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A Level Physics > Module 5: Chapter 19 - Stars > Flashcards

Module 5: Chapter 19 - Stars Flashcards

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1
Q

Where do comets originate from?

A

the Oort Cloud or the Kuiper Belt

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2
Q

Where does the tail of a comet face?

A

Always points away from the sun

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3
Q

What is a Solar Mass (Mₒ)?

A

The mass of the sun

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4
Q

Describe the structure of a Neutron Star

A

A Neutron Star is composed of a solid crust of atomic nuclei roughly 1km in depth. It then has a liquid core composed entirely of Neutrons. The star’s radius is only 10km.

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5
Q

Describe the structure of a black hole

A

A black hole is formed when the core of the star continues to increase in density until it collapses into a point of infinite density called a singularity. It appears black as the gravitational field is so strong that even light cannot escape it. It is surrounded by an event horizon

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6
Q

What is a singularity?

A

A point of infinite density at the centre of a black hole where gravity is so strong that spacetime and our laws of physics break down.

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7
Q

What is an event horizon?

A

A sphere surrounding the black hole where the escape velocity is equal to the speed of light. Nothing that occurs beyond this boundary can ever affect the universe outside it and anything which passes the event horizon will fall into the black hole

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8
Q

Describe what it would look like if an object passes through the event horizon of a black hole

A
  • To an outside observer, an object falling into the black hole slows down as it approaches the event horizon, never quite crossing it
  • From the object point of view, it will cross the event horizon and fall towards the singularity
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9
Q

What is the Schwarzchild radius?

A

The radius at which beyond light cannot escape from the vicinity of a black hole. Inside this radius the escape velocity from the black hole is greater than the speed of light and therefore light (or anything else) cannot escape from the gravitational pull of the black hole. It is the distance from the singularity to the event horizon.

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10
Q

What is the equation for the Schwarzchild radius?

A

r = (2GM)/c²

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11
Q

Explain how the equation for the Schwarzchild radius is derived:

A

To escape from the vicinity of a black hole, an object must have just enough kinetic energy to cancel out the gravitational potential energy:

0.5mv² ≥ GMm/r
r = 2GM/v²
r = 2GM/c²

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12
Q

What is one solar mass in kg?

A

2x10³⁰kg

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13
Q

List colours in decreasing temperature order:

A
  1. Blue
  2. White
  3. Yellow
  4. Orange
  5. Red
  6. Black
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14
Q

What are 3 ways to measure the brightness of light?

A
  • Luminous Emittance
  • Luminous Flux
  • Luminous Intensity
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15
Q

What are the units for Luminous Emittance?

A

Lux (lx)

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16
Q

What are the units for Luminous Flux?

A

Lumen (lm)

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17
Q

What are the units for Luminous Intensity?

A

Candela (cd)

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18
Q

What is one lux?

A

One lux is equal to one lumen per square metre

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19
Q

What is flux?

A

Flux describes any effect that appears to pass or travel through a surface

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20
Q

Why does a more distant star appear dimmer?

A

The vacuum of space does not absorb light so the total amount of light is not reduced as tou get further away, however the area the light is spread over increases decreasing the intensity of light.

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21
Q

What is the intensity of light known as?

A

Luminous Flux

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22
Q

What is the equation for Luminous flux?

A

Flux = P/4πd²

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23
Q

What is the apparent magnitude of a stars brightness?

A

It uses a logarithmic scale to give an idea of the order of magnitude of the stars brightess. It makes it possible to put all celestial objects on a scale from -30 being the brightest object in the sky (the sun) to +30 being the dimmest object visible in the hubble space telescope

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24
Q

What is the equation for the apparent magnitude of a stars brightness?

A

m = -2.5log(F) + constant

m = apparent magnitude, F = flux

The constant is based on the brightness of a comparison star (usually Vega)

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25
Q

What is absolute magnitude of a star’s brightness?

A

It is related to apparent magnitude but removes the effect of distance, giving the apparent magnitude of a star if it was at a standard distrance of 10pc.

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26
Q

What is the equation for the absoute magnitude of a stars brightness?

A

M = m - 5log(d/10)

M = absolute magnitude, m = apparent magnitude, d = distance

d must be measured in parsecs

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27
Q

Compare what apparent magnitude describes vs absolute magnitude

A
  • Apparent magnitude describes flux
  • Absolute magnitude describes luminosity
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28
Q

What is the Hertzprung-Russel (HR) diagram?

A

A chart of a star’s luminosity (absolute magnitude) against temperature

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29
Q

Describe the axis of a Hertzprung-Russel diagram:

A
  • The y axis is luminosity, measured as multiples of the sun’s luminosity
  • The x axis is temperature, measured in kelvin and is a logarithmic scale doubling each x value. It decreases as you progress along the axis
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30
Q

Sketch a Hertzprung-Russel diagram:

A
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31
Q

What is spectral classification?

A

Depending on the relative proportions of elements in a star (found from absorption lines in their spectra) , it can be classified into a letter scheme:
O
B
A
F
G
K
M

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32
Q

Describe the spectral classifications:

A
  • It begins at O where the star contains lots of Helium ions, helium atoms and hydrogen atoms. It is the hottest of all stars and is blue in colour
  • F is in the middle and contains ionized metals, it has a lower temperature and is white in colour
  • M is at the end of the classification classes and contains neutral atoms, it has a much lower temperature and is red in colour
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33
Q

What is the spectral classification of the sun?

A

Class G

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34
Q

What is a standard candle?

A

A candle of a known luminosity

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35
Q

What is the difference between the emission spectra of a filament light bulb and a fluorescent light bulb?

A
  • Filament Light bulbs emit a near continuous spectrum of light
  • A Fluourescent light bulb emits specific frequencies of light, the superposition of which forms white light. This is much more energy efficient

In general, fluorescent lights tend to produce a cooler, bluer light compared to the warmer, yellower light produced by filament lights.

36
Q

Describe the energy levels within an atom

A
  • They are discrete energy levels
  • Electrons cannot exist inbetween energy levels
  • Energy levels are negative (an electron with 0 or positive energy is free from the atom)
  • The energy level which is most negative is nearest the nucleus and is known as the ground state
37
Q

Describe how a fluorescent light bulb works:

A

When energy is transferred to the gas within the light bulb (usually from an electric current) the electrons become excited and move to a higher energy level. When the electron then relaxes and returns to its original energy level it looses energy and gives it off in the form of a specific frequency of photon

38
Q

What happens to frequencies of light not absorbed by an atom

A

They are transmitted

39
Q

Describe how the elemental composition of the outer layers of a star can be determined:

A

An absorption spectrum is formed as light emitted from the stars core passes through its outer layers. It is a set of specific frequencies of electromagnetic radiation, visible as dark lines in an otherwise continuous spectrum. The light emitted from the stars core contains a wide range of wavelengths, however only specific frequencies of photons are absorbed by atoms as they contain discrete electron energy levels. Their electrons are excited between these discrete energy states (corresponding to the amount of energy of the photons as E = hf). Every element has a characteristic line spectrum as they contain unique electron energy levels - allowing the elemental composition the star to be determined.

40
Q

What is deflection?

A

The change in anm object’s velocity as a consequence of contact or collision with a surface or the influence of a potential field

41
Q

What light refracts the most?

A

Violet Light

42
Q

What light refracts the least?

A

Red Light

43
Q

What light diffracts the most?

A

Red Light

44
Q

What light diffracts the least?

A

Violet Light

45
Q

What is a black body?

A

A theoretical object that absorbs all the light that hits it, therefore it appears perfectly black when cold. When heated above absolute zero, it emits light across the whole electromagnetic spectrum, in a distribution that looks like this:

46
Q

What does Wien’s displacement law state?

A

The hotter the black body, the shorter the peak wavelength of the curve

47
Q

What is the equation for Wien’s displacement law?

A

λₘₐₓT = 2.9x10⁻³ mK

48
Q

How can you determine the approximate temperature of a star?

A

By modelling it as a black body and using Wien’s displacement law

49
Q

Estimate the temperature of a star with a peak wavelength of 600nm

A

4833K

50
Q

What is the luminosity of a star?

A

The energy a star radiates in all directions per second, its power output

51
Q

What is Stefan’s law?

A

The luminosity of a star is directly proportional to its surface area and to the fourth power of its temperature

52
Q

What is the equation for Stefan’s law?

A

L = 4πr²σT⁴
L = AσT⁴

53
Q

What is σ in Stefan’s law?

A

The Stefan-Boltzmann constant.
5.67x10⁻⁸ JK⁻⁴m⁻²s⁻¹

54
Q

a grating with spacing d = 1.82x10⁻⁶m is used to find the wavelength of detected light. The first maximum is measured at a maximum of 26°, find the wavelength of the light

A

7.98x10⁻⁷m

55
Q
A

6.58x10⁸m

56
Q
A

3.23x10⁻⁷m

57
Q

What is the universe?

A

Everything that exists within space and time

58
Q

What are stellar nebulae?

A

Clouds of dust and gas (mainly hydrogen), often many times larger than our solar system

59
Q

Explain how a stellar nebula is formed

A

Neublae are formed over millions of years as the tiny gravitational attraction between particles of dust and gas pulls the particles towards each other, forming vast clouds

60
Q

Explain the formation of a star

A

Due to gravitational attraction within a stellar nebula, the dust and gas particles get closer together causing the gravitational collapse to accelerate. Due to tiny variations in the nebula, regions of higher density begin to form within the nebula. These regions pull in more dust and gas, gaining mass and getting denser and therefore hotter as gravitational energy is transferred to thermal energy. Eventually a protostar forms, a very dense sphere of dust and gas. To become a star, nuclear fusion needs to start in the core. Fusion reactions produce energy in the form of kinetic energy. Extremely high pressures and temperatures inside the core are required to overcome the electrostatic repulsion between hydrogen nuclei to fuse them together to form helium nuclei. Once this nuclear fusion begins, a main sequence star is born.

61
Q

Describe how main sequence stars are stable and how they become unstable:

A

Once a star is born, it remains in a relatively stable equilibrium with almost a constant size. Gravitational forces attempt to compress the star, but the radiation pressure from the photons emitted during fusion and the gas pressure from the nuclei in the core push outwards. These forces balance each other out and maintian the equilibrium. These stars remain stable until a star runs low on hydrogen in its core, meaining hydrogen fusion cannot continue.

62
Q

What is radiation pressure?

A

Pressure from the photons in the core of a star, which act outwards to counteract the pressure from the gravitational foce pulling the matter in the star inwards

63
Q

Compare how long different stars remain in the main sequence stage:

A

Larger stars have much hotter cores than smaller stars, this means that the hydrogen fusion occurs much more rapidly than in smaller stars and have a much larger power output. However, this means the hydrogen is depleted much quicker and therefore larger stars spend less time in the main sequence stage

64
Q

What are planets?

A

Planets is an object in orbit around a star with 3 important characteristics:
* It has a mass large enough for its own gravity to give it a spherical shape
* It has no fusion reactions
* It has cleared its own orbit of most other objects

65
Q

What are planetary satellites?

A

A body in orbit around a planet

66
Q

What are comets?

A

Comets are small irregular bodies made up of ice, dust, and small pieces of rock. All comets orbit the sun, many in highly eccentric elliptal orbits. As they approach the sun they develop spectacular tails.

67
Q

What are solar/planetary systems?

A

A system with a star and all the bodies that orbit it

68
Q

What are galaxies?

A

A collection of stars and interstellar dust and gas.

69
Q

What are dwarf planets?

A

Bodies which almost satisfy all the requirements of a planet except they have not cleared their orbit of other objects

70
Q

Whar are asteroids?

A

Objects to small and uneven to be planets, usually in near-circular orbits around the sun and without the ice present in comets

71
Q

What are the 5 life cycle stages of a star between 0.5 Mₒ and 10 Mₒ

A
  1. Stellar Nebula
  2. Protostar
  3. Main Sequence Star
  4. Red Giant
  5. White Dwarf + Planetary Nebula
72
Q

What are the 6 life cycle stages of a star between greater than 10 Mₒ

A
  1. Stellar Nebula
  2. Protostar
  3. Main Sequence Star
  4. Red Supergiant
  5. Supernova
  6. Neutron star (if core has a mass of 2Mₒ) or Black hole (if core has a mass greater than 3Mₒ)
73
Q

Explain what happens to a star of mass between 0.5Mₒ and 10Mₒ when it becomes unstable:

A

Once the star begins to run out of hydrogen in its core, it begins to become unstable. The star will evolve into a red giant. The reduction in energy released by fusion in the core means that the gravitational force is now greater than the reduced force from radiation and gas pressure. Therefore the core of the star begins to collapse, increasing pressure enough to start fusion in a shell surrounding the core. Red giants contain inert cores, there is no fusion in the actual core of the star as very little hydrogen remains and the temperature is not high enough for the helium nuclei to covercome the electrostatic repulsive forces. However, fusion of hydrogen into helium continues in a shell around the core causing the preiphery of the star to expand as the outer layers slowly move away from the core. As these layers expand, they cool causing the star to glow a red colour. Eventually most of the outer layers of the red giant drift off into space, leaving the hot core (known as a white dwarf) surrounded by a planetary nebula. No fusion reactions take place within a white dwarf, they emit energy due to leaking photons created in its earlier evolution

74
Q

What is a planetary nebula?

A

The outer layers of a red giant that have drifted of into space, leaving the hot core begins at the centre as a white dwarf

75
Q

What is a white dwarf?

A

A very dense star formed from the core of a red giant, in which no fusion occurs

76
Q

What is electron degeneracy pressure?

A

A quantum mechanical pressure created by the electrons in the core of a collapsing star due to the pauli exclusion principle

77
Q

Explain how electron degeneracy pressure occurs

A

The pauli exclusion principle states that 2 electrons cannot exist in the same energy state. When the core of a star begins to collapse under the force of gravity, the electrons are squeezed together, creating a pressure that prevents the core from any further gravitational collapse. However, the electron degeneracy pressure is only sufficient to prevent gravitational collapse if the core has a mass less than 1.44Mₒ, if the core is more massive than this the star’s life takes a more dramatic turn.

78
Q

What is the Chandrasekhar limit?

A

The mas of a star’s core beneath which the electron degeneracy pressure is sufficient to prevent gravitational collapse, 1.44 solar masses

79
Q

Why does a white dwarf not undergo gravitational collapse if there is no nuclear fusion in the core?

A

Electron degeneracy pressure

80
Q

Explain what happens to a star of mass greater than 10Mₒ when it becomes unstable (up until supernova):

A

When the hydrogen in the core begins to run low, the core begins to collapse under gravitational forces. However, as the core’s of these stars are much hotter than smaller stars, the helium formed from hydrogen fusion begin to fuse to form heavier elements. The changes in the core cause the star to expand, forming a red supergiant. The temperatures and pressures are high enough to form even massive nuclei together (forming upto iron) forming a series of shells inside the star. This process continues until the star develops an inert iron core. As the star cannot fuse iron nuclei, the star becomes incredibly unstable causing the star to collapse inwards and then all the outer layers rebound off the solid iron core leading to a shockwave that ejects all the core material out into space. This is known as a (type II) supernova. The supernova has such a high power output that it fuses iron nuclei and forms all the heavier elements in the periodic table. The shockwave of the supernova helps distribute these elements throughout the universe.

81
Q

Explain what happens to a star of mass greater than 10Mₒ after a supernova occurs, leaving a remnant core of mass 2Mₒ

A

If the mass of the remnant core is greater than the Chandrasekhar limit, the gravitational collapse continues, forming a neutron star. Neutron stars are made up almost entirely of neutrons and can be very small (just 10km in diameter). They are incredibly dense, having a similar denisty to an atomic nucleus.

82
Q

Explain what happens to a star of mass greater than 10Mₒ after a supernova occurs, leaving a remnant core of mass 3Mₒ

A

If the core has a mass greater than 3Mₒ, the gravitational collapse continues to compress the core. The result is a gravitational field so strong that in order to escape it an object would need an escape velocity greater than the speed of light. Supermassive blackholes with masses of several million Mₒ are believed to be at the centre of most galaxies.

83
Q

What do the layers of a red supergiant look like

A
84
Q

Explain the life cycle of stars with a HR diagram:

A
  • Low mass stars evolve from main sequence stars to red giants. They then gradually eject their outer layers, ending up as white dwarfs
  • Higher mass stars start at X, rapidly consuming their fuel and swelling into red supergiants at Y before going supernova
85
Q

What is Wien’s constant?

A

2.90x10⁻³ mK

86
Q

What is the difference between the flux and luminosity of a star?

A

flux is the energy received over a unit area, luminosity is the total energy output of the star

A Level Physics (29 decks)

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