Chapter 12 - Our place in the universe Flashcards
What does our solar system consist of?
The sun and all objects which orbit it
What shape is the orbit of a comet?
Elliptical
How can radar be used to work out the distance to a stationary object in the solar system?
-A short pulse of radio waves (travelling at the speed of light) is sent out and bounces off the object
-The pulse returns and is detected after an interval of time
-The distance is worked out using speed x time
d = 1/2 ct
Half the time interval is used so it is just the time there and not there and back
How can radar be used to work out the relative velocity between the Earth and an object in the solar system?
- Two radar pulses are reflected off an object with a known period of time between pulses
- The distance of the object at the time of each pulse is found using d = 1/2 ct
- Speed = distance/time is used with change in distance and change in time to find the relative velocity of the object
What assumptions need to be made for radar ranging?
- The speed of light is always constant
- The time taken to reach the object and the time taken to return are the same
For what objects can radar ranging be used?
Objects within the solar system
For what relative velocities can radar ranging be used?
Velocities much less than the speed of light
otherwise there will be relativistic effects
What two things does the brightness of a star depend on?
The luminosity and the distance from Earth
What is luminosity?
How much light energy is given out in a given time period
What is the term for how bright a star looks when seen from Earth?
What does this depend on?
Apparent magnitude
The absolute magnitude and the distance from Earth
For what kind of objects can you use brightness to work out distance?
Standard candles
Objects whose brightness can be calculated directly
What is a standard candle?
An object whose brightness can be calculated directly
Give an example of a standard candle?
Cepheid variable stars
their brightness changes with a known pattern
What units are often used to measure distances within the solar system?
Astronomical units
What is the symbol for astronomical units?
AU
What is an astronomical unit?
The mean distance from the Earth to the sun
What is the symbol for a light year?
ly
What is a light year?
The distance that electromagnetic waves would travel through a vacuum in one year
When doppler shift happen?
When there is a relative velocity between an observer and a source of waves
What happens to the frequency and wavelength when a source of radiation moves towards you?
Higher frequency, lower wavelength
What happens to the frequency and wavelength when a source of radiation moves away from you?
Lower frequency, higher wavelength
What is pitch?
Frequency
What is the formula which relates speed, wavelength and frequency?
v = λ f speed = wavelength x frequency
How can Doppler shift be used to measure the velocity of an object relative to Earth?
The spectrum of the radiation emitted by the object is compared to radiation emitted by the same atoms on Earth
The amount by which the spectrum is shifted gives the doppler shift and so the velocity can be calculated
What is the equation for finding velocity from a doppler shift?
When can this equation be used?
v/c = Δλ/ λ
when v
When does redshift happen?
When an object is moving away from the Earth
When does blueshift happen?
When an object is moving towards the Earth
What is a star’s absorption spectrum?
Radiation is emitted in a continuous spectrum, but atoms in the atmosphere of the star absorb specific wavelengths, depending on what kind of atoms they are.
This produces dark absorption lines in the spectrum.
There is a characteristic pattern for each atom and so the composition of the stellar atmosphere can be found
At what speeds does time dilation happen?
Close to the speed of light
What happens to time for two objects moving relative to each other?
What is this effect called?
Time will run at different speeds for the two different objects
(but this is only noticeable when the relative velocity is close to the speed of light)
RELATIVISTIC TIME DILATION
What is the name for the time interval measured by a stationary observer?
Proper time/wristwatch time
t0 or Τ (tau)
If there is a stationary observer and an observer moving at close to the speed of light, who will measure the longest time interval for the moving observer?
The person moving at close to the speed of light
Why can the effects of relativistic time dilation be ignored when v
The effects aren’t noticeable because γ is very close to 1
What is the equation for relativistic time dilation?
t = t0 x γ
What is the symbol for the relativistic/gamma factor?
γ (gamma)
What is the relativistic factor?
γ = 1 / √(1 - v^2/c^2)
What is the recessional velocity?
How fast a galaxy is moving away from Earth
What is recessional velocity proportional to?
Distance from Earth
What is Hubble’s law?
v = H0 x d (H0 = hubble's constant)
What is the symbol for Hubble’s constant?
H0
What are the units of v, recessional velocity?
kms^-1
ms^-1 in SI units
What are the units of d, distance from Earth?
Mpc (megaparsecs)
m in SI units
What is the symbol for a megaparsec?
Mpc
How many metres are there in one megaparsec?
3.09 x10^22
What are the units of H0, the hubble constant?
kms^1Mpc^-1
What are the SI units of the hubble constant?
s^-1
What value does the rate of expansion of the universe depend on?
Hubble’s constant
Is the universe expanding uniformly?
yes
What is the difference between normal redshift and cosmological redshift?
Cosmological redshift is due to the expansion of space which radiation is moving through (as the universe expands), not due to a relative velocity
If the universe has been expanding at the same rate for its whole life, how old is it?
t = 1 / H0
Why can the exact age of the universe not be worked out?
The exact value of the Hubble constant is not known
Approximately how old is the universe?
13 billion years old
What is the limit on the size of the observable universe?
A sphere with radius (age of the universe) light years
What is the hot big bang (HBB) theory?
The universe started off very hot and very dense (perhaps as an infinitely hot, infinitely dense singularity) which has been expanding ever since
What is cosmic background radiation?
Electromagnetic radiation produced in the very early universe which can still be detected
It has been redshifted to the microwave region and is uniform in every direction
What are the properties of cosmic background radiation?
- continuous spectrum corresponds to temperature 2.7K
- isotropic and homogeneous (the same intensity in every direction)
- Microwave
What causes tiny fluctuations in the temperature of cosmic background radiation?
What are they needed for?
(very small) energy density variations in the early universe
Needed for initial ‘seeding’ of galaxy formation
How does the hot big bang model explain the large abundance of helium in the universe?
The hot dense universe must have been hot enough for hydrogen fusion to produce helium
