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Stars The Electromagnetic Spectrum • What are electromagnetic waves? • Stars, including our sun, emit many types of radiation, which travel to Earth in the form of waves. • There are also some man-made waves in Earth’s atmosphere, such as TV waves. These waves are called electromagnetic waves Electromagnetic Waves • • • • • • • • • Cosmic Rays Gamma (γ) Rays X-Rays Ultraviolet (UV) Radiation Visible Light Infrared Radiation Microwaves TV Waves Radio Waves What is the electromagnetic spectrum? Wavelength – the distance between 2 consecutive crests (tops) of a wave Frequency – the number of crests that pass any given point during a period of time What is the difference between the various types of electromagnetic waves? • The only difference between the various types of waves is the amount of energy the waves carry: The shorter the wavelength, the higher the energy • http://earthguide.ucsd.edu/eoc/special_topic s/teach/sp_climate_change/p_emspectrum_i nteractive.html • https://www.youtube.com/watch?v=bjOGNV H3D4Y What can we determine about a star from its visible light radiation? • We can use a star’s visible light spectrum to tell what that star is made of. • Prisms can be used to separate the colors of white light. The various wavelengths making up white light bend at different angles when they pass through the prism. They separate from each other and the colors of the visible light spectrum become visible. • The visible light spectrum is the series of colored bands broken up and arranged in the order of their wavelengths. The visible light spectrum makes up a small portion of the electromagnetic spectrum. • The following are the seven main colors of the visible light spectrum in order from longest to shortest wavelength: • Red, Orange, Yellow, Green, Blue, Indigo, and Violet (ROYGBIV) • Stars, like all matter in the universe, are composed of chemical elements. Each chemical element absorbs certain wavelengths of light, producing a certain pattern of dark lines. We can take a photograph of the visible light spectrum emitted from a star. The pattern of dark lines that show up in the picture of the spectrum of a star can be used to identify which elements are in that star. • Spectroscope:Instrument used to determine which elements are present in a star by examining the dark spectral lines and comparing them to the spectral lines of known elements. Doppler Shift • How does the movement of an object in space affect how we see the light from that object on Earth? • Light from moving objects in the solar system will appear to have different wavelengths than light from stationary objects, depending on the relative motion of the source and the observer. • There are two types of wavelength changes, depending on the movement of the object emitting light: • 1) Red Shift: If the source of light is moving away from you, then the wavelength of the light is stretched out and the light is shifted toward the red end of the spectrum. • 2) Blue Shift: If the source of light is moving toward you, then the wavelength of the light is compressed and the light is shifted toward the blue end of the spectrum. What is a “Doppler” shift? • Red shift and blue shift are together referred to as Doppler shifts and are named after Christian Doppler, the Austrian scientist who discovered them in 1842. https://www.youtube.com/watch?v=z0EaoilzgGE https://www.youtube.com/watch?v=a3RfULw7aAY The diagram below and to the right shows a galactic star at the bottom left with its spectrum on the bottom right. • Notice the dark absorption lines in the spectrum. • The other three spectra show the spectrum of a nearby galaxy, a medium distance galaxy, and a distant galaxy. Notice how the pattern of absorption lines shifts to the red end of the spectrum as the galaxies become more distant. The numbers above and below the spectra are the measured wavelengths in nanometers. Classification of Stars • 3 Ways to Classify Stars 1.)Temperature (Kelvin (K) = Degrees Celsius + 273) • Coolest Stars: Temperature = 2000 Kelvin • Hottest Stars: Temperature = 40,000 Kelvin Another way to describe temperature of stars is Spectral Class • Spectral Class O stars are the hottest; Spectral Class M stars are the coolest. • Mnemonic device to remember the order of the spectral classes from hottest to coolest: Oh Be A Fine Guy, Kiss Me or Oh Be A Fine Girl, Kiss Me • However, temperature doesn’t tell a star’s whole story and is therefore not the best way to classify them. • For example, Betelguese and Proxima Centauri have the same surface temperature (they are both Spectral Class M stars). However, Betelguese is millions of times brighter (it has a greater or more negative absolute magnitude). Apparent and Absolute Magnitude • Recall from earlier that apparent and absolute magnitude are ways of classifying stars by their brightness or luminosity. H-R Diagrams • Just prior to World War I (1911 – 1913), two astronomers working separately had the same idea. They both determined a way to classify stars by both their temperature and absolute magnitude at the same time. • • These two scientists were Ejnar Hertzsprung (from Denmark) and Henry Norris Russell (from the U.S.). They developed the HertzsprungRussell (H-R) Diagram Based on their temperature and absolute magnitude, stars fall into one of 4 categories on the H-R Diagram: • Main Sequence Stars • About 90% of all stars in the universe are main sequence stars. • The main sequence can be found on a narrow band running from the upper left corner (hot, bright stars) to the lower right corner (cool, dim stars) on the H-R diagram. • The distinguishing characteristic of main sequence stars is that they start off as hot and bright and go until they are cool and dim. • Our sun is a Main Sequence star. It is found in the middle of the main sequence diagonal. • By comparing the temperature of other Main Sequence stars to the temperature of our sun, we can make certain assumptions about their brightness and size relative to our sun: • Example All Main Sequence stars HOTTER than our sun are also BRIGHTER than our sun and LARGER than our sun, such as Sirius. • Example All Main Sequence stars COOLER than our sun are also DIMMER than our sun and smaller than our sun, such as Proxima Centauri • Note: The above statements are true ONLY for Main Sequence stars and do not apply to the remaining 10% of stars not located on the Main Sequence of the H-R Diagram