C19 - Stars

Question 1

What’s a nebulae?

Answer 1

Gigantic clouds of dust and gas (mainly hydrogen).

They are the birthplace of stars.

Question 2

How are nebulae formed?

Answer 2

Over millions of years due to the tiny gravitational attraction between particles of dust and gas pulling the particles towards each other, eventually forming vast clouds.

As the dust and gas get closer, the gravitational collapse accelerated.

Question 3

What is a protostar?

Answer 3

A very hot, dense sphere of dust and gas.

Question 4

How’s a protostar formed?

Answer 4

Tiny vibrations in the nebula cause dense regions to form.
These regions pull in more dust and gas, gaining mass and getting denser.

They also get hotter as gravitational energy is transferred to thermal energy.

Question 5

Why does a protostar get hotter?

Answer 5

As it gains mass and density, gravitational energy is transferred to thermal energy.

Question 6

What is necessary for nuclear fusion to occur?

Answer 6

Extremely high pressures and temperatures are needed to overcome the electrostatic repulsion between hydrogen nuclei for them to fuse.

Question 7

How does a star remain in a stable equilibrium? (Main sequence star)

Answer 7

Gravitational forces act to compress the star but the radiation pressure (from the photons emitted during fusion) and the gas pressure (from nuclei in the core) push outwards.

These balance the gravitational force.

Question 8

What’s a planet?

Answer 8

An object in orbit around a star with:

• a mass large enough for its own gravity to make it round
• no fusion reactions
• an orbit cleared of most other objects

Question 9

What’s a planetary satellite?

Answer 9

A body in orbit around a planet e.g moons and man made satellites.

Question 10

What’s a comet?

Answer 10

Small, irregular bodies made up of ice, dust and small pieces of rock.

They all orbit the sun.

Question 11

What does the solar system consist of?

Answer 11

The sun and all objects which orbit it.

Question 12

What are galaxies?

Answer 12

A collection of stars and interstellar dust and gas.

Question 13

What’s a supernova?

Answer 13

The implosion of a red supergiant at the end of its life, which leads to the ejection of stellar matter into space, leaving an inert remnant core.

Question 14

What is the order of the life cycle of a low mass star?

Answer 14

Protostar
Main sequence star
Red giant
White dwarf
Black dwarf 
(Planetary nebula)

Question 15

What is the order of the life cycle of a large mass star?

Answer 15

Protostar
Main sequence star 
Red supergiant
Supernova
Neutron star OR Black hole

Question 16

What is a red giant?

Answer 16

A stage of the life cycle of a low mass star where gravitational force is now greater than the radiation and gas pressure.

This causes the star core to collapse. As the core shrinks, pressure increases so fusion starts in the shell around the core.

Question 17

What occurs at the core of a red giant?

Answer 17

They have inert cores.

Fusion no longer takes place as very little hydrogen remains and the temperature isn’t high enough for He nuclei to overcome the electrostatic repulsion between them.

However fusion occurs in the shell, causing the periphery of the star to expand and cool, producing the red colour.

Question 18

How is a white dwarf produced?

Answer 18

When the layers of the red giant around the core drift off into space as a planetary nebula, leaving behind the hot core (white dwarf).

Question 19

What’s a white dwarf?

Answer 19

A phase of the life cycle of a low mass star which is very dense and emits energy as it leaks photons.

Question 20

What is electron degeneracy pressure?

Answer 20

Pressure created by electrons.

When the core of a star begins to collapse under the force of gravity, the electrons are squeezed together which creates a pressure that prevents the core from further gravitational collapse.

Question 21

What is the Chandrasekhar limit?

Answer 21

The maximum mass of a stable white dwarf star.

The electron degeneracy pressure is only sufficient to prevent gravitational collapse if the core has a mass less than 1.44 * mass of the sun. (The Chandrasekhar limit).

Question 22

What is the value of the Chandrasekhar limit?

Answer 22

The Chandrasekhar Limit is now accepted to be approximately 1.4 times the mass of the sun; any white dwarf with less than this mass will stay a white dwarf forever, while a star that exceeds this mass is destined to end its life in a supernova.

The electron degeneracy pressure is only sufficient to prevent gravitational collapse if the core has a mass less than 1.44 * mass of the sun.

Question 23

What occurs within a red supergiant?

Answer 23

Temperature and pressure are high enough to fuse massive nuclei, forming shells of elements within the star until it develops an iron core.

Iron nuclei cannot fuse to the star becomes unstable and explodes via a supernova.

Question 24

What cant iron nuclei fuse (within a red supergiant)?

Answer 24

The reactions cannot produce any energy.

Question 25

What is formed after a supernova?

Answer 25

A neutron star

OR

A black hole

Question 26

What’s a neutron star?

Answer 26

A star formed after a supernova if the mass of the core is greater than the Chandrasehkar limit, as the gravitational collapse continues.

Question 27

What’s a black hole?

Answer 27

A phenomena produced after a supernova if mass of the core is approx 3 * mass of the sun.

Gravitational collapse continues to compress the core, producing a gravitational field so strong that not even photons can escape.

Question 28

What does the Hertzsprung-Russell diagram show?

Answer 28

It’s a graph demonstrating the relationship between the luminosity (y axis) and average surface temperature (x axis).

(Temperature is shown decreasing from left to right)?

Question 29

What is the general appearance of the Hertzsprung-Russell diagram?

Answer 29

Generally, as temperature decreases (left to right), luminosity decreases. (main sequence)

However there are anomalies.
- supergiants have high luminosity regardless of temperature.

• white dwarfs only exist at high temperatures with low luminosity.
• giants branch off from the main sequence around 9000K and increase in luminosity as temperature decreases

Question 30

What are features of the energy levels in gas atoms? (4)

Answer 30

Electrons have discrete energies. They cannot have an energy between two levels.

Energy levels are negative as external R is needed to remove an e- from the atom.

An e- with zero E is free from the atom.

The E level with the most negative value is known as the ground level / ground state.

Question 31

What happens when an electron moves from a lower to higher energy level?

Answer 31

It becomes excited and moves up energy levels.

Question 32

What happens when an electron moves from a higher to lower energy level?

Answer 32

It loses energy.
Energy is conserved so, as is moves down energy levels, a photon is emitted from the atom.

This is called de-excitation.

Question 33

What are the 3 types of spectra?

Answer 33

Emission line spectra

Continuous spectra

Absorption line spectra

Question 34

What’s an emission line spectra?

Answer 34

A spectra unique to each element because of its unique set of energy levels.

If atoms are excited, when they drop back down to lower energy levels, they emit photons with discrete frequencies specific to that element. Each photon corresponds to a specific wavelength.

Question 35

How is an emission spectra formed?

Answer 35

If atoms are excited, when they drop back down to lower energy levels, they emit photons with discrete frequencies specific to that element. Each photon corresponds to a specific wavelength.

Question 36

What’s a continuous spectra?

Answer 36

Spectra of all visible frequencies/wavelengths.

Question 37

What’s an absorption line spectra?

Answer 37

A spectra with a series of dark lines against the background of continuous spectra.

The dark lines have the same wavelength as the bright emission spectral lines for the same gas atoms.

Question 38

How are absorption spectra formed?

Answer 38

Formed when light from a source that produces a continuous spectrum passes through a cooler gas.

As photons pass through the gas, some are absorbed by the gas atoms, raising e- up into higher energy levels thus exciting the atom.

Only photons with energy exactly equal to the difference between the energy levels are absorbed (so only certain wavelengths are absorbed).

Although photons are re-emitted when the e- lose energy, they’re emitted in all possible directions so its original intensity is reduced.

Question 39

How is starlight analyses using diffraction gratings?

Answer 39

When light passes, it is split into a series of narrow beams.
Monochromic light will interfere and superpose and form maxima and minima based on the waves’ path difference and phase difference.

Question 40

What’s a black body?

Answer 40

A hot object that absorbs all electromagnetic radiation that shines into it and, when in thermal equilibrium, emits a characteristic distribution of wavelengths at a specific temperature.

Question 41

How is the graph of Wien’s displacement law drawn?

Answer 41

Wavelength across x axis (increasing left to right)

Intensity along y axis (arbitrary units)

Upwards curve to a peak then downwards curve.

Question 42

What is Wein’s displacement law?

Answer 42

Peak / max wavelength is inversely proportional to 1/T (temperature).

Peak wavelength * temperature = constant

(For a black body)

Question 43

How does the temperature of a black body affect its Wein displacement graph?

Answer 43

As temperature increases, the peak of the intensity-wavelength graph becomes sharper and peak wavelength decreases.

Question 44

What is Stefan’s law?

Answer 44

The total power (luminosity) radiated per unit surface area of a black body is directly proportional to the fourth power of the absolute temperature of the black body.

L = 4 πr ² σT ⁴

Question 45

What’s the equation for luminosity, according to Stefan’s law?

Answer 45

L = 4 πr ² σT ⁴

Where L is luminosity, measured in watts

r is the radius of the star, in metres

T is the surface absolute temperature, in kelvin

σ is the Stefan constant

Question 46

What 2 laws can be used to determine the radius of a distant star?

Answer 46

Wien’s displacement law

Stefan’s law