SC 1
A contracting cloud of gas and dust with accretion disks that is heating up due to gravity but has not yet started nuclear fusion in its core
what is a protostar?
a young star that has finished forming as a protostar but has not yet started stable hydrogen fusion in its core
what is a pre-main-sequence (PMS) star?
When it runs out of hydrogen to burn.
When does a star exit the main sequence?
The phase after helium flashes, where the core burns helium steadily into carbon. It's horizontal because nearly all low-mass stars arrive at nearly the same core mass of 0.47 Mo.
The envelope of the star on this branch determines its color: thin -> hot/blue, red -> cool/red.
What is the Horizontal branch?
luminosity-driven winds blow away the outer layers because the stars are very luminous and their outer layers are very loosely bound.
How does mass loss on giant stars occur?
PMS stars gain gravitational energy as they contract under gravity. The contraction converts gravitational potential energy into heat, raising the star's core temperature
PMS stars radiate heat and light (electromagnetic radiation) from their surface into space. This radiation makes the star shine before fusion begins
how do PMS stars gain and lose energy?
the path a fully convective pre-main-sequence star follows on the Hertzsprung-Russell (H-R) diagram as it contracts and cools before reaching the main sequence. Mostly vertical on HR diagram, applies to small mass PMS stars
What is the Hayashi track?
The path a pre-main-sequence star with a radiative core follows on the Hertzsprung-Russell (H-R) diagram as it contracts toward the main sequence. Mostly horizontal on the HR diagram, it applies to high-mass PMS stars
What is the Henyey track?
a late stage in the life of low- to intermediate-mass stars (.8Mo < M < 8Mo) after core helium burning has ended. The star has:
An inert carbon-oxygen core
A helium-burning shell
A hydrogen-burning shell surrounding the core
What is the asymptotic giant branch?
A nucleosynthesis process in AGB stars where nuclei capture neutrons slowly, allowing beta decay between captures, producing many heavy stable elements. The reasons we have our heavy elements.
What is the s-process (slow neutron capture)?
When a star's energy transport form switches from convective to radiative. For stars < .4 Mo, the switch never happens, and they remain on the Hayashi track all the way to their ZAM position
When does the switch between the Hayashi and Henyey tracks occur?
stars that have just begun stable hydrogen fusion in their cores and have reached the main sequence for the first time, and the Kelvin-Helmholtz contraction stops
What are Zero Age Main Sequence (ZAMS) stars?
When the star's central temperature reaches 10^7 K. (for stars < .08 Mo this ignition never happens because the star never actually gets hot enough)
When does hydrogen burning switch on?
1.) inner helium burning shell: He -> C + O
2.) outer hydrogen burning shell: H -> He (CNO)
A small helium layer between the two layers fuels the inner layers and collects from the outer ones.
How do AGB stars generate energy?
1.) The helium shell suddenly releases a huge amount of energy
2.) Energy cannot escape immediately and is absorbed into the thin helium layer between the two shells, and then the layer expands.
3.) expansion cools the hydrogen-burning shell, shutting it down briefly.
4.) helium fuel gets exhausted, luminosity drops
5.) hydrogen shell rebuilds the helium core and the next pulse occurs
What happens during an AGB pulse?
The phase in a star's life immediately after it leaves the main sequence, when it stops fusing hydrogen in its core and begins hydrogen shell burning around an inert helium core. (moves up and to the right on the HR diagram)
what is the subgiant branch?
The phase after the subgiant stage, where the star has an inert helium core and burns hydrogen in a shell, expanding and cooling to become a red giant. (even more vertically up on the HR diagram)
What is the Red Giant Branch (RGB)?
They have a high-density radiative helium core with a high mass and a small radius. A low-density envelope surrounds the core, composed of hydrogen-rich plasma; this spans almost all of the star's radius.
What are characteristics of RGB stars?
When a low- or intermediate-mass star ejects its outer layers at the end of the AGB phase, leaving behind its hot, inert, degenerate core, which becomes a cooling ___ _____.
How do white dwarfs form?
When an AGB star ejects its outer layers, and the exposed hot core ionizes this gas with UV radiation, causing it to glow before dispersing into space
How do planetary nebulae form?
A very strong generation of energy takes place (CNO-cycle), resulting in a very steep luminosity gradient, making the interior luminosity step-like due to the low energy generation in the envelope.
What happens within the hydrogen shell?
a sudden onset of helium fusion in the core of a low-mass star when the inert helium core becomes degenerate and reaches a critical temperature (~100 million K). Because the degenerate helium core of a low-mass red giant cannot expand when it heats up, helium fusion ignites explosively until degeneracy is lifted. Occurs in low mass RG stars. They occur through the triple alpha process.
What are helium flashes and why do they happen?
a situation in which an increase in energy production leads to conditions that cause even more energy to be produced, creating a self-reinforcing, uncontrolled rise in energy output.
What is energy runaway?
radiating away leftover heat: rapid early cooling (neutrinos), slower photon cooling, crystallization that delays cooling, and eventually fading toward a theoretical black dwarf over trillions of years.
How does white dwarf cooling evolve?
cooling carbon-oxygen interior solidifies into a crystal lattice, releasing latent heat and slowing the white dwarf's cooling
What is crystallization of a white dwarf?