Transcript 3 sr -1
Electromagnetic Spectrum 1 Astrophysics Important Relationships Speed of light = frequency times wavelength Energy of a photon = Plank’s constant times frequency where 2 Astrophysics Light: The Cosmic Messenger We can determine many properties of an object based on the light we observe 3 Position Apparent brightness Spectral energy distribution Doppler shift Spectral line broadening Zeeman splitting Temporal variations polarization Astrophysics Doppler Shift 4 Astrophysics The Magnitude Scale First introduced by Hipparchus (160 - 127 B.C.): Brightest stars: ~1st magnitude Faintest stars (unaided eye): 6th magnitude More quantitative: 1 mag. difference gives a factor of 2.512 in apparent brightness (larger magnitude = fainter object!) 1st mag. stars appear 100 times brighter than 6th mag. stars The magnitude scale system can be extended towards negative numbers (very bright) and numbers > 6 (faint objects): Sirius (brightest star in the sky): mv = -1.42 Full moon: mv = -12.5 Sun: mv = -26.5 Magnitudes Magnitudes are a logarithmic measure of the flux from an object. Flux (F) is related to an objects luminosity (L) and distance (r) as: Think-Pair-Share What flux is measured from the sun at the surface of Mars, at a distance of 1.5 AU from the sun? Magnitudes Magnitude Magnitude difference Absolute Magnitude Denoted with capital M magnitude an object would have at a distance of 10 pc. Difference between apparent and absolute magnitude is known as distance modulus. Think-Pair-Share The apparent magnitude of the sun is -26.81. What is its absolute magnitude? Magnitude Zeropoint The star Vega is by definition m = 0. All magnitudes are measured relative to Vega. Photometric Bands Magnitudes are usually measured at a particular wavelength Traditional filter set Photometric Bands The Sloan Digital Sky Survey (SDSS) uses different filters SDSS filter set Blackbody Radiation Blackbody ideal emitter reflects no light Wien’s displacement Law 15 Astrophysics Quantization of Energy Max Planck (1858 - 1947) found empirical fit to blackbody spectra units of B are erg s-1 cm-3 sr-1 or 16 Astrophysics Planck Function Planck quantized energy and found: 17 Astrophysics Blackbody Radiation As T increases, blackbody emits more radiation at all wavelengths. Stefan-Boltzmann Equation 18 Astrophysics Blackbody Radiation For stars: 19 Astrophysics Matlab Exercise Plot a 10,000 K and 5800 K blackbody curve with UBVRI transmission curves. 20 Astrophysics