2 28.1 A CLOSER LOOK AT LIGHTLight is a form of electromagnetic radiation, which is energy that travels in waves.Waves of energy travel at 300,000 km/sec (speed of light)Ex: radio waves and x-raysElectromagnetic waves do not need a medium to travel, they travel through spaceindicate what elements are present and its motion
3 Electromagnetic waves are arranged into a continuum called the electromagnetic spectrum. Wide range of wavelengthsLong wavelengths with low frequencies at one end, short wavelengths with high frequencies at the other endWavelength measured crest to crest/trough to troughFrequency the number of that crests of the same wavelength that pass a point in one second.
4 Visible light portion of the electromagnetic spectrum is studied using a spectroscope. Spectra of a stars allow for astronomers to learn about the star’s elements and motion.
5 Three types of visible spectra Continuous spectrum: unbroken band of colors, emitting all colors of visible lightglowing solids, such as a light bulb filamentglowing liquids, molten ironhot, compressed gases inside a starEmission spectrum: unevenlyspaced lines of different colors,emitting light of only somewavelengthsglowing thin gasesevery element has its own uniqueemission spectrum, element’s finger printAbsorption spectrum: dark lines thatcross a continuous spectrum.forms when light from a glowing object pass through a cooler gas which absorbs some wavelengthscan determine what is present in the cooler gas by comparing emission and absorption spectrum
6 Doppler Effect is the apparent change in the wavelength of radiation or sound due to relative motion between the object and the receiver.
7 Doppler effect applies to lights as well as sound. Shift of the emission spectra can indicate if the object is movingRedshift- shifted towards red end of spectrum; away from EarthBlueshift- shifted towards blue end of spectrum; towards EarthDoppler effect determined that the universe is expanding
8 TELESCOPESOptical Telescopes – gather far more light than an unaided eye and magnify imagersReflectingUses one lens at back to gather and focus lightImage reflected on to a small mirror and then the eye pieceRefractingUses two lensesLens at the front gathers lightEyepiece magnifies image
9 Radio Telescopes Other Technical term for really big satellite dishes Use to detect energy waves at frequencies lower than visible lightOtherDetects energy waves at frequencies higher than visible lightUsually satellites in spaceGamma rayBackground radiationX-rayHubble (infrared)
10 28.2 Stars and Their Characteristics Constellations: human grouping of starsWill change shape over time due to expanding universeMove across sky from east to westBig and Little Dipper used to find the North StarParallax: change in an object’s direction due to a change in the observer’s position
11 North Star Called Polaris Sits directly over the North pole Does not move to the naked eyeVery powerful tool for navigation
12 Circumpolar Constellations – constellations that seem to move around the north star Appear to never “set” or go below the horizon
13 Constellations that dominate nighttime sky change with each month due to the change in Earth’s position around the Sun. (Parallax!)
14 ParallaxThe further the object from the viewer, the less the parallax shift.
15 Distances in SpaceAstronomical Unit (AU- used for distances within our solar systemLight year is the distance light travels in one year (9.5 trillion kilometers)Example light-years means that the light we see has been traveling for 4.2 years before we can see it (4.2 X 9.5 trillion km)Parsec short for “parallax second” equal to light-years
16 Star Composition Star Mass Mainly hydrogen and helium We must estimate, since we cannot measureGravitational pull- the stronger the pull, the larger the objectExpressed in numbers where our Sun is 1Ex: 32 sunsStar Mass
18 Star Temperature Different colors mean different temps Cool stars are redder in colorEx. Betelguese with a surface temperature of 3000oCMid-temperatures are yellower in colorEx. The Sun with a surface temperature of 5500oCHot stars are blue in colorEx. Sirus
19 Star Brightness Luminosity: brightness of a star Depends on size and tempDistance from Earth not a factorIf two stars are the same size the hotter star would be brighterApparent magnitude is how bright a star appears to an observer on EarthDepends on distanceAbsolute magnitude is the measure of how bright the star would be if all stars were the same distance from Earth (10 parsec)
20 Variable Stars Some stars that pulse as they expand and contract Brightness varies over many daysCalled Cepheid Variables