presentsa production STELLAR RADIATION STELLAR TYPES based on the IB Astrophysics option 1
STELLAR RADIATION STELLAR TYPES Nuclear fusion Luminosity Apparent brightness Black body radiation Stefan-Boltzmann law Wien’s law Absorption spectra Spectral classification The HR diagram
StarsStars are formed by dust coming together overa long time through mutual gravitationalattraction.The loss of potential energy is responsible forthe initial high temperature necessary forfusion.The fusion process releases so much energythat the pressure created prevents the star fromcollapsing due to gravitational pressure.
Gravity RadiationGravity pulls outer layers in, Pressure Radiation Pressure pushes them out. Gravitational The more mass the pressure star has, the greater the gravitational pressure and the higher the central pressure
Nuclear fusion The energy the Sun emits is generated by the fusion in its core. In order to begin the fusion process of hydrogen to form helium, a very high temperature is needed: 107 K.
MASS The most important variable for a ‘hydrogen-burning’ star Mass affects itsLUMINOSITY and TEMPERATURE
LuminosityThe LUMINOSITY of a star is how much ENERGY it gives off per second (aka Power) in watts This light bulb has a luminosity of 60 Watts The Sun has a luminosity of 3.90x1026 W (often written as L0)
Apparent brightness The energy that arrives at the Earth is only a very small amount when compared with the total energy released by the Sun. d b is called the apparent brightness of the star Unit is W/m2
Solar Constant Solar luminosity, L0 = 3.90x1026 W Sun-Earth distance, d=1.5 x 1011 m 4πd2 = 2.83 x 1023b = 1378.1 Wm-2 or Js-1 m-2The amount of energy arriving at the Earthevery second per square metre, although it is not evenly spread over the sphere.
Black bodyA black body is a perfect thermal emitter and absorber. Its spectrum depends only on its temperature.
Black body Spectrum A black body with a higher temperature has greater intensities of all wavelengths and its wavelength of maximum intensity is shorter.
Stefan-Boltzmann The area under a black body radiationcurve is equal to the total energy emittedper second (L) per unit of area (A) of theblack body. Stefan showed that this area was proportional to the fourth power ofthe absolute temperature (T)of the body. where, σ = 5.67x10-8 W m-2 K-4
Wien displacement law The wavelength of maximum intensity is inversely proportional to the absolute temperature of the body
A Star’s Temperature This can be calculated from its black body spectrumFor the Sun, max. intensity wavelength = 500 nm
Size of a starWe can use Wien’s Law to find thetemperature of a star from its spectrumThen find the luminosity/ areafrom the Stefan-Boltzmann relationThen find the luminosity from theapparent brightness and distanceFind the radius from the area of the star
Real stellar spectra In the spectrum of a star is evidence for the elements in its outer layersTheoretical Emissionblack body andSpectrum Absorption Lines
Absorption line strengthThe relative strength ofhydrogen absorption linesin stellar spectra dependson the temperature of thestar. The first stellarclassification used thismethod. The Harvard team of ‘computers’ Williamina Fleming
Spectral classificationNowadays, the classification is according to temperature Hotter Cooler
OBAFGKM O Be A Fine Girl/Guy KissClass Spectrum Me Color Temperature O ionized and neutral helium, bluish 31,000-49,000 K weakened hydrogen B neutral helium, stronger blue-white 10,000-31,000 K hydrogen A strong hydrogen, ionized white 7400-10,000 K metals F weaker hydrogen, ionized yellowish white 6000-7400 K metals G still weaker hydrogen, ionized yellowish 5300-6000 K and neutral metalsK weak hydrogen, neutral orange 3900-5300 K metalsM little or no hydrogen, neutral reddish 2200-3900 K metals, molecules L no hydrogen, metallic red-infrared 1200-2200 K hydrides, alkalai metals T methane bands infrared under 1200 K
The HR Diagram This diagram shows a correlation between the luminosity of a star and its spectral type. The scale on the axes is not linear. Luminosity depends on mass and size. Colour depends on temperature.
The HR Diagram This diagram shows a correlation between the luminosity of a star and its spectral type. h ere are The scale on the axes is u Yo not linear. Luminosity depends on mass and size. Colour depends on temperature.
Stars are notrandomlydistributed; theyform groups.MAIN SEQUENCE90% of all starsGIANTS ANDSUPERGIANTSVery large andvery cool for a starWHITE DWARFSSmall and hot stars
Binary starsSirius A and B Many star systems are binary (double stars) Sirius is a visual binary, since we can see both stars (Sirius B is at the ESA/Hubble bottom left).
Eclipsing binary When the plane of orbit is end-on, eachstar will pass in front of the other and dim it Their orbital period and relative sizes can be estimated from the variation in brightness
Spectroscopic binary Due to the Doppler effect, the spectral lines of an approaching starwill be blue-shifted, while the spectral lines of areceding star will be red-shifted leading to double lines from which the orbital speeds can be calculated.
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