20 Questions
What is the source of energy for main sequence stars?
The proton-proton chain and the CNO cycle
What temperature is required for helium to be converted to carbon through the triple-α process?
100 million degrees
What happens to the core of a star when the electrons and helium nuclei become degenerate?
It becomes incompressible and further contraction stops
Why does helium require higher temperatures to fuse compared to hydrogen?
Helium nuclei have more electric charge, making it harder to fuse
What leads to the formation of carbon and oxygen nuclei in stars?
The triple-α process
What prevents expansion of the core despite increased temperature in stars?
Degenerate core
What causes stars to become horizontal branch stars?
Helium burning
What process leads to the formation of dust particles important for interstellar clouds and proto-solar systems?
Stellar wind during the asymptotic giant branch phase
What causes the RGB bump in the evolution of red giant stars?
Discontinuity in hydrogen abundance left behind by deep convection
At what temperatures do stars reach the tip of the red-giant branch at solar metallicity?
Around 3,000 K
What happens to stars initially more massive than 2 M☉ during their evolution?
Perform a blue loop before joining the asymptotic giant branch
What determines the mass and properties of the white dwarfs that form subsequently from red giants?
Mass lost from red giants
What causes a spread of stars along the horizontal branch (HB) at constant luminosity?
Variation in the mass of the hydrogen envelope around the helium core
What are the characteristics of extreme horizontal branch stars?
Temperature of 20,000–30,000K and beyond normal core helium burning stars
What causes the RR Lyrae gap in globular cluster color-magnitude diagrams (CMDs)?
Pulsating stars known as RR Lyrae variable stars
What determines the differences in radii, effective temperatures, and color of the red clump, a population I counterpart to HB stars?
Differences in the helium content and age
What type of stars are powered by helium fusion in the core and by hydrogen fusion in a shell surrounding the core?
Red giants
What initiates fusion more smoothly without a flash in stars with larger helium cores?
Non-degenerate cores
What class of variable stars with amplitudes of a few thousandths of a magnitude and semi-regular periods of 10 – 100 days was discovered through microlensing surveys?
OGLE Small Amplitude Red Giants (OSARGs)
What type of stars experience the initiation of helium fusion through the triple-alpha process, causing a temperature rise and rapid increase in fusion rate?
Stars with masses up to $2.3 M_{igodot}$
Study Notes
Stellar Evolution and Variability in Red Giants
- Mass lost by more-massive stars leaving the red-giant branch before the helium flash is challenging to measure directly.
- Current mass of Cepheid variables like δ Cephei can be accurately measured due to binaries or pulsating stars, with an apparent 20% mass loss, mostly during the blue loop and pulsations.
- Some red giants are large amplitude variables, including Mira variables, semiregular variables, and slow irregular variables, previously considered to be asymptotic giant branch (AGB) or supergiants.
- Studies in the late 20th century showed that all giants of class M were variable, with amplitudes of 10 milli-magnitudes or more, and late K class giants were also likely to be variable with smaller amplitudes.
- Microlensing surveys in the 21st century provided accurate photometry, leading to the discovery of a new class of variable stars called OGLE Small Amplitude Red Giants (OSARGs) with amplitudes of a few thousandths of a magnitude and semi-regular periods of 10 – 100 days.
- Thousands of OSARGs were detected in the Magellanic Clouds and a catalog of 192,643 OSARGs in the direction of the Milky Way central bulge was published.
- Horizontal-branch stars are powered by helium fusion in the core and by hydrogen fusion in a shell surrounding the core, immediately following the red giant branch in stars similar to the Sun's mass.
- After exhausting core hydrogen, stars leave the main sequence, begin fusion in a hydrogen shell around the helium core, and become giants on the red giant branch.
- Stars with masses up to 2.3 times the mass of the Sun experience the initiation of helium fusion through the triple-alpha process, causing a temperature rise and rapid increase in fusion rate.
- Stars initially between about 2.3 M☉ and 8 M☉ have larger helium cores that do not become degenerate, triggering helium fusion before the core becomes degenerate.
- Non-degenerate cores initiate fusion more smoothly without a flash, and the output of the helium flash event is absorbed by the layers of plasma above, not visible from the exterior of the star.
- The evolutionary track of a sun-like star shows the horizontal branch and red clump region after core hydrogen exhaustion, with substantial changes in stellar structure and a reduction in luminosity.
Test your knowledge of stellar evolution and variability in red giants with this quiz. Explore topics such as mass loss in red giants, Cepheid variables, Mira variables, OSARGs, and the evolutionary track of sun-like stars.
Make Your Own Quizzes and Flashcards
Convert your notes into interactive study material.
Get started for free
