To help understand the HR Diagram  More Equations To help understand the HR Diagram 

Is everything appears to what it is?

Temperature - Wein’s Law SURFACE Temperature Based on color Measures spectrum of a star Strongest wavelength tells you the surface temperature using Wein’s Law T= 3 x 106 wavelength

Temperature example

Only Boys and Fearless Girls Kill Mice Spectral Classes Only Boys and Fearless Girls Kill Mice

Spectral Classes Spectra of Stars Can Tell Us: Temperature Motion Composition Rotation Speed Luminosity

Luminosity Luminosity: how bright is a star based on absolute magnitude

Luminosity Depends on 2 factors Temperature Radius Hotter = brighter Bigger = brighter Distance can make bright stars seem dim and dim stars appear brighter

Scenarios 1) If both stars are 7500 K but star A is 5 x times bigger than star B, which is more luminous? Why? 2) If both stars 7500 K but star A is 5 parsecs away and star B is 15 parsecs away, which is more luminous? Why? 3) If star A is 10,000 K and is 20 parsecs away but star B is 3,000 K and is 10 parsecs away, which is more luminous? Why?

Magnitude About 150 B.C., the Greek astronomer Hipparchus measured apparent brightness of stars using units called magnitudes Brightest stars had magnitude 1 and dimmest had magnitude 6 The system is still used today and units of measurement are called apparent magnitudes - how bright a star appears to look to an observer

Magnitude A star’s apparent magnitude depends on the star’s luminosity and distance – a star may appear dim because it is very far away or it does not emit much energy The apparent magnitude can be confusing Scale runs “backward”: high magnitude = low brightness Modern calibrations of the scale create negative magnitudes Magnitude differences equate to brightness ratios: A difference of 5 magnitudes = a brightness ratio of 100 1 magnitude difference = brightness ratio of 1001/5=2.512

Magnitude Astronomers use absolute magnitude to measure a star’s luminosity The absolute magnitude of a star is how bright a star is at 10 parsecs away(32.6 light years) A comparison of absolute magnitudes is now a comparison of luminosities, distance variable is removed! An absolute magnitude of 0 approximately equates to a luminosity of 100 x The Sun

Practice Order the stars based on luminosity and temperature. Temperature Luminosity   Blue Giant Blue Super Giant Neutron Star Red Dwarf Red Giant Red Super Giant White Dwarf Yellow Dwarf