Light and Temperature

1. Light and Temperature Astronomy: The Science of Seeing

2. Mastering Astronomy • Go to MasteringAstronomy.com • Register using: • Access Code: • WSSBCW-SNELL-POIND-SPINA-GIGOT-FLEES • ClassID: NORDGREN2014 • We will start using for reading and homework due next Monday.

3. Goals • What is light? • What are the types of light? • Where does the light we see come from? • Understanding the light of heat. • On a sunny day why is it hot in my car?

4. "[Of celestial bodies] We may determine their forms, their distances, their sizes, and their motions - but we can never know anything of their chemical composition; and much less, that of organized beings living on their surface.“ Philosopher Auguste Comte, 1835 Open Cluster NGC 290: A Stellar Jewel Box Credit: ESA & NASA; Acknowledgement: E. Olszewski (U. Arizona)

5. What you see is all you get! • So you need to squeeze EVERY last drop of information out of the light we get. • This semester we’ll see how we can use light to: • Take a star’s temperature. • Weigh a black hole. • Tell what our Galaxy looks like from the outside. • See the beginning of the Universe.

6. The “Visible” Spectrum • When you think of “light”, what do you think of?

8. Light • Travels at the speed of light (a CONSTANT): c = 3 x 1010 cm/s • The wavelength (l) and frequency (n) are related: c = ln • The energy of light is: E = hn = hc/l

9. E = hc/l

10. Concept Question • Which of the following is true? • Red light has more energy than blue light. • Radio waves travel slower than visible light waves. • Gamma waves have less energy than Radio waves. • Blue light has a lower frequency than red light. • Ultraviolet has smaller wavelengths than infrared.

11. Emission lines Absorption lines Continuum A Spectrum • A spectrum = the amount of light given off by an object at a range of wavelengths.

12. Three Reasons All objects do one or more: • Reflect light because of color or smoothness • Emit light because of their temperature (thermal radiation) • Emit or absorb light because of their composition (spectral lines) A person, house, or the Moon: reflects visible light, and because each is warm, emits infrared light.

13. Reflecting Light

14. Temperature and Light • Warm objects emit light. • Thermal radiation

15. Kelvin Temperature • Kelvin: an absolute scale. • Kelvin is Celsius + 273 degrees. • Water freezes: 0 C  273 K • Water Boils: 100 C  373 K • Room Temp: 80 F  27 C  300 K • Surface Sun: 5800 K

16. Thermal Radiation Laws • Hotter is bluer. • (peak at shorter wavelength) • Hotter is brighter. • (More intense at all wavelengths)

17. visible range Star A Energy Output per second Star B V I B G Y O R Wavelength Concept Test Which of the two stars (A or B) is at a higher temperature? • Star A • Star B • The two stars have the same temperature. • It is not possible to infer this relationship.

18. visible range Star A Energy Output per second Star B V I B G Y O R Wavelength Concept Test Which of the following best describes how Star A would appear compared to Star B? • Star A would appear more red than Star B. • Both stars would appear more red than blue. • Both stars would appear more blue than red. • Star A would appear more blue than Star B. • None of the above.

19. visible range Star A Energy Output per second Star B V I B G Y O R Wavelength Concept Test Which of the following best describes the light from Star A compared to Star B? • More ultraviolet light but less visible light. • More infrared light but less visible light. • More visible light but less infrared light. • Less infrared light and less ultraviolet light. • More ultraviolet light and more visible light.

20. Concept Test • Imagine the Sun’s surface suddenly became much cooler (while the Sun’s size remains the same). Compared to the light it now emits, the Sun would emit: • More ultraviolet light but less visible light. • More infrared light but less visible light. • More visible light but less infrared light. • Less infrared light and less ultraviolet light. • More ultraviolet light and more visible light.

21. Atoms in Motion • Everything is composed of atoms which are constantly in motion.

22. Temperature • The hotter the object, the faster the average motion of the atoms. COOLER HOTTER

23. Atoms and Light • As atoms move they collide (interact, accelerate). • Collisions give off energy. • But light IS energy. E = hc/l

24. Light and Temperature • The hotter the object the faster the average atom and the more energetic the average collision. • The faster the atoms the more collisions there are. HOT COLD

25. Energy and Intensity • The more energetic the average collision the bluer the average light that is given off. • Since E = hc/l • The more collisions that occur the more light that is given off per surface area. • 1. Hotter is bluer. • (peak at shorter wavelength) • 2. Hotter is brighter. • (more intense at all wavelengths)

26. Graphically

27. Thermal versus Reflection • Thermal radiation is light given off because of an object’s temperature. • Don’t confuse with reflected light: • Buses are yellow not because they are hot enough to emit visible radiation but rather they reflect the yellow light given off by the Sun. • What kinds of thermal radiation do we see in our everyday life?

28. The IR World • Everyday objects (at everyday temperatures) emit thermal radiation in the IR, this is why we equate IR with HEAT. http://www.x20.org/library/thermal/blackbody.htm

29. Orion - visible Orion – by IRAS Betelgeuse The IR Universe • Everyday things that are hot radiate in the IR: • Dust – There are interstellar clouds of dust. Orion Nebula

30. The IR Universe Io from IRTF. • Molten Rock – There are lava flows on a moon of Jupiter. Orion – by IRAS

31. The Moon in eclipse. R. Gendler The IR Universe • In eclipse, there is no reflected light. • Only thermal radiation. • Differences in composition lead to differences in temperature. Orion – by IRAS

32. The Greenhouse Effect • Why is my car hot on a summer day? • At T = 6000 K, the Sun radiates mostly visible light. Windshield is transparent to visible light. • Car seat absorbs this visible light and warms up to 400 K. • At T = 400 K, my seat radiates mostly at longer wavelengths in the IR. Windshield is opaque in the IR. • Result: Energy is TRAPPED inside the car!

33. Homework #3 • For Wednesday 22-Jan: • Read Cosmic Perspectives Chapter 5 • Do end of Chapter Questions: 18, 22, 27, 34, 37