1 Light and Atoms Why study the behavior of light and atoms? –It is only through light that we know anything about the Universe. –We can’t experiment on stars and planets. –Light tells us about the position and velocity of a star or planet. It can also tell us about the temperature and the composition. An infrared image of a man holding a match. White is the hottest temperature, Blue/Black are the coolest temperatures

2 Light & Atoms Atoms interact with light by absorbing, emitting and bending light Atoms leave their unique signature or “fingerprint” in the light they emit or absorb –this can tell us what stars and planets are made of –astronomers see these “fingerprints” in light from objects in distant galaxies. Atoms in our own atmosphere can blur and absorb light from distant objects

3 The Nature of Light The speed of light (c) is constant in a vacuum Light has particle-like properties (photons) and wave-like properties (wavelength and frequency) Which way you describe light depends on the kind of observation you are making. Light is an electromagnetic wave

4 The Nature of Light Light has a wavelength and a frequency –wavelength ( ) - distance between wave crests –frequency ( ) - number of wave crests that pass a point in 1 second When you tune your radio you are actually changing the frequency. A piano will emit different frequencies of sound based on which key you strike.

5 Wavelength and Frequency The wavelength ( ) and the frequency ( ) are related to the speed (c) by the formula  = c If the wavelength increases the frequency must decrease for the speed to stay the same

6 Color and Frequency The frequency and color of light are related. Red light for example has a lower frequency than blue light. White light is a mixture of light of many different frequencies. A prism can break light into a rainbow (spectrum) of colors.

7 Wave vs. Photon Wavelength/Frequency –Short wavelength: blue –Long wavelength: red Wave Amplitude –Small amplitude –Large amplitude Photon energy –High energy –Low energy Photon flux (num. photons/area) –Low photon flux –High photon flux

8 Electromagnetic Radiation Visible light is just one form of electromagnetic radiation. Together they form the electromagnetic spectrum. They differ in their wavelength, frequency and energy. However specifying the wavelength, frequency or energy uniquely characterizes the form of electromagnetic radiation.

9 The Energy of EM Radiation The energy (E) carried by electromagnetic radiation is related to the frequency ( ). X-rays have a higher frequency and higher energy than radio waves for example. T < 10 K 10 – 1,000 K 1,000 – 10,000 K 10 4 – 10 6 K 10 6 – 10 8 K T > 10 8 K

10 The Energy of EM Radiation The energy (E) carried by electromagnetic radiation is related to the frequency ( ). X-rays have a higher frequency and higher energy than radio waves for example. Cold gas / Accelerated electrons / Cosmic Background Radiation Cool Stars / Warm dust Planets Planets/Stars/Galaxies Hot massive stars Supernovae / Hot tenuous gas Pulsars / Black Hole Accretion

11 Multi-Wavelength Astronomy Different physical processes produce light at different wavelengths. To better understand a physical system (planet, star, galaxy), observe it in as many energy (wavelength) bands as possible. –Choose the bands to match the energy range for the physical process of interest.

12 Multi-Wavelength Astronomy The Sun Supernova Remnant Cas A Supernova Remnant M1, The Crab Nebula Giant Elliptical Galaxy M87 Peculiar, Interacting Galaxy Centaurus A

13 Wien’s Law Wien’s law allows astronomers to determine the temperature of a star. The wavelength at which a star is brightest is related to its temperature Hotter objects radiate more strongly at shorter wavelengths Blue has a shorter wavelength than red, so hotter objects look bluer. Objects can emit radiation at many different wavelengths. The wavelength at which a star is brightest is related to its temperature. This is Wien’s Law

14 When can you use Wien’s Law? Only for objects that emit light not for those that reflect light Light emitted by hot, solid objects obey Wien’s Law Can not use with gases unless they are of a high density The Sun and other stars obey Wien’s Law since the gases they are composed of remain at a high density (at least up to the outermost layers of the star).