Astrophysics concepts study guide

Order of planets: (including order in mass and radius; smallest to largest.)
Mercury; 1, 1
Venus; 3, 3
Earth;4, 4
Mars; 2, 2
Jupiter; 8,8
Saturn; 7,7
Uranus; 5, 6
Neptune; 6,

CONSTELLATIONS:
Stars that look like they are close together but do not have anything related physically except that they are all bright
STELLAR CLUSTER:
Group of stars that are physically close together rather than looking as they are, formed by the collapse of a gas cloud

Distance light travels in a year, 9.46×10¹⁵

•Galaxy contain stars between 10³ and 10⁵ light years across
•each star aprox 1ly apart
•each galaxy 10⁶ ly apart

Stars and constellations moving east to west, but the relative position of the constellations do not change. Earth rotates on its axis every 24 hours. Earth does full revolution of the sun every 365 days.

Fusion. Stars are composed of mainly hydrogen. Main reaction is that of nuclear fusion where hydrogen is fused into helium providing energy. Fusion process is called a proton-proton chain:
4¹₁H= ⁴₂He+2e⁺+2ve+2γ

•When star is expending fuel it rises in temperature and therefore rises in 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.

Amount of energy radiated by the star per second (sun has luminosity of 3.8×10²⁶W)

Amount of energy per second received per unit area. It is how bright a star seems to us depending on its luminosity and how far away it is.
b=L/(4πd² ) Wm⁻²
4πd² is surface area
(spectroscopic parallax)

L=σAT4.
A=4πr²
The Stefan-Boltzmann constant: σ = 5.67 x 10⁻⁸Wm⁻²K⁻⁴ (given)

the higher the temperature the lower the wavelength at which most of the energy is radiated.
Wein’s displacement law is stated as: λmax=(2.9×10⁻³)/T (given)

•surface temperature of a star is determined by measuring the wavelength at which most of the radiation is emitted.
•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)

CLASS:
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

Acronym: Oh Be A Fair God, Kill Me

MAIN SEQUENCE STARS: centre of HR diagram, 90% of the stars we see.
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.

BINARY STARS:
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.
ECLIPSING BINARY:
•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

Main sequence, red giant, red supergiant, white dwarf and Cepheid stars should be shown, with scales of luminosity and/or absolute magnitude, spectral class and/or surface temperature indicated. Students should be aware that the scale is not linear.
Students should know that the mass of main sequence stars is dependent on position on the HR diagram.

Defined in terms of the angle subtended at the star.
If the distance to a star is 1pc then the angle will be 1 second. dparsec=1/(p(arcsec⁡θ))

Distance found by measuring the angle the telescope is rotated through when moving it from 2 different positions. The distance is very large so the angle will be very small.

If the distance is too big then the angle that will be measured will be too small to calculate a distance.
Needs to be less than 100 parsecs away

idk look for sample problems

•Scale that determines how bright a star is measured from 1 to 6 where 1 is 100 times brighter than 6
•(therefore 2.512=100^⅕ times brighter than the previous)
•gives relative visual brightness from earth

Magnitude of a star viewed from a distance of 10pc
m-M=5 log⁡(d/10)
d=10×10^((m-M)/5) pc (given)

Apparent brightness:
A measure of how bright a star appears to be when you see it from Earth.

Absolute brightness: A measure of how bright the star really is, if all stars were the same distance from Earth.

The most intense wavelength of light emitted from a star is used to find its temperature (using Wien’s law λ_max=(2.9×10⁻³)/T). From the HR diagram we can then find its luminosity.

b=L/(4πd² ) Wm⁻²

only has a limit of about 10Mpc, as stellar distances increase, the uncertainty in luminosity becomes greater and so the uncertainty in distance is creater

Unstable star that undergoes periodic expansions and contractions, leading to a periodic change in the apparent brightness of the star as viewed from Earth.

theres a picture you can look at on http://www.atnf.csiro.au/outreach/education/senior/astrophysics/variable_cepheids.html

•”Standard Candles,” objects with known luminosity, and comparing this with their apparent magnitude we can easily calculate their distance
•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

•locate cepheid in galaxy
•measure period
•use graph to find its absolute magnitude M
•measure how bright it appears (maximum)
•calculate how far away it is

•Infinitely big implying that the gravitational force on each star was the same in each direction holding them in static equilibrium.
•If the universe is static, then the stars will be in the same place forever.
•He also concluded that the universe must be uniform.

1) (If there was an infinite number of stars, why is the sky dark?) There is a finite number of stars and each star has a finite lifetime.
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.

•Light from distant galaxies is red-shifted meaning the wavelength is getting longer suggesting that the universe is expanding.
•Also the further galaxies are moving faster than inwards ones suggesting there was an explosion (big bang).

•Time and space grew out of the 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)

•detected by Penzias and Wilson in 1960’s.
•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

•temperature of universe after big bang was very high but as it expanded it cooled down to about 3K
•wavelength of CMB corresponds to temperature consisten with this cooling down
•red shift due to expansion of universe

The matter and radiation of our present time was initially all packed together into an extremely hot and dense fireball, that exploded giving rise to the Big Bang.
•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.

•formed when huge clouds of gas and dust are compressed
•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

LOW MASS:
•helium synthesis
SUFFICIENT MASS:
•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)

•Star fuses hydrogen→helium
•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

•relationship between luminosity and mass of main sequence star
L∝m^n where 3