Astro 1050 Wed. Feb. 18, 2015

Transcription

1 Astro 1050 Wed. Feb. 18, 2015 Today: Begin Chapter 5: Light the Cosmic Messenger For Friday: Study for Test #1 Be sure to bring green bubble sheet, #2 pencil and a calculator. 1

2 Chapter 5: Light, the Cosmic Messenger Properties of light are fundamental Almost everything we know about the universe outside our solar system comes from interpreting the light from distant objects. 2

3 Light: What is it? Radiation from a Source White Light Composed of Broad Range of Colors Dispersed via a Prism into Colors (Spectrum) Colored Light is Unchangeable by a Prism Colors are Inherent Property of the Light Itself Old-school Film and Modern Detectors Respond to Light Beyond the Visible Spectrum Electromagnetic Spectrum: Towards shorter wavelength and higher energy: Visible light, Ultraviolet light, X-Rays, Gamma-Rays Towards longer wavelength and lower energy: Visible light, Infrared radiation, microwaves, radio waves 3

4 The Electromagnetic Spectrum Lowest energy Radio waves Microwaves Infrared Visible Ultra-violet X-Rays Gamma rays Highest energy From our text: Horizons, by Seeds Atmospheric Windows 4

5 Light is an electromagnetic wave Changing electric fields generates magnetic fields (e.g. radio antenna) Changing magnetic fields generates electric fields Creates a cycle where one field causes the other: The E and B fields oscillate in strength, and the disturbance moves forward. Electrons Respond to the Changing Fields To describe the wave you need to specify Direction it is moving Strength of the fields (its intensity) Frequency or Wavelength of the oscillation (υ and λ are inversely related) Orientation of the electric E field: up or sideways (polarization) Wavelength and Frequency are Related to Speed: c = νλ In a vacuum all lightwaves move at the same speed c = m/s 5

6 More Properties of Light Light has both wave and particle properties Travels like a wave Wave Can t Extend to Infinity Wave Segment (Beginning & End) or Photon Interacts with matter like a particle: photon Full explanation involves quantum mechanics For most cases we can just choose the right model from the above two choices Photons, unlike particles in other kinds of radiation, are particles of pure energy Brighter Light Corresponds to More Photons 6

7 Relationship between Energy and Wavelength of Light Experiments Show Energy of Each Photon Related to Wavelength Short wavelength High energy photons Long wavelength Low energy photons Intensity total energy (per area & per second) (# of photons per area per second) (energy per photon) Example of falling rain: Amount of rain (# of raindrops) (volume per drop) 7

8 Why is energy per photon so important? Modified example: Hailstorm (with your car outside in it) Threshold for damage to car set by size (weight) of individual hailstones Below threshold hailstones cause no dents Above threshold they cause bigger and bigger dents Number of dents = number of hailstones bigger than threshold Very unlikely two small hailstones hit exactly together to cause a dent Real life example: Ultra-Violet light hitting your skin Threshold for chemical damage set by energy (wavelength) of photons Below threshold (long wavelengths) energy too weak to cause chemical or genetic changes Above threshold (short wavelength) energy photons can break apart DNA molecules Number of molecules damaged = number of photons above threshold Very unlikely two photons can hit exactly the same molecule to cause damage 8

9 Numerical Relationship Between Wavelength and Photon Energy E = hc λ Inverse relationship: Smaller λ means more energetic c = speed of light = m/s h = Planck s constant = joules x s Note: Joule is a unit of energy 1 Joule/second = 1 Watt Energy of a single photon of 0.5 µm visible light? E hc = λ joule s m/s 19 = = m joule Seems very small, but this is roughly the energy it takes to chemically modify a single molecule. Photons from a 100 W light bulb (assuming all 100W goes into light) 100 joule/s 100 Watts = 100 joule/s = = joule/photon 20 photon/s 9

10 Composition of Matter Atomic Structure Nucleus (protons + neutrons) ( meters) Cloud of Electrons (~ meters) Atoms Change Energy by Electrons jumping between levels via Absorption or Emission of Photons Only a Photon of the Right Energy is Emitted/Absorbed Each Element has a Unique Spectral Absorption Signature 10

11 Interaction of Light & Matter Hot Solids & Dense Gas Emit Continuous Spectrum (Thermal Spectrum) Cool Gas Produces Absorption Spectrum Hot Gas Emits Emission Spectrum Thermal Radiation Law #1: I(T ) = Area x!t 4 with! = 5.7x10!8 watts/(m 2 x K 4 ) (Radiation Increases Rapidly with Temp.) Thermal Radiation Law #2: I peak = 2900um / T(K) (Radiation Peaks Bluer with Increasing Temp.) Motion of Source Compresses or Stretches Wavelengths! obs!! 0! 0 = v c 11

12 Information Content of Light Brightness For a Given Temp. a Larger Surface Area Emits More Light Temperature Color of Continuous Spectrum (Blackbody) Depends on Temperature (Thermal Law #2) Spectral Line Strengths Depends on Temperature Atomic Collisions Excite Electrons Into Higher Levels Affects Detailed Spectral Line Strengths Presence of Ions Sensitive to Temperature Motion of Source Doppler Shift of Spectral Lines Elemental Abundances Spectral Signature of Different Elements will be Superimposed 12

13 Collecting Light with Telescopes Properly curved lenses and mirrors can form Images All the light leaving one point on object gets reassembled at one point on the image. From our text: Horizons, by Seeds In a camera you put an electronic sensor at the image to record it. In a telescope you put a magnifying glass (an eyepiece) near the image to see a magnified version of it. Most astronomers don t spend much time looking through telescopes they put recording devices at the image and use them as large cameras. 13

14 Refracting vs. Reflecting Telescopes Refracting telescopes use lenses. Reflecting telescopes use mirrors. Nearly all professional optical telescopes are reflecting telescopes. Why? Refracting telescopes suffer from chromatic abberation, and it is expensive and difficult to make large lenses. From the text, Horizons by Seeds 14

15 Why do astronomers need large telescopes? Large telescopes can collect more light Can detect fainter objects Have more light for specialized analysis. Large telescopes can form more detailed images Waves spread out as they go through an opening. The larger the opening, the less they spreads out. The longer the wavelength the more they spread out Maximum resolution θ λ/d where D is Diameter of telescope (Bigger Telescopes Produce Sharper Images!) Radio telescopes have to be much bigger than visible ones From our text: Horizons, by Seeds 15

16 Bigger is Better! Rainfall Analogy: Light from space falls on Earth like rain. Brightness of an astronomical object depends on how fast photons arrive (like inches of rain per hour) Light from astronomical object covers the Earth, you, your car, etc. The brighter the source, the faster (more frequent) photons arrive. Larger telescopes collect more light than small ones from same object. Larger buckets collect more rain than small ones More light means astronomers can study object in more detail. Larger buckets collect more rain in a given amount of time than small ones More light allows fainter objects to be detected and measured More light allows astronomers to slice the light into finer components for more detailed analysis. For example: a higher resolution of its spectrum. 16

17 Kinds of measurements made with telescopes Measure brightness of objects (photometry) Record images using electronic CCD detectors Split it into different wavelengths with spectrometers From our text: Horizons, by Seeds Next chapter: Why atoms and molecules emit and absorb selected wavelengths What that can tell you about the emitting/ absorbing material 17

18 Observing over the entire electromagnetic spectrum Different phenomena produce different wavelength waves Ordinary stars: Visible light Cool planets or dust clouds: Infrared light Moving charged particles, cool molecules: Radio waves Very hot objects: X-Rays and Gamma Rays Quasars: ALL wavelengths Only visible, some IR, and radio make it through atmosphere Need to observe from space for other wavelengths Going into space also lets you obtain more detailed images On Earth telescope size isn t only limit on image resolution Temperature fluctuations in atmosphere cause seeing (blurring) 18

19 Atmospheric Turbulence Blurs Images (Twinkling Stars: seeing ) Bad seeing /Good seeing 19

20 Hubble Space Telescope (HST) (above turbulent atmosphere) 20

21 Adaptive Optics to Correct Atmospheric Turbulence Examples of Adaptive Optics (AO) Starfire Optical Range (NM) Lasers excite upper atmosphere for monitoring turbulence Used to image satellites, monitor tests AMOS (HI) Similar to Starfire Astronomical Uses Major observatories moving into AO Will be included in all future, large telescopes 21

22 Next Generation Telescopes Examples of Future, Large Telescopes TMT: Thirty Meter Telescope Giant Magellan Telescope James Webb Space Telescope Cost will be ~ $1B 5B Staffing of ~

23 Chandra X-ray Observatory From Chandra s Webpage Tycho s Supernova in X-rays 23

24 Radio Telescopes From the Text, Horizons by Seeds 24

25 Infrared Telescopes Near-Infrared Image Optical Image 25

26 Infrared Telescopes SIRTF Space Infrared Telescope Facility WIRO 2.3m 26