2. How do Astronomers know what they know? Almost everything we know about Astronomy was learned by gathering and studying light from distant sources Properties of light 1. Behaves like a wave; has wavelength (distance from crest to crest)  Visible light can be separated into distinct wavelengths by a prism  Radio waves = a few kmGamma rays = less than a billionth of a cm 2. Behaves like a particle; photons – small packet of light energy  Photons from the sun push the tail of a comet away.  Light with shorter wavelengths have more energetic photons!

3. Electromagnetic Spectrum Electromagnetic Radiation(EMR) - the flow of energy at the speed of light in the form of electric & magnetic fields. Radio waves, infrared, visible light, ultraviolet, x-rays, and gamma rays Electromagnetic spectrum – arrangement of EMR according to their wavelengths and frequencies Most wavelengths are too long or short for us to see Continuous spectrum  A spectrum of radiation distributed over an uninterrupted range of wavelengths.

4. Absorption vs. Emission Spectrum Emission spectrum – a series of bright lines of particular wavelengths produced by a hot gas under low pressure releasing energy Absorption spectrum – “dark line spectrum,” produced when white light passes through a gas and certain wavelengths of light are absorbed Most often used by astronomers to identify stars. The specific wavelengths of absorption spectral lines of elements are like “fingerprints” used to identify the specific elements present in a star

5. Spectrum of stars Stars are made of a mixture of elements. The line spectra will contain all the lines from each element in the mixture. Line spectrum of the sun 

7. The Doppler Effect Doppler effect – the perceived change in wavelength of a wave emitted from a source moving away or toward an object (wavelength doesn’t actually change) Sound: Higher pitch when approaching Lower pitch when receding Light: Light source moving away—the wavelength is stretched & causes a Red Shift Spectral lines shift toward red end Light source moving toward—wavelength is compressed & causes a Blue Shift Spectral lines shift toward blue end

8. Tools For Studying Space Telescopes are used to collect the light energy (EMR) from distant galaxies and stars Types of telescopes: Optical, Radio, & Space

9. Optical Telescopes: Properties Optical telescopes – contain mirrors or lenses, collect visible light 1. Light-gathering power – intercepts light from distant objects larger lens or mirror = “see” farther 2. Resolving power – allows for sharp images and fine detail larger lens or mirror = sharper image 3. Magnifying power – ability to make an image larger, depends on focal lengths of the objective and eyepiece

10. Optical Telescopes: Refracting Refracting telescope – use lenses to refract or bend light First used by Galileo Mostly smaller telescopes The objective lens produces an image by bending light from distant objects so that light converges on a focus (central point) The image is inverted as light passes through both the objective lens and the eyepiece.

11. Optical defect of refracting telescopes: Chromatic aberration – light of different colors are focused at different locations. Chroma = colorAberration = to go astray when red light is in focus there is a blue halo when blue light is in focus there is a red halo Optical Telescopes: Refracting (continued)

12. Optical Telescopes: Reflecting Reflecting telescope – uses a concave mirror to focus light Newton built reflecting telescopes Focus is in front of the mirror. Advantages: Glass doesn’t need to be optical quality Can be supported from behind the mirror Can be made much larger Disadvantage: secondary mirror blocks some light. Solution…make it bigger!

13. Detecting invisible radiation Radiation we can’t see: gamma, X-ray, UV, IR, microwave, & radio Most radiation from the sun cannot pass through our atmosphere. Objects can be sent above the atmosphere to detect it (balloons, rockets, satellites) A narrow band of radio waves does pass through our atmosphere and can be detected on Earth’s surface

14. Radio Telescopes Focuses incoming radio waves on an antenna, absorbs and transmits waves to an amplifier Interferometer - several radio telescopes linked together Advantages = can run all the time, cheaper than space telescopes, can detect clouds of gas cooler than visible light Disadvantages = take up a lot of space, hindered by human-made radio interference

15. Space Telescopes Orbit above the atmosphere to produce clearer images Hubble Space Telescope was the 1 st Used to study everything from birth of stars to extrasolar planets (planets orbiting other stars) Chandra X-ray Observatory Studies black holes by collecting X-Rays Compton Gamma-Ray Observatory Studies gamma rays from exploding stars &black holes James Webb Space Telescope (planned space observatory) Will study IR radiation from the oldest stars