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How far away are the stars?. Parallax. Measuring the Distance to the Stars with Parallax. Remember that 1 parsec = 3.26 light years. Visual magnitude scale. Originally, the brightest stars in the sky were considered first magnitude, the next brightest group second magnitude, and so on
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Measuring the Distance to the Stars with Parallax Remember that 1 parsec = 3.26 light years
Visual magnitude scale • Originally, the brightest stars in the sky were considered first magnitude, the next brightest group second magnitude, and so on • Now, magnitude is more precisely defined, so that a m=+1.0 star is 100 times as bright as a m=+6.0 star • Each magnitude is a factor of 2.5
Star Names • Most of the brightest stars have names (e.g., Betelgeuse, Sirius) • Up to 24 stars in each constellation have a name with a Greek letter (e.g., β Tauri) • The rest may only be known by a catalog number (e.g., HDE 226868)
Absolute magnitude • Equivalent to apparent magnitude of the star if it was at a distance of 10 parsecs • The sun is M=+4.8
The brightest stars (M=-10) have luminosities of 106 L • The dimmest stars (M=+17) have luminosities of 10-5 L Luminosity • Measure of the amount of light a star gives off • L=3.83 x 1026 W
“Cool”, red star Warm, yellow star Hot, blue star A star’s color reveals its surface temperature
Photometry • Technique used to accurately measure the color of stars using UBV filters • U - ultraviolet wavelengths • B - blue wavelengths • V - visible wavelength • astronomers compare combinations of U minus B (U-B) and B minus V (B-V) • these combinations are called color indices
Stars are classified by their spectra as O, B, A, F, G, K, and M spectral types
Stars are classified by their spectra as O, B, A, F, G, K, and M spectral types • O B A F G K M • hottest to coolest • bluish to reddish • An important sequence to remember: • Our Best Astronomers Feel Good Knowing More • Oh Boy, An F Grade Kills Me • Oh Be a Fine Girl (or Guy), Kiss Me
The Hertzsprung-Russell diagram identifies a definite relationship between temperature and absolute magnitude Absolute magnitude vs temperature or luminosity vs spectral type
Determining the Sizes of Stars from an HR Diagram • Main sequence stars are found in a band from the upper left to the lower right • Giant and supergiant stars are found in the upper right corner • Tiny white dwarf stars are found in the lower left corner of the HR diagram
Luminosity classes • Class I includes all the supergiants • Class V includes the main sequence stars • For example, the Sun is a G2 V
How do we know the mass of a star? • Binary star systems • Recently, also from extrasolar planets