astrophysics 2

8/3/2019 Astrophysics 2

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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

astrophysics 2

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