Astro IB Questions

Distinguish between solar system and galaxy

galaxy is much larger in size than a solar system

galaxy contains many solar systems

galaxy is more luminous

Distinguish between a comet and a planet

a comet is a small icy body whereas a planet is mostly made up of rock or gas

a comet is often accompanied by a tail while a planet is not

a comet generally has a larger orbit than a planet

a planet must have cleared other objects out of the way of its orbital neighbourhood

Why values of distance might be different

small measurements used thus uncertainty in measurement

parallax angle very small

uncertainties in measuring

values same order of magnitude so not significantly different

Distinguish between constellation and stellar cluster

Constellation stars are not gravitationally bounded while stellar clusters are

Cluster stars are of similar age while those in constellations are not

Constellations form a visible pattern while stellar clusters do not

Stellar clusters are close in space while constellations aren not

Outline processes that produce the change in luminosity with time of Cepheid variable

Cepheid variables contract and expand

surface temperature decreases then increases

surface becomes transparent then opaque

Explain how Cepheid variables are used to determine distances

Peak luminosity depends on period

More luminous Cepheid variables have greater period

Measurements of apparent brightness allow distance determination

Explain why Cephids are used as standard candles

known as luminosity

determined from period-luminosity graph

used to estimate distance of galaxies

Suggest the conditions that will cause the Sun to become a red giant

Fusion of hydrogen in core eventually stops and core contracts

Sun expands and surface begins to cool

Helium fusion begins in the core

Sun becomes more luminous

Outline why Sun will maintain a constant radius after it becomes a white dwarf

electron degeneracy prevents further compression

State why there is a maximum distance that astronomers can measure using a stellar parallax

difficulty in measuring really small angles

Describe how chemical composition of a star can be determined

obtain line spectrum of a star

compare to laboratory spectra of elements

Describe satages of evolution from leaving main sequence until final stable stage

red giant

planetary nebula

white dwarf

Outline how equilibrium of main sequence stars is achieved

outward radiation pressure is balanced by inward gravitational force

Determine why stellar parallax is not suitable for all stars

stellar parallax limited to closer stars

State what is meant by main sequence stars

star fusing hydrogen into helium

State what is meant by a binary star

2 stars orbiting common centre of mass

State 2 characteristics of CMB radiation

black body radiation of approximate temp of 2.7K

uniform throughout (isotropic)

Describe how CMB provides evidence for Big Bang model

universe is expanding so wavelength of CMB in the past is much smaller indicating very high temperature at the beginning

due to expanding universe wavelength becomes longer when measured in the present time

State what is meant by the Big Bang model of the universe

theory in which all space created at a singularity at an enormous temperature

universe has been expanding ever since

State 2 features of CMB radiation consistent with Big Bang model

CMB has black body radiation spectrum

wavelength stretched by explosion

highly homogenous

State the nature if anisotropies observed in the CMB radiation

Temperature of CMB is not constant in different dierections

Identify 2 possible causes of ansiotropies

quantum fluctuations that have expanded

density perturbations that resulted in galaxies and clusters of galaxies

Outline how line spectra of distant galaxies are used to confirm hypotheses about the expansion of the universe

spectra of galaxies redshifted compared to spectra on Earth

redshift interpreted as cosmological expansion of space therefore universe expands

State what is meant by critical density

density of the flat universe

Discuss how scientists use galactic rotation curves to explain how less than 20% of matter can be observed

rotation speed of galaxies is larger than expected away from centre

thus there must be more mass at the edges of the universe than observable thus indicating the presence of dark matter

Light from distant galaxies is redshifted. Explain cosmological origin of this redshift

according to general relativity space expands stretching distances between far away objects

wavelength of photos received than emitted are therefore longer leading to observed redshift

Outline what is meant by dark energy

energy filling all space

resulting in repulsive force opposing gravity

accounts for accelerating universe

State 2 candidates of dark matter

black hole or brown dwarf

Explain evidence that indicated the location of dark matter in galaxies

rotational velocity of stars are expected to decrease as distance from centre of galaxy increases

observed velocity of outer stars is constant

implying large mass on edge which is dark matter

Outline why a hypothesis of dark energy has been developed

data of type 1a supernovae shows inverse expanding at accelerated rate

gravity expected to slow down expansion of universe

dark energy opposes gravity

so universe must be expanding

Outline, with reference with reference to Jeans criterion how Sun is likely to have formed

interstellar gas from earlier supernovae gravitational attraction between particles

if mass is greater than Jean’s criterion

as gas collapses temperature increases leading to nuclear fusion

Suggest how fluctuations of CMB radiation are linked to observation that galaxies collide

fluctuations in CMB due to difference in temperature during inflationary period

leading to formation of galaxies

Show how anisotropies in the CMB distribution are interpreted

CMB anisotropies related to fluctuations in density which are the cause for formation of structures

Describe what is meant by dark matter

invisible

interacts with gravitational force

Explain why rotation curves are evidence for existence of dark matter

rotational velocity of stars expected to decrease outside core of galaxy

flat curve suggests existence of matter that cannot be seen - dark matter

Outline how Hubble’s law is related to z

redshift of galaxies proportional to distance from Earth

Explain the observed galactic redshift in cosmological terms

redshift due to expansion of universe, not recessional speed

expansion of universe increases distance between objects so wavelength increases leading to observed redshift

Explain how observations of spectra from distant galaxies provide evidence that the universe is expanding

all distant galaxies exhibit red shift

Describe the mechanism of formation of type I a supernovae

formed in collision of 2 white dwarf stars where shock waves from collision rip both stars apart

Describe the mechanism of formation of type II supernovae

red supergiant explodes when the core collapses

why type I a supernovae used to measure distances

necessary to measure distance of very distant objects more accurately

act as standard candle

measure apparent brightness

Explain why presence of visible matter cannot account for velocity of stars when r>R

velocity is supposed to decrease

if mass is constant and r is increasing then velocity should be decreasing, however it’s not due to differences in mass

v-r graph is contradicted

r - process

r-process involves «rapid» neutron capture✔

in r-process further neutrons are captured before the beta decay occurs ✔

r-process occurs in type II supernovae ✔

r-process can lead to elements heavier than bismuth/lead/polonium

s - process

s-process involves «slow» neutron capture ✔

in s-process beta decay occurs before another neutron is captured ✔

s-process occurs in giant stars «AGB stars» ✔

s-process terminates at bismuth/lead/polonium