Astro Flashcards
How is a star formed?
- Dust cloud of hydrogen gas are pulled together due to gravitational attraction.
- This causes gravitational collapse, increasing temperature and kinetic energy.
- This causes the fusion of hydrogen nuclei to helium.
- Energy is released during fusion reactions.
- A stable star is formed when gravitational pressure is equal to radiation pressure.
Death of a star BELOW Chandrasekhar’s limit (1.4x mass of the sun)
- Fusion stops when all hydrogen fuses into helium.
- Less fusion = less radiation pressure. This means there is a net gravitational attraction force.
- The hydrogen core contracts, increasing kinetic energy and therefore temperature. Hydrogen in core fuses.
- Results in expansion into red giant, which then cools and contracts, gaining more kinetic energy which fuses helium into oxygen and carbon.
- Helium fusion finishes, star collapses and outer layer of gas is then ejected.
- White dwarf is left over. Very dense and very hot. Stabilised by electron degeneracy.
Death of a star ABOVE Chandrasekhar’s limit?
- Core contracts when fusion ends.
- Temperature/kinetic energy increases.
- Fusion of heavier elements occurs.
- Iron core builds up and then collapses on itself.
- GPE is transferred to KE producing intense heating. Gravitational pressure forces the electrons and protons to combine to create neutrons. This produces huge amounts of energy, a supernova.
- Temperature is so high that the fusion of elements heavier than iron can occur.
- Neutron star in the core is left over.
What is luminosity?
the amount of energy emitted by a star per unit time.
What is one arcsecond?
1/3600 degrees
What is a parsec?
The distance a star must be in order for the mean of earth’s radius to subtend an angle of one arc second.
What is hubble’s law?
Recessional speed of galaxy is proportional to its distance.
What is the cosmological principle?
The universe is large, isotropic and homogenous.
Why is dark matter presumed to exist?
Mass calculated via luminosity and mass via redshift and rotational speed, galaxies have more mass than expected.
What does homogeneous mean?
Uniform distribution of matter (same density)
What does isotropic mean?
No matter where you look in the universe, it looks the same
What happened during the big bang and after?
- infinitely dense, small and hot.
- expansion led to cooling
- Quarks, leptons and their anti-particles are now present.
- more matter than antimatter - annihilation of antimatter
- further cooling = quarks combining to make hadrons
- Cooling causes hadrons to combine, helium nuclei form
- Atoms form as electrons become bound to nuclei.
- Gravitational force attracts matter and forms stars and galaxies.
- expansion and cooling leads to gamma radiation redshifting into cosmic background radiation with temperature 2.7K
What is the evidence for the big bang?
- Galaxies are moving apart/universe is expanding.
- If galaxies are moving away from each other, they must have been closer together and started from a point.
- cosmic background radiation of 2.7k suggests that gamma from big bang’s intense heat has redshifted as the universe has expanded and cooled.
- Further away the galaxy the faster the speed of recession.
- There is more helium in the universe than expected.
How to calculate critical density?
KE = GPE
1/2mv^2 = GMm/R
v = Hod
Input into v^2
Density = mass/volume
Volume is a sphere so 4/3pir^3
Rearrange for Mass
Input as M
Rearrange for density
What happens when density of university is the same as the critical density?
Flat universe - expansion continues but slows.
