Chapter 18 - The Nature Of The Universe

Question 1

Learn hertzsprung Russell diagram

Answer 1

.

Question 2

Define 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

What are the three facts of the cosmological principle?

Answer 3

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

Question 4

How many stars/galaxy, galaxies and tf stars are there?

Answer 4

10^11 stars/Galaxy
10^11 galaxies
10^22 stars

Question 5

Define gravitational collapse?

Answer 5

The gathering of dust and gas due to gravitational forces.

Question 6

Explain the five steps behind the birth of our sun(/stars)?

Answer 6

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

Question 7

Explain why interstellar dust heats as it compresses?

Answer 7

Fall in GPE, tf rise in KE, tf rise in collisions tf rise in temperature

Question 8

What is hydrogen burning?

Answer 8

The fusion of hydrogen to helium.

LEARN EQUATIONS

Question 9

Define radiation pressure?

Answer 9

An outward pressure created by the photons produced from fusion reactions in a star.

Question 10

Explain the ageing of stars less than three solar masses up to the point of the red giant? (4 steps)

Answer 10

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

Question 11

7 elements in a star like the sun from core to surface?

Answer 11

Iron
Silicon
Oxygen
Neon
Carbon
Helium
Hydrogen

Question 12

Explain the ageing of stars less than three solar masses up to the point of the White dwarf from a red giant? (3 steps)

Answer 12

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

Question 13

Define red giant?

Answer 13

A star characterised by its cooler surface and extremely large surface area

Question 14

Define planetary nebula?

Answer 14

Hot gases and dust blown away from a low mass star as it evolves into a White dwarf

Question 15

Define White dwarf?

Answer 15

The very hot remnant of a low mass star

Question 16

5 characteristics of a white dwarf?

Answer 16

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

Question 17

Define fermi/electron degeneracy pressure?

Answer 17

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

Question 18

Explain the ageing of stars greater than 3 solar masses up to supernova? (3)

Answer 18

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

Question 19

After supernova, what happens to lighter stars?

Answer 19

forms a neutron star (entirely neutron core, very dense)

Question 20

After supernova, what happens to heavier stars? (2)

Answer 20

Neutron star so large it collapses in on itself

Tf forms a black hole

Question 21

What different measures should be used for different distances?

Answer 21

Solar system - AU
Stars in our galaxy - light year
Stars and galaxies - parsecs

Question 22

Define and distance of AU?

Answer 22

The average distance from the Earth to the Sun.

150x10^9m

Question 23

Define and distance of light year? (ly)

Answer 23

The distance travelled by light in one year.

9.5x10^15m

Question 24

Define and distance of parsec? (pc)

Answer 24

The distance that gives a parallax angle of one arc second.

3.1x10^16m

Question 25

Define parallax? (p)

Answer 25

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.

Question 26

1 arcsecond = ?

Answer 26

1/3600 degrees

Question 27

Parsec equation?

Answer 27

Distance (pc) = 1/arcsecond(p)

pc=1/p

Question 28

Define Olber’s paradox?

Answer 28

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

Question 29

What does static mean?

Answer 29

Neither expanding or contracting

Question 30

Define redshift?

Answer 30

A term used to describe the shifting of the entire spectrum of a receding source to longer wavelengths.

Question 31

Redshift wavelength = ?

Answer 31

λ + Δλ

Δλ = distance moved by source in the time it takes to emit one wave

Question 32

Define Doppler effect?

Answer 32

The change in frequency and wavelength of a wave caused by the relative movement between source and detector.

Question 33

Doppler equation?

Answer 33

Δλ/λ = v/c

Question 34

Hubble’s Law, graph and equation?

Answer 34

Speed of recession of Galaxy is proportional to the distance to the Galaxy

Graph v-x straight line through origin

v=H0x

Question 35

Define Hubble constant and value?

Answer 35

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

H0 = 70kms^-1Mpc^-1

Question 36

What can we deduce from Hubble’s Law?

Answer 36

That once all galaxies were concentrated in a very small dense space

Question 37

Define Big Bang?

Answer 37

An event that describes a very hot explosion from which all of time and space evolved.

Question 38

How can we predict the age of the universe?

Answer 38

1/H0 = 14billion years

Tf we can only observe universe of distance 14billion ly away

Question 39

4 conclusions of universe?

Answer 39

• 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)