Stars: The End of a Star

(or is just the beginning?)

CARBON BURNING: Death Up to this point, most of the events of stellar evolution are well documented. What happens to a star after the red-giant phase is not certain. We do know that a star, regardless of its size, must eventually run out of fuel and collapse. In theory, GRAVITY WINS. With this in mind, we will consider the death of stars and group them into three categories according to mass:   Low-Mass Stars (0.5 solar mass or less) Medium-Mass Stars (0.5 solar mass to 3.0 solar mass) Massive Stars (3.0 solar masses or larger)   Low-mass stars   A low mass star becomes a white dwarf Low mass stars (0.08-5 SM during main sequence) will go the planetary nebula route. A low mass core (,1.4 SM) shrinks to white dwarf. Electrons prevent further collapse. The size of the white dwarf is close to that of earth, and the outer layers are planetary nebula. Click here to learn more about how white dwarves are formed.   Medium-mass stars become neutron stars This supernova was first observed in 1987 by the Hubble Telescope (NASA) A higher mass core (between 1.4-3 SM) shrinks to neutron star. Supernova happens when a neutron star is created. Neutrons prevent further collapse. The size of a neutron star is about that of a large city. Click here to learn more about how neutron stars are formed. More Massive Stars These stars are so massive (10-20 solar masses) that the hydrogen burning and helium burning phases occur relatively quickly when compared with smaller stars. These stars utilize carbon burning. Interactive Lab Carbon Burning Process The overall reactions that occur for carbon burning occur so rapidly and with so much energy that the star blows apart in an explosion called a supernova. The outer layers of the star blast into space, and the core is crushed to immense densities. Carbon burning occurs when the helium in the core is gone. The core needs to maintain temperature to keep the gas pressure up; otherwise the star cannot resist gravity. When carbon burning does occur, iron is formed. Iron is the most stable of all nuclei, and ends the nuclear fusion process within a star. When these heavier elements form in the core, they take away energy rather than release it. With the decrease in fuel for fusion, the temperature decreases and the rate of collapse increases. Just before the star totally collapses, there is a sudden increase in temperature, density, and pressure. The pressure and energy compact the core further, squeezing it like “Charmin.” The compact core becomes a rapidly whirling ball of neutrons, and that’s why now this star is termed a neutron star. The largest mass stars may become black holes The highest mass star has a core that shrinks to a point. On the way to total collapse it may momentarily create a neutron star and the resulting supernova rebound explosion. Gravity finally wins. Nothing holds it up. Space so warped around the object that it effectively leaves our space – black hole! Interactive Lab This activity shows what happens to different size stars at the end of their life cycles.