E.5 Fusion and Stars Flashcards
Why are main sequence stars stable?
There is an equilibrium between outward thermal or radiation pressure and inward pressure due to gravitational forces
Low and medium mass stars - major source of outward pressure
Thermal pressure (expansion of heated gas)
High mass stars - major source of outward pressure
Radiation pressure (transfer of momentum from photons)
Why does nuclear fusion occur within a star?
High temperatures and densities within the core of a star
Why does nuclear fusion occur within a star due to high temperatures?
The nuclei posses enough kinetic energy to overcome the repulsion between positively charged nuclei
Why does nuclear fusion occur within a star due to high densities?
Creates a high pressure that force nuclei close together, increasing the chance of collision between them
First stage of stars lifecycle
Protostar is formed from interstellar dust and gas
End of main sequence star
Most of the hydrogen is fused to helium, outward pressure in reduced and star shrinks, increases in temp. Outward pressure increases as hydrogen in outer layers can fuse, so becomes a red giant. Helium is fused to C/O in core.
What happens after all helium is fused in the core?
Core contracts further and depending on its mass turns into a planetary nebula or supernova.
Small stars (less than 4M) after main sequence
Planetary nebula - outer layers are ejected leaving a hot core, core is known as a white dwarf which cools over time and forms a brown then black dwarf.
Medium stars (4-8 M) after main sequence
Further stages of fusion with neon, sodium and magnesium. Outer layers are ejected and leave a white dwarf.
Chandrasekhar limit
White dwarves have a max mass of 1.4M due to electron degeneracy pressure
Electron degeneracy pressure
Electrons cannot be packed in any closer by gravity. Above 1.4M, gravity would overcome electron repulsion and the star would collapse.
Large stars (Over 8M) after main sequence
Continue to fuse heavier elements, outer layers continue expanding to create a red supergiant. Eventually all silicon is fused to iron and fusion stops. Core contracts rapidly and outer layers are ejected as a supernova. Core turns into a neutron star or black hole depending on mass
What happens inside a neutron star?
Protons and electrons are forced together by gravity
Why do neutron stars have a maximum mass of 3M?
Oppenheimer-Volkoff limit - neutron degeneracy limit - neutrons cannot be packed any closer by gravity. Above 3M, gravity would overcome neutron repulsion and the star would collapse to form a black hole.
Hertzsprung-Russell diagram
Graph that allows comparisons between stars
Hertzsprung-Russell diagram x and y axis
y axis is a logarithmic scale of luminosity
x axis is a scale of decreasing temperature
Red giants characteristics
Red in colour, cool in temp, large mass and surface area
White dwarf characteristics
White in colour, hot temp, small mass and surface area
Instability strip
Region of stars that vary in size and luminosity due to periodic expansion and contraction
What is stellar parallax used to determine?
Distance of stars relatively close to Earth
Parallax
Apparent movement of stars due to Earth orbiting the sun
Parallax angle
Half of the observed angular displacement of the star
