Understanding Light in Astronomy: Properties, Laws, and Spectra

1. Light Astronomy 315 Professor Lee Carkner Lecture 4

2. Scale Exercise • What is scale for solar system (0.0016 ly)? • measure ball diameter = • real/model = scale • scale = 0.0016/2 = • What is the model value for the distance to Sirius (9 ly)? • real/scale = model • 9 /0.0008= 11250 cm = • Distance to other end of Science Building • Change scale so that ball equals 9 ly • new scale = 9/2 = • Find size of galaxy in model (100000ly) • 100000/4.5 = 22222 cm = • Distance to Old Main

3. Disturbing the Universe • Can’t visit directly or send probes • Would take ~100000 years to get to nearest star • Can do some simulations in the lab • But how do we know if they are right?

4. Light • What is light? • How do these properties give us information about the object that emitted the light?

5. What is Light? • EM radiation can be thought of in two different ways: • As a stream of photons (particle) • Light is both a particle and a wave • We use what ever formulation is most useful

6. Properties of Light • When we examine a light emitting object, what do we want to know? • Energy • Photon Flux • How much total energy is emitted by an object depends on how much energy each photon has and how many of them are emitted

7. Wavelength • Each photon has a wavelength • Energy is inversely related to the wavelength (l) • Long wavelength = • Short wavelength = • We will often measure wavelength in meters or nanometers (1 billionth of a meter, or 1X10-9 m)

8. Waves

9. Speed and Frequency c = 3 X 108 m/s = 186,000 miles/s • We can use this speed to write the frequency: c = lf • Frequency is directly related to energy • High frequency = high energy • Low frequency = low energy

10. Color • This is called visible light • Short wavelength, high energy = blue • Long wavelength, low energy = red

11. A Spectrum

12. Star Colors • Stars come in 4 basic colors

13. How is Light Produced? • Every object in the universe emits blackbody radiation that depends on its temperature • Given in degrees Kelvin • Room temp = 300 K • Higher T means more radiation

14. Spectrum • The radiation is a continuum of wavelengths called a spectrum • We can describe the spectrum as a curve on the intensity versus wavelength diagram

15. Peak Wavelength and Temperature • A higher temperature produces a spectrum that peaks at shorter wavelengths • Wien’s Law: lmax = 3,000,000/T • Where T is in Kelvin and l is in nanometers

16. Intensity and Temperature • A higher temperature means more total energy emitted • Stefan-Boltzmann law: P = seAT4 • s is the Boltzmann constant (5.67 X 10-8 W/m2 K4) • A is the surface area of the object (in m2) • T is the temperature in Kelvin

17. Using Radiation Laws • Wien’s Law • If you can find the peak wavelength you can find the temperature • Stefan-Boltzmann law • Hot objects emit more energy then cool objects • The intrinsic brightness of a star depends on both its temperature and size

18. Alberio • This is the double star Alberio • Two stars orbiting around each other • Both are the same distance from Earth • Size of star image proportional to brightness • What is the relative temperature and size of the stars?

19. The Electromagnetic Spectrum • Light can have a wide range of wavelengths • This corresponds to a wide range in energies • Today we call the range of wavelengths the electromagnetic spectrum

20. The EM Spectrum

21. The EM Spectrum and You • You see in visible light, feel infrared as heat and get a sunburn from ultraviolet • Microwave and radio have long wavelengths and low energy

22. Next Time • Read Chapter 5.1-5.8