Astrophysics Flashcards
Solar System Order
Sun Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune
Mercury
Rocky
closest to sun
smallest planet
no moons
no atmosphere
Venus
Rocky
hottest planet (nearly 500 degrees c)
thick atmosphere
Mars
rocky
the red planet due to large amounts of iron oxide in the surface
Jupiter
gas
the biggest planet
Saturn
gas
has rings
Asteroids
lumps of rock
most are found between mars and jupiter in the asteroid belt
Comets
made of ice and dust
takes hundreds of years to orbit the sun in a highly elliptical orbit
as they get close to the sun they melt (hence leaving a tail)
Planetary system
a group of planets orbiting a star
Binary stars classification
visual - can be seen as two different stars
eclipsing - detected by periodic variations in brightness as one star obstructs the other
spectroscopic - detected by changes in the wavelength of light received from each star (due to Doppler effect)
Galaxies
a galaxy is a large scale collection of stars, gas and dust held together by gravity
Stellar cluster
a close group of gravitationally bound stars, gas and dust
Galactic cluster
a group of galaxies gravitationally bound together
larger groupings of galactic clusters are called superclusters
Nebula
is an intergalactic cloud of dust and gas
Constellations
certain stars appear to make patterns in the sky
these stars are not necessarily close together
one AU
mean distance from the centre of the earth to the centre of the sun
light year
distance light travels in a year
Method of Stellar parallax
- the stars apparent position relative to background stars is noted at 6 month intervals
- using trigonometry tanp= earth-sun distance/sun-star distance
- as p is a very small angle tan p = sin p = p in radians
- this means that the smaller the angle p, the greater the distance to the star
- the angle p is so small that it is measured in arc-seconds (1/3600th of a degree)
- the distance to the star (d) and the parallax angle (p) are related by the following formula d(parsec) = 1/p(arc seconds)
parallax angle
half of the observed angular displacement of the star
parsec
the distance at which the angle subtended by the radius of the earth’s orbit is one arc-second
Limitations of Stellar parallax
at greater distances, the parallax angle becomes too small to be accurately measured (i.e. the uncertainty becomes larger than the value)
Why are stars stable?
there is an equilibrium between the outwards radiation pressure and inwards gravitational pull
Luminosity
the total amount of energy emitted per second by a star
Apparent brightness
is the amount of energy from the star recieved per unit area by an observer
Stellar spectra
atoms in cooler atmospheres are exited absorbing photons of certain energies these transitions appear as dark absorption lines
this can tell us:
- what elements the star is made of
- the temperature of the star
- how the star is moving
Red giants
red in colour (cool)
large mass
large surface area
are in a late stage of the stars life cycle
Red supergiants
red in colour (cool)
very large mass
very large surface area
are in a late stage of the stars life cycle
White dwarfs
white in colour
small mass
small surface area
final stage in the life cycle of smaller stars
no longer undergoing fusion (cooling)
Cepheid variables
unstable
regular variations in luminosity (and brightness) due to periodic variation in size
late stage in the life cycle of some stars
Cycle:
He absorbs radiation from fusion
He heats up and expands (radiation pressure > grav force)
He cools as it expands
So star contracts (grav force > rad pressure)
Determining distance using cepheid variables
- find period
- determine luminosity
- use b and L to calculate d
Life cycle of a star up to formation of a red giant
gas clouds called nebulae come together due to gravity
as the particles get closer they:
- lose Ep
- gain Ek (therefore increasing temp)
the temperature can cause ionization of the gas and light is emitted
a protostar is formed
further contraction increases temp and causes fusion of H to He
this is called nucleosynthesis (creation of different elements by fusion)
main sequence star is formed
this is stable as forces are equal
Eventually the H core is converted to He, this happens faster is larger stars
the core contracts but H fusion continues in the outer layers and they expand
forming a red giant
the core gets hotter and can now undergo fusion into C and O
what happens next is dependent on the mass of the star
Small stars (initial mass < 4Mo)
the outer layers are ejected (called a planetary nebula) leaving a hot core behind
this is a white dwarf
Medium sized stars (initial mass 4-8Mo)
have further stages of fusion with Ne, Na and Mg being formed
eventually the outer layers are ejected once again leaving behind a white dwarf
Chandrasekhar Limit
white dwarfs have a maximum mas of 1.4Mo
this is due to electron degeneracy pressure
above this mass, gravity would overcome electron repulsion and the star would collapse further
Large stars (initial mass > 8Mo)
continue to form more elements through fusion
heavier elements are formed in the core while lighter elements are formed in the outer layers
the outer layers keep expanding creating a red supergiant
eventually silicon is fused to form Fe
once the core is all Fe, the core contracts and the outer layers are ejected as a supernova
the remaining core forms a neutron star or a black hole
Oppenheimer-Volkoff Limit
neutron stars have a maximum mass of 3Mo
above this mass, gravity would overcome neutron repulsion and the star would collapse further forming a black hole
Pulsars
neutron stars cool over time as they no longer produce heat
they will emit energy as radio waves
if they are rotating they will emit energy in bursts
Mass of star determines…
fusion products
Mass of stars core determines …
eventual fate
Newton’s model of the universe
- infinite in time and space
- uniform
- static
Oblers Paradox
is Newton’s model of the universe was correct and there is an infinite number of stars then the night sky would not be dark
Evidence for Big Bang theory
galaxies are red shifted (meaning that they are moving away as the absorption lines due to the elements found in stars were found at longer wavelengths than expected)
leads us to the idea of a singularity (everything in the universe was once located at one point
the temperature of the early universe was very high an it has been cooling as it has expanded (cosmic microwave background)
Cosmic microwave background radiation
microwave radiation was coming towards earth from every direction
essentially isotropic (same in all directions) implying matter of the early universe was distributed uniformly throughout space
detailed analysis revealed tiny fluctuations (ansiotropies) in temp in dif. regions of space, thought to have caused by small differences in density which led to the development of structures within the universe as it expanded
Big bang theory and Olber’s paradox
suggests a universe that is not infinite in time or space
the night sky is dark because:
- light from very distant galaxies has not reached us yet
- as galaxies are moving away from us and as their light is red shifted, the light from very distant galaxies is shifted to the infrared which is not visible
Hubble’s Law
the recession speed of galaxies is proportional to their distance from earth
Conversion of Mpc to km
1Mpc = 3.08 x 10^19 km
Cosmic scale factor (R)
is a way of quantifying the expansion that has taken place in the universe
type Ia supernova
type of supernova that occurs in a binary star system where one of the stars is a white dwarf
the white dwarf may gain additional mass from the companion star and fusion can begin again
however, the rate of fusion increases rapidly causing the white dwarf to supernova once the mass exceeds the Chandrasekhar limit
all have similar maximum luminosity and emits a known amount of light
10^10 times the luminosity of the sun
maximum luminosity reached rapidly then falls off over 6months or so
no hydrogen lies in spectrum but has a strong silicon line
Why was accelerating universe proposed?
the distance of type Ia supernovae can be accurately calculated
these calculations showed that these galaxies were further away than originally thought
suggesting that the rate of expansion of the universe has been increasing
therefore there must be an outwards accelerating force to counteract an inwards gravitational pull on the universe (dark energy)
Why are Cepheid variables known as standard candles?
because their luminosity can be determined and then used to calculate distance
Types of Stellar clusters
there are two types:
- globular: symmetrically arranged, more closely packed in the centre, contains old stars
- open: contains several hundred stars (much smaller) and is irregular in shape, young stars
Jeans Mass (Mj)
the minimum mass required for a gas cloud to collapse
these values depend on the temp and radius of. the gas cloud
Jeans Criterion
if the mass of the gas cloud is greater than the Mj, the gas cloud may collapse and initiate star formation
Fusion process in small stars
process is the three stage proton-proton chain
Fusion process in stars with mass > 6Mo
fusion process is in 6 stages known as the CNO cycle
this requires much higher temperatures than the proton-proton chain in order to overcome the Coulomb repulsion and so can only occur in large stars
Formation of larger nuclides
formed by neutron capture
although this initially does not create a new element, just a different isotope of the same element however the newly formed nuclide may be unstable and then decay by beta -ve decay whereby a neutron turns into a proton and a new element is formed
S-process
slow neutron capture
in the later stages of a massive star’s life cycle
a large range of elements are able to be formed (up to bismuth-209)
slower process and there is time for beta decay between captures
R-process
rapid neutron capture
occurs in type II supernovae
high neutron flux means that many neutrons can be added to a nuclide in a short space of time with no time for beta decay to occur
over time beta decay occurs until a more stable nuclide is formed
a smaller range of elements can therefore be formed but these elements are heavier than bismuth, have an excess of neutrons and are radioactive
Lifetimes of Main Sequence Stars
the more massive a star the shorter its lifetime
the higher temps and pressure within the core of massive stars means that fusion proceeds at a faster rate than in lower mass stars
lifetime of a star is proportional to the M^(-2.5)
Type 1 Supernovae
do NOT have hydrogen lines
produced by old low mass stars (H has been used up)
sub-catagorised (e.g. type Ia)
Type II supernovae
- have hydrogen lines
- produced by young massive stars
1. result from the rapid collapse and violent explosion of a massive star
2. when fusion of Si into Fe has stopped at the end of its lifetime the star implodes due to gravity overcoming radiation pressure
3. this collapse is stopped by neutron degeneracy pressure which creates an outward shockwave
4. the shockwave causes further fusion in the outer layers of the star resulting in the formation of elements heavier than Fe
5. once the shockwave reaches the edge of the star, the star explodes blowing material off as a supernovae - 10^9 times the luminosity of the sun
- luminosity falls a little before reaching a slight plateu for several days then falls off rapidly
Cosmological Principle
when viewed on a large scale, the universe is:
- homogenous (it is the same everywhere)
- isotropic (looks the same in all directions)
Density parameter
is the ratio of actual density of the universe to the critical density
Flat universe outcome
if the actual density is equal to the critical density
density parameter = 1
gravity slows expansion but takes an infinite time to do so
Closed universe outcome
if the actual density > critical density
density parameter > 1
gravity causes the universe to collapse back on itself
Open universe outcome
if the actual density is less than the critical density
density parameter < 1
gravity slows expansion but is not strong enough to stop it
Accelerating universe
rate of expansion continues to increase as a result of dark energy
Relationship of cosmic scale factor and temperature
Wien’s law states that wavelength max T = 2.9 X 10^-3
this means that as the wavelength emitted by the universe has increased its temp must have decreased
cosmic scale factor (R) gives a measure of the relative size of the universe so
wavelength is proportional to R
therefore
T is inversely proportional to R
the faster the rate of expansion of the universe the greater the rate that the temperature falls
Orbital velocities of stars of differing distances to the centre of the galaxy
stars close to the centre of a galaxy have an orbital velocity proportional to its distance r from the centre of the galaxy (formula given)
for a far away star the star can be considered to be orbiting with the grav. force provided by the mass of the entire galaxy
so the orbital velocity is inversely proportional to the square root of its distance from the centre of the galaxy
Reasons for believed existence of dark matter
observations for the orbital velocities of stars within galaxies have shown that stars far away from the galactic centre do not have the expected reduced velocities instead there is an initial increase in orbital velocity with distance (as expected) but the orbital velocities then stay roughly constant as distance increases implying that M is proportional to R
hence there must be undetected mass forming a halo around the outer rim of galaxies
as this matter emits no radiation it has been called dark matter
it is this mass that maintains the high orbital velocities of stars far from the galactic centre
Possible candidates for dark matter
MACHOS (Massive compact halo objects) - high density compact stars at the end of their lives (e.g. black holes, neutron stars), questionable whether these exist in sufficient numbers to provide the necessary amount of dark matter
WIMPS (Weakly interacting massive particles) - these are subatomic particles that are not made up out of ordinary matter, would need to be a huge number of them
What is Dark energy
fills all space and opposes the attractive force of gravity between galaxies and so is responsible for the accelerating expansion of the universe
