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A LEVEL PHYSICS > Astrophysics > Flashcards

Astrophysics Flashcards

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1
Q

What is the luminosity of a star

A

The total energy emitted per second

Sun abt 4e26

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2
Q

Apparent magnitude m

A

depends on luminosity and distance from Earth.

Magnitude 1 star has 100x intensity of magnitude 6 star, so a 1m was 2.51x brighter than 2m

brightest objects in the sky have negative apparent magnitudes

I2/I1 = 2.51^(m1-m2)

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3
Q

Hipparchus scale

A

Brightest stars given apparent magnitude of 1, while dimmest given apparent magnitude of 6.

Considering visible radiation emitted by object - visible luminosity - important when using optical telescopes

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4
Q

Parallax

A

Imagine ur moving - stationary objects in the foreground seem to move faster/more than objects further away. Apparent change in position is parallax. Greater the angle of parallax, closer the object is

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5
Q

Definition of parsec

A

1 Parsec is distance at which 1AU subtends an angle of 1 arcsecond

remember arcsecond is angular separation/2

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6
Q

What is the astronomical unit (AU)
What is a light year

A

The average distance from the Earth to the sun, Earth’s orbit is an ellipse

Light year - distance light (EM radiation) travels in a vacuum in one Earth year

Distance in light years tells you how long ago light left object

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7
Q

Defining parallax angle

A

Consider Earth orbiting sun. There’s a star far away enough such that its relative motion is negligible (stays stationary).
Draw right angle triangle where distance from earth to star forms hyp, radius of Earth’s orbit forms opposite. In 6 months, the Earth would have moved relative to the star, and would have swept out an angle of 2x the parallax angle.

parallax is the apparent shift of position of any nearby star against the background of distant stars- distant stars have virtually fixed position

Greater angle of parallax, closer body is

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8
Q

Defining parsec in terms of AU and arcseconds

A

1 parsec is the distance at which 1AU subtends an angle of 1 arcsecond

parsec is between sun n star

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9
Q

What is absolute magnitude
+eq

A

The apparent magnitude that an object would have if it was viewed from 10 parsecs away

m-M=5log(d/10)

doesnt depend on distance from earth

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10
Q

What is a standard candle

A

An astronomical object with a known absolute magnitude - can directly calculate luminosity

e.g. type 1a supernovae

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11
Q

Definition of a perfect black body

+ stuff + peak wavelength eq

A

A body that can absorb and emit all wavelengths of electromagnetic radiation
Black body since they don’t reflect

Graph of intensity vs wavelength varies with temperature - higher temperature, higher peak intensity at a lower wavelength, also less broad of a peak. Higher temp causes intensity to increase at every wavelength, however not an equal increase, shorter wavelengths have greater increases

λT = k where k = 2.9e-3 metre kelvins

as energy decreases becomes more red, as energy increases becomes more violet

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12
Q

Stefan’s law + intensity

A

Power output/Luminosity of a star is related to surface area and T by

P=σAT^4 where A = 4π r^2

I = P/4πd^2 inverse sq law (d is distance between Earth and star)

Assuming star is a perfect sphere
radiation spreads out and becomes “diluted” so intensity decreases

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13
Q

Wien’s displacement law

A

λmaxT = k where k = 2.9e-3 metre kelvins where λmax is the peak wavelength of a black body curve

and T is the abs temp of the outer layer (photosphere)/surface

KELVIN KELVIN KELVIN

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14
Q

Useless things that might be useful

A

There’s almost a million million stars in the milky way
light from sun takes 500s
Observable universe - region we can observe via em radiation (observed temporal edge but not spatial)

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15
Q

arc minute and arc second

A

arc minute = 1/60 a degree
ac second = 1/3600 a degree

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16
Q

Why would very small parallax angles be hard to measure

A

Smearing effect of Earth’s atmosphere - limits Earth based telescope

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17
Q

Method used to measure distances greater than 300 parsecs

A

Standard candles

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18
Q

Relating D, radius of Earth’s orbit and angle of parallax

A

D=1/theta
D in parsecs
Theta in arcSECONDS

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19
Q

Calculating angle subtended as viewed from Earth

A

Calculate normal angle subtended then x 2 since accounting for motion

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20
Q

Brightness is

A

A subjective scale of measurement

Stars are bright since they emit EM radiation

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21
Q

Intensity is

A

Effective brightness of a star, follows inverse sq law wrt star (Assuming star gives out equal amount of energy in each direction)

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22
Q

Luminosity vs intensity

A

Luminosity - power output, intensity - apparent brightness

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23
Q

Reasons why brightness is subjective

A

Air pollution, atmospheric distortion and human interpretation

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24
Q

Remember for apparent magnitude scale

A

Vega set as 0 point w m=0. However all shtick is only true for visible light, different stars have different apparent magnitudes for different parts of em spectrum

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25
Q

1 Parsec

A

= 3.26 Light years

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26
Q

Examples of black bodies

A

Stars, Stoves, furnaces, warm blooded animals

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27
Q

Magnitudes and power

A

Two stars w the same absolute magnitude have the same power output - relate w Stefan’s law

Ax:Ay = Ty:Tx = (dx/dy)^2

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28
Q

Assumptions when analysing stars

A

Assuming star is p black body, no light is absorbed/scattered by material between star and observer.

ALSO NOTE a hotter star might not appear brighter than a cooler one as it may not emit as much visible EM radiation - important to use optical and non optical telescopes

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29
Q

Significance of emission and absorption spectrum

A

Unique for each element, can therefore be used to identify elements when unsure of contents of a substance

Each line in emission spectrum due to a photon of specific energy hf. Absorption line due to absorption

Excite sample, let sample de excite and record emission, compare w known elements. Doesn’t matter where sample is excited spectrum always the same

Can also observe redshift/blueshift

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30
Q

For hydrogen absorption line to occur in visible part of spectrum

A

Electron must be in n=2 state

Visible absorption lines caused by electrons moving from higher energy level to n=2 state

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31
Q

Why hydrogen atoms in n=1 state can’t absorb visible photons

A

Visible photons don’t have sufficient energy to cause excitation from n=1

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32
Q

Balmer series

A

Series of lines corresponding to wavelengths of visible part of hydrogen’s absorption spectrum

SPECIFICALLY seen when light from a star has been absorbed by hydrogen in atmosphere while passing through

For this to occur, electrons in H atom have to exist in n=2 state, happens at hot temperatures, where collisions between atoms give electrons more energy. If energy is too high, some electrons may reach n=3, which would result in fewer Balmer transitions. So INTENSITY of Balmer line depends on temperature

Also balmer lines only give info of surface properties, not core (core doesn’t contain electrons)

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33
Q

What causes absorption lines in spectrum from star

A

Due to “corona”/atmosphere of hot gases surrounding the star above photosphere, Photosphere emits continuous spectrum. Atoms/ions/molecules in hot gases absorb photons of diff wavelengths

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34
Q

How many temperatures possible for a given intensity of Balmer lines

A

2 due to nature of graph - curve up then peak then down. To combat use absorption lines of other atoms/molecules

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35
Q

Stellar class system from hottest to coldest

A

OBAFGKM

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36
Q

Spectral class O

A

Blue
Between 25,000 - 50000 K
He+ion,He and H

(most atoms in n=3)

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37
Q

Class B

A

Blue
between 11,000-25,000 K
He,H (balmer)

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38
Q

Class A

A

Blue-white
between 7,500-11000 K
Strong H, ionised metals

Many atoms in n=2

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39
Q

Class F

A

White
6000-7500 K
Ionised metals

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40
Q

Class G

A

Yellow white
5,000-6,000 K
Ionised metals, neutral atoms (metals)

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41
Q

Class K

A

Orange
3,500 - 5,000 K
Neutral atoms

42
Q

Class M

A

Red
<3,500 K
neutral atoms, TiO

Star is cool enough to form molecules

Red giants have lowest surface temperature, bc their intensity of emission is so low as their surface area is so high

43
Q

Explaining intensity of Balmer line for each spectral class

A

O - weak
B - slightly stronger - stars are too hot so more atoms likely to be ionised

A - strongest - perfect temperature for n=2

F - weak - too cool for n=2 excited state
GKM - very weak/none - too cool to excite. + very little atomic hydrogen

44
Q

Dwarf star

A

Star much smaller in diameter than sun

45
Q

Giant star

A

Star much larger in diameter than sun

46
Q

Two stars w same surface T but unequal absolute magnitudes

A

One with greater power output has larger surface area - larger diameter

47
Q

Two stars w same M but unequal surface temperatures

A

Hotter star has smaller surface area - smaller d kek

48
Q

Plotting axes for H diagram

A

Y axis absolute magnitude
-15 at top, 10 at bottom
X axis surface temperature
50,000 K at origin, 2500 at other end
(very hot lhs very cool rhs)
Scale is non linear
can also be classes on x axis

Also HR diagram significant as it tells us there are fundamentally different types of stars -allowed discovery of life cycle of star

49
Q

Each quadrant in HR diagram

A

Top left - bright and hot
bottom left - dim and hot
Top right - cool and bright
Bottom right - cool and dim

50
Q

Groups on HR diagram

A

Giants in M0
Supergiants around top right from B at M=-10
White dwarfs at 10<M<15 around bottom left corner

Main sequence form approximately diagonal from O-10 to M15

SUN IS AT G5
VEGA - approx A0
BETELGEUSE - approx M-5

Approximated that all stars have same power output

At top of main sequence are hot and luminous blue, bottom are cool and dim reds

51
Q

Why is more energy required to fuse larger elements together

A

Larger nuclei have more protons so greater electrostatic repulsion - greater force required over distance

Fusion of heavier elements occurs in the core - requires a larger gravitational pressure as nuclei have more protons, opp for lighter elements.

52
Q

Formation of star up till Main sequence

A

Dust and gas clouds in space contract under gravity becoming denser, forming a PROTOSTAR
In collapse, GPE converted to thermal as gas atoms gain Ek, interior of collapsing matter becomes hotter.

If enough matter’s there, temperature becomes high enough for nuclear fusion to occur
(If there’s not enough star doesn’t heat up enough and eventually cools when it stops contracting

Energy released from fusing H to He increases core temperature. Outer layer of protostar becomes hot and light emitting layer (photosphere) formed - now a star.

New star reaches internal equilibrium as inward gravitational force = outward radiation pressure. Star stable with constant luminosity. M depends on its mass, greater mass greater M - luminosity. Remains in main sequence for most lifetime. Emitting light due to H burning in core.

Sphere is the most stable shape in the universe

During main sequence, when fusing H, mass before>mass afterwards, energy lost to radiation

Fusion pressure from fusion products acts outwards

Larger stars need greater rate of fusion, spend less time on the main sequence

53
Q

Red giant phase

A

Most H in core converted to He, fusion pressure decreases, core contracts, increasing temperature of core - makes star hot enough to carry out fusion of He and other elements - star increases in size as fusion pressure»gravity(larger fragments pushing outwards). Red giants fuse elements larger than H

Temperature is lower than main sequence as even though there is greater energy, there is greater area so more spread out + Stefan’s law maintaining constant power output.

Wavelength at at peak intensity increases

Red giant phase about 1/5 lifetime

54
Q

Core vs Shell burning

A

Core - fusion in inner layer
Shell - fusion in outer layer

In red giant phase, core burning heats up outer layer allowing shell burning

55
Q

White dwarf phase`

A

When nuclear fusion in core ceases, star cools and core contracts, ejecting outer layer which form new planetary nebulae

Mass between 4 and 8 solar masses, core heats up enough to cause energy release via fusion forming nuclei as heavy as iron. Stops when all fuel (light nuclei) is used up

After ejecting layer, star is a little bit more than core, which is white due to release of GPE. If mass is under 1.4 solar masses contraction of core stops as electrons in core can’t be forced any closer (as they must exist in shells). Star now stable and is a white dwarf - cools as it radiates thermal energy into space and eventually becomes invisible. If mass greater than 1.4 solar masses, explodes in supernova

electrons repel each other as they must exist in shells - stops atoms from contracting further. Repulsive electron force = gravitational force

White dwarf is core of carbon
must have high T according to Stefan’s law

56
Q

When does fusion cease

A

When star runs out of fuel (light nuclei) or iron formed - more stable than other nuclei so can’t be fused to be made more stable

energetically unfavourable - highest binding energy per nucleon

57
Q

Red supergiants

A

Occur above 8 solar masses, red giant swells into supergiant which explodes in supernova. Can fuse up to iron. Unopposed gravitational force causes star to implode into core, increases pressure and temperature causing supernova - enough energy to fuse iron + releasing elements formed in giant phase forming new nebula

58
Q

Energy output of a supernova

A

10^44 joules

59
Q

Characteristics of supernova

A

Rapid increase in M (between -15 and -20). Increase in luminosity occurs in abt 24 hours, then gradual decrease over time scale in order of years

Increase in luminosity usually corresponds to change of abt 20 magnitudes

Can cause intense outflow of neutrinos and gamma photons

Light curve shows sharp initial peak then gradual decrease
M on y axis days on x

60
Q

Type 1a supernovae

A

All undergo same light curve. Show strong absorption line due to silicon. Peak luminosity abt 10^9 times the sun then decrease. (Thought to occur when white dwarf in binary system attracts matter from companion star)

Reach a KNOWN peak luminosity, known as standard candles

Peak is -20

61
Q

Type I supernovae

A

Have no strong hydrogen lines present and divided into 1a 1b and 1c

62
Q

Death of a high mass star

A

Core mass greater than 1.4 solar masses, electrons in core can no longer prevent further collapse as they are forced to react w protons to form neutrons. p+e—n+v

Sudden collapse makes core denser until neutrons can’t be forced any closer - core density abt same as atomic nuclei. Suddenly becomes rigid, and matter surrounding the core hits it and rebounds as a shock wave, propelling the surrounding matter outwards into space in an explosion. Releases so much energy it may outshine host galaxy. Enough energy to form elements heavier than iron

Also causes outflow of neutrinos + gamma neutrons

63
Q

Neutron stars

A

Core of a supernova after surrounding mater has been expelled - small compared to the Sun.

Same density of nucleus - virtually no space between - neutrons can’t be completely compressed as strong force is repulsive at 0.5fm

Produced in supernova - cons of momentum may cause it to rotate
very quick - emits radio waves in narrow beams due to charged particles spiralling in intense mg field around poles of star. Mg and rotational axis are different. Radio waves sweep past Earth like lighthouse

Diameter abt 20km, very large gravitational field, high escape velocity. Form as neutrons are the most stable particle in a large system

64
Q

What would happen is a GRB occurred close to the Earth

A

Would destroy ozone layer - mass extinction

65
Q

Pulsars

A

Pulsating neutron stars emitting beams of radio radiation

66
Q

Black holes

A

If core of neutron star becomes so dense that not even light can escape - black hole, can’t continue to absorb mass from its surroundings. Doesn’t emit photons, absorbs all incident photons. Event horizon is sphere surrounding black hole from which nothing can escape.

Attracts and traps surrounding matter, drawn towards a singularity at centre. May be charged, may be rotating, any property carried by in falling matter is lost. Information lost

67
Q

Calculating Schwarzchild radius

A

R=2GM/c^2

Distance between event horizon and singularity

Also defined as the distance at which escape velocity = speed of light

68
Q

Supermassive black holes

A

Thought to exist at centres of many galaxies. As at centres, stars are much closer together than they are at the edges. Supermassive black holes can pull in millions of stars, can therefore gain enormous amounts of matter

69
Q

Gamma ray bursts

A

Short lived - 0.01 to 1s from black holes, due to merger of neutron stars or neutron star falling into one

Long lived - 10s to 1000s from collapse of massive star in supernova

total energy from typical GRB about the same as total energy of sun over entire lifetime

70
Q

Doppler effect caveat

A

Observational effect - no change in actual properties of wave, only observed wave

71
Q

Doppler effect example

A

Ambulance moving towards you, wavelength decreases f increases
Moving away opposite

72
Q

Doppler effect

A

When wavelength/frequency of a wave is altered by relative motion between source and observer - can be experienced by any wave

73
Q

Red shift

A

Doppler effect with light

74
Q

How to determine if a star is red/blueshifting

A

Compare spectral lines to known spectrum. Can be done as spectral lines for element are the same regardless of motion, location,temperature or anything

75
Q

Eq for amount of redshift Z

A

Z = Δλ/λ Δf/f or v/c

only works when v«<c

CONSIDERED TO BE ABSOLUTE CHANGE IN WAVELENGTH, BUT APPROXIMATE CHANGE IN FREQUENCY DUE TO RELATIVISTIC EFFECTS

positive z means redshift, negative means blue

Can also give time light takes to reach us. Z=0 present

76
Q

Cosmological redshift

A

Detected from all distant galaxies except andromeda. Proof of expansion of universe + hot big bang model

Not andromeda as it’s gravitationally bound to milky way

77
Q

Stellar redshift

A

Detected from nearby stars, pairs of stars called binary stars, allowing to determine properties

78
Q

Binary star system

A

Two stars orbiting a common centre of mass

79
Q

Light curve for eclipsing binary

A

M on y, time on x
highest luminosity when both are visible, higher dip when hotter is in front of cooler, lower dip when cooler is in front of hotter

80
Q

Graph of changing wavelength for one star in binary

A

Sinusoidal - peak when star is receding with max radial/recessional velocity
(two stars next to each other)

81
Q

Redshift to determine rotational motion

A

Consider a rotating star, part moving towards us blueshifted, part moving away redshifted

82
Q

Evidence for HBB model

A

CMBR + abundance of H and He

83
Q

Hubble’s Law

A

Recessional velocity of a distant galaxy is proportional to its distance from Earth

v=Hd v is recessional velocity, H Hubble’s constant, D distance from Earth

also as graph is a straight line, assumes that rate of expansion is constant - now thought not to be true

H = 65Km/s per Mpc

Kilos per second per Megaparsec

84
Q

Controversy around H

A

I shld assume H is constant, however astronomers disagree on value since value has changed over last 60 years

85
Q

Cosmological redshift

A

Space in between galaxies expanding, effect is that both galaxies observe the other to be moving away

Greater space in between them, greater rate of expansion

86
Q

Calculating age of universe with H

A

v=Hd T=d/v = 1/m

Convert H from Km/s to m/s and d from Mpc to m

Assumed universe has expanded constantly, believed not to be true
Also H not assumed to be constant, which leads to an overestimate of age of universe. As we think expansion has been increasing

87
Q

Dark energy

A

Unknown form of energy theorised to be responsible for increasing expansion of universe

88
Q

Why aren’t the solar system or milky way expanding

A

Due to gravitational forces of sun/planets/stars opposing expansion

Also it’s possible for a galaxy to be receding from us so fast that their light never reaches us. c>v

89
Q

Observable universe

A

Part of universe in which we can detect objects via em radiation

90
Q

Radius of observable universe

A

Maximum distance that light can travel in age of universe

diameter about 46billion light years, however age only 13.7billion years. This is because the expansion of the universe increasing the size of the observable universe.

91
Q

Big bang model

A

Universe had a beginning
Started off very hot and dense and has been expanding ever since. All matter was created at the start, and matter density of universe has been decreasing over time

at start all matter concentrated into size smaller than an atom

92
Q
A
93
Q

CMBR as evidence for HBB

A

HBB predicts that lots of EM radiation produced in early stages of universe, which should still be observable today - As wavelengths have been stretched out this cosmic background radiation is in microwave region.

radiation is largely isotropic and homogeneous, which confirms the cosmological principle (same in all directions)

The background radiation also shows a Doppler shift

There are very tiny fluctuations in temperature,due to
tiny energy-density variations in the early universe, and are needed for the initial ‘seeding’ of galaxy formation.

94
Q

Amount of He as evidence for HBB theory

A

The early universe had been very hot, so at some point it must have been hot enough for hydrogen
fusion to happen. By studying how much helium there is compared to hydrogen, we can
work out a time frame for this fusion. Together with the theory of the synthesis of the heavier
elements in stars, the relative abundances of all of the elements can be accounted for

Universe is 74% H and 24%He

95
Q

Cosmological principle

A

Universe is homogenous (every part looks like the other part), and isotropic (the same in all directions) so it doesn’t have a centre

96
Q

Quasars

A

Most distant measurable objects
Powerful galactic nucleus containing a supermassive active black hole (taking in matter around it) at the centre of a galaxy. Black hole surrounded by mass of whirling gas which produces the
light in the same way as a pulsar, magnetic fields produce jets of radiation streaming out from poles

As matter accelerated towards blackhole, causes gamma rays to be emitted

97
Q

Detecting Quasars

A

Shot out jets of material, and were active radio sources (also show large optical redshift)

produced a continuous spectrum that was nothing like a black body radiation curve and instead
of absorption lines, there were emission lines of elements that astronomers had not seen before. But ended up being the Balmer series redshifted crazily

98
Q

Exoplanets

A

Planets not in the solar system
Orbiting stars other than them much brighter than them so outshone. Too small to distinguish from nearby stars (angular separation between planet and star smaller than min, if not then too far away so dim)

helps if planet orbits a brown dwarf

Only a few rlly large and hot ones, far away from stars can be seen using special telescopes

99
Q

Detecting exoplanets using Doppler shift

A

Planet and star orbit common centre of mass. As star>planet com is close to star, causes tiny variations in star’s orbit.
Causes very small red/blueshifts which can be detected and hint at exoplanet. Minimum mass of exoplanet can also be calculated.

movement
needs to be aligned with the observer’s line of sight — if the planet orbits
the star perpendicular to the line of sight then there won’t be any
detectable shift in the light from the star

Also exoplanet mass needs to be close to that of star

100
Q

Detecting exoplanets using transit method

A

As the exoplanet crosses in front of the star, some of
the light from the star is blocked from Earth’s view.This leads to a dip in the light curve observed on Earth. From this, the radius of the exoplanet can be found.

Chances of the planet’s path being perfectly lined up so that it crosses the line of sight between the
star and the Earth is incredibly low.
This means you can only confirm observed exoplanets,
not rule out the locations of any.

Also need periodic dips to confirm existence of exoplanet, as dips can be caused by other things e.g. sun spots. Also period may be so long

101
Q

Comological redshift as proof of big bang

A

Due to space between galaxies expanding
Extrapolating backwards, mechanism is the big bang
Production of all matter and energy produced expansion force

102
Q

Why is space not at absolute 0

A

Heated by CMBR, which will eventually become cosmic radio wave background radiation due to expansion.

CMBR started off as gamma at beginning

A LEVEL PHYSICS (17 decks)

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