Chapter 17 ± Star Stuff
x All main-sequence stars are in gravitational equilibrium ± outward push of gas
pressure balancing the inward pull of gravity.
x Because the force of gravity is greater within more massive stars, the pressure inside
must also be greater.
x To provide this greater pressure, the cores must be hotter and denser, leading to a
greater fusion rate.
x With higher core temperatures and higher fusion rates, massive stars burn through
their hydrogen gas much faster, explaining why they live shorter.
x Low-mass stars are stars born with less than about the mass of our Sun.
x Intermediate-mass stars have birth weights between about 2 and 8 solar masses.
x High-mass stars are those stars born with masses greater than about 8 solar masses.
Red Giant Stage
core. The star becomes out of balance, gravity shrinking its core size.
x 7KHVWDU¶VRXWHUOD\HUVKRZHYHUwill expand outward, becoming a subgiant.
DIAGRAM 17.4 ± why stars expand in size.
vicinity still holds some hydrogen.
x Gravity then shrinks both the non-burning helium core and the surrounding shell
of hydrogen, allowing for hydrogen to sustain.
x Hydrogen burning shell will proceed at a greater fusion rate, creating a greater
x Size increase because energy transport within its interior cannot keep pace with
this larger energy-generation rate, so thermal pressure will build up and push the
x Helium fusion occurs only when nuclei slam into one another at much higher
speeds than those needed for hydrogen fusion, meaning that a higher temperature
x The helium fusion process converts three helium nuclei into one carbon nucleus.
x Degeneracy pressure takes part in counteracting gravity, so the core does not
x The rising temperature causes the helium fusion to soar upward, called helium
flash, releasing an enormous amount of energy into the core, allowing thermal
pressure to dominate.
x As a helium-burning star, size is smaller and is hotter, dropping its life track.