Modelling The Universe Flashcards
Hubble’s Constant
The ratio of the speed of a receding galaxy to its distance from the observer. =v/x
Olber’s paradox
For an infinite, uniform and static universe, the night sky should be bright because of light received in all direction from stars
What is the cosmological principle?
A principle that states that the universe has the same large-scale structure when observed from any point in the universe
Define critical density
The density of the university that will give rise to a flat universe.
What is a black hole?
The remnant of a massive star formed as a result of matter collapsing to a singular point.
The density of a black hole is so intense that even light cannot escape from it.
Chandrasekhar limit
The maximum mass of a white dwarf - about 1.4 solar masses
Closed universe
A model of the universe in which gravitational force is strong enough to halt its expansion and reverse the process towards a Big Crunch
Dark matter
Matter that emits or reflects little EM radiation making it very difficult to detect
Define electron degeneracy pressure
Pressure created by closely packed electrons in a white dwarf due to Pauli’s exclusion principle
Define a light year
The distance travelled by light in one year.
9.5x10^15 m
Define redshift
The shifting of the entire spectrum of a receding source to longer wavelengths
How large is 1 astronomical unit?
1.496 x 10^11m
How large is 1 Parsec (pc)?
3.086 x 10^16 m
How large is 1 light year (ly)?
9.461 x 10^15 m
Describe the formation of a star
Slightly denser areas of the universe have a slightly higher gravitational pull
This pulls in material, making it denser and giving it a greater gravitational pull, so it pulls in more material
This is known as gravitational collapse
As it collapses, it becomes hotter because it is losing potential energy and gaining kinetic energy
This ball of gas and dust is known as a protostar
Eventually the core of the material becomes hot enough for hydrogen fusion to take place forming helium nuclei. This process is know as hydrogen burning
The fusion creates radiation pressure which balances out the gravitational force
Once they are equal, the star will stop collapsing and continue to burn for millions of years.
It is now called a main sequence star
Describe the death of a star that is less than 3 solar masses
When it runs out of hydrogen to fuse, the star starts to collapse in on itself under its own gravity, heating up the core further
The material surrounding the core still has some hydrogen. And it’s temperature now becomes hot enough to fuse the hydrogen - shell hydrogen burning
The core keeps contracting until it is hot enough to fuse helium into carbon and oxygen - core helium burning
Core helium burning releases a lot of energy and creates lots of radiation pressure which pushes the outer layers of the star outward forming a red giant
When the helium runs out, the outer core will continue to collapse, however it’s doesn’t get hot enough for any more fusion to occur
Eventually electron degeneracy pressure stops it from contracting further and it’s outer layers spread out into space, forming a planetary nebula, leaving behind a white dwarf
Describe the death of a star greater than 3 solar masses
When a star runs out of hydrogen to fuse, the star will collapse under its own gravity
This heats up the core further and in the largest stars it becomes hot enough for fusion to occur all the way up to iron
This releases even greater amounts of energy and radiation pressure which pushes the outer layers of the star outwards, forming a super red giant
When the star runs out of nuclei to fuse it starts to collapse again
Because the star is so big, it’s gravity will overcome the electron degeneracy pressure
The electrons are forced to combine with protons to form a solid neutron core
The outer layers of the star collapse and rebound against the solid neutron core in an explosion called a supernova
All that remains of the star is the incredibly dense neutron core called a neutron star
For the biggest and heaviest stars (>10 solar masses) gravitational collapse continues until it collapses into a singularity, forming a black hole
What assumptions are made with Olber’s paradox?
The universe is infinite, homogenous and static
How does Hubble’s law resolve Olber’s paradox?
Because the universe is expanding, so it is not static
Most galaxies show __________ indicating that the universe is ___________
Redshift, expanding
What is the evidence for the Big Bang?
- Redshift and Hubble’s Law
- When we look at distant galaxies, we are looking back in time.
These galaxies consist of stars entirely made of hydrogen and
helium, the only elements around in the first years of the universe - Cosmic background radiation
Describe the different fates of the universe
If the universe does not have enough mass in it, it will keep on expanding forever as the gravitational forces would not be enough to slow it down. This is called an open universe, and the ‘big rip’
If the universe has enough mass in it, the gravitational forces will be enough to slow the expansion of the universe and eventually shrink it. This is called a closed universe and the ‘big crunch’
If the universe has just the right amount of mass, the universe will expand forever at an ever decreasing rate. This would be a flat universe
Explain what is meant by fusion
Fusing of lighter nuclei to form heavier nuclei
Mass decreases in this reaction and this is transformed into energy
Explain the conditions necessary for fusion to occur in the core of a star
High temperatures
High pressure
The protons repel each other, so strong nuclear force comes into play when the nuclei are closed to each other
Explain why galaxies do not collapse on each other
The galaxies are moving away from each other
Other galaxies may be pulling them in opposite directions
The acceleration is too small to collapse (other than over a long period of time)
Describe the evolution of the universe immediately after the Big Bang to the present day
It was very hot and dense
All forces were unified
Expansion led to cooling
Quark soup
More matter than antimatter
Quarks combine to form hadrons
Imbalance of protons and neutrons
Atoms formed
Gravitational force responsible for galaxies forming
Current temperature 2.7K
Describe an absorption spectrum
A spectrum of dark lines against a background of continuous spectrum