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For IB Physics
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By the end of the section you will be able to: E2.1 State that fusion is the main energy of stars E2.2 State that in a stable star there is an equilibrium between
radiation and gravitational pressure E2.3 Define the LUMINOSITY of a star
E2.4 Define APPARENT BRIGHTNESS and state how it ismeasured
E2.5 Apply the Stefan-Boltzmann law to compare the luminositiesof different stars
E2.6 State Weins (displacement) law and apply it explain theconnection between the colour and temperature of stars
E2.7 Explain how atomic spectra may be used to deduce chemicaland physical data for the stars E 2.8 Describe the overall classification system of spectral classes E 2.9 Describe different types of star E2.10 Discuss the characteristics of spectroscopic and eclipsing
binaries
E2.11 Identify the general regions of star types in the Hertzsprung-Russell (HR)diagram
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The sun radiates 1026J/s!Process is FUSION
(not burning!)
2 H atoms become 1
Helium 1% of the mass is
converted to energy E=mc2
Mass lost of sun is4x109kg/s
(calc energy) Some stars have
different reactions
(see later)
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Gas and radiation isemitted at a huge rate
Radiating for the last 4.5billion years
Gravitation pull drags theparticles back Forces stable at the
moment Some escape in
prominences and solar
wind This type of nuclear
reactions will carry on forabout another 5 billionyears
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Defined as the TOTAL ENERGY EMITTED PER SECOND J/s. This of course is the POWER of the star so the unit is the
Watt (W)
A very luminous star could be faint to the naked eye if it is along way away The hotter and bigger the star the more Luminous Least Luminous star known :Proxima Centauri
- Type=M5, Magnitude=11.0, Distance=4.22 ly This dim red dwarf is the nearest star to the Sun, and it is a
member of the Alpha Centauri system lies 0.24 light years from the main pair of stars, requiring over one
million years to orbit them. It is also a flare star - capable of brightening a magnitude or more
in minutes.
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A team of astronomers from the University of Floridahave found what could be the brightest star ever seenin the Universe.
Located 45,000 light years away across our galaxy, LBV1806-20 could be 40 million times brighter and 150times larger than our own Sun.
This gigantic and bright star isnt long for the Universe;however, its only a couple of million years old, andwill blow up as a supernova in a few million more.
But dont expect to find the star which is at least 5million times brighter than the sun in the night sky.
Dust particles between Earth and the star block out allof its visible light. Whereas the sun is located only 8.3light minutes from Earth, the bright star is 45,000 lightyears away, on the other side of the galaxy.
It is detectable only with instruments that measureinfrared light, which has longer wavelengths that canbetter penetrate the dust
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If 2 stars are the same distance from the Earth the onewith the greatest luminosity would be brightest It is possible for 2 different stars to have the samebrightness how? Double the distance square the area
Distance inversely proportional tobrightness Called inverse square law
(Solve for distance and Luminosity)
Brightness measured byusing digital camera and
measuring the pd & energyAstronomers usemagnitude scale tocompare
- 1 is brighter than 3
(Calculations!)
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Hot objects emit light If an object is a perfect
emitter all its energy isseen as radiation BlackBody Radiation
Stars are almost perfectemitters Peak wavelength (max
energy) is related totemperature by Wiensdisplacement Law
We can then calculate thetemperature of the star byanalysing the radiation
Once we have thetemperature of thestar we can calculatethe Luminosity wehave the by Stefan-
Boltzmann Law:
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A hot object radiates its energy as radiation over the continuous spectra Atoms of a gas have electrons in specific energy levels The electrons jump to higher energy levels when heated and become
excited They fall back to the original level giving out particular packages, or
quanta of energy of specific wavelength If the gas is between the light source and the earth the lines are subtracted
from the continuous spectra We can analyse the gases in the outer levels of the sun or star the radiation
from the core shines through it
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The thousands of Fraunhoferlines see in the Suns spectra cantell us what substances arepresent
Also by studying the lines wecan identify the energy neededto produce them and thereforethe temperature of the gas
The same can be done for starswhich show quite differentspectral lines particularlylooking at the hydrogen lines
We can therefore analyse thelight from a star to get chemicalcomposition, surfacetemperature, luminosity andsurface area of the star. Onlydistance to go!
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The spectra can show evidence of theDoppler Effect Redshifted stars aremoving away from usThe amount of change of thecharacteristic absorption lines thefaster it is travelling
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By spectral class we can work out the properties of differentclasses of stars Giants and Super giants
Very large in size Red Comparatively cool The later stages in the life of a star
By spectral White Dwarfs
Small in size and white in colour As they are white they are very hot They are at the almost the final stage fusion is no longer happening is a
remnant cooling down They cool enough for light not to be seen a brown dwarf
Main Sequence stars 90% of stars in the prime of their life our Sun is an example
Variables Have changing luminosity due changing size. Cepheid variables have a link between the period of variation and average
luminosity This means the absolute luminosity in nearby galaxies can be calculated and from
this the distance.
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A log scale of theluminosity of thestar (or absolutemagnitude) isplotted against the
spectral class (ordecreasingtemperature nonlinear scale)
There is a banddown the middle
with 90% of thestars mainsequence. Theseburn hydrogen
Stars at the end oftheir life are on
either side
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Many stars are binary andorbit each other about acommon centre of mass
A visual binary can be seenas 2 stars close together in atelescope - Sirius A&B. 10%of stars seen from Earth
A spectroscopic binary canbe detected by the blue andred shifting of its lightperiodically
An eclipsing binary gets inthe way of its partner andthere is a dip in brightnessperiodically-Algol
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Orbiting binaries follow Newtons laws of motion(see Fields and Forces topic)
We can calculate their masses from these formulas
If they are viewed as satellites in a circular orbit:GMm/r2= mw2r
If they are equal mass
mA + mB = 42
r3
/GT2
Where m is their masses, r the radius, T time period andG the gravitational constant, w is 2/T
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