Stellar Physics Flashcards
Apparent brightness
b
(Wm-2)
Luminosity
L
(W)
b = L/4πd^2
b = Apparent brightness (Wm-2) L = Luminosity (W) π = pi d = distance to star (m)
P/A = σT^4
P/A = Power per unit area (Wm-2)
σ = Stefan Boltzmann’s Constant (5.67 x 10 -8) (Wm-2
K-4)
T = Temperature of Body (K)
L = 4πrσT^4
L = Luminosity (W) π = pi σ = Stefan Boltzmann's Constant (5.67 x 10 -8) (Wm-2 K-4) T = Temperature of Body (K) r = radius of star (m)
Stars are formed in
interstellar clouds when GRAVITATIONAL forces overcome THERMAL pressure, and cause a molecular cloud to contract until the core becomes hot enough to sustain nucleaur fusion, which then provides thermal pressure that BALANCES the gravitational force.
The stages of proton-proton chain in stellar fusion
Stage 1 - Deuterium formed ejecting one neutrino and one positron
Stage 2 - Helium -3 formed ejecting one positron and a gamma-ray
Stage 3 - Helium formed
One example of a p-p chain is
2 1 3
H + H —> He + y
1 1 2
HR Diagrams are a representation of the classifications of stars
Check jotter for diagram on classifications and position
Fusion occurs in
the core of stars in the main sequence of a HR diagram
That hydrogen fusion in the core of a star supplies
the energy that maintains the star’s outward thermal pressure to balance inward gravitational forces.
When the hydrogen in the core becomes depleted, nuclear fusion in the core ceases. The gas surrounding the core, however, will still contain hydrogen.
Gravitational forces cause both hydrogen to shrink. In a star like the Sun, the hydrogen shell becomes hot enough for hydrogen fusion in the shell of the star. This leads to an increase in pressure which pushes the surface of the star outwards, causing it to cool.
At this stage, the star will be in the giant or supergiants region
In a star like the sun the core
shrinks and will become hot enough for the helium in the core to begin fusion
What determines the lifetime of a star
the mass
Every star ultimately becomes
a white dwarf
a neutron star
or a black hole
the mass will determine it’s fate
The sun is in a state of equilibrium
between the gravitational attraction holding it together and the outward thermal pressure resulting from the hydrogen fusion pushing outwards.
