astrophysic pestel Flashcards
Explain, with reference to the possible fate of the universe, the significance of the critical density of matter in the universe.
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
Suggest one reason why it is difficult to estimate the density of matter in the universe.
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;
parsec
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;
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.
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;
critical density.
this is the value of density for which the universe will begin to contract after an infinite amount of time;
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.
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
(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.
a universe whose density is equal to the critical density;
(ii) the mutual gravitational attraction would slow the expansion down;
(ii) Define the luminosity of a star.
(ii) the (total) power radiated/emitted/produced (by the star);
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.
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;
Explain how the cosmic microwave background (CMB) radiation is consistent with the Big Bang model.
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;
big bang
space and time originated from a single point in a large explosion / an expanding universe that originated from a single point / OWTTE;
nebula
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
z
it is a measure of cosmological redshift
binary star system.
two stars orbiting about a common centre «of mass/gravity»
Explain why Cephids are used as standard candles.
Cepheids have a definite/known «average» luminosity
which is determined from «measurement of» period
main sequence star.
stars fusing hydrogen
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.
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
State the most abundant element in the core and the most abundant element in the outer layer.
core: helium
outer layer: hydrogen
Outline one reason for the difference in wavelength.
space expanding
comet
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;
Suggest how the discovery of the CMB in the microwave region contradicts Newton’s assumption of the static universe.
current low temperature observed is a result of expansion; (expansion) has caused cooling from high temperatures;
planet / star
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;
standard candle.
(i) object of known luminosity/power;
