Stars
By Cosmology Horizon
Fragmentation
Clouds
~10ly - 300ly
~100 - ~1,000,000 M☉；
………
Accretion disks
2,000Au – 20,000Au
~1 M☉
Star-forming complex
Static clouds
Gravitational Collapse
元婴期 —— Protostar
500,000 years
10,000 °C
Features
Accretion disk
Jet
Ignite nuclear fusion� 15,000,000 °C���Repelling gases� Contain the growth of prostar����Theoretical limit:150M☉ � Higher M, higher L & T
To a normal star
Hydrostatic equilibrium
Inward
Gravitational force
Outward
Heat & radiation
Hydrogen bomb vs Star
Core as powerhouse
< 1.5 M☉: Proton-proton chain
H -> release energy -> He
Expected: 10 billion years
More massive: CNO cycle
More efficient for massive stars
C,N,O as catalysts
Help turning H into He
2. Most stars
H–R diagram
1.Harvard spectral classification
Spectral type + 0-9.9 temperature (e.g. G2)
2.MK classification
Hypergiants, Supergiants, Giants, Subgiants…
Oh Be A Fine Guy/Girl Kiss Me
Why no green stars?
T4
Stellar evolution
Main-sequence star
Hydrogen fusion
Red dwarfs: < 0.6 M☉, M & K
Medium-mass stars: 0.6-8 M☉, G —— B
Massive stars: > 8 M☉, B —— O
Low-mass stars
Low rate of fusion
- Low mass & pressure
Main sequence
- Six to twelve trillion years
White dwarf
- Trillions of years to cool down
Mid-sized stars (0.6-8 M☉)
subgiant
red-giant-branch
horizontal branch
asymptotic-giant-branch
Subgiant phase (~108 years) �
Inert He core
H shell fusion
Hotter & brighter
Red-Giant-Branch phase�(1-2 B)
1. Inert core heated
2. Stronger H shell burning
3. The star continues to expand
4. He fusion ignited
Horizontal Branch (100M)
Equilibrium state
Helium flash
Sensitive to temperature change
End of the horizontal branch
The He is also depleted
Asymptotic-Giant-Branch (1-10M)
Planetary nebula
Masses ejected
Thermal pulsation
White dwarf in the center
Massive stars (> 8M☉)
Supergiant phase
Immense size and luminosity
103-6 L ☉
Undergo a sequence of fusion processes
Concentric layers fusing
The iron core reaches Chandrasekhar limit (≈1.4 M☉) inducing supernova
Supernova
Detailed process
The iron core collapses within 1 sec (>1.4 M☉ )
Protons and electrons are pushed together
A neutron core (30km in diameter) forms / Black hole
The falling substances (at 70,000km/s) bounce,
forming a shock wave
The shock wave blows out the outer layer of the star
A supernova remnant: N 63A ↓
How powerful is a supernova?
Luminosity
Outshine the whole galaxy
Matter ejection
Supernova can eject matter up to 10% of the speed of light
Energy output
1046 J (take the sun billions of years to emit)
Duration of brightness
Remain bright for weeks or months due to the radioactive decay of elements formed
Gradually fades as matter disperses
Neutron star
Densest body
Extreme gravitation
Extreme rotation
Stellar metallicity
Stellar populations
Population I
Old Metal poor
More terrestial planets found
*Often found on disk around galactic center
Population II
Young Metal rich
More gas giants found
*Often discovered in galactic halo and bulge
Higher opacity
1. Stronger Stellar winds
Outer layer is easier to be driven by radiation
2. Longer Main-sequence lifetime
Less efficient in radiating energy
Outer layer traps more heat
Larger radius and lower surface temperature
Less central pressure
Slower rate of H fusion
Higher metallicity results in higher opacity
Binary stars
Classification
1. Based on method of observation
Visual / Spectroscopic / Eclipsing / Astrometric
2. Based on configuration of the system
Detached / semidetached / contact
A visual binary star
Visual binary
Observed as double stars in telescope
large enough angular seperation (resolution)
High relative brightness
Appendix
A 🡪 Brighter – primary star
B 🡪 Dimmer – secondary star
e.g. Sirius A/B
Spectroscopic binary
Very close to each other
Not possible to detect directly
Detected by the doppler effects
Calculate the radial velocity
Periods derived
Double stars are seen 🡪
Only the primary one visible 🡪
Eclipsing binary
Variable stars
Variance in brightness
Can also be visual or spectroscopic
Properties from the light curve:
Relative sizes
Orbiting period
Detection of a third mass
Astrometric binary
Only orbit of brighter component
Sirius
The dimmer component can be weighted approximately
Configurations
1. Detached binaries (most binary)
2. Semidetached binaries
Mass transfer
3. Contact binaries
A common envelope
Final merger
Thanks for listening!