Modelling The Universe

Question 0

Hubble’s Constant

Answer 0

The ratio of the speed of a receding galaxy to its distance from the observer. =v/x

Question 1

Olber’s paradox

Answer 1

For an infinite, uniform and static universe, the night sky should be bright because of light received in all direction from stars

Question 2

What is the cosmological principle?

Answer 2

A principle that states that the universe has the same large-scale structure when observed from any point in the universe

Question 3

Define critical density

Answer 3

The density of the university that will give rise to a flat universe.

Question 4

What is a black hole?

Answer 4

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.

Question 5

Chandrasekhar limit

Answer 5

The maximum mass of a white dwarf - about 1.4 solar masses

Question 6

Closed universe

Answer 6

A model of the universe in which gravitational force is strong enough to halt its expansion and reverse the process towards a Big Crunch

Question 7

Dark matter

Answer 7

Matter that emits or reflects little EM radiation making it very difficult to detect

Question 8

Define electron degeneracy pressure

Answer 8

Pressure created by closely packed electrons in a white dwarf due to Pauli’s exclusion principle

Question 9

Define a light year

Answer 9

The distance travelled by light in one year.

9.5x10^15 m

Question 10

Define redshift

Answer 10

The shifting of the entire spectrum of a receding source to longer wavelengths

Question 11

How large is 1 astronomical unit?

Answer 11

1.496 x 10^11m

Question 12

How large is 1 Parsec (pc)?

Answer 12

3.086 x 10^16 m

Question 13

How large is 1 light year (ly)?

Answer 13

9.461 x 10^15 m

Question 14

Describe the formation of a star

Answer 14

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

Question 15

Describe the death of a star that is less than 3 solar masses

Answer 15

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

Question 16

Describe the death of a star greater than 3 solar masses

Answer 16

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

Question 17

What assumptions are made with Olber’s paradox?

Answer 17

The universe is infinite, homogenous and static

Question 18

How does Hubble’s law resolve Olber’s paradox?

Answer 18

Because the universe is expanding, so it is not static

Question 19

Most galaxies show __________ indicating that the universe is ___________

Answer 19

Redshift, expanding

Question 20

What is the evidence for the Big Bang?

Answer 20

• 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

Question 21

Describe the different fates of the universe

Answer 21

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

Question 22

Explain what is meant by fusion

Answer 22

Fusing of lighter nuclei to form heavier nuclei

Mass decreases in this reaction and this is transformed into energy

Question 23

Explain the conditions necessary for fusion to occur in the core of a star

Answer 23

High temperatures

High pressure

The protons repel each other, so strong nuclear force comes into play when the nuclei are closed to each other

Question 24

Explain why galaxies do not collapse on each other

Answer 24

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)

Question 25

Describe the evolution of the universe immediately after the Big Bang to the present day

Answer 25

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

Question 26

Describe an absorption spectrum

Answer 26

A spectrum of dark lines against a background of continuous spectrum