Semester 2 - Definitions Flashcards
(95 cards)
What is the Virial Theorem?
The Virial Theorem relates the potential energy and total kinetic energy in a self-gravitating sphere in hydrostatic equilibrium.
How do we know that stars evolve?
Change in stars is inevitable due to the finite energy source they possess.
Define the Jeans mass and its role in stellar evolution.
The Jeans mass is the maximum mass of gas stable against gravitational contraction. It determines the minimum mass required for a gas cloud to collapse and form a star.
What are the timescales involved in stellar collapse and evolution?
The timescales include the freefall timescale (for collapse), Kelvin-Helmholtz timescale (for radiation-powered contraction), and nuclear timescale (for energy generation via nuclear fusion).
What are the different types of opacity and their significance in stellar evolution?
Opacity types include electron scattering, free-free, bound-free, and bound-bound. They affect the transport of radiation within stars and influence their temperature profiles and evolution.
What is initial free-fall collapse?
Protostars are initially accreting mass from their host molecular clouds and shrinking under their own gravity. They are not initially in H.E.
Describe the pre-main sequence sources.
These low-mass objects that are bright in the optical and lie above the theoretical zero-age main sequence (ZAMS) are not fusing H-He yet. There are also signs of a protoplanetary disk.
What are evolutionary tracks in stellar structure and evolution?
Evolutionary tracks represent the paths that stars take through the Hertzsprung-Russell (HR) diagram as they evolve. They help in understanding the changes in stellar properties over time.
What are protostars and what evidence supports their existence?
Protostars are young stellar objects still undergoing gravitational contraction. Evidence for their existence comes from observations of redshifted absorption lines in molecular spectra.
What are T Tauri stars and what distinguishes them from main sequence stars?
T Tauri stars are pre-main sequence low mass stars with high luminosity and are found in nebulae or young clusters. They are brighter than main sequence stars of similar spectral types and are often surrounded by accretion disks left over from stellar formation.
What are Herbig Ae/Be Stars?
Herbig Ae/Be stars are pre-main sequence higher-mass counterparts of T Tauri stars. They are located to the right of the main sequence.
What prevents equatorial winds in Herbig Ae/Be stars?
A disk of gas and dust, rotating and accreting onto the star, prevents equatorial winds.
Explain the process of collapse in a gas cloud.
Gas clouds collapse under self-gravity when M > MJ. Initially the collapse is free fall but matter the gas cloud approaches H.E.
Describe the adiabatic contraction phase.
In the adiabatic contraction phase, radiation produced within the cloud becomes trapped due to increasing opacity as ionization occurs. The cloud heats up, and contraction becomes adiabatic as the cloud moves towards H.E.
What is the isothermal contraction phase and when does it end?
Isothermal contraction is when T~const. as potential energy released is either radiated away or absorbed without increasing T. The isothermal phase ends once all H and He in the cloud is dissociated and ionised.
How does the Jeans mass change during adiabatic contraction and what is the line of stability?
During adiabatic contraction, temperature increases, leading to an increase in the Jeans mass as density increases. The line of stability, determined by the adiabatic index, separates the regime of collapse from expansion.
What circumstances lead to instability in the adiabatic regime?
Instability in the adiabatic regime occurs when the adiabatic index is less than or equal to 4/3. This can happen during phase changes or with polyatomic molecules with s degrees of freedom.
Describe the onset of hydrostatic equilibrium and the emergence of a protostar.
Once a collapsing cloud becomes opaque enough to trap radiation and completes dissociation and ionization, temperature and pressure start to rise. An outward pressure gradient halts contraction, leading to the establishment of H. E and the emergence of a protostar.
Describe what temperature H.E. is established?
H. E is established at a T ~ 30,000K which is determined by the virial theorem.
Describe the effect on a molecular cloud in the presence of magnetic fields?
Magnetic fields in molecular clouds exert pressure on the gas, resisting contraction.
How does rotation influence the evolution of a rotating gas cloud?
Rotation introduces an additional centrifugal force opposing gravity.
What are Hayashi tracks, and how do they relate to stellar evolution?
Hayashi tracks represent evolutionary paths on the HR diagram followed by fully convective protostars.
Describe the pre-MS evolutionary tracks for low mass stars.
Low-mass stars have long Hayashi tracks and remain fully convective throughout their pre-main sequence lifetimes. Low mass stars do not develop a radiative core and arrive on the hayashi track fully convective.
What marks the arrival of a star on the main sequence, and how does it vary with mass?
The arrival on the main sequence occurs when nuclear fusion begins in the core. For low-mass stars, this transition occurs smoothly, while high-mass stars may exhibit deviations in their tracks due to brief periods of burning before settling onto the main sequence.