Chapter 2 Read Pages 7-17 Continuous Radiation from Stars

2.1 Brightness of Starlight n What two major factors determine how bright a star appears to us? n Answers: F power output F distance from us

n m = Apparent Magnitude n M = Absolute Magnitude n L = Luminosity n f = Energy Flux What quantities are used to describe how bright a star is?

Ptolemy AD n n Ptolemy divided the stars visible to the naked eye into six classes of brightness call magnitudes. n n The magnitude scale is a logarithmic scale.

n n Energy flux (f) is the energy per unit area per unit time received from a star. n n A difference of five magnitudes corresponds to a factor of 100 in energy flux.

2.2 The Electromagnetic Spectrum n Thomas Young - wave nature of light n Albert Einstein - photons n James Maxwell - electromagnetic theory

The Electromagnetic Spectrum (Seven Forms of Light) n Radio Waves - communication n Microwaves - used to cook n Infrared - “heat waves” n Visible Light - detected by your eyes n Ultraviolet - causes sunburns n X-rays - penetrates tissue n Gamma Rays - most energetic

The Visible Spectrum

Wave Speed = Frequency  Wavelength c =  m/s Hz m

Questions n In which of the seven forms of light…. u …does our Sun have its peak intensity? u …does our eyes have the greatest sensitivity? u …is the Earth’s atmosphere fairly transparent?

2.3 Colors of Stars Quantifying Color I( ) = Intensity Function versus Wavelength I( ) = “ “ “ Frequency

2.3.2 Blackbodies n...objects that are ideal radiators when hot n...perfect absorbers when they are cool n Blackbody Examples: u light bulb filament u stove or horseshoe u stars (not perfect blackbodies)

Wien’s Law n “Hotter bodies radiate more strongly at shorter wavelengths.”

n Star temperatures range from about 3000K to about 50,000K.

Star Colors F Reddish  coolest star F Orange-ish F Yellowish F White F Bluish  hottest star

Stefan-Boltzmann Law - a star of temperature T radiates an amount of energy each second equal to  T 4 per square meter Stefan-Boltzmann Law - a star of temperature T radiates an amount of energy each second equal to  T 4 per square meter n Luminosity - the amount of energy per second (or power) given off by a star

What is the luminosity of the Sun? n T = 5800K n R = 7 x cm  = 5.7 x erg/(cm 2 K 4 s)  = 5.7 x erg/(cm 2 K 4 s)

2.4.1 Planck’s Law n Rayleigh-Jeans Law n Planck’s Equation

2.4.2 Photons n Photon energies are proportional to their frequencies.

2.5 Stellar Colors n Color Index allows astronomers to quantify color. n Negative values for the color index (B-V) correspond to blue stars.

2.6 Stellar Distances n Parsec - the distance from our Sun at which the angle between the Earth and the Sun subtends an angle of one arcsecond n 1 arcsecond = 1/3600 degrees n 1 parsec = 3.26 light years n Light-year - the distance that light travels in one year

n Inverse-Square Law - the apparent brightness of a star decreases with increasing distance from Earth Measuring A Star’s Brightness

Measuring a Star’s Distance n Parallax - the apparent change in the position of a star due to the motion of the Earth n Nearby objects exhibit more parallax that remote ones.

2.7 Absolute Magnitudes n Distances to stars can be found from the distance modulus,