Astronomy Test 2 (Chapter 12)

What is a nova?

A star that suddenly increases in brightness, often by a factor of as much as 10,000, then slowly fades back to its original luminosity. The result of an explosion on the surface of a white dwarf star, caused by matter falling onto its surface from the atmosphere or a binary companion

What elements are created by fusion reactions in massive stars?

All elements up to a including iron

What is concealed by the atmosphere of a red giant?

At stage 13, formerly concealed by the atmosphere of a red giant, the carbon core becomes visible as a white dwarf as the envelop recedes

What is hydrogen shell-burning?

Fusion of hydrogen in a shell that is driven by contraction and heating of the helium core. Once hydrogen is depleted in the core of the star, the hydrogen burning stops and the core contracts due to gravity, causing the temperature to rise, heating the surrounding layers of hydrogen in the star, and increasing the burning rate there The pressure exerted by this enhanced hydrogen burning causes the stars non-burning outer layers to increase in radius

What is dense matter?

The stage representing the maximum compression that a star can acquire; supported by the resistance of its electrons to further compression, the contraction of the core stops

What is the horizontal branch?

region of the H-R diagram where post-main-sequence stars again reach hydrostatic equilibrium. At this point, the star is burning helium in its core and fusing hydrogen in a shell surrounding the core

The Cycle of Stellar Evolution

1. STAR FORMATION stars form when interstellar clouds collapse under their own gravity 2. STELLAR EVOLUTION the most massive stars evolve the fastest (producing iron, silicon, and other heavy elements). Low-mass stars evolve more slowly, creating most of the carbon, nitrogen, and oxygen that makes life possible 3. SUPERNOVA + HEAVY ELEMENTS some stars eventually explode, throwing out heavy element debris that enriches interstellar space 4. INTERSTELLAR MEDIUM stellar recycling begins within dark interstellar clouds

What is a Type 1 supernova?

A carbon detonation supernova. A white dwarf in a binary system can accrete enough mass that it cannot support its own weight. The star collapses and temperatures become high enough for carbon fusion to occur. fusion begins throughout the white dwarf almost simultaneously and an explosion results

What is a Type II supernova?

A core-collapse supernova. A highly evolved stellar core rapidly implodes and then explodes, destroying the surrounding star

What is a helium flash?

an explosive event in the post-main-sequence evolution of a low-mass star. When helium fusion begins in a dense stellar core, the burning is explosive in nature. It continues until the energy released is enough to expand the core, at which point the star achieves stable equilibrium again

What is a planetary nebula?

the ejected envelope of a red-giant star, spread over a volume roughly the size of our solar system. The evolution of low-mass stars is the source of virtually all of the carbon-rich dust observed throughout the plane of our galaxy

What is a black dwarf?

the endpoint of the evolution of an isolated, low-mass star. After the white dwarf stage, the star cools to the point where it is a dark "clinker" in interstellar space

What is core hydrogen burning?

the energy-burning stage for main sequence stars, in which the helium is produced by hydrogen fusion in the central region of the star. A typical star spends up to 90% of its lifetime in hydrostatic equilibrium brought about by the balance between gravity and the energy generated by core hydrogen burning

What is the accretion disk?

the flat disk of matter spiraling down onto the surface of a neutron star or black hole. Often, the matter originated on the surface of a companion star in a binary system

What is turnoff mass?

the mass of the star that is just evolving off the main sequence at any moment

What are supernova remnants?

the scattered glowing remains from a supernova that occurred in the past. The Crab Nebula is one of the best studied supernova remnants

What is the redgiant branch?

the section of the evolutionary track of a star corresponding to intense shell burning, which drives a steady expansion and cooling of the outer envelope of the star. As the star gets larger in radius and its surface temperature cools, it becomes a red giant

What is the subgiant branch?

the section of the evolutionary track of a star that corresponds to changes that occur just after hydrogen is depleted in the core, and core hydrogen burning ceases. Shell hydrogen burning heats the outer layers of the stars which causes a general expansion of the stellar envelope

What is main-sequence turnoff?

the special point on the H-R diagram for a cluster, indicative of the cluster's age. If all stars in the cluster are plotted, the lower-mass stars will trace out the main sequence up to the point where stars begin to evolve off the main sequence toward the red giant branch. The point where stars are just beginning to evolve off is the main-sequence turnoff.

What is the asymptotic giant branch?

to distinguish the second visit to the red giant region, the star's evolutionary track during Stage 11 is referred to as the asymptotic giant branch