# 3 Light and Temperature Astronomy: The Science of Seeing.

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3 Light and Temperature Astronomy: The Science of Seeing

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?

3 Open Cluster NGC 290: A Stellar Jewel Box Credit: ESA & NASA; Acknowledgement: E. Olszewski (U. Arizona)ESANASAE. OlszewskiU. Arizona "[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

3 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: 1.Take a star’s temperature. 2.Weigh a black hole. 3.Tell what our Galaxy looks like from the outside. 4.See the beginning of the Universe.

3 The “Visible” Spectrum When you think of “light”, what do you think of?

3

3 Light Travels at the speed of light (a CONSTANT): c = 3 x 10 10 cm/s The wavelength ( ) and frequency ( ) are related: c = The energy of light is: E = h = hc/

3 E = hc/

3 Three Reasons All objects do one or more: 1.Reflect light because of color or smoothness 2.Emit light because of their temperature (thermal radiation) 3.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.

3 Reflecting Light

3 Temperature and Light Warm objects emit light. –Thermal radiation

3 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

3 Thermal Radiation Laws 1.Hotter is bluer. –(peak at shorter wavelength) 2.Hotter is brighter. –(More intense at all wavelengths)

3 Concept Test Which of the following best describes how Star A would appear compared to Star B? 1.Star A would appear more red than Star B. 2.Both stars would appear more red than blue. 3.Both stars would appear more blue than red. 4.Star A would appear more blue than Star B. 5.None of the above. V I B G Y O R visible range Star A Star B Energy Output per second Wavelength

3 Concept Test Which of the two stars (A or B) is at a higher temperature? 1.Star A 2.Star B 3.The two stars have the same temperature. 4.It is not possible to infer this relationship. V I B G Y O R visible range Star A Star B Energy Output per second Wavelength

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

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

3 Atoms and Light As atoms move they collide (interact, accelerate). Collisions give off energy. But light IS energy. E = h c 

3 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. COLD HOT

3 Energy and Intensity The more energetic the average collision the bluer the average light that is given off. –Since E = h c  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)

3 Graphically

3 peak  const/T

3 Result If everything with a temperature gives off light, why doesn’t we see everything glowing? peak = k/T peak = (3 x 10 -3 m/K) * 1/ 300 K peak = 10 -5 m IR

3 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?

3 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

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

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

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

3 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!

3 Venus and Earth Certain gases act the same way as your windshield: Carbon Dioxide (CO 2 ). Venus – Runaway greenhouse effect. Earth – Could that happen here?