Astrophysics Flashcards
(50 cards)
1
Q
Nebula
A
- A cloud of dust & gas in space (largely hydrogen)
- Collapses under gravitational forces
- GPE transfers to KE & Thermal energy, increasing temp
- Moves to the main sequence when temp & pressure are high enough for hydrogen fusion to begin
2
Q
Main sequence
A
- Hydrogen fuses to helium in the core
- Stable star, radius does no change, as weight is balanced by thermal expansion (pressure due to fusion)
- High mass stars stay on the main sequence for less time than low mass main sequence
- When the star (begins) to run out of hydrogen, the core collapses
- Moves to the red giant / red supergiant when temp & pressure are high enough for helium fusion to begin
3
Q
Red giant / red supergiant
A
- Helium fuses to heavier elements in the core
- Stable star, radius does no change, as weight is balanced by thermal expansion (pressure due to fusion)
- Lower surface temp than the main sequence phase
- When the star (begins) to run out of helium, the core collapses
4
Q
White Dwarf
A
- When a red giant (begins) to run out of helium, the core collapses
- Leaving behind a nebula & a white dwarf
- A white dwarf is hot, dense & no longer fusing
5
Q
Supernova
A
- When a red supergiant (begins) to run out of helium, the core collapses
- Temp & pressure of the core increases until fusion of heavy elements begins
- The sudden release of energy leads to an explosion
- The explosion leaves behind a nebula & a neutron star, or a black hole for the highest mass stars
6
Q
Describe the life cycle of a massive star, beginning from a cloud of dust & gas
A
- Star forms from a nebula drawn together by the force of gravity (to form a protostar)
- As the dust & has are drawn together, star gets denser & hotter
- Eventually temperature gets hot enough for hydrogen nuclei to undergo nuclear fusion to form helium & star becomes a main sequence star
- When the star runs out of hydrogen, it expands & turns red & forms a red supergiant
- Red supergiant explodes in a supernova
- Supernova throws outer layer of dust & gas into space, leaving behind either a very dense neutron star
- Or an even denser black hole depending on their size
7
Q
Describe how a main sequence star become a white dwarf (4)
A
- Hydrogen fusion stops (in core)
- Core collapses
- Core temperature of star increases
- Star expands to become a red super giant
- Surface temperature of star decreases
- Helium fusion begins
- White dwarf formed when helium fusion stops
- The rest of the star is released as a planetary nebula
8
Q
Describe how a main sequence star is formed from a nebula (3)
A
- Nebula is a cloud of dust & gas
- Gravity causes cloud to collapse
- Temperature of cloud increases
- Creating a protostar
- Main sequence star created when fusion starts
9
Q
What is the universe?
A
A large collection of billions of galaxies
10
Q
What is a galaxy?
A
A large collection of billions of stars
11
Q
Where is the solar system?
A
In the Milky Way Galaxy
12
Q
When does gravitational field strength (g) increase?
A
- When the mass of the source increases.
- When the distance from the source decreases
13
Q
How does gravitational field strength compare between Earth, the Moon, and Callisto?
A
- gₑₐᵣₜₕ > gₘₒₒₙ (Because Earth has a much greater mass.)
- gₘₒₒₙ > g꜀ₐₗₗᵢₛₜₒ (Because Callisto has a larger radius than the Moon.)
- The Sun’s mass is much higher than Earth’s, but we feel Earth’s gravity more because we are closer to its center.
14
Q
Why do objects in orbit move in circles?
A
Objects in orbit cannot escape gravity
15
Q
How does direction behave in orbit?
A
Direction is different at all points.
16
Q
How does velocity behave in orbit?
A
Velocity is different at all points.
17
Q
How does speed behave in orbit?
A
Speed is constant at all points.
18
Q
When comets orbit closer to the sun..
A
- Gravitational field strength is higher
- so force on comet is higher
- so orbit is tighter
19
Q
When comets orbit far from the sun..
A
- Gravitational field strength is higher
- so force on comet is higher
- so orbit is tighter
20
Q
What is the orbit of a comet?
A
orbits are elliptical highly
21
Q
What is the orbit of a planet & moon?
A
all orbits are circular or elliptical (slightly squashed circle)
22
Q
Equation for orbital speed:
A
orbital speed = 2(PI) X orbital radius / time period
- orbital speed (m/s)
- orbital radius (m)
- time period (s)
23
Q
How can stars be classified?
A
- Stars can be classified depending on their colour
- The temperature of the stars determines its colour
- The more blue the star is, the hotter it is
24
Q
What do all stars emit?
A
Visible light
25
For a star to remain stable:
Thermal expansion (gas & radiation pressure) = weight
26
For a star to expand (main sequence to red giant):
Thermal expansion > weight
27
For a star to contract (red giant to white dwarf):
Thermal expansion < weight
28
What is thermal expansion?
- the outward force from a star’s heat that balances gravity.
29
PAPER 2 What is absolute magnitude?
A measurement of the brightness of a star at a standard distance (10 Parsecs / 32.6 light years)
30
PAPER 2 What is the relationship between absolute magnitude and brightness?
Absolute magnitude is a reverse scale: 5 = very dim, 1 = dim, -1 = bright, -5 = very bright
31
PAPER 2 Hertzsprung-Russell (HR) diagram
-used to classify stars
-x-axis = temperature or colour (scale backwards)
-y-axis = absolute magnitude or luminosity
32
PAPER 2 Top right on HR diagram
Red giants
33
PAPER 2 Bottom left on HR diagram
White dwarfs
34
PAPER 2 Middle on HR diagram
Main sequence
35
PAPER 2 Describe the past evolution of the universe & the main arguments in favour of the Big Bang theory
- The universe began 14 billion years ago
- Big bang created energy, space & time
- followed by an expansion & cooling of space
- The tiny hot dense point that it began with is called a singularity
36
PAPER 2 Evidence for the Big Bang:
- Cosmic Microwave Background Radiation
- Red Shift of galaxies
37
PAPER 2 Cosmic microwave background (CMB)
- It is background radiation from all over the Universe &
- This implies all parts of the universe were in contact at one point
- These used to be gamma rays, but the wavelength has increased over time
- This demonstrates that the universe was significantly hotter a long time ago
38
PAPER 2 Stationary wave source..
emits at a constant wavelength & frequency
39
PAPER 2 If a source moves towards an observer…
Wavelength decreases, frequency increases
40
PAPER 2 If a source moves away from an observer..
Wavelength increases, frequency decreases
41
PAPER 2 Equation for redshift:
Redshift = Change of wavelength/Reference Wavelength = Speed of a galaxy/Speed of light
42
PAPER 2 The greater the distance from Earth…
- The faster a galaxy moves
- The greater the red shift
43
PAPER 2 Explain why the red-shift of galaxies provides evidence for the Universe expanding
- In general, the further away a galaxy is, the greater the redshift
- The greater the redshift, the faster the galaxy is moving
- Therefore as distance increases, speed increases
- This implies expansion from a single point (like an explosion)
44
What orbits the sun?
Planets, comets & asteroids
45
What orbits a planet?
The Moon
46
What orbits the Earth?
Artificial satellites
47
List similarities in the orbital motion of planets.
Similarities:
- Slightly elliptical orbits
- Orbit in the same plane
- Move in the same direction
48
List differences in the orbital motion of planets
Differences:
- Different distances from the Sun
- Different orbital speeds
- Different orbital times
🔹 The further from the Sun → slower speed and longer orbit time
49
Describe the motion of moons around planets.
- Orbit planets in circular paths
- Some planets have multiple moons
- Closer moons → faster and shorter orbits
50
How are comet orbits different from planets?
- Orbits are highly elliptical or hyperbolic
- Speed increases near the Sun
- Speed decreases far from the Sun
- May not orbit in the same plane
- May orbit in the opposite direction
