astrophysic pestel

Question 1

Explain, with reference to the possible fate of the universe, the significance of the critical density of matter in the universe.

Answer 1

if less than critical density, universe expands without limit
if equal to critical density universe stops expanding after an infinite amount of time;
if greater than critical density, universe expands first then contracts

Question 2

Suggest one reason why it is difficult to estimate the density of matter in the universe.

Answer 2

there is matter that cannot be detected;
which is likely to consist of dark matter/neutrinos;

difficulty of measuring volume accurately;
because of difficulty of measuring distances accurately;

matter is not evenly distributed;
so density may vary from place to place;

Question 3

parsec

Answer 3

distance at which 1 AU subtends an angle of 1 arcsec / distance at which the angle subtended by the radius of Earth’s orbit is 1 arcsec;

Question 4

Light from distant galaxies, as seen by an observer on Earth, shows a red-shift. Outline why this observation suggests that the universe is expanding.

Answer 4

because of the Doppler effect;
light from sources moving away from an observer is observed to have a lower frequency than from the sources when stationary.
redshift indicates motion away from observer/Earth;

Question 5

critical density.

Answer 5

this is the value of density for which the universe will begin to contract after an infinite amount of time;

Question 6

A Cepheid star and non-Cepheid star both belong to the same distant galaxy. Explain, stating the quantities that need to be measured, how the luminosity of the non-Cepheid star may be determined.

Answer 6

measure period of Cepheid to determine its (average) luminosity;
compare the apparent brightness of the star and Cepheid to find L using L∝b;

measuring the apparent brightness of the star gives the luminosity (since distance is now known) from L=4πd2b

Question 7

(i) State, by reference to critical density, the condition that must be satisfied for the universe to be flat.

(ii) In a flat universe, the rate of expansion would be slowing down. Suggest a reason for this.

Answer 7

a universe whose density is equal to the critical density;

(ii) the mutual gravitational attraction would slow the expansion down;

Question 8

(ii) Define the luminosity of a star.

Answer 8

(ii) the (total) power radiated/emitted/produced (by the star);

Question 9

Distances to galaxies may be determined by using Cepheid variable stars. By considering the nature and properties of Cepheid variable stars, explain how such stars are used to determine galactic distances.

Answer 9

the (outer layers of the star) undergo a (periodic) expansion and contraction

which produces a (periodic) variation in its luminosity/apparent brightness;
the (average) luminosity depends on the period of variation;
by measuring the period, the luminosity can be found;
by then measuring its apparent brightness, its distance from Earth can be found;

Question 10

Explain how the cosmic microwave background (CMB) radiation is consistent with the Big Bang model.

Answer 10

the CMBR fills all of space / is uniform / is distributed equally, consistent with an “explosion” (at start of universe); the temperature of the radiation (2.7 K) is consistent with cooling due to expansion/redshift;

temperature of the universe immediately after the Big Bang was very high; as it expanded it cooled down;
the wavelength of the CMB corresponds to a temperature consistent with this cooling down / OWTTE;
red shift is due to expansion of universe;

Question 11

big bang

Answer 11

space and time originated from a single point in a large explosion / an expanding universe that originated from a single point / OWTTE;

Question 12

nebula

Answer 12

made of dust and/or gas
formed from supernova
can form new stars
some radiate light from enclosed stars
some absorb light from distant stars

Question 13

z

Answer 13

it is a measure of cosmological redshift

Question 14

binary star system.

Answer 14

two stars orbiting about a common centre «of mass/gravity»

Question 15

Explain why Cephids are used as standard candles.

Answer 15

Cepheids have a definite/known «average» luminosity

which is determined from «measurement of» period

Question 16

main sequence star.

Answer 16

stars fusing hydrogen

Question 17

The Sun and Theta 1 Orionis will eventually leave the main sequence. Compare and contrast the different stages in the evolution of the two stars.

Answer 17

the Sun will evolve to become a red giant whereas Theta 1 Orionis will become a red super giant

the Sun will explode as a planetary nebula whereas Theta 1 Orionis will explode as a supernova

the Sun will end up as a white dwarf whereas Theta 1 Orionis as a neutron star/black hole

Question 18

State the most abundant element in the core and the most abundant element in the outer layer.

Answer 18

core: helium

outer layer: hydrogen

Question 19

Outline one reason for the difference in wavelength.

Answer 19

space expanding

Question 20

comet

Answer 20

icy/dusty object;
moving around the Sun on a (highly) elliptical orbit;
when close to Sun likely to display atmosphere (coma)/tail;
when far from Sun (ice re-freezes and) atmosphere no longer present;

Question 21

Suggest how the discovery of the CMB in the microwave region contradicts Newton’s assumption of the static universe.

Answer 21

current low temperature observed is a result of expansion; (expansion) has caused cooling from high temperatures;

Question 22

planet / star

Answer 22

stars, and not planets, have cores undergoing fusion;

stars have much greater mass/luminosity/absolute magnitude/temperature than planets;

planets reflect starlight rather than emit;

Question 23

standard candle.

Answer 23

(i) object of known luminosity/power;