Our Place in the Universe Flashcards
What are the orbit-shapes of comets?
Ellipsis
What are the orbit shapes of planets?
Circles
How can a distance be measured using radar?
By firing a brief pulse of radio waves at the object, then measuring the amount of time it takes for them to return. The distance from the earth to the astronomical object will be equal to half the speed of light multiplied by this time.
How can the velocity of an astronomical object be measured using radar?
By measuring the distance of the earth from the object twice, and calculating half the time difference between the measurements. The difference in distances of the earth from the object over this time is equal to object’s velocity.
What are the assumptions of the radar astronomical distance measuring technique?
The speed of the radio waves is constant. The time taken for the signal to reach the object is the same as the time taken for it to return. That the speed of the object is much less than the speed of light.
What two factors is the brightness of an object dependent upon?
Luminosity (Light energy per second (J/s)) and distance
What is the apparent magnitude of a star?
How bright it appears in observation.
What is the absolute magnitude of a star?
How bright the star actually is.
How can the distance of a galaxy be calculated using star brightness?
By observing the behaviour of cephid variable stars within that galaxy.
What is 1 AU in metres?
1.5 x 1011m
What is 1 light year in metres?
What is the approximate diameter of the Milky Way?
100 000 light years
9.46 x 1020m
What is the distance to Earth’s nearest star, proxima centauri?
It is 4.2 light years to proxima centauri.
What is the distance to the edge of the observable universe?
14 x 109 light years
What is the doppler effect?
The shifting of a wave’s wavelength as the source of the wave and an observer move relative to one another.
How is it possible to measure the velocity of an object relative to the earth using doppler shift?
By comparing the absorption spectrum of the object with a spectrum of a stationary object of the same elemental composition. The difference in wavelength between these two spectra, over the original (unshifted) wavelength of the radiation is equal to the ratio between the velocity of the object and the speed of light (v/c = Δλ/λ).
If the wavelength of a wave becomes shorter, is the wave being blueshifted or redshifted?
Blueshifted ( Blue ~475nm )
If the frequency of a wave is decreased by wave-shifting, has the wave been redshifted or blueshifted?
What is blueshift?
The post-emission decrease in a wave’s wavelength.
What is redshift?
The post-emission increase in a wave’s wavelength.
What part of a star is radiation emitted from?
The photosphere - a very hot region.
How can the composition of a star be deduced?
By comparing the absorption spectra of that star (atoms in the star’s atmosphere absorb some of the radiation emitted in the photosphere) with each element’s absorption spectrum.
What is the assumption of doppler-shift distance measurements? Why does this assumption need to be made?
That the velocity of the object is much less than the speed of light. This assumption needs to be made because of special relativistic effects (namely, time dilation).
What is time dilation?
The difference in the elapsed time measured by one observer moving relative to another. The moving observer is determined to be experiencing time more slowly than the ‘stationary’ observer. The stationary observer is as determined by an inertial reference frame.
What is it that suggests galaxies are moving away from the earth? Are the galaxies actually getting further away?
Redshifting of radiation emitted by those galaxies. The galaxies are getting further away, although it is not because they are moving away from the earth; it is because the space between them and the earth is expanding.
What is the relationship between recessional velocity and galaxy distance?
v = H0d
Where:
v = recessional velocity [kms-1]
H0 = Hubble’s constant [kms-1MPc-1]
d = distance of the galaxy from the earth [MPc]
What is 1 mega parsec [MPc] in metres?
3.09 x 1022m = 1MPc
What causes cosmological redshift?
The expansion of space.
What is cosmological redshift?
The redshifting of radiation as a result of the expansion of space.
What determines the rate of expansion of the universe?
Hubble’s constant, H0.
What is the age of the universe?
1/H0 ~13 billion years
What evidence is there for the Hot Big Bang theory?
Cosmological redshift - the expansion of the universe suggests that the universe was initially infinitely dense and infinitely hot and has been expanding ever since.
What is cosmic microwave background radiation?
The HBB theory predicts that lots of EM radiation was produced by the Big Bang. This radiation is also predicted to have been stretched by cosmological redshifting and is now in the microwave region. Hence, it is called cosmic microwave background radiation.
What are the features of cosmic microwave background radiation?
It is a continuous spectrum corresonding to a temperature of 2.73K
It is isotropic and homogeneous (same in all directions and all the same).
There are tiny temperature fluctuations in it which reflect its varying energy density.
Why are the fluctuations in energy density of cosmic microwave background radiation important?
Because these fluctuations allow galaxies to form.
Why is Helium abundant in the universe? How were the other elements formed?
The HBB theory states that the Big Bang was hot enough for hydrogen fusion, which evolves helium.
The other elements were/are synthesised by stars.
