Chapter 19 - Stars

Question 1

Nebulae

Answer 1

Clouds of gas and dust, often hundreds of times larger than our solar system. They compress due to the gravitational attraction and become hotter

Question 2

Planets

Answer 2

An object in orbit around a star that:

i) Has a mass large enough for its own gravity to give it a round shape
ii) Has no fusion reactions
iii) Has cleared its orbit of most other objects

Question 3

Planetary Satellites

Answer 3

A body in orbit around a planet

Question 4

Comets

Answer 4

Small, irregular bodies made up of ice, dust and small pieces of rock. Orbits a star in strongly elliptical orbits.

Question 5

Solar Systems

Answer 5

Contain stars and all of the objects that orbit them

Question 6

Galaxies

Answer 6

A collection of stars and interstellar gas and dust

Question 7

Lower mass stellar evolution

Answer 7

Nebula -> Protostar -> Main sequence -> Red giant -> Planetary nebula -> White dwarf
(0.5-10)M☉

Question 8

Higher mass stellar evolution

Answer 8

Nebula -> Protostar -> Main sequence -> Red supergiant -> Supernova -> Neutron star or Black hole
M > 10M☉

Question 9

M☉

Answer 9

1 solar mass

Question 10

Star birth

Answer 10

It starts with a cloud of gas and dust (nebula) where the particles are pulled together by gravitational attraction
Denser regions form where more gas and dust are pulled in and gravitational energy is transferred to thermal energy, increasing the temperature
A hot and dense protostar will form in one part of the cloud

Question 11

Main sequence

Answer 11

The main phase of a star’s life, outward gas pressure and radiation from the fusion is balanced by the inwards gravitational force
The duration is dependent on the mass of the star, a larger star has a hotter car that releases more energy but has a shorter life

Question 12

Red Giant

Answer 12

As the hydrogen runs low, the gravitational forces overcome the gas pressure and it begins to collapse
It increases the pressure so fusion happens in shells around the core but not in the core as the temperature is too low to overcome the electrostatic repulsion between helium atoms
The star expands and cools, causing the red colour

Question 13

White Dwarf

Answer 13

As a red giant gets larger, the outer layers drift off leaving a planetary nebula
Leaves an extremely hot and dense core made of helium
No fusion takes place and the only energy emitted comes from leaking photons produced by earlier reactions

Question 14

Electron Degeneracy Pressure

Answer 14

When a star is compressed, it creates a pressure that prevents any further collapse. Wolfgang Pauli

Question 15

Chandrasekhar Limit

Answer 15

The maximum mass of a white dwarf, 1.44M☉. The maximum mass such that the electron degeneracy pressure prevents any further collapse

Question 16

Red Supergiant

Answer 16

Have enough heat to overcome the electrostatic repulsion between helium so it can fuse into heavier elements
They fuse in shells around the core as enough energy is released to fuse further elements
Continues until there is an inert iron core and non-fusing hydrogen on the surface

Question 17

Supernova

Answer 17

The inactive iron core makes the star incredibly unstable and it violently implodes
Creates a shockwave that ejects all of the core material into space, ‘explosion’ is called a type II supernova
Elements above iron are formed and scattered through the universe

Question 18

Neutron Star

Answer 18

M > 1.44M☉
The collapse continues and a neutron star is formed almost exclusively made of neutrons
Diameter 10km and mass 2M☉

Question 19

Black Hole

Answer 19

M > 3M☉
Gravitational collapse continues past the point of a neutron star
The result is a gravitational field such that an object would need an escape velocity greater than the speed of light
Vary in mass up to several million M☉

Question 20

Schwartzchild Radius

Answer 20

The radius of an imaginary sphere sized so that if all the mass of an object were compressed into it the escape velocity would be greater than the speed of light

Question 21

Schwartzchild Radius Formula

Answer 21

r = 2GM/c^2

s

Question 22

Hertzsprung-Russell diagram

Answer 22

A classification system for stars, graphs the luminosity against surface temperature for all stars in our galaxy
The temperature decreases
from left to right

Question 23

Luminosity

Answer 23

The total radiant power output

Question 24

1 Ls

Answer 24

The luminosity of the sun (3.85 x 10^26 W)

Question 25

Appearance of a HR diagram

Answer 25

A tan shaped curve from the top-left to bottom right, fairly narrow - main sequence stars
At a low surface temperature and average luminosity, a circle branches containing giants
An oval all the way along the top of supergiants
A cluster in the bottom left for white dwarfs

Question 26

HR axes

Answer 26

y: Luminosity 0.001 to 100000 Ls
x: Temperature: 40000 to 2500 K
Logarithmic scale

Question 27

Gas Atoms

Answer 27

Electrons bound to an atom can only exist in discrete energy levels
The energy levels are negative as external energy is required to remove an electron from an atom
An electron with 0 energy is free from the atom

Question 28

Ground State

Answer 28

The energy level with the most negative energy

Question 29

Moving between energy levels

Answer 29

When an electron moves to a higher energy level it is excited
When an electron moves from a higher energy to a lower one it loses energy
The electron moves down from its excited state, releasing the energy gained as a photon with an energy equal to the difference in energy between the two energy levels
Each atom has its own energy levels

Question 30

Moving up multiple levels

Answer 30

It could settle at any or none of the remaining energy levels until the ground state, releasing a photon for each it stops at

Question 31

Emission line spectra

Answer 31

Show a series of lines at different wavelengths to show the wavelengths that each element emits (the difference between energies of energy levels an electron moves between)

Question 32

Continuous spectra

Answer 32

A spectrum showing all the wavelengths of visible light, produced by atoms of a heated solid metal

Question 33

Absorption line spectra

Answer 33

A continuous spectrum with a series of dark spectral lines at specific wavelengths overlayed, the wavelengths of light absorbed by the star (the same as an element’s absorption lines)

Question 34

Making absorption line spectra

Answer 34

Light from a source that produces a continuous spectrum passes through a cooler gas
Photons with energies equal to the difference between energy levels are absorbed
When they are re-emitted, it is in all directions with a lower intensity
These can be compared to an elements emission spectra to determine the composition of gases around a star

Question 35

Diffraction grating vs double slit

Answer 35

A diffraction grating has sharper fringes and a clearer and brighter diffraction pattern
Brighter as more light passes through

Question 36

Grating equation

Answer 36

sin(θ) = nλ/d

where d is the distance between adjacent bright fringes and n is the order of the maxima

Question 37

Calculating the highest order maxima

Answer 37

d/λ

Question 38

Black Body

Answer 38

An idealised object that absorbs all EM radiation that hits it and then re-emits as a distribution of wavelength dependent on their temperature, almost any object can be modelled as a black body

Question 39

Wien’s Displacement Law

Answer 39

λmax T = constant

The peak wavelength of light emitted from a black body

Question 40

Higher temperature link to peak wavelength

Answer 40

Lower peak wavelength, sharper peak on intensity-wavelength graph

Question 41

Stefan Boltzmann Law

Answer 41

L = 4ℼr^2σT^4

Question 42

Stefan constant (σ)

Answer 42

5.67 X 10^-8 W m^-2 K^-4