CHAPTER 28 STARS AND GALAXIES

28.1 A CLOSER LOOK AT LIGHT Light 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-rays Electromagnetic waves do not need a medium to travel, they travel through space indicate what elements are present and its motion

Electromagnetic waves are arranged into a continuum called the electromagnetic spectrum. Wide range of wavelengths Long wavelengths with low frequencies at one end, short wavelengths with high frequencies at the other end Wavelength measured crest to crest/trough to trough Frequency the number of that crests of the same wavelength that pass a point in one second.

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.

Three types of visible spectra Continuous spectrum: unbroken band of colors, emitting all colors of visible light glowing solids, such as a light bulb filament glowing liquids, molten iron hot, compressed gases inside a star Emission spectrum: unevenly spaced lines of different colors, emitting light of only some wavelengths glowing thin gases every element has its own unique emission spectrum, element’s finger print Absorption spectrum: dark lines that cross a continuous spectrum. forms when light from a glowing object pass through a cooler gas which absorbs some wavelengths can determine what is present in the cooler gas by comparing emission and absorption spectrum

Doppler Effect is the apparent change in the wavelength of radiation or sound due to relative motion between the object and the receiver.

Doppler effect applies to lights as well as sound. Shift of the emission spectra can indicate if the object is moving Redshift- shifted towards red end of spectrum; away from Earth Blueshift- shifted towards blue end of spectrum; towards Earth Doppler effect determined that the universe is expanding

TELESCOPES Optical Telescopes – gather far more light than an unaided eye and magnify imagers Reflecting Uses one lens at back to gather and focus light Image reflected on to a small mirror and then the eye piece Refracting Uses two lenses Lens at the front gathers light Eyepiece magnifies image

Radio Telescopes Other Technical term for really big satellite dishes Use to detect energy waves at frequencies lower than visible light Other Detects energy waves at frequencies higher than visible light Usually satellites in space Gamma ray Background radiation X-ray Hubble (infrared)

28.2 Stars and Their Characteristics Constellations: human grouping of stars Will change shape over time due to expanding universe Move across sky from east to west Big and Little Dipper used to find the North Star Parallax: change in an object’s direction due to a change in the observer’s position

North Star Called Polaris Sits directly over the North pole Does not move to the naked eye Very powerful tool for navigation

Circumpolar Constellations – constellations that seem to move around the north star Appear to never “set” or go below the horizon

Constellations that dominate nighttime sky change with each month due to the change in Earth’s position around the Sun. (Parallax!)

Parallax The further the object from the viewer, the less the parallax shift.

Distances in Space Astronomical Unit (AU- used for distances within our solar system Light year is the distance light travels in one year (9.5 trillion kilometers) Example - 4.2 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 3.258 light-years

Star Composition Star Mass Mainly hydrogen and helium We must estimate, since we cannot measure Gravitational pull- the stronger the pull, the larger the object Expressed in numbers where our Sun is 1 Ex: 32 suns Star Mass

Star size comparison

Star Temperature Different colors mean different temps Cool stars are redder in color Ex. Betelguese with a surface temperature of 3000oC Mid-temperatures are yellower in color Ex. The Sun with a surface temperature of 5500oC Hot stars are blue in color Ex. Sirus

Star Brightness Luminosity: brightness of a star Depends on size and temp Distance from Earth not a factor If two stars are the same size the hotter star would be brighter Apparent magnitude is how bright a star appears to an observer on Earth Depends on distance Absolute magnitude is the measure of how bright the star would be if all stars were the same distance from Earth (10 parsec)

Variable Stars Some stars that pulse as they expand and contract Brightness varies over many days Called Cepheid Variables