Our Place in the Universe

Question 1

What are the orbit-shapes of comets?

Answer 1

Ellipsis

Question 2

What are the orbit shapes of planets?

Answer 2

Circles

Question 3

How can a distance be measured using radar?

Answer 3

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.

Question 4

How can the velocity of an astronomical object be measured using radar?

Answer 4

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.

Question 5

What are the assumptions of the radar astronomical distance measuring technique?

Answer 5

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.

Question 6

What two factors is the brightness of an object dependent upon?

Answer 6

Luminosity (Light energy per second (J/s)) and distance

Question 7

What is the apparent magnitude of a star?

Answer 7

How bright it appears in observation.

Question 8

What is the absolute magnitude of a star?

Answer 8

How bright the star actually is.

Question 9

How can the distance of a galaxy be calculated using star brightness?

Answer 9

By observing the behaviour of cephid variable stars within that galaxy.

Question 10

What is 1 AU in metres?

Answer 10

1.5 x 10¹¹m

Question 11

What is 1 light year in metres?

Question 12

What is the approximate diameter of the Milky Way?

Answer 12

100 000 light years

9.46 x 10²⁰m

Question 13

What is the distance to Earth’s nearest star, proxima centauri?

Answer 13

It is 4.2 light years to proxima centauri.

Question 14

What is the distance to the edge of the observable universe?

Answer 14

14 x 10⁹ light years

Question 15

What is the doppler effect?

Answer 15

The shifting of a wave’s wavelength as the source of the wave and an observer move relative to one another.

Question 16

How is it possible to measure the velocity of an object relative to the earth using doppler shift?

Answer 16

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).

Question 17

If the wavelength of a wave becomes shorter, is the wave being blueshifted or redshifted?

Answer 17

Blueshifted ( Blue ~475nm )

Question 18

If the frequency of a wave is decreased by wave-shifting, has the wave been redshifted or blueshifted?

Question 19

What is blueshift?

Answer 19

The post-emission decrease in a wave’s wavelength.

Question 20

What is redshift?

Answer 20

The post-emission increase in a wave’s wavelength.

Question 21

What part of a star is radiation emitted from?

Answer 21

The photosphere - a very hot region.

Question 22

How can the composition of a star be deduced?

Answer 22

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.

Question 23

What is the assumption of doppler-shift distance measurements? Why does this assumption need to be made?

Answer 23

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).

Question 24

What is time dilation?

Answer 24

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.

Question 25

What is it that suggests galaxies are moving away from the earth? Are the galaxies actually getting further away?

Answer 25

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.

Question 26

What is the relationship between recessional velocity and galaxy distance?

Answer 26

v = H₀d

Where:

v = recessional velocity [kms^-1]

H₀ = Hubble’s constant [kms^-1MPc^-1]

d = distance of the galaxy from the earth [MPc]

Question 27

What is 1 mega parsec [MPc] in metres?

Answer 27

3.09 x 10²²m = 1MPc

Question 28

What causes cosmological redshift?

Answer 28

The expansion of space.

Question 29

What is cosmological redshift?

Answer 29

The redshifting of radiation as a result of the expansion of space.

Question 30

What determines the rate of expansion of the universe?

Answer 30

Hubble’s constant, H₀.

Question 31

What is the age of the universe?

Answer 31

1/H_{0 ~}13 billion years

Question 32

What evidence is there for the Hot Big Bang theory?

Answer 32

Cosmological redshift - the expansion of the universe suggests that the universe was initially infinitely dense and infinitely hot and has been expanding ever since.

Question 33

What is cosmic microwave background radiation?

Answer 33

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.

Question 34

What are the features of cosmic microwave background radiation?

Answer 34

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.

Question 35

Why are the fluctuations in energy density of cosmic microwave background radiation important?

Answer 35

Because these fluctuations allow galaxies to form.

Question 36

Why is Helium abundant in the universe? How were the other elements formed?

Answer 36

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.