EXAM 3 REVIEW SHEET

Astronomy 110: Fall 09: Monday Novenber 2, 2009

ISM AND STELLAR BIRTH

Gas Clouds are called Nebulae. 3 types: dark, reflection, emission
Dark - dense and cold, seen in silhouette
Reflection - seen by light reflected off the gas; blueish
Emission - bright, most emitted light is in emission lines from hydrogen (reddish)
Stars are born in dark clouds (often embedded in emission nebulae)
Interstellar reddening: blue light scatters, red light passes through (reddens background stars)
Star formation: role of external shock wave (comes from explosion of a nearby star)
low mass stars slow to be born, high mass stars born more quickly
ZAMs; star is born means hydrogen fusion starts in core

STELLAR STRUCTURE

Stellar laws: (2 important ones) conservation of energy, hydrostatic equilibrium
Hydrostatic equilibirum = balance pressure and inward gravitational weight
Energy transport in stars: convective, radiative, conductive (stars do NOT use conduction)
Massive stars have convective cores and radiative envelopes (no granulation at photosphere)
Low mass stars, like the sun, have radiative cores and convective envelopes
Very low mass stars are convective throughout

STELLAR EVOLUTION

Evolution depends on mass: low mass stars (< 8M_sun), high mass stars >8M_sun
Evolution governed by changes in the star's core as nuclear fusion progresses
Star adjusts to core changes and outside properties change (luminosity, radius, surface temperature)
Outside changes result in changed location on the HR diagram as star ages -> evolutionary tracks
Low mass stars, whimper death, create planetary nebula around them
End result of low mass star = White Dwarf (WD), left over inert carbon/oxygen core
Chandrashekar limit: 1.4M_sun, no WD can have mass greater than this amount
High mass stars, explode in supernovae, create fast expanding gas cloud
End result of high mass star = [1] nothing, [2] neutron star (NS), and [3] black hole (BH)

Low mass evolution: 7 stages
Main Sequence (hydrogen core burning)
Red Giant (inert helium core growing, hydrogen shell burning)
Helium Flash (helium core starts burning, hydrogen shell fusion continues)
Horizontal branch (helium core burning + hydrogen shell burning)
Super Giant (inert carbon/oxygen core growing, helium shell + hydrogen shell burning)
Planetary Nebula (helium shell instability, star sheds outer envelope)
White Dwarf (left over inert carbon/oxygen core)

High mass evolution
core continually burns heavier and heavier elements, builds up shells like onion layers
no inert cores as each step progresses, until iron builds up in the core
when iron core reaches Chandrashekar limit, core implodes; star explodes

HR Diagram and Ages of Star Clusters
Assumptions: [1] stars are co-evolutionary, [2] stars all the same distance away
Key points: "O" stars live the shortest, "M" stars take longest to be born
We examined HR diagrams for 1, 10, and 100 million yrs, and for 1 and 10 billion (review these)
Main sequence turnoff, used as a tool to measure ages of clusters (be able to explain)
Presence of horizontal branch = very old (10 billion yrs)

WHITE DWARFS, NEUTRON STARS, AND BLACK HOLES

WD are about the size of the Earth; larger mass WD have smaller sizes
WD are hot core remnants of a low mass stars
WD density is about 1 million times that of water, 10⁶ g cm^-3
NS are about the size of a small city; size does not depend upon the mass
NS density is about 100 trillion times that of water, 10¹⁴ g cm^-3
NS can be Pulsars (chance alignment of beamed radiation)
BH are about the size of a college campus
BH characterized by singularity, event horizon, and photon sphere
Event horizon, region wihin which nothing can escape, even light
Space and time are "bent" around a BH
Gravitational lenses can result from BHs

EXTRA CREDIT ON EXAM

be able to write a short response to the following questions:
What if you fell into a BH? What would you see and experience? What if you observed your friend falling into a BH? What would you see?