected gases, producing a planetary nebula.