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SL Physics > Option D: Astrophysics > Flashcards

Option D: Astrophysics Flashcards

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1
Q

What is our solar system made up of?

A
  • A star (the Sun)
  • 8 planets
  • Dwarf plants (eg. Pluto)
  • Asteroids
  • Comets
  • Moons
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2
Q

List the planets in our solar system

A

My Very Educated Mother Just Served Us Noodles
In increasing distance from the Sun
- Mercury
- Venus
- Earth
- Mars
- Jupiter
- Saturn
- Uranus
- Neptune

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3
Q

Give the names and information on “The Rocky Planets”

A

1) Mercury
- Closest to the Sun
- The smallest planet
2) Venus
- Hottest planet (approx. 500°C)
3) Earth
4) Mars
- Red bc of iron oxide on the surface
- Largest volcano in our Solar System

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4
Q

Give the names and information on “The Gas Planets”

A

1) Jupiter
- The biggest planet
2) Saturn
- Rings made of ice and dust
3) Uranus
4) Neptune
The coldest planet (-245°C)

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5
Q

Compare asteroids and comets

A

Asteroids are lumps of rock (i.e. mini planets) whereas comets are made of ice and dust. They start to melt when they come close to the Sun which makes a tail. Comets have a more elongated orbit.

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6
Q

Define a planetary system

A

A group of planets orbiting a star.

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7
Q

Define a star

A

A star is a luminous sphere of plasma held together by its gravity.

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8
Q

Define binary stars and give their classification

A

Binary stars are two stars orbiting a common centre.
They are classified visually, by eclipsing (i.e. periodic variations in brightness when one star obscures the other) or spectroscopically (changes in the wavelength of light received from the stars due to the Doppler effect).

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9
Q

Define a stellar cluster and compare the two types

A

A stellar cluster is a close group of gravitationally bound stars, gas and dust (within a galaxy).

There are two types: globular and open.
- Globular stellar clusters contain 10^4-10^5 stars, are symmetrically arranged and contain old stars.
- Open stellar clusters contain several hundred stars, are irregularly arranged and contain young stars.

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10
Q

Define a galaxy

A

A galaxy is a large-scale collection of stars, gas and dust held together by gravity. Can be spiral or elliptical.

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11
Q

Define a galactic cluster

A

A galactic cluster is a group of galaxies gravitationally bound together, orbiting a common centre of gravity.

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12
Q

Give the arrangement of groups beyond the solar system

A

Stars
Stellar clusters
Galaxy
Galactic cluster
Galactic supercluster

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13
Q

Define nebulae

A

An intergalactic (between galaxies) cloud of dust and gas.

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14
Q

Define constellations

A

Patterns of stars in the sky that are NOT gravitationally bound. They are a human construct and the stars are not necessarily close to each other.

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15
Q

Define a light year

A

The distance light travels in one year.

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16
Q

What is stellar parallax used for?

A

To determine the distance of stars that are relatively close to Earth.

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17
Q

What is the limitations of stellar parallax?

A

If the star is too far away from Earth (> 100pc) then the parallax angle becomes too small to be measured accurately. The uncertainty of the measurement becomes greater than the angle itself.

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18
Q

How is the parallax angle measured?

A

The apparent position of the star is recorded six months apart. The parallax angle is measured from the angular shift of the star relative to distant background stars.

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19
Q

Define the parallax angle

A

Half of the observed angular displacement of the star.

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20
Q

Define the parsec

A

The distance at which the angle subtended by the radius of the Earth’s orbit is one arc-second.

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21
Q

What can the stellar spectra tell us?

A
  • What elements make up the star (absorption spectrum)
  • The temperature of the star (using the wavelength max formula)
  • How the star is moving (Doppler shift)
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22
Q

Why are main sequence stars stable?

A

During this phase, stars are stable because there is an equilibrium between outwards radiation pressure and inwards gravitational pull.

23
Q

Define luminosity

A

The total amount of energy emitted per second by a star (W). Depends on the temperature and size of the star (relates to formula).

24
Q

Define apparent brightness

A

The amount of energy received per second per unit area by an observer. (Wm-2)

25
Q

What type of stars does the mass-luminosity relationship apply to?

A

Only main sequence stars.

26
Q

Give information about Red Giants/Supergiants

A

Red Giants:
- Red in colour (cool)
- Large mass
- Large surface area
- At a late stage in a star’s life cycle

Red Supergiants are as above but bigger.

27
Q

Give information about White Dwarfs

A
  • White in colour (hot)
  • Small mass
  • Small surface area
  • Final stage in a star’s life cycle
  • No longer undergoing fusion (cooling down)

*They have a max mass of 1.4M (Chandrasekhar limit)

28
Q

Give information about cepheid variables and the process by which they expand/contract

A

Information:
- Unstable
- Regular variations in luminosity (and brightness) due to a periodic variation in surface area
- Late stage in the life cycle of some stars

Process:
- Helium atoms absorb radiation (from fusion)
- Helium atoms heat up and expand
(radiation pressure > gravitational force)
- Helium atoms cool as it expands
- So the star contracts (as gravitational force > radiation pressure)
- And the cycle begins again

29
Q

How are stars formed?

A

Over millions of years, gas clouds (nebulae) come together due to gravity. As the particles get closer they lose potential energy and gain kinetic energy, therefore increasing in temperature. The temperature causes ionization of the gases in the nebulae and light is emitted (this is the formation of a protostar).

30
Q

How are main sequence stars formed?

A

Further contraction of a protostar increases the temperature of the core and nuclear fusion (H->He) occurs. Thus, a main sequence star is formed. This is a stable phase as inwards gravitational pull = outwards radiation pressure.

31
Q

How are red giants formed?

A

Eventually all the H atoms in the core of main sequence stars are converted to He atoms. The core contracts and H fusion continues in the outer layers. The star expands and a red giant is formed. The contracting core gets hotter and He undergoes fusion to C and O. Over time all the He will be used up. The eventual outcome depends on the mass of the star.

32
Q

What happens to small stars after they become red giants?

A

Small stars (<4 solar masses)
The outer layers are ejected as a planetary nebula. The hot core left behind is a white dwarf. Over time, it will cool down as there is no more heat being produced in nuclear fusion.

33
Q

What happens to medium stars after they become red giants?

A

Medium stars (4-8 solar masses)
Further stages of fusion to form Ne, Na and Mg. The outer layers are ejected as a planetary nebula. The hot core left behind is a white dwarf. Over time, it will cool down as there is no more heat being produced in nuclear fusion.

34
Q

What happens to large stars after they become red giants?

A

Large stars (>8 solar masses)
More elements are formed in nuclear fusion. The outer layers keep expanding forming a red supergiant. Eventually the core is all iron. Once this occurs, the core contracts quickly and the outer layers are ejected as a supernova. The remaining core forms a neutron star or a black hole.

35
Q

What is the Chandrasekhar limit?

A

White dwarfs have a maximum mass of 1.4 solar masses. This is because of electron degeneracy pressure. Above the mass, gravity would overcome electron repulsion and the star would collapse further.

36
Q

What is the Oppenheimer-Volkoff limit?

A

Neutron stars have a maximum mass of 3 solar masses. This is because of neutron degeneracy pressure. Above this mass gravity overcomes neutron repulsion and the star collapses further forming a black hole.

37
Q

What does the mass of the star determine?

A

Its eventual fate.

38
Q

On a Hertzprung-Russell Diagram where are White Dwarfs, Main Sequence and Red Giants/Supergiants drawn?

A

White Dwarfs
10^-2 to 10^-4 Luminosity

Main Sequence
10^4 to 10^-4 Luminosity

Red Giants
10^2 to 10^3 Luminosity

Red SuperGiants
To the right end of Red Giants at 10^6 Luminosity.

39
Q

What does R/Ro tell us?

A

R represents the age of the universe now and Ro represents the age of the universe then. It tells us by what factor the universe has increased in size by since light was originally emitted.

40
Q

What was Newton’s model of the universe?

A

Newton believed that the universe was:
- infinite in space
- infinite in time
- uniform
- static

Otherwise it would collapse under its own gravitational force.

41
Q

What is Olbers’ Paradox?

A

If Newton’s model of the universe is correct and the universe is infinite there would be an infinite number of stars so the sky would be infinitely bright. But it’s not, so a different model of the universe is needed.

42
Q

What is the Big Bang theory?

A

The Big Bang theory is the idea that 15 billion years ago everything in the universe was located at one point and it exploded. Since then the universe has been expanding, creating space as it does.

43
Q

What is evidence for the Big Bang theory?

A
  • Light produced by galaxies was red-shifted (the absorption lines due to the elements in stars were found at longer wavelengths than expected). This shows that galaxies are moving away from Earth.
  • Cosmic microwave radiation was found to be coming towards Earth from every direction. This radiation had a wavelength that corresponded to that emitted by an object of 2.76K. Calculations show that the temperature of the universe would have cooled to 2.76K after 15 billion years of expansion.
44
Q

What does the Big Bang theory show?

A

The sky is dark because. . .
- Light from very distant stars has not reached Earth yet
- As galaxies are moving away from Earth and their light is red-shifted, it is shifted into infra-red which is not visible to our eyes

This provides a resolution to Olbers’ Paradox as it suggests that a universe is neither infinite in time nor space.

45
Q

What does a positive and negative z value show?

A

A positive z value means there is a redshift (i.e. the galaxy is moving away from Earth).

A negative z value means there is a blueshift (i.e. the galaxy is moving towards Earth).

46
Q

What is the relationship between recession speed and distance from Earth?

A

Recession speed is directly proportional to the distance from Earth (i.e. a linear line on a graph).

47
Q

How do we find Hubble’s constant?

A

By finding the gradient of a recession speed-distance graph.

48
Q

How to convert kms^-1 Mpc^-1 to years

A

1Mpc -> pc -> ly -> m -> km
Times value by 1/Ho.

Value in seconds so ÷ by (60x60x24x365) to get in years.

49
Q

Define the unit AU

A

The mean distance from the centre of the Earth to the centre of the Sun.

50
Q

Why are cepheid variables known as standard candles?

A

Because the luminosity of a Cepheid variable can be determined from its period. If its apparent brightness is measured then its distance from Earth can be calculated. If a distant galaxy contains a Cepheid variable star then the distance to that galaxy can be found.

51
Q

What type of graph are Cepheid variables?

A

Shark fin curve with a rapid expansion and a slower contraction.

52
Q

What are Type 1a supernovae?

A

Type 1a supernovae is a type of supernova that occurs in binary star systems where one of the stars is a white dwarf. All Type 1a supernovae have a similar max luminosity. If their apparent brightness is measured, the distance to the supernovae can be calculated.

53
Q

What did evidence from Type 1a supernovae show?

A

The Big Bang theory assumed that the rate of expansion of the universe was constant. However, in the late 1990’s distance measurements calculated using Type 1a supernovae showed that the distance to galaxies was further than was thought, suggesting that the rate of expansion of the universe is increasing.

54
Q

What is dark energy?

A

The outwards accelerating force thought to be counteracting the inwards gravitational pull on the universe.

SL Physics (9 decks)

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