4.3 Our Place in the Universe Flashcards
Name the planets of our solar system, in order.
Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
What are comets made of? Describe their orbit.
Rock, ice and dust. Elliptical.
Explain how you would use radar to measure large distances from the Earth.
Send a short pulse of radio waves to be reflected back to Earth. Measure the time between emission and detection, t and use the equation 2d = c t to find d.
How can you measure the average speed of distant objects using radar?
Send two pulses repeated by a time interval. Find the difference in distance between the intervals and use it to calculate average speed.
State two assumptions made when using radar to measure large distances.
The radio waves are always travelling at c and the time taken to reach the object is equal to the time taken to return. (Also no relativistic effects)
Name two factors that affect the brightness of a star.
Its luminosity (energy given out in a given time) and its distance from us.
Give two factors that affect how bright a star appears to us.
Absolute magnitude and how far away it is.
What is apparent magnitude?
How bright a star appears from Earth.
Give the name and an example of objects that you can calculate the brightness of directly.
Standard candles, for example Cepheid variable stars.
Why are Cepheid variable stars standard candles?
Because their brightness changes in a predictable pattern.
What is the definition of an astronomical unit, AU?
The mean distance between the Earth and the Sun.
How many metres are there in an astronomical unit?
1.50 x 10^11
What is the definition of a light-year?
The distance that electromagnetic waves travel though a vacuum in one year.
How many AU in one light-year?
63000
How many metres are there in one light-year?
9.46 x 10^15
What is the distance between the Sun and the nearest star, Proxima Centauri in light-years?
4.2ly
What is the width of the Milky Way in light-years?
100,000ly
How can you calculate the size of the observable universe?
Multiply the age of the universe by the speed of light.
When a car is moving away from you, what happens to the sound waves and therefore the frequency of the sound heard from the car?
They are stretched out, giving a longer wavelength and there a lower frequency when they reach you.
How does speed of the emitter change the sound wave being observed?
The greater the speed, the greater the change in wavelength.
Explain the principle that allows us to use the Doppler effect to measure the speed of distant objects.
Atoms will emit and absorb radiation with same characteristic spectrum wherever they are. You can measure how much it has been shifted to find the object’s speed.
When is an object’s emission spectrum redshifted?
When it is moving away from the observer.
When is an object’s emission spectrum blueshifted?
When it is moving towards the observer.
Give the equation used to find the velocity of an object from the radiation it emits.
v / c = ∆λ / λ
Where is a continuous spectrum of radiation emitted from?
A very hot region of a star called the photosphere.
Why are characteristic absorption spectra created by each atom?
Because every atom absorbs different parts of the spectrum.
How can you use the pattern of absorption lines from a star to find its speed?
Record an absorption spectrum in a lab and compare the difference in wavelength.
What is time dilation?
Time runs at different speeds for observers moving relative to each other. This is only noticeable at speeds close to c.
What is the equation for “stopwatch time”, τ in terms of time measured by a moving observer, t?
t = 𝛾 τ
Give the equation used to find the relativistic factor, 𝛾.
𝛾 = 1 / √(1 - (v^2 / c^2))
What is recessional velocity proportional to?
Distance from Earth/
The more redshifted light from a distance star is…
The faster the star is moving away from us and the further it is away from us.
What is Hubble’s law?
v = H0 d (v is in km s^-1, d is in Mpc and H0 is in km s^-1 Mpc^-1)
How many metres in one Mpc?
3.09 x 10^22
What is the range of generally accepted values for Hubble’s constant, H0?
50 to 100 km s^-1 Mpc^-1
What are the SI units for H0?
s^-1
Why is Hubble’s law strong evidence for the Hot Big Bang Theory.
All objects are moving away from each other so they are likely to have originally come from one point.
What is cosmological redshift caused by?
Space expanding and light waves being stretched along with it.
What illusion is caused by the universe expanding uniformly?
The idea that the observer is at the centre of the universe.
What is the age of the universe in terms of Hubble’s constant?
1 / H0
What is the approximate age of the universe and the value of Hubble’s constant to go with it?
13.7 billion years. 75 km s^-1 Mpc^-1
What is the Hot Big Gang Theory?
The theory that the universe started off very hot and dense and has since been expanding.
How does the HBBT predict Cosmic Microwave Background Radiation?
It predicts that lots of EM radiation was produced in the very early universe. It should still be observed today, with it’s wavelengths redshifted to the microwave region.
Give some properties of CMBR.
Continous spectrum corresponding to 2.73K. Isotropic and homogenous (same in every direction). Small fluctuations in temperature. Doppler shift caused by the Great Attractor.
What are the tiny fluctuations in CMBR temperature caused by and what do they mean?
Tiny energy density variations which seeded galaxy formation.
How does the HBB model account for the amount of helium in the universe?
The early universe was hot enough for hydrogen atoms to fuse.
