Stars are ubiquitous objects in our Universe.
Many Sun-like and low-mass stars are at the centers of rich exoplanet systems discovered in recent
years.
...and the rarest, most massive stars end their lives through spectacular supernovae explosions.
Populations of stars residing in galaxies like our own Milky Way not only emit most of the light
we observe from the distant Universe
but they also play a major role in shaping their environments,
ultimately influencing the evolution of galaxies through cosmic time.
In order to better understand and interpret these systems and phenomena, we need to improve our
understanding of what is at the heart of it all: stars, and in particular, how different types of
stars evolve over time.
Fundamentally speaking, stars are simple, gravitationally-bound balls of gas powered by nuclear
fusion. We run computer simulations to model their evolution, which we can then compare to
observations. Any discrepancies can be investigated in more depth and what we learn from this
process can be used to help improve the models.
The following page will guide you through the life of a star that is only slightly more massive
than our own Sun.
We will use something known as the Hertzsprung-Russell diagram (HR-diagram) to guide our
exploration.
The HR-diagram is a powerful visualization and diagnostic tool frequently used by
stellar astronomers. It is a graph that shows the brightness of a star (luminosity) on the y-axis
and its temperature on the x-axis.
Since the star's brightness and temperature change over the course of its lifetime, we can graph
its life trajectory on the HR-diagram.
Different types of stars show similar trends and behaviors when placed on the
HR-diagram, and we can make use of that fact to draw some physical meaning and intuition.
We will step through each evolutionary phase of the star's life in the following pages.
The timeline on top will serve as a guide through our journey. You can click on any stage to move
directly there. You can also move your cursor to the star to learn more about its properties.
Pre-Main Sequence
On the PMS, the star undergoes contraction as it descends the nearly vertical track on the
Hertzsprung-Russell diagram known as the Hayashi track. The central density and temperature are
not sufficiently high at this stage for hydrogen fusion to take place in the center. All PMS
stars start out as fully-convective objects (energy is transported by large-scale turbulent motions, akin
to boiling water), but for those that are more massive than ~0.3 solar masses, a radiative region
(energy is carried away by photons) develops in the interior as they approach the MS
Main Sequence
The MS phase is the longest-lived stage of evolution leading up to the formation of a white dwarf.
For the Sun, this is expected to last ~10 billion years. On the MS, the star steadily fuses
hydrogen to helium, which generates thermal energy and provides support against gravity.
Hydrogen-fusion occurs primarily via proton-proton chain for stars like the Sun, but in more
massive stars, the CNO-cycle dominates. Moreover, this process takes place in a radiative core
for stars like the Sun, but for more massive stars, it occurs in a convective core instead. Stars
harboring convective cores show a characteristic 'hook' morphology as they leave the MS.
Red Giant Branch
Once the star runs out of its hydrogen fuel in the center, it can no longer produce enough energy
to hold up its own weight. As a result, the helium core begins to contract, releasing gravitational
energy in the process. This extra energy is deposited outside the helium core, which heats up the
layer and triggers hydrogen fusion in a shell. This so-called shell-burning is accompanied by an
expanding, cool, convective outer envelope. The star is now a red giant star and it continues to
become brighter and larger at a roughly constant temperature, tracing its path upward the Hayashi
track. This phase is expected to last only ~1.5 billion years for the Sun.
Core Helium Burning
During the Red Giant Branch phase, hydrogen shell-burning deposits freshly-fused helium on the inert helium
core as it continues to contract, making it both denser and more massive. Eventually, the core
becomes massive, dense, and hot enough for the onset of helium fusion. In stars below ~2 solar masses,
ignition proceeds violently through a phenomenon known as the helium flash: the helium core is so
dense that the most central region becomes degenerate, which suffers an unstable, explosive
thermonuclear runaway reaction with the sudden ignition of helium. In stars above ~2 solar masses, helium
ignites in a non-degenerate core and thus proceeds in a quiescent manner. Regardless of the
ignition condition, the star leaves the Hayashi track and is no longer a red giant, and helium
fusion ensues along with hydrogen shell-burning beyond the core. This phase is expected to last
only ~0.1 billion years for the Sun.
Asymptotic Giant Branch
Eventually, the star runs out of helium fuel in the center. The carbon/oxygen core contracts and
the outer layers expand to form an AGB star, analogous to the RGB phase. In stars less massive
than ~8 solar masses, the core cannot become dense and hot enough for carbon and oxygen to fuse into
heavier elements, and the central regions become degenerate. However, the contracting core releases
gravitational energy and heats up the layer above the carbon/oxygen core, which sets of helium
shell-burning. In turn, helium shell-burning deposits energy in the hydrogen layer above,
triggering hydrogen shell-burning as well. Throughout the AGB phase, hydrogen and helium
shell-burning alternate, giving rise to the well-known thermal pulses. These pulses are associated
with dense winds and a rich array of heavy elements synthesized through successive
neutron-captures called the s-process. Eventually, the AGB star sheds its envelope entirely,
leaving behind its hot carbon/oxygen core.
Post-Asymptotic Giant Branch + White Dwarf
At this post-AGB phase, the bare, degenerate core is shrouded deep inside shells of gas it ejected
during the AGB phase. The core is hot and thus produces copious amounts of high energy photons
that ionize and light up the surrounding gas. Observationally, these objects correspond to
planetary nebulae. Though initially very hot, the central object quickly cools and can no longer
light up the surrounding gas that is also gradually dispersing into the interstellar medium. The
compact, central object that is left behind is now a white dwarf, which is supported against
gravity by electron degeneracy pressure. White dwarfs are notable for their incredibly high
density: approximately a solar mass's worth of material is packed inside a sphere the size of the
Earth.
