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Physics OCR A'level > Ch19&20: Astrophysics and Cosmology > Flashcards

Ch19&20: Astrophysics and Cosmology Flashcards

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1
Q

Define a planet.

A

An object with enough mass for their gravity to make them spherical, no nuclear fusion and with an orbit clear of other objects.

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2
Q

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

A

A dwarf planet is typically smaller with an orbit not clear of other objects.

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3
Q

What are planetary satellites?

A

Bodies that orbit a planet.

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4
Q

Define an asteroid.

A

An object too small and uneven to be a planet with a near circular orbit.

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5
Q

What is the difference between asteroids and comets?

A

Comets have elliptical orbits, and are irregularly shaped. Asteroids are more spherical and have circular orbits.

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6
Q

Define a solar system.

A

Systems containing stars and orbital objects.

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7
Q

Define a galaxy.

A

A collection of stars, interstellar dust, and gas held together by mutual gravitational force.

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8
Q

Define emission spectra.

A

A set of specific frequencies of electromagnetic radiation, visible as bright lines on a dark spectrum, created by excited atoms giving off specific photons as they transition between energy levels. Every element has a unique line spectrum.

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9
Q

Define a continuous spectrum.

A

A spectrum in which all visible frequencies of light are present.

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10
Q

Define absorption line spectra.

A

A set of specific frequencies of electromagnetic radiation, visible as dark lines on a continuous spectrum, they are absorbed by electrons moving between energy levels in the form of photons.
Every element has a unique line spectrum.

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11
Q

How many arcseconds in 1 degree?

A

3600

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12
Q

What quantity do arc seconds measure?

A

Angles.

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13
Q

What is Wein’s law?

A

The blackbody radiation curve for different temperatures peaks at a wavelength inversely proportional to the temperature of the object.

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14
Q

What is Stefan’s law?

A

Total radiant heat energy emitted by a blackbody is proportional to the 4th power of its absolute temperature.

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15
Q

What is the equation for Stefan’s law?

A

L=4πr²T^4σ

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16
Q

Define a light year.

A

The distance travelled by light in one year. Speed of light x Seconds in a year

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17
Q

What is 1 Au?

A

The average distance from the Earth to the sun. 1.5 x 10^11m

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18
Q

What is the first stage in the life cycle of a star?

A

Nebula, Collection of dust, gas and rock.

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19
Q

How is a protostar formed from a nebula?

A

Gravity clumps the nebula together, the gpe turns into heat at the centre of the protostar.

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20
Q

What is the approximate make up of a nebula?

A

75% hydrogen, 25% helium.

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21
Q

What is the requirement for a protostar to become a main sequence, and why?

A

When the temperature is above 15 Million Kelvin. This is the minimum energy needed to overcome electrostatic force for nuclear fusion.

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22
Q

What keeps a main sequence star stable?

A

The balance of Gravitational forces inwards and radiation pressure outwards.

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23
Q

What happens to main sequence stars below a mass of 10☉as they run out of hydrogen?

A

They become red giants. The reduction in energy causes the star to begin to collapse, this increase in pressure allows fusion to take place in shells.

24
Q

What happens to main sequence stars above 10☉?

A

They become red supergiants, fusing heavy elements up to and including iron.

25
Q

What happens to a red giant when fusion stops?

A

Outer layers escape as new nebulae and the hot dense core becomes a white dwarf.

26
Q

What is the maximum mass of a white dwarf?

A

1.44M☉

27
Q

Why is there a limit on mass of White dwarf, what is this limit called?

A

The chandrasekhar limit. If mass is any higher, electron degeneracy pressure will not be great enough to sustain the star from collapse.

28
Q

What causes supernova of a red supergiant?

A

The pressure of fusion in the core no longer withstands the gravitational pressure, and the star collapses.

29
Q

What happens to a star after supernova?

A

Either becomes a neutron star, for masses under 3M☉ or a blackhole, for masses over 3M☉.

30
Q

What is a neutron star?

A

A star made almost entirely of neutrons, with a mass of around 2M☉ and a diameter of around 10km.

31
Q

What is a blackhole?

A

Very massive objects with gravitational fields so strong, light cannot escape them.

32
Q

What are the two axis of the Hertzsprung-Russell diagram? What is irregular about them?

A
  • Luminosity (y) against Temperature (x)

- Temperature decreases along axis

33
Q

What is the cosmological principle?

A

The universe is isotropic and homogenous.

34
Q

Define Isotropic.

A

Looks the same in all directions when viewed from any point.

35
Q

Define homogenous.

A

Uniformly distributed with the same density over large volumes.

36
Q

What is the doppler effect?

A

When wavelengths of moving objects appear to change because they are moving toward/away from us.

37
Q

Define ‘red shifted’.

A

Wavelength is appearing to increase, an object is moving away from us.

38
Q

Define ‘blue shifted’.

A

Wavelength is appearing to decrease, the object is moving toward us.

39
Q

How can the age of the universe be determined using Hubble’s law?

A

T = 1/Ho

40
Q

What are the two main pieces of evidence for the Big Bang theory?

A
  • Red Shift

- Microwave background radiation.

41
Q

How can a parsec be derived?

A
  • 1 Parsec = 1 Au/tan(1 arcsecond)
  • tan(small angle)=angle
  • so, 1 Pc=1 Au/1 Arcsecond
42
Q

What is the equation used to determine distances using stellar parallax? What units are used?

A

d = 1/p

d: parsecs
p: arcseconds

43
Q

Describe the state of the universe at the time of the Big Bang.

A

Universe is a hot, dense, singularity.

44
Q

Describe the state of the universe at a time of 10^-35 seconds.

A

Inflation, no matter yet, only gamma photons and electromagnetic radiation. Temperature is around 10^28 K

45
Q

Describe the state of the universe at a time of 10^-6s.

A

First fundamental particles gain mass through a mechanism involving the Higgs Boson.

46
Q

Describe the state of the universe at a time of 10^-3 seconds.

A

First hadrons created from quarks, most mass in universe created through pair production.

47
Q

Describe the state of the universe at a time of 1 second.

A

Creation of matter stops, temperature of around 10^9K

48
Q

Describe the state of the universe at a time of 100s

A

Rapid expansion continues, protons and neutrons form to create helium and deuterium nuclei, no heavier elements than lithium or beryllium. About 25% of all matter is helium nuclei.

49
Q

Describe the state of the universe at a time of 380,000 years.

A

Cool enough for atoms to form, nuclei capture electrons. Electromagnetic radiation from this stage is what is detected today as background radiation.

50
Q

Describe the state of the universe at a time of 30 million years.

A

First stars appear, through fusion they create the first heavier elements.

51
Q

Describe the state of the universe at a time of 200 million years.

A

Our galaxy forms.

52
Q

Describe the state of the universe at a time of 9 billion years.

A

Our solar system forms, then our sun forms, then Earth around 2 billion years after.

53
Q

At what age of the universe do the first particles gain mass?

A

10^-6 seconds.

54
Q

At what age of the universe do atoms form?

A

380,000 years.

55
Q

Define intensity.

A

Energy per unit area.

56
Q

State Hubble’s law.

A

Recessional speed of a galaxy is proportional to its distance from us.

57
Q

State two properties of a black hole.

A
  • Very dense and very small

- Traps light, slows time, high escape velocity.

Physics OCR A'level (19 decks)

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