1. Electromagnetic Spectrum
Astrophysics

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

3. 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

4. Doppler Shift
Astrophysics

5. The Magnitude Scale First introduced by Hipparchus (160 - 127 B.C.):
First introduced by Hipparchus
( B.C.):
Brightest stars: ~1st magnitude
Faintest stars (unaided eye): 6th magnitude
More quantitative:
1 mag. difference gives a factor of in apparent brightness (larger magnitude = fainter object!)
1st mag. stars appear 100 times brighter than 6th mag. stars

6. 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

7. 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:

8. 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?

9. Magnitudes
Magnitude
Magnitude difference

10. 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.

11. Think-Pair-Share
The apparent magnitude of the sun is What is its absolute magnitude?

12. Magnitude Zeropoint The star Vega is by definition m = 0.
All magnitudes are measured relative to Vega.

13. Photometric Bands
Magnitudes are usually measured at a particular wavelength
Traditional
filter set

14. Photometric Bands
The Sloan Digital Sky Survey (SDSS) uses different filters
SDSS
filter set

15. Blackbody Radiation Blackbody Wien’s displacement Law ideal emitter
reflects no light
Wien’s displacement Law
Astrophysics

16. Quantization of Energy
Max Planck ( )
found empirical fit to blackbody spectra
units of B are erg s-1 cm-3 sr or
Astrophysics

17. Planck Function
Planck quantized energy and found:
Astrophysics

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

19. Blackbody Radiation
For stars:
Astrophysics

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