Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW1Questions of the Day• What do the spectra of stars tell us about their compositions and temperatures?• How can a star made almost entirely of Hydrogen show no absorption lines of Hydrogen?• What is the difference between luminosity and apparent brightness?• How does apparent brightness depend on luminosity and distance?Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UWAdministrative stuff•Please remember to add “Astro 101” in the subject of your email!•Remember: Lots of help is available during office hours! You can go to any TA for help!2Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW3A simple stellar spectrum• Almost entirely Hydrogen absorption! Just like we’d expect for a universe made mostly of Hydrogen.HydrogenH!H"H# H$Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW4But, obviously not true for this star!• How do these stars relate to each other??• Are different stars made of different elements in different proportions (“chemical composition”)?This was the situation at the turn of the century (1900, not 2000), when astronomers wondered, “What the heck?”….Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW5The Harvard “Computers”Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW6Annie Jump CannonDiscovered that stars fell into a natural order, based upon their spectra.Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW7Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW8The Spectral Sequence•Previously, Williamina Fleming had classified stars based upon the strengths of their Hydrogen lines.A – Strongest Hydrogen linesB – 2nd strongestC – 3rd strongest(etc, down to O)• Cannon figured out there was a different order which made more sense, and which condensed the classes down to 7: OBAFGKM, with numerical subdivisions 1-9 (O1,O2,O3,…,O9,B1,B2,…,M9)Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW9H!H"H#Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW10Only Boring Astronomers Find Gratification Knowing Mnemonics Like ThisUsed to be: “Oh be a fine girl/guy, Kiss me!”, but astronomers discovered new types of stars which were beyond M-type stars.NEW!Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW11The spectral sequence in Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW12Same spectra, plotted as graphs of brightness vs wavelength.BLUEREDAstronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW13There is a systematic change in color across the spectral sequence!BLUEREDO stars – BlueM stars - RedAstronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW14Temperature!HOT!(B star)Cool!(K star)Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW15Temperature varies incredibly strongly across the spectral sequence…Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW16But can temperature changes produce such large variations in the absorption lines?Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW17Cecilia Payne-GaposchkinAstronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW18Why do lines come and go?_ _ _No bound electrons to absorb photons!Completely ionized elements cannot produce absorption lines!Temperature too high!Some atoms completely ionized!Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW19Why do lines come and go?Can’t produce absorption lines that start above ground levelIf there are no high energy photons to first push the electrons out of the ground state, absorption lines from the high energy levels will never occur.[Need to go 1!2, before going 2!3]Temperature too low!All electrons in the ground stateOKNo!Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW20“Line Strength” depends upon temperatureToo hot, few bound electronsToo cool, few electrons in Level 2Line is Strongest at this TemperatureAstronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW21But, different elements behave differently ! accurate thermometerSimilar analysis responsible for the T=5770K measurement of temperature for the Sun.Ionized elements can still produce absorption lines, as long as they still have some electrons left (i.e. even if helium loses one electron to ionization, it still has one more left!).Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW22The spectral sequence in order…Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW23Molecules play a role as well…•Molecules can absorb (and emit) photons.•The different “energy levels” correspond to the amount of rotation and vibration.•These levels are also quantized!Important in cool M, L, and T stars, where the temperature is low enough that molecules aren’t destroyedAstronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW24When the temperature of a star varies:• “Spectral Type” (OBAFGKM) varies• Color varies• Surface brightness varies ! " # T4 ! " = #T4Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW25But, higher surface brightness does not necessarily mean the star is brighter.•Luminosity is not the same as Surface Brightness!Hot.Cool.Cool, but big!Luminous!Same LuminositySame Surface BrightnessAstronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW26•Luminosity: Energy per second being emitted, total.more energy coming out in photons per secondLow LuminosityHigh LuminosityAstronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UWXThe “per second” matters!Which is more luminous, slowly burning the gas, or exploding it?The energy of the gas in the can is FIXED, but you can release it quickly or slowly.Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW27What are the units of Luminosity?1 L" = 1 Solar Luminosity = 3.9 X 1026 Watts = 3.9 X 1026 Joules / Second = 3.9 X 1026 (kg m2/s2) / sRemember that Luminosity is Energy Per Second.Walking: 106 Joules/hour The US: 1020 Joules/year[The unit of energy is the “Joule”, equal to the kinetic energy that 1 kg has if it’s moving at 1 m/s]Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW28•Surface Brightness is Luminosity per Area: ! SurfaceBrightness=LuminosityArea" # $ % & ' Surface brightness is the energy per second passing through the squareTo get the total luminosity, multiply the
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