Option D Astrophysics (SL) Flashcards
how many planets are there in our solar system
8
name the planets in order
Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune
My Very Enthusiastic Mother Just Served Us Noodles
what is the hottest planet
venus
what are asteroids
lumps of rock mostly found between Mars and Jupiter in the astroid belt
what are comets
- they are made of dust and ice
- take hundreds of years to orbit the sun
- they start to melt when they get close to the sun, this make a tail
What is a planetary system
a group of planets orbiting a sun
what are binary stars
a pair of stars orbiting their common centre of mass
how can binary stars be detected
- visual - can be seen as two seperate stars
- eclipsing - detected by periodic variations in brightness as one star obscures the other
- spectroscopic - detected by changes in the wavelength of light received from each star (due to Doppler effect)
what is a stellar cluster
a close group of gravitationally bound stars, gas and dust
what is a globular stellar cluster
- contains about 10,000 – 100,000 stars
- symmetrically arranged and more closely packed in the centre.
- contains OLD stars
what is open stellar cluster
- contains several hundred stars
- irregular in shape
- contains YOUNG stars
Average distance between starts within a galaxy/average distance between galaxies =
10^-6
10^-6 =
Average distance between starts within a galaxy/average distance between galaxies
how can stars exist
singly or as binary stars
what is a galactic cluster
a group of galaxies gravitationally bound together
what is a supercluster
a larger group of galactic clusters
what is nebulae
an intergalactic cloud of dust and gas
what are constellations
- where certain stars appear to make patterns in the sky.
- it is important to realise that these stars are not necessarily close to each other.
what measurement do we use in the solar system
astronomical unit (AU) = 1.5x10^11m
what measurement do we use outside the solar system
light year = 9.46x10^15m
what is stellar parallax used for
- to find the distances to stars we see in the night sky
- it is used to determine the distance of stars relatively close to Earth (100 parsecs is max distance)
how does stellar parallax work
- the star’s apparent position relative to background stars is noted at 6 month intervals
- using trig: tan p = earth-sun distance÷sun-star distance
- as p is a very small angle, tan p = sin p = p (in radians)
- the angle p is so small it is measured in arc-seconds (1/3600th of a degree)
- using d (parsec) = 1/p(arcsecond)
definition of parsec
the distance at which the angle subtended by the radius of the Earth’s orbit is one arc-second
what is parallax angle
half of the observed angular displacement of the star
definition of luminosity
the total amount of energy emmitted per second by a star
(unit: W)
this depends on the size and temperature of the star.
definition of apparent brightness
the amount of energy received per second per unit area by an observer (unit: Wm^–2)
can you do luminosity ratio questions?
Yes, continue
No, pg 15 booklet
what is stella spectra
a continuous spectrum of colour, however some wavelengths are missing, leaving an absorption spectrum
how can we use stella spectra
it can be used to find what elements make up a star as every element will adsorb a different wavelength of light, therefore the black lines are what elements make up that star.
what can stella spectra tell us
what elements make up the star (absorption spectrum)
the temperature of the star (wavelength max)
how the star is moving (doppler effect)
describe where red supergiants, red giants, white dwarfs and main sequence stars are on a hertzsprung-russell diagram
red giants: cool + luminous, top right
red giants: cool + luminous, middle-top right
main sequence: through the middle, a slight curvature
white dwarfs: hot + dim, bottom left
what are the axis on a hertzsprung-russell diagram
y-axis: logarithmic luminosity scale
x-axis: decreasing temperature
definition of main sequence stars
90% of all stars, “normal” stars, they fuse hydrogen into helium, difference between them is their masses, larger stars are more luminous
where does the sun lie in the hertzsprung-russell diagram
luminosity of 1
temperature of 5,400 kelvin
mass-luminosity relationship questions
see pg 20
describe red giants
- red in colour
- large mass
- large surface area
- are at a late stage in a star’s life
describe red supergiants
- red in colour
- larger mass than red giant
- larger surface area than red giant
- are at a late stage in a star’s life
- fusing Si into Fe
describe white dwarfs
- white in colour (relatively hot)
- small mass
- small surface area
- final stage in the life cycle of smaller stars
- no longer undergoing fusion (cooling down)
describe cepheid variables
- unstable
- regular variations in luminosity (and brightness) due to a periodic variation in size
- late stage in the life cycle of some stars
why do cepheid variables vary in size?
helium absorbs radiation
––> helium heats up and expands (radiation pressure > gravitational force)
––> helium cools as it expands
––> grav force > rad pressure so star contracts
––> repeat
what is a ‘standard candle’
cepheid variable star, because its luminosity can be determined and then used to calculate distance
“shark fin” curve
- rapid expansion
- slower contraction
how to find distance using cepheid variables?
1) find period: measure variation in brightness of the cepheid variable star over time to determine its period
2) determine luminosity, the period is related to its average luminosity
3) use brightness and L to calculate d, use brightness formula to calculate d
how is a protostar formed?
- nebulae come together due to gravity
- they lose potential energy and gain kinetic energy (temperature increases)
- the temperature can cause ionisation of the gas
- light is emitted ––> protostar is formed
from protostar, how is a main sequence star formed
further contraction increases the temperature of core and fusion of H to He can occur, main sequence star is formed
stable phase as:
inwards gravitational pull = outwards radiation pressure
what is nucleosynthesis
creation of different elements by fusion
how is red giant formed?
- eventually all the H in core of main sequence star has been converted to He via nucleosynthesis.
- the larger the star the faster this happens.
- the core contracts but H fusion continues in the outer layers and they expand creating a red giant
- the contracting core gets hotter so He can now fuse into C and O.
how do small stars, (initial mass < 4 solar masses) become their late stage
- layers are ejected, called planetary nebula
- leaving a very hot core behind
- produces a white dwarf which cools over time
how do medium stars, (initial mass 4–8 solar masses) become their late stage
- further fusion with neon, sodium and magnesium
- before ejecting outer layers (planetary nebula) leaving white dwarf
define the Chandrasekhar limit
maximum mass of white dwarf is 1.4 solar masses. Due to electron degeneracy pressure (the electrons cannot be packed any closer by the gravitational force), any star above this mass, gravity would overcome electron repulsion and will start to collapse further
how do large stars, (initial mass > 8 solar masses) become their late stage
- form more elements through fusion, heavier in core, lighter in outer
- creating red supergiant
- silicon ––> iron, once the core is all iron, it contracts rapidly
- outer layers are ejected as supernova.
remaining core forms neutron star or a black hole (if star was very large)
maximum mass of neutron star
3 solar masses
Oppenheimer-volkoff limit
the maximum mass of neutron star which is 3 solar masses
rotating neutron stars
pulsars:
- neutron stars cool over time as they are no longer producing heat (like white dwarf)
- they will emit energy as radio waves in bursts
drawing a star’s life cycle on an H-R diagram
for low mass star:
for high mass star:
see pg 27
newton’s model of the universe
- infinite in space
- infinite in time
- uniform
- static
Olber’s Paradox
if newton’s model of the universe was correct, then as there are an infinite number of stars why is the sky dark at night?
no matter how far away a star is, they will all be equally bright
redshift definition
absorption lines due to the elements found in stars were found at longer wavelengths than expected
the Big Bang model
- everything in the universe was located at one point - a singularity
- this point exploded creating space and time
- since then the universe has been expanding, creating space as it does
singularity definition
at some point in time, everything in the universe was located at one point.
what is cosmic microwave background radiation (CMB)
a microwave radiation was coming towards Earth from every direction in space. CMB was found to have a wavelength that corresponded to that emitted by an object of 2.76K.
shows that the universe would have cooled to 2.76K after almost 15 billion years
why is the sky dark due to the Big Bang theory
- light from very distant stars has not yet reached us yet
- as galaxies are moving away from us and their light is red-shifted, the light from very distant galaxies is shifted into the infra-red which is not visible to our eyes
if the redshift ratio (z) is positive, …
if z is negative, …
+ve means that there is a redshift (galaxy moving away from Earth)
–ve shows a blueshift (galaxy moving towards Earth)
Hubble’s law
the speed at which galaxies are moving away from Earth is directly proportional to their distance from Earth
explain cosmic scale factor (R)
a way of quantifying the expansion that has taken place in the universe:
- assume a galaxy emitted light of wavelength λº at some time in the past when the cosmic scale factor had a value of Rº
- due to expansion of the universe the wavelength of this light when detected today would have increased to a new value λ, increasing by Δλ
- the cosmic scale factor would have changed over this time period from Rº to R
- this means that space has stretched by a factor of ΔR in the time the wavelength has increased by Δλ
λº––>λ (increase Δλ)
Rº––>R (increase ΔR)
rate of expansion of the universe is:
a) increasing
b) decreasing
c) the same
a) increasing
evidence was found from Type Ia supernovae
why is the universe expansion rate increasing?
dark energy
what does tells you the universe is expanding?
light produced by galaxies is red-shifted. this showed that all galaxies observed were moving away from Earth
proton-proton chain
stars like the sun undergo fusion from hydrogen to helium
this process is called proton-proton chain
CNO cycle
stars with mass > 4 solar masses undergo hydrogen to helium
this is a different process called CNO cycle
s-process
slow neutron capture
- neutrons captured individually
- time for beta-negative decay between captures
- large range of heavier elements formed (up to bismuth)
- occurs in the later stages of a massive star’s lifecycle
r-process
rapid neutron capture
- large number of neutrons captured
- no time for beta-negative decay between captures
- more limited range of elements formed (beyond bismuth)
- occurs in type II supernovae
time-mass relationship
t∝M^(-2.5)
formation of type Ia supernovae
formed when a white dwarf in a binary system gains mass from its companion star and explodes due to fusion restarting
- used as a standard candle due to a known amount of light it emits, allowing distance to be worked out.
formation of type II supernovae
- result from the rapid collapse and violent explosion of a massive star (>8 solar masses)
distinguishing type Ia supernovae on Earth
- 10^10 times luminosity of sun
- max luminosity reached rapidly and then gradually falls over 6 months or so
- no hydrogen lines in spectrum but has a strong silicon line
distinguishing type II supernovae on Earth
- 10^9 times luminosity of sun
- luminosity falls a little before reaching a slight plateau for several days then falls off rapidly
- hydrogen lines in spectrum
isotropic
same everywhere
homogenous
looks the same in all directions
critical density (pc)
density required for flat universe (gravity slows expansion but takes an infinite time to do so)
density parameter (Ωo)
actual density / critical density
Ωo=1 flat (gravity slows expansion but takes an infinite time to do so)
Ωo>1 closed (gravity causes the universe to collapse back in on itself - the Big Crunch)
Ωo<1 open (gravity slows expansion but is not strong enough to stop it)
derive pc=3Ho^2/8πG
Et= Ek+Ep
= 1/2 mv^2 – GMm/r
substitute M=4/3π r^3 p, v=Hor
then simplify
**Ho are in s^(-1) to convert to kms^-1Mpc^-1, ÷ by 3.24 x 10^–20
temperature - comic scale factor
T∝1/R
where is most matter located in a galaxy
centre
derive v=√(4πGp/3)
Fg=Fc
GMm/r = mv^2/r
(M= 4/3π r^3 p)
v^2=4πGp/3
for stars close to the centre of galaxy, v and r relation
v∝r
evidence of dark matter
v∝r not v∝1/√r
so matter that is not observable must exist
candidates for dark matter
MACHOS, WIMPS
universe consists of
68% dark energy
27% dark matter
5% ordinary matter
cosmic microwave background (CMB) contradictions
isotropic but can’t be as then there would be no structure
(matter uniformly distributed throughout space)
