IB Physics Topic Option E Astrophysics
•has sufficient mass for its self gravity
•has cleared the neighborhood around its orbit
•loose particles of ice and rock that are blown off by the solar wind forming a tail
Mercury 1 1
Venus 3 3
Earth 4 4
Mars 2 2
Jupiter 8 8
Saturn 7 7
Uranus 5 6
Neptune 6 5
Stars that look like they are close together but do not have anything related physically except that they are all bright
Group of stars that are physically close together rather than looking as they are, formed by the collapse of a gas cloud
distance at which 1AU subtends an angle of 1 arc sec
p=distance of earth from sun/distance of star from sun
between stars within a galaxy and between galaxies, in terms of order of magnitude.
•each star aprox 1ly apart
•each galaxy 10⁶ ly apart
example, our Sun), there is an
equilibrium between radiation
pressure and gravitational pressure.
•required to keep a balance between the great force of gravity compress the star.
•Gravitation force can collapse the star
•radiation pressure which can make the star expand.
•equilibrium is gained through nuclear fusion which provides the energy the star needs to keep it hot so that the star’s radiation pressure is high enough to oppose gravitational contraction.
b=L/(4πd² ) Wm⁻²
4πd² is surface area
The Stefan-Boltzmann constant: σ = 5.67 x 10⁻⁸Wm⁻²K⁻⁴ (given)
Wein’s displacement law is stated as: λmax=(2.9×10⁻³)/T (given)
•Most stars essentially have the same chemical composition, yet show different absorption spectra as they have different temperatures.
•Absorption spectra gives information about the temperature of the star and its chemical composition.
•Doppler shift information of speed relative to earth (red shift→longer wavelength, blue shift→shorter wavelength)
O 60 000K-30 000K Blue
B 30 000K-10 000K Blue-white
A 10 000K- 7 500K White
F 7 500K- 6 000K Yellow-white
G 6 000K- 5 000K Yellow
K 5 000K- 3 500K Orange
M 3 500K- 2 000K Red
GIANTS: cool star that gives out a lot of energy, large mass, luminosity 100 times that of our sun
SUPERGIANT: very big cool star, luminosity 106 times greater than sun, radii up to 1000 times that of the sun
WHITE DWARFS: small hot star, hotter than sun but only size of earth, low luminosity
VARIABLE STAR: has changing luminosity so position on HR diagram is not constant.
pairs of stars that orbit each other (or more accurately, around their common centre of mass)
VISUAL BINARY STARS:
•one that can be distinguished as two separate stars using a telescope
SPECTROSCOPIC BINARY STARS:
•can be identified from its spectrum.
•Over time the system shows a spectrum that oscillates, being doppler shifted towards the blue and red with a regular period.
•Orientation of orbit causes them to periodically pass between the earth and eachother then they eclipse each other.
•Causes a reduction in the stars apparent brightness (diagram is light curve)
SEE NOTES FOR DIAGRAMS
Students should know that the mass of main sequence stars is dependent on position on the HR diagram.
SEE NOTES FOR DIAGRAM
If the distance to a star is 1pc then the angle will be 1 second. dparsec=1/(p(arcsecθ))
Needs to be less than 100 parsecs away
•(therefore 2.512=100^⅕ times brighter than the previous)
•gives relative visual brightness from earth
d=10×10^((m-M)/5) pc (given)
b₁/b₂ =[2.512^(-m₁ )]/[2.512^(-m₂)]
spectroscopic parallax is limited to measuring stellar distances less than about 10 Mpc.
•Period of variation in luminosity for cepheid variable is related to average absolute magnitude. The greater the period the greater the maximum luminosity.
•Used when measuring distances greater than 10Mpc
•use graph to find its absolute magnitude M
•measure how bright it appears (maximum)
•calculate how far away it is
•If the universe is static, then the stars will be in the same place forever.
•He also concluded that the universe must be uniform.
2) The universe has a finite age and stars that are beyond the event horizon have not yet had time for their light to reach Earth.
3) The radiation received is redshifted and so contains less energy.
•Also the further galaxies are moving faster than inwards ones suggesting there was an explosion (big bang).
•Universe is expanding really means that space is growing rather than spreading into the nothingness that surrounds it.
Calculating Red shift: ∆λ/λ=v/c (given)
•big bang theory predicts that CMB corresponds to the black body at 3K λmax=b/T
•Was thought to be uniform but satellite detected very small variations that were just enough to show that the early universe was not completely uniform enabling galaxies to form.
•CMB radiation same in all directions, characteristic of black body radiation
SEE NOTES FOR BLACK BODY RADIATION CURVE
•wavelength of CMB corresponds to temperature consisten with this cooling down
•red shift due to expansion of universe
•Within seconds, matter was accelerated through 3 dimensions, expanding and developing very rapidly.
•Time became a measure of the rate of that expansion, the necessary 4th dimension.
FLAT: the rate of expansion tends to zero at infinite time
CLOSED: stops expanding and starts to contract
•It is very unlikely that this is the way the universe is, since just one extra electron would make it contract.
•density of universe for which expansion rate slow to zero but never rest (produce flat universe)
•less that ρc universe expand for evermore
•greater than ρc universe expand then contract (closed)
universe determines the development of the universe.
•If all the stars and gas clouds in a galaxy were measured the total mass is not big enough to give the gravitational attraction to hold it together (its only 4%) the rest is called dark matter.
MACHOS: massive astronomical compact halo objects
WIMPS: weakly interacting massive particles
investing significant resources into researching the nature of the universe.
•understanding the nature of the universe sheds light on fundamental philosophical questions
•one of most fundamental, interesting and important area for mankind
•gives rise to technology that may eventually improve quality of life
•travel to new planets if life on earth becomes impossible
•money could be spend on more useful earth bound needs
•money better spent on other areas of research such as medical
•better to fund less expensive projects
•is information really worth the cost
•cannot form on their own as gravitational force not big enough to pull particles together
•as cloud comes together, GPE→KE→temperature
•temperature causes outward pressure that pushes against gravitational attraction, however as atoms get closer together, the gravitational attraction increases so gas continues to collapse
•eventually dense core formed surrounded by cloud of dust and gas
•heats up until fusion of hydrogen takes place
•red giant continues to fuse higher and higher elements
•fusion ends with nucleosythesis of iron (iron has the highest binding energy per nucleon of all nuclei, will no longer shine)
•eventually hydrogen will run out so fusion reaction happens less causing star to not be in equilibrium →core collapses, increasing the temperature
•now fusion of helium possible→star increases massively in size→expansion means outer layers are cooler hence RED GIANT
L∝m^n where 3
•’critical mass’ of the initial star which dictates its evolution
•value is 1.4 ⨉ solar mass(mass of sun M⦿)
•no white dwarf can be more massive than this limit
•any degenerate object more massive must collapse into a neutron star
•largest mass for a neutron star
•value is 3 solar masses (mass of sun M⦿)
•Neutron degeneracy pressure also has a mass limit, above which it cannot support the star
•once used up all helium, core continues to contract, radiating energy as it shrinks
•reaches electron degeneracy
•radiation from core blows away outer layers of star, exposing core
•this cools, becoming WHITE DWARF
•eventually fuse to form iron which forms central core
•fusion stops (highest binding energy) and core compresses until electron degeneracy prevents further contraction
•reaches point where it contains only neutrons packed closely to the nucleus
•outer layers rush in, bouncing off core, flying back out producing an explosion
•all that remains is neutron core (neutron star)
•forms a planetary nebula and then becomes a white dwarf
•a white dwarf is stable due to electron degeneracy pressure
•experiences a supernova and becomes a neutron star or collapses to a black hole
•a neutron star is stable due to neutron degeneracy pressure.
•pulsates at a very rapid and precise frequency
•believed to be rotating neutron stars
•a rotating neutron star expected to emit an intense beam of radio waves in a specific direction
•since rotating, signal received comes at regular pulses
•found light from all galaxies are red shift
•furthest galaxies shifted more than close ones (implies universe is expanding)
• astronomers know that dark energy is causing our universe to expand at an accelerating rate
H₀=recession velocity/separation distance
•since there are gravitational attraction between galaxies, speed of recession should be decreasing
•assume recession velocity is constant
H₀=recessional velocity/separation distance
→This is the same as 1/H₀
So the age of the universe=1/H₀
•But first need to convert distance into km
•calculation assumes velocity is constant (since we know gravitational attraction slows down galaxies, recession velocity much smaller than it was.
•as universe expanded, it cooled down until it reached a temperature of 4000K (which is equivalent to particle energy 0.4eV which is not enough to ionize hydrogen)
•electrons then started combining with protons to form atoms.