Chapter 20: Stellar Evolution - The Life and Death of a Star
As hydrogen is consumed
the star eventually leaves main sequence.
Lagrangian point is:
where gravity acts equally between the two stars.
Our Sun will not become a nova because this only happens to stars:
with a binary companion.
During Stage 8, Subgiant Branch:
• Increased energy output increases luminosity. • Increased energy from core also causes radius to increase. This spreads outer layers out, causing surface temperature to decrease to ~4000 K.
The Hydrogen Shell Burning Stage occurs during Stage 8, Subgiant Branch and:
• Once hydrogen is completely converted into helium in the deepest part of the core, fusion ceases. Radiation pressure becomes weaker and gravity takes over. The core shrinks. • Temperatures increase due to shrinking, but not enough to start helium fusion. Layers containing hydrogen, surrounding the helium core, begin to fuse more rapidly.
Stage 12: A Planetary Nebula
•There is no carbon fusion in the center, only hydrogen and helium burning in surrounding shells. This burning becomes unstable. • Helium-shell flashes occur, which produce fluctuations in the intensity of energy emission from the star. • Layers pulsate as envelope repeatedly is heated, expands, cools, and contracts. • Eventually the envelope is ejected right from the core. We are left with a core of carbon ash.
Depending on how close the stars are:
There are different types of binary star systems.
What temperature is needed to fuse helium into carbon?
100 million K.
Stage 9, Red Giant Branch: Core is VERY dense -
10^8 kg/m3 (while non-burning layers have a density of 10-^3 kg/m3)
Stage 10, Helium Fusion: Once the core temperature has risen to 100,000,000 K, the helium in the core starts to fuse, through a triple alpha process:
4^He + 4^He → 8^Be + energy 8^Be + 4^He → 12^C + energy
Hydrogen vs. Helium at star birth:
A star's helium aboundance at birth is about 10%.
Stage 9, Red Giant Branch: Helium core is tiny:
A thousandth the size of the star.
White Dwarf
After becoming a planetary nebula, core eventually cools and becomes a white dwarf. Very small - size of Earth (or smaller) with mass of the Sun or less
Current Age of Main Sequence Turn-off Stars =
Age of Cluster!
Noting the mass of the main sequence turnoff stars in a star cluster allows you to determine its:
Age.
Luminosity increases as a result of what during the asymptotic giant branch?
As a result of the increased energy being produced in the core.
At what points on the H-R Diagram do all the stages occur at?
At Points 7-8: Subgiant/Subgiant Branch. At Points 8-9: Red Giant Branch At Point 9: The Helium Flash At Points 10-11: Asymptotic-Giant Branch At Points 11-12: Planetary Nebula At Point 13: White Dwarf At Point 14: Black Dwarf
At what temperatures does helium fusion occur at?
At least 10^8 K & Higher (100,000,000 K+)
What temperature does carbon fusion occur at?
At least or over 600,000,000 K.
At what stage is a star called a subgiant?
At stage 8, it travels along the subgiant branch at this time as well.
Current Age of Main Sequence Turn-off Stars = Age of Cluster! Why is this true?
Because, exactly enough time has passed for stars of this mass to burn all of their hydrogen and begin moving away from the Main Sequence.
Refer to the figure below. What is the name of the star at the point labeled 14?
Black dwarf. (It falls off the H-R Diagram.)
Cooled-off white dwarfs that no longer glow visibly are what kind of star?
Black dwarfs.
Helium Flash: Once thermal pressure dominates once again:
Core expands, density drops, and equilibrium is restored.
The star travels along the asymptotic giant branch during what conditions?
Core shrinks, central temperatures increase, which increases the rates of helium and hydrogen fusion. Outer layers expand, surface temperatures decrease, but luminosity increases as a result of the increased energy being produced in the core.
What happens to the electrons' pressure in Helium Flash?
Electron Degeneracy.
Helium Flash: What is Electron Degeneracy?
Electrons are easily squeezed together until the point of contact, but are virtually incompressible thereafter.
Refer to the figure below. What is the name of the stage at the point labeled 9?
Helium Flash.
Refer to the figure below. What is the name of the star at the point labeled 10?
Horizontal branch.
Hydrogen vs. Helium at the end of a Sun-like star's life (10 billion years):
Hydrogen mostly; yet all gone at center.
When a star's inward gravity and outward pressure are balanced, the star is said to be:
In hydrostatic equilibrium.
Helium Flash: The electron degeneracy is an upper limit, what does that mean for the helium flash?
Increases in temperature will not allow for pressure adjustments or any other adjustments.
A star is on the horizontal branch of the HR diagram. Which statement is true?
It is burning both hydrogen and helium.
Summary of the end states of stars:
Less than 0.08 SM: (Hydrogen) brown dwarf 0.08-0.25: Helium white dwarf 0.25-8: Carbon-oxygen white dwarf 8-12~*: Neon-oxygen white dwarf Greater than 12*: Supernova (Chapter 21) *Precise numbers depend on the (poorly known) amount of mass lost while the star is on, and after it leaves, the main sequenc.e
The Algol binary system consists of a 3.7 solar mass B main sequence star and a 0.8 solar mass red subgiant. How is the less massive star more evolved than the more massive B star?
Likely because the subgiant was once more massive and evolved first. When it became a subgiant, it expanded outside of its Roche lobe, and the B star - initially less massive, "stole" the subgiant's envelope.
A star (no matter what its mass) spends most of its life as a:
Main sequence star.
Mass is the main determinate of where stars end up on the main sequence.
Mass also determines how long a star will live on the main sequence.
Are stars always determined to become a supernova if their mass may fit?
No, because the star may lose mass, and eventually not have enough to produce a supernova.
Globular clusters are;
Older than open clusters.
At this point of stable helium fusion, the star resides where?
On the horizontal branch.
Moves toward the red supergiant region of the HR diagram, when do stars do so?
Once an element is depleted, core contracts and fusion of next element begins
Hydrogen vs. Helium at Sun's age (5 billion years):
Only a small amount of hydrogen changed into helium.
Theory vs. Reality
Our evolutionary tracts, predicted by models of stellar interiors, agree with observations of stars in the globular cluster M80.
Refer to the figure below. What is the name of the path between the points labeled 11 and 12?
Planetary nebula.
Refer to the figure below. At what numbered point on the graph does the helium flash occur?
Point 9; Helium Flash.
What is a black dwarf?
Proceeding a white dwarf barren of nuclear fusion, it cools and dims, becomnig a black dwarf, falling off the HR Diagram.
Which of the following lists, in the correct order, a possible evolutionary path for a star?
Red Giant, Planetary Nebula, White Dwarf
Refer to the figure below. What is the name of the path between the points labeled 8 and 9?
Red giant branch.
Mass transfer in binary systems occurs when one giant sweels and exceeds its:
Roche lobe.
Compared to less massive stars, more massive stars experience:
Shorter lifetimes.
Stage 9, Red Giant Branch: Red giant is HUGE:
Size of Mercury's orbit.
Stages of post-main sequence stellar evolution for a Sun-like star:
Stage 7: Main-sequence star Stage 8: Subgiant Branch Stage 9: Helium Flash Stage 10: Horizontal Branch Stage 11: Asymptotic-Giant Branch Stage 12: Carbon Core -> Planetary Nebula Stage 13: White Dwarf Stage 14: Black Dwarf
Convection causes surface temperature to remain fairly constant during what stage?
Stage 9.
Stellar Mass < 8 solar masses —
Star dies "gently"
Stellar Mass > 8 solar masses —
Star dies catastrophically
Main sequence lifetime of Main Sequence Turn-off Stars calculated directly from mass:
Stellar lifetime is proportional to 1/(stellar mass) to the third power.
In the evolution of a Sun-like star:
Such stars never become hot enough for carbon fusion to occur.
Why are high temperatures and densities neccessary in Helium Fusion?
The 8^Be nucleus is highly unstable and will decay in about 10-12 seconds unless an alpha particle fuses with it first.
Stars form from the collapse of a massive dust cloud.
The collapse of several fragments allows for the formation of individual stars.
What is a planetary nebula?
The core after finishing burning the last of its fuel in Stage 12 emits UV radiation causing an ejected envelope to glow.
Planetary nebula:
The core finishes burning the last of its fuel. It heats up and eventually emits ultraviolet radiation, which causes the ejected envelope to glow. The ejected envelope is a planetary nebula.
Stage 9, Red Giant Branch: ~25% of the star's mass is packed into where?
The core.
Refer to the figure below. What is the star on at the point labeled 7?
The main sequence.
During the hydrogen burning shell phase:
The star grows more luminous.
Stage 14: A Black Dwarf
The white dwarf has no nuclear fusion within it, so it continues to cool and dim, and eventually becomes a black dwarf. It falls off the HR diagram.
Helium Flash: Helium fuses ferociously, but eventually temperature rises enough to allow:
Thermal pressure to dominate once again.
Blue stragglers gained their mass from mergers between smaller mass stars, meaning:
They indeed did not have time to evolve yet. (They are an example of a rare exception of stars changing spectral classes.)
Blue stragglers appear to be main sequence stars that do not follow the theory because
They should have evolved long ago with the others of their spectral type.
What happens if stars never become hot enough for carbon fusion to occur?
They won't produce elements heavier than carbon.
Meanwhile, the core in Stage 9, Red Giant Branch, continues to shrink and non-burning layers continue to expand.
This path of nearly constant temperature on the HR diagram is called the red giant branch. The star is a red giant.
Helium Flash: When the helium starts fusing, the pressure cannot adjust:
This pressure is almost independent of temperature.
At stage 8, star becomes opaque to outward radiation:
Thus, source of energy output is convection.
A star cluster contains the same composition, age, and distance away, and primarily differ in their masses.
Thus, we can use clusters to test stellar evolution theories.
Stage 11: Back to the Giant Branch:
Very soon, helium and hydrogen burning shells form around a core of carbon ash (no carbon burning occurs at this stage).
The order of evolutionary stages of a star like the Sun would be main sequence, red giant, planetary nebula, and finally:
White Dwarf.
Refer to the figure below. What is the name of the star at the point labeled 13?
White dwarf.
The eventual fate of our Sun is to:
become a steadily cooling white dwarf.
On the main sequence, stars undergo core hydrogen burning:
conversion of hydrogen into helium via nuclear fusion (NOT burning as in a typical fire on Earth, no chemical reactions).
Once an element is depleted,
core contracts and fusion of next element begins
Stellar mass > 8 solar
core temperatures become hot enough to allow carbon fusion, oxygen fusion, and more
In a detached binary,
each star has its own Roche lobe.
For a white dwarf to become a nova it is necessary for it to:
have a binary companion. (This is why the Sun will not become supernovae, it is isolated.)
Stellar mass > 2.5 solar:
helium fusion begins smoothly - no helium flash
High mass stars evolve much more quickly;
higher masses and gravities generate more heat
The balance of the hydrostatic equilibrium is shifted as
hydrogen is consumed.
In a contact binary,
much of the mass is shared between the two stars.
Stars spend the majority of their lives
on the main sequence.
In a semi-detached binary,
one star can transfer mass to the other.
Roche lobes are:
regions around stars where gravity will attract matter and pull it toward the central star.
Rapid mass transfer can occur in
star systems where the subgiant was once more massive and evolved first.
Black holes are formed by:
supernovae from the most massive stars (stars with a large mass >8ms)