# Black Body Radiation Physics 105 Goderya. Need for Quantum Physics Classical mechanics and relativity cannot explain Blackbody Radiation –The electromagnetic.

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Black Body Radiation Physics 105 Goderya

Need for Quantum Physics Classical mechanics and relativity cannot explain Blackbody Radiation –The electromagnetic radiation emitted by a heated object Photoelectric Effect –Emission of electrons by an illuminated metal Spectral Lines –Emission of sharp spectral lines by gas atoms in an electric discharge tube

Black Body Radiator A hypothetical object that emits Electromagnetic radiation and whose spectrum is continuous with a peak in the wavelength that corresponds to the temperature of the object Wavelength Energy Peak wavelength

Blackbody Radiation Graph Experimental data for distribution of energy in blackbody radiation As the temperature increases, the total amount of energy increases –Shown by the area under the curve As the temperature increases, the peak of the distribution shifts to shorter wavelengths

Black Body Radiation The light from a star is usually concentrated in a rather narrow range of wavelengths. The spectrum of a star’s light is approximately a thermal spectrum called a black body spectrum. A perfect black body emitter would not reflect any radiation. Thus the name “black body”.

Two Laws of Black Body Radiation 2. The peak of the black body spectrum shifts towards shorter wavelengths when the temperature increases.  Wien’s displacement law :  max ≈ 3,000,000 nm / T K (where T K is the temperature in Kelvin). 1. The hotter an object is, the more luminous it is: L = A*  *T 4 where  = Stefan-Boltzmann constant A = surface area;

Color and Temperature Orion Betelgeuse Rigel Stars appear in different colors, from blue (like Rigel) via green / yellow (like our sun) to red (like Betelgeuse). These colors tell us about the star’s temperature.

The Amazing Power of Starlight Just by analyzing the light received from a star, astronomers can retrieve information about a star’s 1.Total energy output 2.Mass 3.Surface temperature 4.Radius 5.Chemical composition 6.Velocity relative to Earth 7.Rotation period

Sun’s Temperature The sun =500 nm T = 3 x 10 6 /500 = 6000 K 10,000 F Wein’s Law gives the surface temperature

Sun’s Luminosity The sun: T= 6000 K, R=7 x 10 8 meters. What is its Luminosity? L = 4x 3.14 x (7 x 10 8 ) 2 x 6 x 10 - 8 (6000) 4 = 5 x 10 26 Watts Compare with 40 watts light bulb

Energy What we Know –600,000,000 tons of hydrogen every second –100 million nuclear bombs every heartbeat

Gravity and Sun Gravity of Sun attracts the matter, What keeps the sun from collapsing onto itself?

How to understand the H-R diagram?

The heart of astrophysics H-R diagram Temperature Luminosity

Hertzsprung and Rusell Sun –Mass –Radii

The Structure of an H-R Diagram Main Sequence stars e.g. Sun Red Giant stars Blue Giants White Dwarfs

How a Star is born Interstellar cloud Gas and dust Several light years across

Example Orion Nebulae Source: NASA, STSI

I2-2a Betelgeuse Source: NASA, STSI

Red Giants Swallowing the planets Source: NASA, STSI

Nova Explosions Nova Cygni 1975 Hydrogen accreted through the accretion disk accumulates on the surface of the WD  Very hot, dense layer of non-fusing hydrogen on the WD surface  Explosive onset of H fusion  Nova explosion

Example Spirograph IC 418 Source: NASA, STSI

Example Eskimo NGC2392 Source: NASA, STSI

Example Ring Nebulae M57 Source: NASA, STSI

Example Stingray Hen 1357 Youngest know planetary nebulae Source: NASA, STSI

I3-5 The Sun as a White Dwarf? Source: NASA, STSI

Why an H-R diagram is important? Evolutionary track –Sun Age of the star

Chandrashekar Limit Low mass stars: Fusion through p-p chain. H to C High mass stars: Fusion through p-p chain and CNO process. H - Fe M core White Dwarf 1.4M sun Neutron Star 3.0 M sun Black Hole

White Dwarfs Degenerate stellar remnant (C,O core) Extremely dense: 1 teaspoon of WD material: mass ≈ 16 tons!!! White Dwarfs: Mass ~ M sun Temp. ~ 25,000 K Luminosity ~ 0.01 L sun Chunk of WD material the size of a beach ball would outweigh an ocean liner!

Where do Chemical Elements come from? Nucleosynthesis

The Famous Supernova of 1987: SN 1987A BeforeAt maximum Unusual type II Supernova in the Large Magellanic Cloud in Feb. 1987

Example Supernova 1987A Source: NASA, STSI

Example Supernovae Remnant Source: NASA, STSI

Properties of Neutron Stars Typical size: R ~ 10 km Mass: M ~ 1.4 – 3 M sun Density:  ~ 10 14 g/cm 3  Piece of neutron star matter of the size of a sugar cube has a mass of ~ 100 million tons!!!

Black Holes Just like white dwarfs (Chandrasekhar limit: 1.4 M sun ), there is a mass limit for neutron stars: Neutron stars can not exist with masses > 3 M sun We know of no mechanism to halt the collapse of a compact object with > 3 M sun. It will collapse into a single point – a singularity: => A Black Hole!

Lighthouse Model of Pulsars A Pulsar’s magnetic field has a dipole structure, just like Earth. Radiation is emitted mostly along the magnetic poles.

The Crab Pulsar Remnant of a supernova observed in A.D. 1054 Pulsar wind + jets

Curvature of Space Thus if the space-time near a massive body is not flat, then the straight line path of light (and other objects) near that body will appear curved.

Black Holes

Black Hole Detection Since light can’t escape, black holes must be detected indirectly. Mass falling into a black hole would emit x rays:

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