Evolution of low-mass stars
Sun is a low mass star.
High mass stars do not last for too long, only a few millions years, which is much
shorter than the life span of our Sun (10 billion years). A few million years is too
short for any life form to evolve or even for planets to form. High mass stars are
quite hot and luminous, high energy radiation is detrimental to any living
The lives of low-mass stars
Sun slowly and steadily fuses hydrogen into helium in its core via the proton-proton
chain. The Sun shines steady since the whole star is in the hydrostatic equilibrium.
Hydrogen fusion supplies the thermal energy that maintains a star’s thermal
pressure and hold gravity at bay. However, fusion in the core of a low-mass star
reduces the number of hydrogen atoms in the core; each fusion reaction converts
four independent protons into just one helium nucleus. Slowly reduce the number of
hydrogen nuclei available to fuse up. Up to the point where the star’s core hydrogen
supply depletes and cease to fusion. With no fusion to supply thermal energy and
maintain the interior pressure, the star will be out of balance. Cannot resist gravity,
Made out of entirely helium. “ash”. Gas surrounding the core will contain plenty of
fresh hydrogen. Gravity shrinks both the non-burning helium core and surrounding
shell of hydrogen, the hydrogen shell soon becomes hot enough for hydrogen shell
burning- hydrogen fusion in a shell around the core. Increase in energy output will
cause a buildup of thermal pressure inside a low-mass star, push its surface outward
causing the star to expand and increase its luminosity. The star will move upward
on the H-R diagram. Increase of the star’s size and its luminosity marks the point in
life of low-mass star when star becomes a red giant.
While the star’s outer layers are expanding, the buried core of the star is shrinking
and growing hotter and denser until the temperature of the inert helium core
reaches about 100 million K. Hot enough for helium nuclei to begin to fuse together.
Fusion operating, regains the same sort of balance it had as a main-sequence star,
except now it is helium fusion that keeps the central temperature steady.
How does a low-mass star die?
Fuses all its core helium into carbon. Fusion will again cease. Go out of balance.
Shrink once again under the crush of gravity. Cause the Sun to expand once again.
Trigger for expansion is the helium fusion in a shell around the inactive carbon core.
Only a few million years. Carbon fusion is only possible at 600 million K, and
temperatures cannot be reach.
Once the core of the star is made out of carbon ash, fusion reactions will cease again
and the core will start to contract. Once it reaches a density of 1000kg/cm ,3
electrons in the core refuse to move closer together and the core stops contracting.
Same charges repel each other. Can only be packed up to a certain point. Above that
density, electrons will not be pushed any further, electron degeneracy pressure.
Helium shell burning generates enough energy to blow the outer layers of the star
off into space. Huge shell of gas expanding away from inactive carbon core. The exposed core will still be very hot and will therefore emit intense ultraviolet
radiation, ionizing the gas and making it glow brightly as a planetary nebula. The
glow of the planetary nebula will fade as the exposed core cools and the ejected gas
disperses into space. The nebula will eventually fully disappear, leaving behind the
star’s cooling carbon core as a white dwarf.
Small in radius and often quite hot because some of them were only recently in the
center of a star and have not yet had time to cool much. As long as no mass is added
to the white dwarf from some other source (companion star in a binary system),
neither the strength of gravity nor the strength of the degeneracy pressure that
holds gravity at bay will ever change. A white dwarf is therefore little more than a
decaying corpse that will cool for the indefinite future, eventually disappearing from
view as it becomes too cold to emit any more visible light. Composition reflects the
products of the star’s final nuclear-burning stage. The white dwarf left behind by a
star similar to our Sun will be made mostly of carbon, fuse helium into carbon at
final stage. Cores of very low-mass stars will never become hot enough to fuse
White dwarf has the mass of the Sun compressed into a volume of the size of the
Earth. Density. Upper limit of the mass of a white dwarf of about 1.4 times the mass
of the sun. Electrons will move at the speed of light. Increasing this mass will make
electrons move faster than the speed of light, which is not possible.
A single white dwarf will never again shine as brightly as their star it once was. No
source of fuel, simply cool with time into a cold, black dwarf. Size will never change,
electron degeneracy pressure will forever keep it stable against the crush of gravity.
But a white dwarf in a close binary system can gradually gain mass if its companion
is a main-sequence or giant star. The infilling matter forms a whirlpool-like disk
around the white dwarf known as an accretion disk because the process in which
material falls onto another body is called accretion. Provides a new energy source,
as long as its companion keeps feeding matter into the accretion disk. If white dwarf
gains enough mass, approach the 1.4 solar mass limit. Cannot go further. As a white
dwarf approaches this upper limit of 1.4 solar masses, its temperature will start to
rise, igniting carbon fusion reactions throughout the white dwarf, creating a “carbon
bomb” detonation. Blows itself apart in an event we call Type I or a white dwarf
White dwarf supernovae provide one of the primary means by which we measure
large distances in the universe. Because white dwarf supernovae always occur in
white dwarfs that have just reached the 1.4 solar mass