Exam2 – Flashcards

1 solar-mass star -The lowest mass star lives longest.

more than about 8 times the mass of our Sun -These stars are massive enough that they will die in supernova explosions.

protostar, main-sequence star, red giant, planetary nebula, white dwarf -But remember that these stages are not equal in length; the star spends much more time as a main-sequence star than in any other stage of life. (Once dead, it remains a white dwarf, though it gradually cools and therefore dims with time.)

The core shrinks while the rest of the star expands. -Gravity shrinks the core until hydrogen shell burning begins; the shell burning produces so much energy that the outer layers of the star expand, and the star becomes a subgiant and then a red giant.

hydrogen fusion in a shell surrounding the central core -Hydrogen shell burning produces energy at an even greater rate than it was produced by core burning when it was on the main-sequence. This excess energy causes the star's outer layers to expand.

Three helium nuclei fuse to form one carbon nucleus. -Each helium nucleus has an atomic mass number of 4 (2 protons and 2 neutrons). The resulting carbon nucleus therefore has an atomic mass number of 3 x 4 = 12 (6 protons and 6 neutrons).

The sudden onset of helium fusion in the core of a low-mass star. -Note that helium flash occurs only in low-mass stars, not in high-mass stars.

They generate energy through both hydrogen fusion and helium fusion. -This is true: Horizontal branch stars are those in which helium fusion has already begun in the core, while hydrogen fusion continues in a shell around the core.

Gas ejected from a low-mass star in the final stage of its life. -Note that, despite its name, it has nothing to do with planets.

become a white dwarf that will slowly cool with time -This will happen about 5 billion years from now.

a red giant -A red giant has fusion occurring in shells around the core.

These stars manufactured most of the carbon atoms in our bodies. -While most heavier elements come from higher-mass stars, carbon is thought to come primarily from red giants: the carbon is produced by helium fusion in the core, then dredged up to the surface by convection and blown out into space by winds and by the ejection of a planetary nebula.

helium and helium -These two nuclei each have two protons (and two neutrons - but neutrons are electrically neutral) and therefore each have a charge of +2. Since these two nuclei have the greatest total charge between them, they will feel the strongest electromagnetic repulsion.

a 10 solar mass star -According to present understanding, high-mass stars always die in supernovae.

a set of steps by which four hydrogen nuclei fuse into one helium nucleus -The CNO cycle is one of two primary pathways to hydrogen fusion. The proton-proton chain is the other pathway and it is used by low-mass stars, while high-mass stars fuse hydrogen via the CNO cycle.

its mass -Mass determines whether the star's core will ever be compressed enough to fuse oxygen.

Iron cannot release energy either by fission or fusion. -Therefore, a star with an iron core has no way to generate the energy needed to counteract the crush of gravity. This is the crisis that, in a fraction of a second, initiates the supernova.

may be either a neutron star or a black hole

It is the nearest supernova to have occurred at a time when we were capable of studying it carefully with telescopes. -Supernova 1987A was in the Large Magellanic Cloud, about 150,000 light-years away, making it the nearest one detected since Kepler's supernova in 1604.

The two stars should be the same age, so we'd expect the subgiant to be more massive than the main-sequence star. -A subgiant has already completed its main-sequence life, and massive stars live shorter lives. Thus, we'd expect the subgiant to be more massive than a star that is still a hydrogen-burning, main-sequence star.

it is produced during the supernova explosions of high-mass stars -Fusion in the core produces elements as heavy as iron; heavier elements like gold are produced during the supernova itself.