Chapter 18 - The Nature Of The Universe Flashcards
Learn hertzsprung Russell diagram
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Define cosmological principle?
A principle that states that the universe has the same large-scale structure when observed from any point in the universe.
What are the three facts of the cosmological principle?
1) universe is homogenous - on large-scale it is same at all places
2) universe is isotropic - same in all directions (same microwave intensity everywhere)
3) laws of physics are universal - same laws apply everywhere
How many stars/galaxy, galaxies and tf stars are there?
10^11 stars/Galaxy
10^11 galaxies
10^22 stars
Define gravitational collapse?
The gathering of dust and gas due to gravitational forces.
Explain the five steps behind the birth of our sun(/stars)?
1) gravitational collapse
2) interstellar dust heats as it compresses
3) centre gets hot+dense, at 10^7K fusion of H starts
4) hydrogen burning (E=mc^2)
5) start reach a stable size due to equilibrium between gravitational forces and radiation pressure
Explain why interstellar dust heats as it compresses?
Fall in GPE, tf rise in KE, tf rise in collisions tf rise in temperature
What is hydrogen burning?
The fusion of hydrogen to helium.
LEARN EQUATIONS
Define radiation pressure?
An outward pressure created by the photons produced from fusion reactions in a star.
Explain the ageing of stars less than three solar masses up to the point of the red giant? (4 steps)
1) hydrogen gets used up, other elements begin to fuse
2) fusion slows, radiation pressure falls, core starts to collapse under gravity
3) collapsing results in thin shell of H and He around core heats, allowing H to fuse again and He to fuse to produce C and O2
4) rise in power -> rise in radiation pressure -> increase in size and fall in temp. tf RED GIANT
7 elements in a star like the sun from core to surface?
Iron Silicon Oxygen Neon Carbon Helium Hydrogen
Explain the ageing of stars less than three solar masses up to the point of the White dwarf from a red giant? (3 steps)
1) core continues to collapse tf increase in temp. increase in density and huge increase in He fusion
2) HELIUM FLASH - materials surrounding core ejected as a planetary nebula
3) bright central core is left tf WHITE DWARF
Define red giant?
A star characterised by its cooler surface and extremely large surface area
Define planetary nebula?
Hot gases and dust blown away from a low mass star as it evolves into a White dwarf
Define White dwarf?
The very hot remnant of a low mass star
5 characteristics of a white dwarf?
1) No H fusion (glows due to photons from pas reactions)
2) Very dense
3) As a star becomes denser, plasma (freely floating electrons) forms
4) White dwarfs max mass = 1.4 solar masses (Chandrasekhar limit)
5) White dwarf prevented from further g collapse due to fermi pressure
Define fermi/electron degeneracy pressure?
Pressure created by closely packed electrons in a white dwarf due to Pauli’s exclusion principle
Pauli’s exclusion principle states that only 2e can exist in the same quantum state, and further g collapse would require this to happen tf it can’t collapse further
Explain the ageing of stars greater than 3 solar masses up to supernova? (3)
1) same as before
2) swells up to super red giant, forms white dwarf>1.4sm tf collapses further
3) fermi pressure overcome, outer shells of neutron core collapse and rebound against solid neutron core, surface layers of star explode as supernova
After supernova, what happens to lighter stars?
forms a neutron star (entirely neutron core, very dense)
After supernova, what happens to heavier stars? (2)
Neutron star so large it collapses in on itself
Tf forms a black hole
What different measures should be used for different distances?
Solar system - AU
Stars in our galaxy - light year
Stars and galaxies - parsecs
Define and distance of AU?
The average distance from the Earth to the Sun.
150x10^9m
Define and distance of light year? (ly)
The distance travelled by light in one year.
9.5x10^15m
Define and distance of parsec? (pc)
The distance that gives a parallax angle of one arc second.
3.1x10^16m
Define parallax? (p)
The apparent shifting of the position of a star relative to the background of distant stars when observed from different positions in the Earth’s orbit round the Sun.
1 arcsecond = ?
1/3600 degrees
Parsec equation?
Distance (pc) = 1/arcsecond(p)
pc=1/p
Define Olber’s paradox?
For an infinite, uniform and static universe, the night sky should be bright because of light received in all directions from stars.
What does static mean?
Neither expanding or contracting
Define redshift?
A term used to describe the shifting of the entire spectrum of a receding source to longer wavelengths.
Redshift wavelength = ?
λ + Δλ
Δλ = distance moved by source in the time it takes to emit one wave
Define Doppler effect?
The change in frequency and wavelength of a wave caused by the relative movement between source and detector.
Doppler equation?
Δλ/λ = v/c
Hubble’s Law, graph and equation?
Speed of recession of Galaxy is proportional to the distance to the Galaxy
Graph v-x straight line through origin
v=H0x
Define Hubble constant and value?
The ratio of the speed (v) of a receding galaxy to its distance (x) from the observer.
H0 = 70kms^-1Mpc^-1
What can we deduce from Hubble’s Law?
That once all galaxies were concentrated in a very small dense space
Define Big Bang?
An event that describes a very hot explosion from which all of time and space evolved.
How can we predict the age of the universe?
1/H0 = 14billion years
Tf we can only observe universe of distance 14billion ly away
4 conclusions of universe?
- not static - probably expanding
- finite age and finite speed of light means there is probably light yet to reach us
- universe is probably not infinite
- distant receding galaxies have dimmer light due to redshift (lower energy ER)
