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 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
dwarf. Electrons prevent further collapse. The size
of the white dwarf is close to that of earth, and the outer
layers are planetary nebula.
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.
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
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!