Electromagnetic Spectrum Astrophysics

Important Relationships Speed of light = frequency times wavelength Energy of a photon = Plank’s constant times frequency where Astrophysics

Light: The Cosmic Messenger We can determine many properties of an object based on the light we observe Position Apparent brightness Spectral energy distribution Doppler shift Spectral line broadening Zeeman splitting Temporal variations polarization Astrophysics

Doppler Shift Astrophysics

The Magnitude Scale First introduced by Hipparchus (160 - 127 B.C.): 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

Sirius (brightest star in the sky): mv = -1.42 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 Wien’s displacement Law ideal emitter reflects no light Wien’s displacement Law 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 Astrophysics

Planck Function Planck quantized energy and found: Astrophysics

Blackbody Radiation As T increases, blackbody emits more radiation at all wavelengths. Stefan-Boltzmann Equation Astrophysics

Blackbody Radiation For stars: Astrophysics

Matlab Exercise Plot a 10,000 K and 5800 K blackbody curve with UBVRI transmission curves. Astrophysics