5.5 AstroPhysics Flashcards

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1
Q

Nuclear Fusion

A

The process of two nuclei joining together and releasing energy from a change in binding energy

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2
Q

Planet

A

Large bodies that move in circular or elliptical orbits around a star

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3
Q

Planetary Satellite

A

A smaller body than a planet that orbits the planet (e.g the Moon)

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4
Q

Comets and where they come from

A

Rocky ice balls that travel in highly elliptical orbits around the Sun. Come from the Oort cloud

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5
Q

Solar System

A

A star (or a binary star pair) orbited by one or more planets

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6
Q

Galaxy

A

A cluster of many millions of stars

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7
Q

The Universe

A

The space in which everything exists

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8
Q

Gravitational Collapse

A

The inward movement of material in a star due to the gravitational force caused by its own mass

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9
Q

When gravitational collapse happens

A
  • When a star is formed the cloud of gas undergoes gravitational collapse
  • In a mature star when the internal gas and radiation pressure can no longer support the stars own mass
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10
Q

Radiation Pressure

A

An outwards pressure caused by the momentum of photons released in fusion reactions

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11
Q

Gas Pressure

A

An outwards pressure caused by the movement of the high energy gas particles inside a star

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12
Q

Main Sequence Star

A

A star in the main part of its life cycle, where it is fusing hydrogen to form helium in its core. The star is stable since the gas pressure and radiation pressure counteract the gravitational force

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13
Q

Red Giant

A

A star in the later stages of its life that has nearly exhausted its hydrogen supply and is now fusing helium. It is larger than a main sequence star and the outer layers are cooler, giving it its red colour

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14
Q

White Dwarf

A

The end product of the life cycle of a low mass star. It is very dense but does not undergo any fusion and will slowly cool down. It does emit light as photons from past fusion reactions leak away

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15
Q

Planetary Nebula

A

An expanding shell of ionised hydrogen and helium ejected from a red giant star at the end of its life

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16
Q

Electron Degeneracy Pressure

A

An outwards acting pressure that prevents stars of mass beneath the Chandrasekhar limit from collapsing further. Since two electrons cannot occupy the same states in an energy level of an atom, when electrons are being pulled into the star due to gravity they will reach a point where they cannot be added to the volume of the star. This has the effect of exerting an outwards force.

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17
Q

Chandrasekhar limit

A

1.4 times the mass of our Sun.

The mass at which a Star will collapse further than a white dwarf and become either a neutron star or a black hole

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18
Q

Red Super Giant

A

A Red Giant that has a mass much higher than that of our Sun

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19
Q

Supernova

A

An explosion produced when the core of a red super giant collapses

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20
Q

Neutron Star

A

The remnants of the core of a red super giant after it has undergone a supernova explosion. It is very dense and composed mainly of neutrons

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21
Q

Black Hole

A

The core of a massive star that has collapsed almost to a point. They are incredibly dense and their gravitational field is so strong that, past the event horizon, not even light can escape

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22
Q

Hertzsprung-Russel diagram

A

A luminosity-temperature graph

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23
Q

Luminosity

A

The total energy that a star emits per second

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24
Q

How stars form

A

Dust and gas come together through gravitational attraction. The work done on moving these particles increases their kinetic energy resulting in an increase in temperature. This large core is called a protostar. The gravitational field of the protostar will attract more matter until the temperature and the pressure in the core is enough for the hydrogen to fuse and create helium. The gravitational pressure will become balanced with the gas pressure and radiation pressure from the fusion. It is now a main sequence star.

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25
Q

Overall fusion reaction

A

4 proton goes to

  • helium-4 (2 proton + 2 neutron)
  • 2 positrons
  • 2 neutrinos
  • gamma rays
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26
Q

Lifecycle of a star of mass beneath the Chandrasekhar limit

A

Stellar Nebula -> Main Sequence
Hydrogen runs out and fusion halts, stopping the outwards pressure and causing the core to collapse and the outer layers to expand and cool as the star becomes a red giant. Core collapses further until helium fuses into carbon and oxygen preventing the core from collapsing further. Once all the helium is fused the core collapses further and ejects its outer layers which form a planetary nebula. The remaining core is a white dwarf which is stable as gravitational forces are counteracted by the electron degeneracy pressure

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27
Q

Lifecycle of a star of mass greater than the Chandrasekhar limit (up to supernova)

A

Stellar Nebula -> Main Sequence
Hydrogen runs out and fusion halts, stopping the outwards pressure and causing the core to collapse and the outer layers to expand as the star becomes a red super giant. As the core collapses, heavier elements are created by fusion. At each stable fusion stage, the further collapse of the core is prevented by the electron degeneracy pressure and the radiation pressure. Once an iron core has built up the fusion will stop and the core will undergo further gravitational collapse. The immense gravitational forces force protons and electrons to combine to form neutrons, which releases an incredible amount of energy causing a supernova as the outer shell is blown off. During a supernova heavier elements the iron can be formed when the nuclei fuse with neutrons.

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28
Q

What happens after a supernova

A

Depending on the mass remaining in the core a neutron star may be formed. These are very small and have very high density and are composed mostly of neutrons. The magnetic field of the neutron star can cause the star to emit vast amounts of high energy radiation from its poles. This is called a pulsar.
If the neutron star is massive enough the pressure can become so large that the neutron star would collapse to a point and become a black hole

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29
Q

Continuous spectrum

A

A spectrum that contains all wavelengths over a wide range

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30
Q

Energy Level

A

Discrete energies that electrons can have when occupying specific orbits.

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31
Q

Emission line spectrum

A

The spectrum of frequencies emitted due to electron transitions from a higher energy level to a lower one

32
Q

Absorption line spectrum

A

The pattern of dark lines that would appear on a continuous spectrum when the spectrum is shone through a medium that would absorb some frequencies of light

33
Q

Energy of energy levels

A

0 at infinity]
-13.6eV at the lowest energy level
varying between these but the energy level is always negative

34
Q

Ground State

A

The lowest energy level

35
Q

Emission spectrum source and how they appear

A

They come from hot gasses when an excited electron moves to a lower energy level and emits a photon.

36
Q

How the frequency of an emitted photon is calculated

A

delta E = hf

where delta E is the change in energy resulting from an electron changing energy level

37
Q

How are emission spectra and absorption spectra related

A

The same wavelengths are absorbed as are emitted

38
Q

How many photons does it take to change the energy level of an electron

A

Always just one

39
Q

How elements in stars can be identified

A

By looking at the spectrum of light from the star. It is assumed that the star emits a continuous spectrum and thus any dark lines in the spectrum will be from that wavelength being absorbed by the elements in the stars atmosphere. Therefore by matching the dark lines to known samples of all the elements we can determine the makeup of the stars atmosphere

40
Q

How come the absorption lines coming from a star aren’t filled in by those same elements emitting light of that wavelength

A

The gasses in the atmosphere are not hot enough to produce emission spectra

41
Q

How to determine the wavelength of light from a star

A

Using a transmission diffraction grating to diffract the light and then using nλ = dsinθ

42
Q

How to determine the number of maxima that will be produced

A

nλ = dsinθ
sinθ <= 1
nλ = d
n = d / λ

43
Q

Wiens displacement law

A

λmax ∝ 1/T

where λmax is the most common wavelength emitted by a star and T is the peak surface temperature

44
Q

How a graph of intensity against wavelength looks like for stars of different temperatures

A

Peak at λmax. Much higher area under the stars of higher temperature

45
Q

Luminosity

A

The total energy emitted per second

46
Q

Stefans law

A
L=4σπr^2T^4
where
L is luminosity
σ is stefans constant
T is temperature
r i s radius
47
Q

Why does a star get brighter when it becomes a red giant even though it becomes cooler

A

Because its radius and thus its surface area increases. And luminosity is proportional to surface area

48
Q

λmax = kT. what is the value of k

A

2.89*10^-3

49
Q

How the measured intensity on Earth can be used to discern information about a star

A
I = L/A
where
I is intensity
L is luminosity
A is area
the area in this case would be the area of a sphere with a radius equal to the distance from the star to earth since the energy has spread out over that distance
50
Q

AU

A

Astronomical Unit

The mean distance between the Earth and the Sun

51
Q

Light Year

A

The distance travelled by light in a vacuum in one year

52
Q

Parsec

A

A unit of distance that gives a parallax angle of 1 second of arc using the astronomical unit as the base of the right angled triangle

53
Q

Stellar parallax

A

The shifting in position of a star viewed against a background of distant stars when viewed from different positions, for instance when viewed from Earth at different points of Earths orbit

54
Q

Arc second

A

1/3600 of a degree

55
Q

How to calculate a distance using parallax

A

Measure the difference in angle between the two extremes of earths orbit. The parallax angle will be half of this. Construct a right angled triangle and do some trig ey

56
Q

Simple relation between parallax angle and distance

A

p=1/d
where p is the angle in arc seconds and d is the distance in parsecs
This works because tan(x) = x at very small x and because parsecs are defined in terms of AU and arc seconds (this only works when a distance of 1AU has been used)

57
Q

Doppler effect

A

The change in wavelength caused by the relative motion between the wave source and the observer

58
Q

Red Shift

A

The apparent increased in wavelength of electromagnetic radiation observed when the source is moving away from the observer

59
Q

Hubbles law

A

The recessional speed of a galaxy is directly proportional to its distance from us

60
Q

Doppler equation

A

Relative change in frequency = Relative change in wavelength = v/c

61
Q

How the change in wavelength from a star can be calculated

A

By looking at the absorption spectrum of the star and seeing how much it has shifted

62
Q

How to estimate the age of the universe

A

1/H

63
Q

Cosmic microwave background radiation

A

Microwave radiation received from all over the sky originating from after the Big Bang. As the universe has expanded the wavelength of the radiation has increased to just a faint microwave radiation with a peak wavelength corresponding to 2.7K

64
Q

The Big Bang Theory

A

The universe was created from a single point where all the mass and energy was situated. Since then the universe has expanded from a small dense point to a large and comparatively cool universe. Time and space were both created at the instance of the Big Bang

65
Q

Cosmological Principle

A

On a large scale the universe is isotropic and homogeneous and the laws of physics are universal

66
Q

Isotropic

A

The same in all directions

67
Q

Homogeneous

A

Of uniform density when considering a large enough volume

68
Q

Experimental evidence for the Big Bang

A

The microwave background at a temperature of 2.7K

69
Q

Experimental evidence for the expanding universe

A

The red shift and that galaxies further away are more redshifted than those that are closer

70
Q

Timeline of the universe

A

Universe is created and is incredibly small and dense and it begins to rapidly expand. Matter and Antimatter are formed in the form of quarks, leptons and photons. There is slightly more matter than antimatter and as they annihilate they leave a universe dominated by particles not antiparticles. Soon the universe cools enough so that quarks can come together and form protons and neutrons. Many high energy photons are released from matter-antimatter annihilation The temperature cools enough that helium and lithium nuclei can form and then electrons gradually become attached to the present protons. This is called decoupling as the universe becomes transparent and photons can move freely. This is where the photons that are now the CMB originate. Over billions of years the small irregularities that existed at the beginning of the universe become stars, planets and galaxies.

71
Q

What the big bang started

A

space-time (a 4 dimensional property that combines the three dimensions of space and the fourth dimension of time)

72
Q

Dark Matter

A

Matter which cannot be seen and does not emit or absorb electromagnetic radiation. It is detected by its gravitational effects

73
Q

Dark Energy

A

An energy that opposes the attractive force of gravity and exerts negative pressure, causing the expansion of the universe to accelerate

74
Q

How we can observe dark matter

A

The centripetal acceleration of spinning galaxies is much higher than would be expected from the mass that we can see. There must be more mass that we cannot see that is causing this

75
Q

Compostion of the universe

A

Dark Energy - 68%
Dark Matter - 27%
Ordinary Matter - 5%

76
Q

Hubbles law equation

A

v=Hd

77
Q

Inaccuracies with hubbles equation

A

Galaxies may be subject to local gravitational effects