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A LEVEL PHYSICS > 19 - Astrophysics > Flashcards

19 - Astrophysics Flashcards

1
Q

units for measuring light

A

lux (lx), lumen (lm), candela (cd)
- using a light metre

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2
Q

hottest to coolest colours of stars

A

blue - white - yellow - orange - red - (black)

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3
Q

1 lux is equal to

A

1lm/m^2
= 1cd sr/m^2
sr is a spherical radian

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4
Q

flux

A

how many vectors go through an area per second

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5
Q

why can’t we visibly see which stars are brighter

A

unknown distances - the more distant, the dimmer due to inverse square law

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6
Q

inverse square law

A

I = P/4πr^2
I - intensity, flux, power over area
P - luminosity

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7
Q

inverse square law - as distance doubles, intensity…

A

quarters

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8
Q

what is apparent magnitude

A

apparent brightness of star when viewed from earth, using a logarithmic scale to give order of magnitude.

it is compared to the brightness of Vega

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9
Q

absolute magnitude

A

logarithmic scale describing luminosity

m-M - 5log(d/10)
m - apparent
M - absolute

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10
Q

what is the HR diagram a chart of

A

star’s luminosity (energy output) or absolute magnitude against temperature or spectral type

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11
Q

axes on HR diagram are what scale

A

logarithmic scale

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12
Q

spectral classification

A

Depending on the relative proportions of hydrogen, helium and the rest, a star can be classified according to a letter scheme: OBAFGKM

these also correspond to the temperatures of the stars and also their colour

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13
Q

why is it more likely for life to evolve with cooler stars

A

cooler stars burn slower and so longer lifetime and more time for life to evolve

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14
Q

range of luminosity axis

A

(Lsun)
10-4 10-2 1 102 104 106

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15
Q

range of temp axis

A

(K)
40000 20000 10000 5000 2500

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16
Q

what happens when electrons get excited

A

when an electron is given enough energy, it gets excited as moves up towards an upper energy level. these distinct energy levels are different for each element. the electron gains energy, absorbing a photon of light with a frequency corresponding to the energy change (can be related using E=hf). only photons with the exact energy of the energy change will be absorbed.

for a particular element, this will produce an ABSORPTION spectra with dark lines against a continuous spectra with the specific wavelengths of the photons absorbed

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17
Q

what happens when electrons relax

A

when an electron relaxes, it loses energy and moves down to lower energy level, emitting a photon of light. the change in energy corresponds to a particular frequency of light for one photon that is emitted. only photons with the exact energy of the energy change will be emitted.

this produces a unique EMISSION line spectrum with each spectral line correspnding to the specific wavelenghts of photons emitted.

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18
Q

spectra - brighter lines show

A

more prominence

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19
Q

what is deflection of light

A

change in objects velocity after contact or collision with a surface

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20
Q

what is diffraction

A

the spreading out of a wave as it passes through a gap/aperture or when it encounters an obstacle

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21
Q

change in wave properties during diffraction

A

speed, wavelength and frequency remain unchanged

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22
Q

conditions for diffraction

A

diffraction will only occur if the wavelength of the wave is similar size to the size of the aperture. effects are stronger when the wavelength is similar size to the gap.

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23
Q

diffraction and specific colours

A

in diffraction, red is diffracted the least and violet is diffracted the most

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24
Q

diffraction grating pag equation

A

dsinϑ=nλ
d - distance between two slits
ϑ - angle from zero order maximum
n - number of maximum
λ - wavelength of light

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25
Q

life cycle of stars - beginning to main

A

nebula is a cloud of dust and hydrogen gas. the force of gravity pull the dust and gas together and friction causes heat and forms a protostar after more mass and pressure
protostar - no fusion occurs. eventually hot enough so hydrogen is able to fuse in helium atoms causing a pressure outwards
main sequence stage - pressure caused by fusion equals the gravitational force inwards - equilibrium, star is stable and obeys Ideal Gas Laws (hot, bigger)

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26
Q

life cycle of stars - ending for below C. limit

A

when the hydrogen fuel runs out, the star becomes a red giant. the size of the star does not surpass the Chandrasekhar limit and so is unable to overcome electron degeneracy pressure. the star gets unstable after running out of helium fuel,expelling outer layers and becomes a white dwarf held by electron degeneracy pressure (no fusion)

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27
Q

life cycle of stars - ending for above C.limt

A

star goes from main sequence star to red super giant when fuel runs out. star surpasses Chandrasekhar limit therefore can overcome electron degeneracy pressure. fuel runs out and star collapses causing supernova. star then either becomes neutron star or black hole. neutron stars are made of neutrons and spin very quickly. black holes have infinite density and gravitational force so high light cant escape.

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28
Q

what is the Chandrasekhar limit

A

the theoretical limit to the size of a star that will become a White dward (1.44Ms) - 1.44 solar masses

29
Q

what is the event horizon

A

the boundary around a black hole in which nothing affects the universe outside it - anything in the boundary can’t escape the black hole’s vicinity due to large grav field

30
Q

blach hole also known as

A

singularity with infinite density

31
Q

what is Schwarzschild radius

A

the event horizon for light around a black hole. any light within this boundary can’t escape the vicinity of the black hole

32
Q

difference between asteroid, meteorite, meteor, comet

A

asteroid - rock
meteorite - rock in earth’s atmosphere
meteor - shooting star
comet - ice and gas

33
Q

examples of dwarf planets

A

pluto, ceres

34
Q

stars, plasma

A

stars are composed of plasma - no electrons

35
Q

nuclear fusion process

A

small hydrogen atoms fuse together to create larger helium nuclei, causes pressure outwards

36
Q

what is the cosmological principle

A

the idea that the universe has the same large scale structure when observed from any point within it

37
Q

three assumptions of cosmological principle

A

universe is homogenous (same density everywhere)
universe is isotropic (same in all directions)
laws of physics are universal and can be applied anywhere in the universe

38
Q

what is 1 AU

A

average distance from earth to sun in m

39
Q

parallax distance equation

A

distance (pc) = 1/parallax(arcseconds)

40
Q

diffraction grating equation

A

dsinϑ=n𝝀
d - distance between two slits (calculate from diffraction grating reading)
ϑ - angle from zero order maximum
n - number of maximum used

41
Q

why is a laser used for diffraction grating PAG

A

coherent - same frequency and equal phase difference

42
Q

why are interference patterns created on screen

A

due to waves superimposing - constructive and destructive interference cause max and mins

43
Q

sunspots

A

sunspots are cooler and fire atomic dust. magnetic field directs towards poles creates auroras such as Northern lights

44
Q

for something to appear a certain colour it…

A

absorbs everything else but reflects that certain colour

45
Q

white..

A

reflects all light

46
Q

what is a black body

A

a theoretical object that absorbs all light that hits it, appearing black when cold. when heated above absolute 0, emits light across whole EM spectrum

46
Q

black…

A

absorbs all light

47
Q

Wiens displacement law

A

the black body radiation curve for different temperatures peaks at a wavelength inversely proportional to the temperature of the object

the hotter the black body, the shorter the peak wavelength

𝝀max ∝ 1/T
𝝀maxT=constant
constant = 2.9x10^-3 (mK)

𝝀max is light of max wavelength and max intensity and T is the absolute surface temperature of object

48
Q

luminosity is equal to

A

energy star radiates per second (power output)

49
Q

Stefans Law

A

luminosity of a star is directly proportional to its surface area and the 4th power of its temperature

L=AσT^4=4πr^2σT^4

50
Q

temp to kelvin

A

0C = 273K

51
Q

power equals

A

luminosity

52
Q

what is a continuous spectra?

A

all visible wavelengths/ frequencies are visible. can be produced by heating of solid metal like a lamp filament.

53
Q

what is a light year

A

the distance light travels in one year in m

54
Q

define stellar parallax

A

the apparent shift in a stars position against a backdrop of other stars

55
Q

define one parsec

A

the distance at which a radius of 1AU subtends an angle of one arcsecond

56
Q

in one degree there are…

A

60 arcminutes and 3600 arcseconds

therefore 1arcsecond = 1/3600 of a degree

57
Q

electrons bounded to atoms can only

A

exist in discrete energy levels.

58
Q

an electron with zero energy

A

is free from the atom

59
Q

the energy level with the most negative value is referred to as

A

the ground state

60
Q

why are energy levels negative in an electron

A

external energy is required to remove an electron from the atom. (also indicate electrons are bound to the positive nuclei.)

61
Q

emission spectra

A

each element produces a unique emission line spectrum because of its unique set of energy levels

62
Q

absorption spectra and light from stars created by

A

light from a source moving through a cooler gas - photons are absorbed by atoms in the gas. in stars they are absorbed by atoms of cooler gas in the outer layers of a star so light from stars produce absorption spectra

63
Q

why does white line being shone on disc split up

A

the disc acts as a diffraction grating to diffract the light ,spliting it into beams of different colours and causing an interference pattern

64
Q

how are maxima formed using diffraction grating

A

superposition of diffracted waves, dependent on path difference and phase difference waves from all slits.

65
Q

dark energy

A

energy that fills up space and accelerates expansion (around 68% pf total energy in universe)

66
Q

dark matter

A

assumed that as distance from centre of galaxy increased, velocity would decrease due to lower grav field but observations showed the velocity to not decrease, leading to the idea that mass is not concentrated in the centre of the galaxy - another type of matter called dark matter exists around 27%of mass in the universe. - particles suggested include axions and gravitinos.

67
Q

future of universe

A

open - continue to expand
closed - stop expanding and collapse in on itself
flat - end to expansion and universe remains size
outcome is dependent on density of universe

68
Q
A

A LEVEL PHYSICS (16 decks)

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