Unit 3

 Much of the information we get in astronomy is carried by “light”.

 Location of light in the sky...gives position of a planet, moon, or star.  Color…gives temperature, speed, and direction of motion, chemical composition.  Brightness…gives distance, composition, periods of motion (from changes).  Size of light…gives size and distance information.

 Visible light, radio waves, x- rays… are all different forms of the something – all “electromagnetic radiation”.

 Speed of light in empty space is constant  C=3*10^8 m/s = 670 billion mph  This finite speed has a real effect on information carried by light

 It takes 1 ¼ sec for light to reach us from the moon.  It takes 8 1/3 minutes for light to reach us from the sun.  It takes 4 1/3 years for light to reach us from the nearest star.

 Think of light as a wave

 Wavelength – the distance between wave crests.  Wavelength is indicated by the Greek symbol “λ” in the figure above.  Frequency = number of wave crests per second passing a point.

We assume that each eave is traveling at the same speed in this figure

 Color is determined by wavelength or frequency of light  We can divide light or electromagnetic radiation by wavelength (color), we create a spectrum.  This is called the Electromagnetic Spectrum

 Visible light is actually a very small part of the spectrum  We can see the colors within the sunlight because each color is bent differently when it passes through a prism.

 1. Sun emits most strongly at these wavelengths  2. Earth’s atmosphere blocks many other wavelengths  3. Human eye efficiently detects available light.

 Sometimes light acts like a particle – “Photon”  Sometimes light acts like a wave  It is really both or neither, we just don’t have the right words to describe it completely.

 Energy carried by light depends on wavelength (color and frequency).  Short wavelength – More energy  Long wavelength – Less energy ▪ Blue light carries more energy than red light. ▪ Thus, X-Rays carry more energy than Radio waves.

 Mathematically, we can express this as:  E = hc/ λ  Where E is the energy, λ is the wavelength, and h and c are constant numbers.

 All objects emit electromagnetic radiation from heat.  Warm – Dull Red  Hot – Bright Red  Really Hot – “White Hot”

 Properties of this from heat:  Covers a range of wavelength  Emits more at one single wavelength (peak wavelength)  Peak wavelength gets shorter for higher temperatures

 Hot star …emits mainly in ultraviolet  Sun…emits mainly in the yellow- green part of the spectrum  Cool star… emits mainly in the red part of the visible  Planets, People…emits mainly in the infrared

 Do not confuse this emitted light with the reflected light, which allows our eyes to see other people, planets, etc.

Grading Rubric

 Gamma Waves  Waves emitted from radioactive nuclei, highly penetrating, very harmful to humans  X-Rays  Waves emitted when high energy electrons bombard metal, used to diagnose internal injuries and treat certain cancers- overexposure is harmful to humans.

 Ultraviolet  Waves which cause sunburn-ozone is our atmosphere converts UV radiation to heat, can cause skin cancer in humans.  Visible Light  Wavelengths detected by the human eye, caused by electrons moving between orbitals in atoms, violet is the shortest wavelength and red is the longest.  Violet-Indigo-Blue-Green-Yellow-Orange-Red

 Infrared  Waves produced by hot bodies, they are readily absorbed and cause the material’s atoms to vibrate, generating heat- usefully in astronomy and in physical therapy.  Radio Waves  Waves produced by charged matter being accelerated, types include AM/FM radio, TV, Microwaves, and radar – useful in astronomy, communication, and have domestic applications.

 Continuous Spectrum  White light spread out by a prism showing all the wavelengths of visible light.  Emission Lines  Specific wavelengths emitted when an atom’s electrons are excited and then jump back to their lowest energy level – unique for each type of atom.

 Fraunhofer Lines  Discovered in 1817 – absorption lines in a star’s spectrum  Absorption Lines  Wavelengths missing from a spectrum caused by the absorption of those wavelengths  Shows the COMPOSITION of the object being studied.

 Doppler Effect  The compression or stretching of an object’s absorption spectrum caused by the relative movement of the object – First identified with the sound waves  Shows DIRECTION  Red Shifting – Moving away  Blue Shifting – Moving towards

 Amount of Shifting  Is used to tell the relative SPEED of the object being studied – the greater the shift, the greater the relative speed.  Composition of Star  Indicates the star’s TEMPERATURE

 Composition is indicated by spectral lines  Shifting of lines indicates relative direction of movement  Amount of shift indicates relative speed  Composition also indicated temperature