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Physics Year 13 > Chapter 19 - Stars > Flashcards

Chapter 19 - Stars Flashcards

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1
Q

what are nebulae

A

giant clouds of gas and dust

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2
Q

how does a protostar form

A
  • nebulae form over millions of years
  • the gas and dust are gravitationally attracted
  • they become denser and gravitational energy is transferred to thermal energy
  • in part of the nebula a protostar will form
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3
Q

how does a protostar become an actual star

A
  • it must have a large enough mass to get the centre hot and dense enough for nuclear fusion to occur
  • this is the point where hydrogen particles have sufficient energy to overcome their electrostatic attraction
  • for it to become a star fusion must occur
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4
Q

what makes a star stable

A
  • gravitational forces compress the star
  • radiation pressure from photons and gas pressure from nuclei act outwards
  • these forces cancel making the star stable and spherical
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5
Q

what are planets/their key features

A
  • they have sufficient mass to be spherical
  • no fusion occurs
  • it has cleared its orbit of other objects
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6
Q

what are planetary satellites/their key features

A
  • a body in orbit around a planet

- includes both man-made and natural satellites

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7
Q

what are comets/their key features

A
  • few hundred metres - few tens of kms in diameter
  • small irregular bodies made from ice dust and small rocks
  • all orbit the sun with very elliptical orbits
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8
Q

what are solar systems/their key features

A
  • A star and all objects that orbit them
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9
Q

what are galaxies/their key features

A
  • a collection of stars/solar systems and interstellar gas and dust
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10
Q

what is a solar mass

A

solar mass

M(dot) is the mass of our sun = 2x10^30 kg

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11
Q

what do smaller mass stars form after their main sequence and what are the masses that do this

A

any stars of 0.5-10 solar masses form red giants after their main sequence

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12
Q

how do red giants form

A
  • as the hydrogen runs out the energy released by the core decreases
  • so gravity becomes the resultant force
  • the core collapses gravitationally
  • as the core shrinks, the temperature and pressure increase enough to start fusion again in a shell around the core
  • the core is inert and contains very little hydrogen
  • the star expands and the outer layers cool
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13
Q

what do red giants tend to form after they have undergone all of their fusion

A
  • white dwarfs
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14
Q

how do white dwarfs form from red giants and what prevents them from collapsing further

A
  • most of the outer layers of the red giants cool and drift into space forming a planetary nebula
  • the hot core remains as a white dwarf
  • this is very dense but no fusion occurs
  • the pauli exclusion principle prevents electrons from existing in the same energy state
  • this creates an electron degeneracy pressure which prevents further gravitational collapse
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15
Q

what are the features of a white dwarf

A
  • very dense but no fusion occurs
  • surface temperature around 30,000K
  • it doesn’t emit ‘new’ energy but ‘leaks’ photons from past fusion
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16
Q

what is the Chandrasekhar limit

A
  • 1.44 solar masses, this is the mass of a star’s core such that for any mass is greater than this, the electron degeneracy pressure is not sufficient to prevent further gravitational collapse
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17
Q

what is the general life cycle of a lower mass star

A

nebula –> protostar —> main sequence —> red giant —> planetary nebula and white dwarf

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18
Q

why do larger stars spend less time in their main sequence phase

A
  • the fusion in larger stars occurs quicker
  • because they have a greater mass
  • so their cores are hotter so hydrogen atoms have more kinetic energy so can fuse more easily
  • the cores are also at a higher pressure so the frequency of collisions is higher
  • so they use up their hydrogen more quickly
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19
Q

what classes as a higher mass star and what do they form after their main sequence

A
  • anything with a mass greater than 10 solar masses

- they form red supergiants after their main sequence phase

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20
Q

how do red supergiants form

A
  • as the hydrogen in higher mass stars runs out, the core begins to collapse
  • this makes the core hotter and denser
  • so particles have a greater kinetic energy and more fusion can occur (helium atoms are then able to fuse)
  • these changes cause the outer parts of the star to expand and cool
  • inside the the pressures and temperatures continue to increase as the star collapses and heavier and heavier elements are made in a series of shells
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21
Q

what occurs in supernovae

A
  • when the red supergiant has an iron core, the star becomes unstable
  • outer layers implode and bounce off the core and are ejected into space
  • heavier elements can be made in this process
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22
Q

what are the two options for what occurs to a core after a supernova

A
  • if the mass of the core is more than the Chandrasekhar limit but less than 3 solar masses then they form a neutron star
  • if the mass of the core is greater than 3 solar masses then gravitational collapse continues and they form a black hole
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23
Q

what is a neutron star/its features

A
  • stars formed almost entirely out of neutrons
  • approx 10km in diameter
  • mass around 10 solar masses
  • density similar to an atomic nucleus
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24
Q

what are black holes/their features

A
  • occur where the gravitational collapse of a core after a supernova continues
  • creates an object so dense that its escape velocity is greater than the speed of light
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25
Q

what is the general life cycle for higher mass stars

A

nebula —> protostar —> main sequence star —> red supergiant —> supernova —> neutron star or black hole

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26
Q

what is the Schwartzchild radius

A

the Schwartzchild radius of an object is the radius such that if all the mass of that object was condensed into a sphere that size, the gravity of it would form a black hole

27
Q

what does the Hertzsprung-Russel (HR) diagram show and what should we remember about it

A
  • shows all the stars in our galaxy on a plot of luminosity (on Y-axis) against temperature (on X-axis)
  • temperature increases right to left not left to right
  • we often use log scales because values can vary massively
28
Q

what is luminosity and what is a common measurement for it

A
  • the luminosity of a star is its total radiant power output
  • can be measured in Watts or solar luminosities
  • 1 solar luminosity = 3.85x10^26 W
  • related to both brightness and size
29
Q

what do you find in the top left of an HR diagram

A

the hottest, most luminous stars

30
Q

what do you find in the bottom right of an HR diagram

A

the coolest, least luminous stars

31
Q

what do you tend to find on the line going from top left to bottom right on an HR diagram

A
  • the main sequence stars, the red giants appear on a sort of branch off of this on the diagonal
32
Q

what do you find along the top of an HR diagram

A

the most luminous stars

the supergiants, (red supergiants in top right)

33
Q

what do you find in the bottom left part of an HR diagram

A

hot, not very luminous stars, usually the white dwarfs

34
Q

what are energy levels and what are the 4 things we need to remember about energy levels/electrons in energy levels

A
  • energy levels are orbits around a nucleus in which an electron sits
  • electrons cannot have a quantity of energy between two energy levels
  • energy values for energy levels are negative because work must be done to remove an electron
  • an electron with 0 energy is free from the atom, the most negative level is the ground state/level
35
Q

what happens when an electron is excited and what can cause this

A
  • when an electron gains energy, it becomes excited and moves up energy levels
  • this can occur from a potential difference, thermal energy or absorption of a photon of the correct energy
36
Q

what usually occurs following an electron becoming excited, how does this link to conservation of energy

A
  • the atom becomes less energetically stable
  • so the electron drops back down to a lower energy level
  • a photon of energy equal to the change in energy between energy levels is emitted
  • so conservation of energy holds
37
Q

which two equations can we use to calculate the energy/frequency etc. of the electron

A 
deltaE = hf
deltaE = hc/(lambda)
38
Q

what are the three types of spectra

A

Emission Line Spectra
Continuous Spectra
Absorption Line Spectra

39
Q

Define an Emission Line Spectrum

A

Emission line spectra show a spectrum of unique frequencies of light for each atom. These frequencies of light are produced when excited electron drop back to a lower energy level releasing a photon of a specific energy

40
Q

define a continuous spectrum

A

A continuous spectrum is a spectrum that shows all the frequencies of visible light

41
Q

how are continuous spectra usually produced

A

from heating a solid metal such as sodium

42
Q

define an absorption line spectrum

A

An absorption line spectrum is a continuous spectrum containing dark lines where photons of those frequencies have been absorbed to excite an electron in an atom

43
Q

explain what emission line spectra show and how they are formed

A
  • Electrons are excited (usually through heating), they move to a higher energy level
  • they then drop back to a lower energy level and emit a photon
  • the photon’s energy is equal to the change in energy between the energy levels the electron moved between
  • this produces a set of discrete frequency emissions according to the element’s energy levels
  • these emissions form an emission line spectrum
44
Q

Are emission/absorption line spectra specific to an element

A

yes

45
Q

explain how absorption line spectra are formed and what they show

A
  • light from a source which produces a continuous spectrum passes through a cooler gas
  • photons at the correct energy (equal energy to the energy change between energy levels) excite an electron to a higher energy level and are absorbed
  • this produces dark lines on the continuous spectrum
46
Q

what should we note about what happens when an absorption line spectrum is formed

A
  • the photons absorbed are re-emitted but in all directions rather than one, this makes it a lower intensity so a dark line still occurs
47
Q

what is the link between emission spectra and absorption spectra

A

they are negatives of each other

48
Q

how can we analyse star light using spectra

A
  • light from stars produces specific absorption line spectra
  • because some photons are absorbed by cooler gases in the outer layers of the star
  • these missing wavelengths can be checked against known element’s spectra to find the chemical makeup of the star
49
Q

what is a diffraction grating

A

” a diffraction grating is an optical component with regularly spaced slits/lines that diffract and split light into beams of different colours, travelling in different directions”

50
Q

what can be the issue with using a double slit, what do we use instead and why

A
  • the fringes produced by passing light through a double slit are not very sharp and it can be difficult to determine the centre of each maximum
  • to solve this we use a diffraction grating instead
  • the greater number of slits means more light is diffracted and a clearer pattern is produced
51
Q

what happens when white light passes through a diffraction grating

A
  • the amount by which the light diffracts when passing through a diffraction grating depends on its wavelength
  • therefore the different components of light diffract different amounts and the white light splits into its component colours
52
Q

why do maxima occur for light that has passed through a diffraction grating

A
  • as the light is diffracted, at certain points the waves meet in phase or with a path difference of n(lambda)
  • this means when the waves superpose at these points, constructive interference can occur and maxima form
53
Q

how can we derive the equation for diffraction gratings

A
  • the angle theta is the angle between the normal line to the grating and the diffracted light ray/line to the maxima
  • it is also the angle between the grating face and the shortest line from one slit to the other wave involved in superposition
  • the opposite face for this mini triangle must be n(lambda)
  • the hypotenuse is d (slit seperation)
  • therefore n(lambda) = dsin(theta)
54
Q

what is the diffraction grating equation and when can it be used

A 
n(lambda) = d sin(theta)
- for monochromatic light
n = order number
lambda = wavelength
d = slit separation
theta = angle from normal to maxima
55
Q

what is the practical we can do to find the wavelength of monochromatic light

A

1) shine monochromatic light (e.g. a laser) normal to a diffraction grating onto a white screen
2) calculate d, slit seperation
3) measure D and x (distance from grating to screen and distance from central order maximum to chosen maximum)
4) calculate theta using tan(theta) = x/D
5) repeat for each maxima
6) plot n against d sin(theta)
7) gradient = wavelength

56
Q

what is black body

A
  • idealised object
  • absorbs all EM radiation that shines on it
  • when at thermal equilibrium it emits a certain distribution of wavelengths and intensities
57
Q

what is Wien’s displacement law, state the proportionality

A

A law that relates the absolute temperature of a black body, T, to the wavelength of its radiation at which intensity is at a maximum, Lambda(max)

Lambda(max) = k/T

58
Q

what is the constant of proportionality in Wien’s Displacement law

A

Wien’s constant = 2.9 x 10^-3 mk (metres-kelvin)

59
Q

how does the distribution of an object’s radiation change as the object’s temperature changes

A

As temperature increases:

  • peak moves left i.e. wavelength (max) decreases
  • distribution becomes higher with a sharper peak
60
Q

state Stefan’s law

A

” Stefan’s law states that the total power radiated per unit surface area of a black body is directly proportional to the fourth power of its absolute temperature”

61
Q

state the equation for Stefan’s law and what Stefan’s constant is

A

L = 4 (pi) (r^2) (sigma) (T^4)

Stefan’s constant is sigma = 5.67 x 10^-8 Wm^-2K^-4

62
Q

what are the proportionalities that go with Stefan’s law

A

Luminosity is directly proportional to:

  • radius squared, r^2
  • surface area, 4(pi)(r^2)
  • (absolute temperature)^4, T^4
63
Q

how will a more distant star’s spectrum be different

A
  • LOWER INTENSITY

- more red-shifted because moving quicker

64
Q

state how an emission line is formed

A

Electron makes a transition to a lower energy level and emits a photon

Physics Year 13 (15 decks)

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