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Physics: the unnecessary science > Stars and Cosmology > Flashcards

Stars and Cosmology Flashcards

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1
Q

Define planet.

A

An object in orbit around a star with:

  • A large enough mass to have its only gravity (round).
  • No fusion reactions.
  • Cleared orbit of most objects.
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2
Q

What is the difference between a planet and a dwarf planet?

A

Dwarf planets have not cleared their orbit of other objects.

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3
Q

Define planetary satellites.

A

A body in orbit around a planet (eg moon, satellite).

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4
Q

Define comet.

A
  • Small, irregular bodies of dust, ice and rock.

- Elliptical orbit around Sun.

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5
Q

Define solar system.

A

Planetary system with a star and at least one planet in orbit around it.

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6
Q

Define galaxy.

A

A collection of stars and interstellar dust and gas bound together by gravitational force.

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7
Q

What are the stages when forming a main-sequence star?

A
  • Nebula
  • Protostar
  • Main sequence
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8
Q

What are the stages when forming a neutron star?

A
  • Red giant
    • White dwarf
    • Neutron star
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9
Q

What are the stages when forming a black hole?

A
  • Red supergiant
  • Supernova
  • Black hole
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10
Q

How is a nebula formed?

A

Tiny gravitational attraction between dust and gas pull particles together, forming vast clouds.

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11
Q

How is a protostar formed?

A
  • Gravitational collapse accelerates as dust and gas get closer together.
  • Denser regions form and pull in dust and gas -> gains mass and is denser.
  • Hotter because GPE is transferred to thermal energy.
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12
Q

How is a star formed?

A
  • Extremely high pressures and temperature in the core to overcome electrostatic repulsion between H nuclei.
  • Nuclear fusion takes place (H nuclei forced together to make He nuclei)
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13
Q

Explain why nuclear fusion in the core of a star prevents further gravitational collapse.

A

Gas and radiation pressure released pushes against gravitation collapse.

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14
Q

Explain why nuclear fusion in the core of a star prevents further gravitational collapse.

A
  • Fusion produces gas and radiation pressure

- Pressure pushes outwards against gravitation collapse.

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15
Q

Describe the similarities and differences between planets and comets.

A

Similarities:

  • In orbit of a star
  • Elliptical orbit

Differences:

  • P: regular and C: irregular
  • Comets cannot clear their orbit of most objects
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16
Q

How does a main-sequence star remain in stable equilibrium?

A
  • Fusion produces gas and radiation pressure
  • Pressure pushes outwards against gravitation collapse.
  • Forces balance, maintaining equilibrium.
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17
Q

Explain why larger stars spend less time in their main-sequence phase.

A
  • Greater rate of fusion

- Depletes Hydrogen in a shorter time period

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18
Q

Name two factors that affect the stability of a star.

A
  • Size

- Mass

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19
Q

What is the astronomical unit (AU)?

A

Average distance from the Earth to the Sun.

1.50 X 10^11 m

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20
Q

What is a light-year (ly)?

A

Distance travelled by light in a vacuum in one year

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21
Q

How many arcminutes and arcseconds are there in 1°?

A

60 arcminutes

3600 arcseconds

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22
Q

Define parsec (pc).

A

The distance at which a radius of 1AU subtends an angle of 1 arcsecond.

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23
Q

How can you determine the value of 1pc with tan?

A

tan (1 arcsecond) = 1 AU / 1 pc

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24
Q

What is stellar parallax?

A
  • Technique used to determine the distance to stars less than 100pc from the Earth.
  • Relies on the apparent shift in position of nearby stars against the fixed background of distant stars.
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25
Q

Give the equation that relates parallax angle (p) and parsecs (d).

A

d (parsecs) = 1 / p (arcseconds)

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26
Q

Equation for intensity?

A

I = P / A

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27
Q

What is the Doppler effect?

A

The change in frequency and wavelength of waves from an object moving relative to the observer.

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28
Q

Why does a driver not experience any Doppler shifts of sound while driving?

A
  • No relative motion between driver and wave source.

- Freq and wavelength received are the same.

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29
Q

What is happening when a galaxy is blue-shifted?

A
  • Galaxy is moving towards the Earth.

- Smaller wavelength

30
Q

What is happening when a galaxy is red-shifted?

A
  • Galaxy is moving away.

- Stretched wavelength

31
Q

What is the Doppler equation?

A

∆ λ / λ = ∆ f / f = v / c

Only use when galaxy is moving a lot slower than speed of light

32
Q

Suggest how measuring the redshift of different sides of a rotating galaxy can find the speed of rotation.

A
  • Each side of the galaxy will have a different velocity so different redshift.
  • Side rotating away will have a greater velocity.
  • Use Doppler equation to find relative velocities.
  • Difference in velocities = speed of rotation
33
Q

What is Hubble’s law?

A

The recessional speed of a galaxy is directly proportional to its distance from Earth.
v ∝ d

34
Q

How does the velocity of a galaxy, Hubble’s constant and distance from galaxy link?

A

v = H₀ d

35
Q

Give two units for Hubble’s constant.

A

s^-1

km s^-1 MPc^-1

36
Q

Give two observations Hubble found that prove that the universe is expanding.

A
  • Light from galaxies is mainly red-shifted.

- Galaxies have a relative velocity away from Earth.

37
Q

What is the Cosmological principle?

A

The assumption that when viewed on a large enough scale, the Universe is homogeneous, isotropic and the laws of physics are universal.

38
Q

What does homogeneous mean?

A

Matter is uniformly distributed- density of the universe is the same

39
Q

What does isotropic mean?

A

The universe is the same in all directions to every observer, and it has no centre or edge.

40
Q

What is the Big Bang theory?

A

The universe started from a single point which then expanded.

41
Q

Give two pieces of evidence for the Big Bang theory.

A
  • Hubble’s law

- Cosmic microwave background radiation

42
Q

Does Hubble’s law support the Big Bang theory? Argue for and against.

A

For: Hubble’s law shows that the universe is expanding through the red-shift of light and it must have originated from a single point.

Against: the expanding universe can be explained through other theories so is a weak argument (more evidence needed).

43
Q

How does microwave background radiation support the Big Bang theory?

A
  • young, hot universe was saturated with high energy gamma photons -> as universe expanded, space stretched over time -> stretched wavelength of photons to microwave radiation
  • young, dense and hot universe expanded -> temperature of space dropped to 2.7K -> universe is a black body radiator -> peak wavelength at 2.7K corresponds to microwave radiation.
44
Q

How do you work out the age of the universe?

A

t = 1 / H₀

45
Q

What proof is there that the expansion of the universe is accelerating?

A

Light from supernova was less intense than expected

46
Q

What evidence is there for dark energy?

A

Light from supernova was less intense than expected -> expansion of universe is accelerating -> must be gaining energy from dark energy in order to accelerate

47
Q

Explain why dark matter has been predicted.

A
  • usually the velocity of an object decreases as distance increases from the centre.
  • stars do NOT slow down
  • mass of a galaxy is not concentrated at the centre
  • there must be dark matter
48
Q

What are the axis of the HR diagram?

A

x axis- average surface temperature (3000K to 40,000K)

y axis- luminosity

49
Q

What is the luminosity of the Sun?

A

3.85 x 10^26 W

50
Q

What are the equations linking energy and Planck’s constant?

A 
E = hf
E = hc / λ
51
Q

What is solar luminosity?

A

1 L☉ = 3.85 x 10^26 W

52
Q

What is an emission line spectrum?

A

Each element produces a unique emission line spectrum due to the unique set of energy shells.
(Black screen with coloured lines to represent wavelengths)

53
Q

What is a continuous spectrum?

A

All visible frequencies or wavelengths present.

Rainbow

54
Q

What is an absorption line spectrum?

A

Dark lines appear where the wavelengths of atoms are present due to the unique set of energy shells.
(Rainbow screen with dark lines)

55
Q

How is an emission line spectrum produced?

A
  • Gas atoms are heated.
  • Electrons are excited and move to a higher energy level.
  • Electrons drop down to lower energy levels, emitting photons with discrete frequencies that correspond to the element.
  • Each line on the emission line spectrum corresponds to a specific wavelength.
56
Q

How is an absorption line spectrum produced?

A
  • Light from a source that produces a continuous spectrum passes through a cooler gas.
  • Photons with the exact energy equal to the different energy levels are absorbed by the gas.
  • Electrons move to a higher energy level and excite the atom.
  • When specific wavelengths (photons) are absorbed, dark lines are created in the spectrum.
57
Q

How can you detect elements within a star?

A

Compare the line spectra of elements and the absorption line spectra of the stars.

58
Q

Why are some wavelengths on the absorption line spectrum of a star not detected?

A

Some photons are absorbed by cooler atoms on the surface so not detected.

59
Q

Describe the difference between continuous spectra and emission line spectra.

A

Emission only has certain frequencies and wavelengths present, depending on the unique set of energy levels.

Continuous shows all frequencies and wavelengths.

60
Q

Why do the wavelengths of the emission lines have the same wavelengths as dark absorption lines?

A
  • Each element has a unique set of energy levels.

- Photons released when the atoms are excited have a specific wavelength.

61
Q

What is the grating equation?

A

dsinθ = n λ

62
Q

What is a black body?

A

Object that absorbs all the electromagnetic radiation that shines onto it AND emits specific wavelengths at a specific temperature.

63
Q

What is Wein’s displacement law?

A

λ max ∝ 1 / T

64
Q

What is the equation that links Wein’s constant, absolute temperature and λ max?

A

Wein’s constant = 2.90 x 10^-3 m K

λ max T = Wein’s

65
Q

What is Stefan’s law?

A

The total power radiated per unit surface area of a black body is directly proportional to the fourth power of the absolute temperature.

L = 4 π r^2 σ T^4

66
Q

What does Stefan’s law show?

A

Luminosity is directly prop to:

  • radius
  • surface area
  • surface absolute temp
67
Q

What is Stefan’s constant?

A

5.67 x 10^-8 W m^-2 K^-4

68
Q

Describe the formation of the Sun and its probable evolution.

A
  • Dust and gas drawn together by gravitational forces.
  • GPE -> KE and temperature increases.
  • Hydrogen nuclei fuse together
  • A stable star is formed when gravitational pressure = radiation pressure
  • When the star runs out of hydrogen nuclei, the outer layers of the star expand.
  • Red giant is formed.
  • Core will become a white dwarf.
69
Q

Describe the evolution of the universe from the Big Bang to the present day.

A
  • At the start, the universe was an infinitely hot, dense singularity.
  • Universe expands and cools
  • There’s a sea of quarks and leptons (antimatter)
  • Universe cools so quarks combine to form particles like protons and neutrons.
  • Atoms then form
  • Gravitational force forms stars and galaxies.
  • Temperature of the universe is 2.7K and the universe is saturated with CMBR.
70
Q

Explain the process of nuclear fusion in the core of the Sun, refer to the conditions necessary.

A
  • There is electrostatic repulsion between protons
  • High temperatures of 10^7K needed
  • Protons travel at fast speeds and come close enough for the strong nuclear force to take effect.
  • Protons produce He NUCELI
71
Q

Name the two forms of energy produced in thermonuclear reactions.

A
  • Kinetic

- Electromagnetic

Physics: the unnecessary science (19 decks)

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