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Physics IB > Chapter 16 - Astrophysics > Flashcards

Chapter 16 - Astrophysics Flashcards

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1
Q

What is the order of the planets?

A

Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.

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2
Q

What is an asteroid?

A

A small rocky body that drifts around the solar system.

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3
Q

What is a meteoroid?

A

An asteroid on a collision course with another planet.
Small meteors can be vaporised due to the friction within the atmosphere.
Larger ones can land on Earth.

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4
Q

What are comets?

A

They are mixtures of rocks and ice in vey elliptical orbits around the sun. Their tails always point away from the sun.

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5
Q

What are nebulae?

A

Interstellar clouds of dust, hydrogen, helium and other ionised gases.

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6
Q

How does our perspective affect how we see constellations?

A

Stars in constellations are not necessarily close by it’s just our perspective.

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7
Q

What are stellar clusters?

A

Stars grouped together, and these can be open containing 10^3 stars, or globular containing 10^5 stars.

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8
Q

Why is our sun stable and how much mass is it losing?

A

The sun is stable because there is a hydrostatic equilibrium between this outward pressure and the inward gravitational force.
The sun is losing mass at a rate of 4x10^9 kgs^-1. This takes place in the core of the sun.

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9
Q

What are visual binary stars?

A

It is binary star that can be distinguished as two separate stars using a telescope.

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10
Q

What are spectroscopic binary stars?

A

They are identified from the analysis of the spectrum of light from the ‘star’. Over time the wavelengths show a periodic shift or splitting in frequency. This links to The Doppler effect.

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11
Q

What is an eclipsing binary star?

A

They are identified from the analysis of the brightness of the ‘star’. Over time the brightness shows a periodic variation. The explanation for the dip in brightness is that it is a result of it’s orbit as one star gets in front of the other. If the stars are of equal brightness this would cause the total brightness to drop by 50 per cent.

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12
Q

What is the parallax movement?

A

As you move from one position to another objects change their relative position. This change means that a close star will have an apparent movement when compared with a more distant set of stars. The closer a star is to the Earth the greater the parallax shift

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13
Q

What is the conversion between parsecs and light years?

A

1 parsec = 3.26 ly

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14
Q

What is the conversion between meters and parsecs?

A

1 m / 3.08 x 10^16

One second of arc must be 3.08x10^16m away

Parsec= parallax angle of one second

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15
Q

What is the equation for distance and the parallax angle?

A

d=1/p

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16
Q

How is the parallax angle measured?

A

By observing the changes in a star’s position over the period of a year.

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17
Q

What can the distance from the Earth to the sun be described as?

A

One astronomical unit

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18
Q

What is the limitation of the parallax method?

A

It can only be used to measure stellar distances of less than 100 parsecs, because further than that the angles become to small to measure accurately.

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19
Q

What is the definition of luminosity?

A

The total power radiated by a star, with the SI units watts

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20
Q

What is the definition of apparent brightness?

A

The power received per unit area, with the SI units Wm^-2

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21
Q

How is distance related to brightness?

A

As distance increases then the brightness decreases since light is being spread over a bigger area. Two stars can have the same apparent brightness even if they have different luminosities, as it depends on the distance they are at.

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22
Q

What is the equation for apparent brightness?

A

b = L/4(Pi)r^2

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23
Q

What is a black body?

A

They are perfect absorbers and emitters of radiation, and emit every type of electromagnetic wave depending on their temperature.
Wavelength x temperature = Wien’s constant

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24
Q

What do absorption lines show?

A

They show ‘fingerprints’ of the elements present.
It shows the missing wavelengths, which are from electrons absorbing photons and transitioning between different energy levels.

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25
Q

What’s the difference between red and blue shifted?

A

Red shifted: moving away from us

Blue shifted: moving towards us

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26
Q

What is the classification so the stars?

A 
O < 30,000-50,000> blue 
B <10,000-30,000> blue-white
A <7,500-10,000> white 
F <6,000-7,500> yellow-white 
G <5,200-6,000> yellow 
K <3,700-5,200>orange 
M <2,400-3,700>red
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27
Q

What is the Stefan-Boltzmann Law?

A

Total power radiated = (Stefan-Boltzmann)x Area x T^4

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28
Q

What does the Hertzsprung-Russel diagram show?

A

Vertical axis: luminosity of the star compared with the luminosity of the sun. It’s a logarithmic scale because it is such a large range.
Horizontal: scale of decreasing temperature.

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29
Q

Where are the main sequence stars on the Hertzsprung-Russel diagram and what are they?

A

They are shown on the diagram from going top left to bottom right, with our sun in the middle(ish). They are the stable stars and the only different between them is their mass. They are funding hydrogen to helium.

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30
Q

Where are the red giants and white dwarf stars on the diagram?

A

The red giants are above the line of main sequence stars, and the white dwarfs are below the line. Red super giants are higher up and more left than the red giants.

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31
Q

What is the mass-luminosity relationship for main sequence stars?

A

L is proportional to M^3.5

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32
Q

What is a standard candle?

A

A star with a known luminosity.

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33
Q

What is a Cepheid Variable?

A

A star of which it’s outer layers undergo a periodic compression and contraction and this produces a periodic variation in its luminosity. This is because as a star gets older the balance between the compression and outwards pressure decreases. Contractions created increased luminosity, compressions crest lower luminosity.

34
Q

How can we use Cepheid variables to Calculate the distance?

A

Calculate the variation of brightness over a given time.
Use Luminosity-period relationship for Cepheids to estimate average luminosity.
Use average luminosity, & average brightness for the apparent brightness equation of:
b=p/4(pi)r^2

35
Q

When do red giant stars start to form and how do their masses affect how long they live for?

A

When the amount of hydrogen runs out to produce more energy. The mass-luminosity relationship can be used to calculate the time different masses of stars will take before the hydrogen fuel is used.
If a star is 10 times more massive than our sun, it will be (10)^3.5 times more luminous. Therefore, will have that much more fuel to use but will use it at a quicker rate. Stars with larger masses live shorter.

36
Q

What occurs to a star once the hydrogen in the core becomes rare?

A

The fusion reactions creating hydrogen to helium will occur less. The star is then no longer in equilibrium and the gravitational force will cause the core to collapse. This collapse will increase the temperature of the core and now helium fusion is now possible. This expansion means it becomes a red giant star.

37
Q

What occurs is a red giant has sufficient mass?

A

The red giant can continue to fuse higher and higher elements and the process of nucleosynthesis can continue. It ends with the nucleosynthesis of iron. The fusion of iron to form higher mass nucleus would take in energy rather than release it, due to its high binding power.

38
Q

What impacts what a star forms into after leaving the main sequence?

A

Its initial mass

39
Q

What is the Chandrasekhar limit?

A

It’s equal to approximately 1.4 times the mass of our sun. Below this limit a process called electron degeneracy pressure prevents the further collapse of the remnant.

40
Q

What occurs if a star is below the Chandrasekhar limit?

A

If a star has less than 4 solar masses, it’s remnant will be less than 1.4 solar masses. In this case the red giant forms a planetary nebula and becomes a white dwarf.

41
Q

What occurs if a star is above the Chandrasekhar limit?

A

If the star is greater than 4 solar masses, it’s remnant will have a greater mass than 1.4 solar masses. Thus, it’s above the limit and electron degeneracy pressure is not sufficient enough to prevent collapse. In this case the red supergiant experiences a supernova and either becomes a neutron star, or a black hole.

42
Q

What is a key feature of a neutron star and a similarity it has to white dwarfs?

A

A neutron star is stable due to neutron degeneracy pressure. White dwarfs and neutron stars do not have a source of energy to fuel their radiation; they must be losing temperature all the time. However because the masses and therefore the temperatures are so high they can still exist of millions of years.

43
Q

What’s distinguishes a red supergiant becoming a neutron star or a black hole?

A

The largest mass a neutron star can have is called the Oppenheimer-volkoff limit and is 2-3 solar masses. Remnants above this limit will form black holes.

44
Q

What is a pulsar?

A

They are cosmic sources of very weak radio wave energy that pulsate at a very rapid and precise frequency. Neutron stars can emit an intense beam of radio waves in a specific direction. Due to the stars rotation, this beam moves around and causes the pulsations we receive on Earth.

45
Q

What are Quasars?

A

They are point like sources of light and radio waves that are very far away, with large red shifts. This places them at the limit of our universe, thus emitting large amounts of power. The energy radiated could be due to whole stars ‘falling’ into the black hole.

46
Q

What are rotation curves?

A

Galaxies rotate around their centre of mass.
A rotation curve for a galaxy shows how this orbital speed varies with distance from the galactic centre. Most galaxies show:
1. Initial linear increase in orbital speed with distance within the galactic centre.
2. A flat or slightly increasing curve showing a roughly constant speed of rotation away from the galactic centre.

47
Q

Why do rotation curves provide evidence for black matter?

A

Orbital velocities are not observed to decrease with distance away form the centre as one may expect. Instead the velocity is roughly constant whatever the radius. Due to v^2=GM/r, the total mass that is keeping the star orbiting in its galaxy must be increasing. However this isn’t the visible mass, so there must be dark matter.

48
Q

What are the 3 outcomes of a star’s life cycle?

A

White Dwarfs - which will become brown dwarf when it ceases to give out any light and becomes sufficiently cold.
Neutron stars - rotating neutron stars have been identified as pulsars.
Black holes

49
Q

Why are Cepheid variables useful?

A

There is a link between the period of brightness variation and their average luminosity. This can then be used by astronomers to calculate the distance to some galaxies.

50
Q

What is step 1 in nucleosynthesis?

A

This overall reaction takes place in the sun.
First process is called the proton-proton (p-p) cycle.
Two protons have to come close enough to interact and form a neutron and proton, giving off a positron and also a neutrino. These positivity charged particles will want to repel one another so high temperatures must be present.

51
Q

What is step 2 in nucleosynthesis?

A

The result from the reaction in step 1 (a neutron and a proton) have to react with another proton. This will create an isotope of helium 3 with two protons and one neutron, and will also give off a gamma ray.

52
Q

What is step 3 in nucleosynthesis?

A

For step 3 to occur, step 2 will need to occur twice, and then both can react together to form helium, which is two neutrons and two protons. It will also give off 2 protons which can then continue the proton-proton cycle.

53
Q

Where does nucleosynthesis take place and how does is star to happen?

A

It takes place in a large cloud of hydrogen, which is it’s hot enough these nuclear reactions can take place spontaneously. The power radiated by the star is balanced by the power released in these reactions - the temperature is effectively constant.
The cloud becomes a high temperature in the first place by:
Loss of GPE as the cloud comes together means an increase in KE, hence temperature.
Once this has started the star can remain stable for billions of years.

54
Q

What is the jeans criterion?

A

How big does a collapsing cloud need to be to form a star.
In symbols the Jeans criterion is that a collapse can start if M>M(J)
M(J) being the critical mass.

55
Q

What makes a cloud more likely to collapse?

A

For a given cloud of gas, M(J) is dependent on the cloud’s density and temperature. The cloud is more likely to collapse if it has:
Large mass
Small size
Low temperature

56
Q

How do we know that the universe is expanding, and in what direction?

A

Almost all galaxies show red shift, suggesting that they are moving away from us.
At first we thought we were in the middle of the expansion, but this is only because it was we who worked out the velocities of the other galaxies.
Everything is moving away from everything else.

57
Q

What are the Key ideas about the Big Bang theory from the concept of expansion?

A

All matter would have been together at the same point roughly 15 billion years ago.
At this point there was an extremely high density and temperature, which mean that the temperature and density have been decreasing since the expansion.
It was the creation of space and time.

58
Q

What is Cosmic microwave background radiation?

A

They discovered CMB radiation coming towards us from all directions in space. The radiation was the same in all directions (isotopic) and did not seem to be linked to a source. It was also a really good match to theoretical black-body radiation by an extremely cold object.

59
Q

What is the first way one can understand CMB?

A
  1. All objects give out electromagnetic radiation. Frequencies can be predicted using the theoretical model of black body radiation. The background radiation is from the universe itself which has now cooled down to an average 2.73 k.
60
Q

What is the second way one can understand CMB?

A

Some time after the Big Bang, radiation became able to travel through the universe. It has been travelling towards us all this time. During this time the universe has expanded - thus the wavelength of this radiation will have increased.

61
Q

What are the key features of galaxies?

A

They aren’t distributed randomly in space, they to be found clustered together. They can turn into superclusters of galaxies, which normally involve galaxies arranged together in joined ‘filaments’ (or bands) that are arranged throughout random empty space.

62
Q

What is Hubble’s Law?

A

As the distance of galaxies increases, so does the the recessional velocity.
The equation for Hubble’s Law is: v=H x d, with H being the Hubble’s constant.
The SI units of the Hubble’s constant are s^-1 but kms^-1Mpc^-1 is also used.

63
Q

How do you calculate the age of the universe?

A

Time = 1/H
There is a graph for the upper limit for the age of the universe. The gravitational attraction between galaxies predicts that the speed of recession decreases all the time.

64
Q

How can one use the Doppler red shift equation?

A

It can only be used when the recessional velocity is small in comparaison to the speed of light. However there are objects like quasars which the observed red shift is greater than 1, implying that their speed of recession is greater than the speed of light.

65
Q

What is the cosmic scale factor?

A

The cosmic scale factor r, can quantify the expansion of the universe that has taken place. All measurements will have increased by a ratio of R/R(0), with R= R(0) + change in R
Thus:
Z = (R/ R(0)) - 1

E.g if the measured red shift is 4, it means that R/R(0) = 5. This means that if R is considered the present size of the universe, then the light must have been emitted when the universe was 1/5 of its current size.

66
Q

What is a supernova and its key features?

A

They are rare catastrophic explosions, that an observer on Earth would be able to see a rapid increase in brightness. Huge amounts of radiated energy are emitted in a short period of time. At its peaks the apparent brightness often exceeds many local stars or individual galaxies.

67
Q

What does the cosmic scale factor show about the expanding universe?

A

R is increasing, and the rate at which it’s increasing is also getting larger as time passes. This means the universe is accelerating and this must involve an outward accelerating force to contract the inward gravitational pull.

68
Q

How does the helium burning process change depending on the stars mass?

A

High mass stars: helium process can begin gradually and spread throughout the core
Small mass stars: this process starts suddenly

69
Q

What is slow neutron capture (s-process):

A

When neutrons are released from reactions that take place in core of stars, and since they are natural it’s easy for a nucleus to capture it. The resulting nucleus is said to be neutron rich, which cou,d then undergo beta decay where a neutron changes into a proton. The overall result is a new element. It takes place during the helium burning stage of a red giant star.

70
Q

What is a rapid neutron capture?

A

This is where neutrons are presented in such vast numbers there isn’t sufficient time to undergo beta decay. Thus, a very heavy nuclei is created. This normally takes place during a supernova. Elements heavier then iron e.g uranium and thorium can only be created this way at very high temperatures and densities.

71
Q

What is the brightness of type 1 supernova?

A

It quickly reaches a maximum brightness (equivalent luminosity of 10^10 suns) which then gradually decreases over time.

72
Q

What is the brightness of type 2 supernova?

A

They often have a lower peak luminosities than a type 1 supernova, it being equivalent to roughly 10^9.

73
Q

How are the types of supernova distinguished and what does type 1 entail?

A

By the analysis of their light spectra. Type 1 do not include the hydrogen spectrum, but for the subdivisons are based on a more detailed spectral analysis:
Type 1a - presence of singly ionised silicon
Type 1b - presence of non-ionised helium
Type 1c - no presence of helium

74
Q

What does analysis of type 2 supernova light spectra entail?

A

They all show presence of hydrogen

75
Q

What’s the cause of a type 1 supernova and a type 2 supernova?

A

Type 1a: white dwarf exploding

Type 2: large mass red giant star collapsing

76
Q

What is the process for each type of supernova?

A

Type 1a: gravity field from white dwarf attracts material from the red giant star, thus increasing the mass of the white dwarf.
Type: iron centre collapsing under its own gravity (due to not being able to fuse another element), creating a neutron star.

77
Q

How is the explosion caused for the type 1 supernova?

A

Type 1: extra mass gained by the white dwarf means it becomes over the Chandrasekhar limit, and electron degeneracy level is not sufficient to stop the gravitational collapse. Nuclear fusion of heavier elements starts and results in a sudden release of energy causing the explosion.

78
Q

How is the explosion caused for type 2 of the supernova?

A

Electron degeneracy pressure isn’t sufficient to stop the gravitational collapse of the core, but neutron degeneracy pressure is and the core becomes a stable and rigid neutron star. The rest of the infalling material bounces off the core creating shock waves moving outwards, causing all outer layers to be ejected.

79
Q

What is the cosmology principle?

A

It’s a pair of assumptions that the universe is isotopic and homogenous.
Isotopic universe is one that looks that same in every direction, which seems to be true about the large scale structure of the universe but it doesn’t apply to all observers on Earth.
A homogenous universe is one where the galaxies and galaxy clusters turn out to be the same distribution in all regions.
However recent discoveries of large scale structures in the universe cause some astrophysicists to question this principle.

80
Q

How did Einstein use the cosmological principle?

A

To develop a model of the universe in which it was static. He proposed that the gravitational attraction between galaxies would be balanced by a yet-to-be discovered cosmological repulsion. The principle is also linked to 3 possible models of the future of the universe.

81
Q

What made physicists think that the universe wasn’t static?

A

Hubble’s observation, CMB and the Big Bang theory.

Physics IB (7 decks)

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