Textbook Notes (368,035)
Canada (161,583)
Physics (160)
PHYS 183 (51)
Tracy Webb (12)
Chapter 17

Chapter 17

4 Pages
76 Views
Unlock Document

Department
Physics
Course
PHYS 183
Professor
Tracy Webb
Semester
Winter

Description
PHYS 183 – The Milky Way Inside and Out – Tracy Webb Winter 2013 17: Star Stuff 17.1 Lives in the Balance  stars with large masses have greater fusion rates since they attain high core temperatures easier  rate of fusion is very sensitive to temperature so massive stars achieve equilibrium with fusion higher than lower mass stars  mass of main sequence star determines luminosity & lifetime  star’s mass also determines what happens when star exhausts core supply of H  low mass star: stars born with lees than 2 solar masses (2M sunf material  intermediate mass star: star with birth weight between 2 and 8 solar masses  high mass star: stars born with masses greater than about 8 solar masses  intermediate mass stars are more similar to high mass stars 17.2 Life as a Low Mass Star  the sun is a low mass star  currently a main sequence star, in the middle of its 10 billion year life  sun shines steadily due to self regulating processes (solar thermostat)  energy in low mass stars moves through a combination of radiative diffusion & convention from core to surface  depth of convention zone depends on internal temperature & therefore mass  radiative diffusion occurs deep inside the star  convection occurs in sun’s outer layers only  cooler temperature so photons more difficult to transport  transition from radiative diffusion to convection occurs 70% of way from centre to surface  stars smaller than sun have cooler interiors so deeper convection zones  highest mass stars have no convection zones near surfaces, but have convective cores since energy produced at rate that radiative diffusion can’t transport in enough time  convection plays role in determining whether star has activity similar to sunspot cycle on sun  most active stars are very low mass  flare stars: stars that have field lines that suddenly snap & reconfigure themselves which releases energy from the magnetic field in the form of flares  as number of particles in star drops, core shrinks & heats up to keep pressure in balance with gravity  raises fusion rate & luminosity  core shrinks due to gravity when nuclear fusion ceases but outer layers expand  subgiant: star that has ceased nuclear fusion and moves to the right on the H-R diagram as it grows in size  red giant: star that grows in size and luminosity with expansion of outer layer  at the end of the red giant stage, it’s 100 times larger in radius & 1000+ times brighter in luminosity  gravity will then shrink the inert helium core & surrounding shell of H, so H shell will become hot enough for hydrogen shell burning  core & shell continues to shrink & heat up until temperature reaches 100 million K which is hot enough or He to fuse together  large amounts of mass will escape via solar wind since increasing radius means weaker pull of gravity PHYS 183 – The Milky Way Inside and Out – Tracy Webb Winter 2013  in very low mass stars, degeneracy pressure halts collapse in inert He cores so not hot enough to fuse He  “dead” cores become white dwarfs  He has 2 protons so greater positive charge than single proton so they repel each other more than H  helium fusion: occurs when nuclei slam into one another at higher speeds that those needed for hydrogen fusion; also called triple alpha; 3 He  1 C + energy  thermal pressure in He too low to counteract gravity, so degeneracy pressure works (doesn’t depend on temperature)  helium flash: rapid rise in temperature of core due to helium fusion and degeneracy pressure; releases energy into core until thermal pressure high enough to combat gravity (no longer need degeneracy pressure) & core begins to expand which pushes hydrogen burning shell outward (lowers temperature & fusion rate)  size and luminosity declines as it becomes a helium burning star  outer layers of stars have different masses depending on how much mass expelled through stellar wind o expelled more mass = smaller radii + higher surface temperature  the longer a star undergoes hydrogen shell burning, the larger & more luminous it becomes  horizontal branch: horizontal line of H-R diagram that consists of helium burning stars since all have the same luminosity but differ in surface temperature  when He core is exhausted, fusion will cease again and star will be out of balance  core will begin to shrink again under crush of gravity but general size of star will increase  star will become a double shell burning giant since H sell will burn atop He layer  both
More Less

Related notes for PHYS 183

Log In


OR

Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Sign up

Join to view


OR

By registering, I agree to the Terms and Privacy Policies
Already have an account?
Just a few more details

So we can recommend you notes for your school.

Reset Password

Please enter below the email address you registered with and we will send you a link to reset your password.

Add your courses

Get notes from the top students in your class.


Submit