Lecture 15: The Death of Stars Flashcards
1
Q
Star Clusters and Stellar Evolution
A
- Globular Clusters are old
- HR diagrams show red giant branch
- good agreement between models and observations
2
Q
H,He abundance
A
- changes as a result of fusion
3
Q
Eventually star’s main sequence life ends
A
- inner core contracts and gets hotter
- outer core heats up and fusion rate increases
- envelope heats up and expands
- luminosity may increase
- surface temperature may change
- Sun will become a red giant
4
Q
Post-MS evolution
A
- succesive fusion cycles
- each core burning stage is much more temperature sensitive with smaller fuel supply and get progressively shorter
- each stage includes core collapse, shell heating and fusion reactions in shell increase, envelope expansion
- eventually envelope is ejected leaving the core behind
- differences at all stages largely due to mass
5
Q
Core He fusion
A
- If the contracting core gets hot enough, helium will fuse to make carbon
- in stars with masses of 2.5Msun or lower, the core is so dense that it is almost an explosive event, called the helium flash
6
Q
Helium flash effect on HR diagram
A
- star moves to horizontal branch
- sometimes referred to as the helium main sequence because helium is burning in the core
7
Q
After He fusion done, then what? (Below 8Msun)
A
- core collapses but does not get hot enough so no further fusion reactions in core
- two fusion shells around inert core, H burning and He burning
- Envelope expands as star moves toward Red Giant stage again, called Asymptomatic Giant Branch (AGB)
- Envelope ejected as a planetary nebula leaving a white dwarf
8
Q
Planetary Nebulae
A
- Named because they appear to be round disks in telescopes
9
Q
After He fusion done, then what? (Above 8Msun)
A
- moves towards the giant branch, two shells, H burning and He burning
- C,O fusion can happen
- Each successive stage happens at higher central temperature
- Structure of high mass star ends up being composed of many shells of different types of fusion
- Iron core cannot be fused to produce energy
- core contracts, and envelope expands
- star explodes as core implodes, supernova
- Protons and electrons in core turn to neutrons
- massive amounts of neutrinos emitted
- neutron star is what’s left, supported by neutron degeneracy pressure
10
Q
Origin of the elements
A
- After a supernova there is lots of energy available and many high energy particles, especially neutrons flying through stellar material
- Allows for many nuclear reactions to occur that are not favourable
- Nuclei bombarded with neutrons may capture some and form a new isotope, however too many neutrons makes it unstable and so a neutron can be converted to a proton, making a new element
11
Q
Stellar Remnants
A
- All are small and dense with no fusion
- masses range from less than 1 solar mass to more than 10 solar masses
- radii are constant
- white dwarfs, neutron stars, black holes
12
Q
White Dwarfs
A
- no internal energy source
- Cool over time
- supported by electron degeneracy pressure, which isn’t dependant on pressure, only on density and number of electrons
- the more massive the white dwarf, the smaller it is
- star must be less than 1.4 solar masses to form a white dwarf
13
Q
Neutron stars
A
- core remnant of supernova
- smaller, denser than white dwarf
- supported by neutron degeneracy
- 1 solar mass neutron star only a few km in diameter
- first observed neutron star is pulsar in crab nebula SN remnant
14
Q
Pulsars
A
- Produced by rapidly-rotating, magnetized neutron stars
- radio source that pulses about once a second
15
Q
Stellar Evolution and Binary Stars
A
- Binary companions are important in later stages
- As a star expands, mass transfer is possible
- 3 consequences
- System with massive star and white dwarf, where white dwarf initially had more mass
- nova, sudden ignition of surface fusion of accreted material from companion
- Type 1a supernova, white dwarf mass exceeds Chandrasekhar limit, cataclysmic ignition of C burning
