Class 16 : Other stars Beyond the solar system – other stars The distance to other stars. Properties of other stars. The Hertzsprung-Russell (HR) diagram.
I : The distance to other stars Recap… 1 AU = 150 million km. How far are the nearest stars? Look at brightness… Brightness of something drops with the square of the distance. Brightest stars are about 10 11 times fainter that the Sun… If same luminosity, this means that they are about 300,000 times further away (i.e. 300,000 AU, or about 5 light years).
Stellar parallax Parallax: Stars appear to wobble as the Earth moves around Sun. Can use this to measure distance to stars (since Earth-Sun distance known well).
If star wobbles with amplitude of 1 arc-second (1/3600 th of a degree), then it is at distance of 1 parsec (definition of parsec). 1 pc = 3.26 light years. In general, Find that nearest star is indeed about 1.2 parsecs (250,000 AU; 4 light years) from us. Method works out to 1000 pc.
II : Stellar properties Stars have various… Luminosities Feeble star = 0.0001 L . Powerful star = 50,000 L . Masses (~0.1 to 100 M ) Biggest stars usually the most luminous. Colors Depends on surface temperature of star. Cool star (RED) = 3000 K. Hot star (BLUE) = 30,000 K.
Beta Cygni (double star, also called Albireo) “topaz & sapphire”
Don’t bother copying… Stellar temperature/color also gives rise to “Spectral Classes.” O (> 30,000 K). B (10,000 – 30,000 K). A (7,000 – 10,000 K). F (6,000 – 7,000 K). G (5,000 – 6,000 K) – the sun! K (4,000 – 5,000 K). M (< 4,000 K).
Knowing luminosity and temperature, we can calculate the radius using the “Stephan- Boltzmann law”: L = 4R 2 T 4 - or - (L/L ) = (R/R ) 2 (T/T ) 4. Important points – = 5.6710 -8 W/m 2 /K 4 is a number called the Stephan-Boltzmann constant. Luminosity increases very rapidly with temperature (2 T gives 16 L) and radius (2 R gives 4 L).
II : The Hertzsprung-Russell diagram Very important plot in astronomy. Plot of luminosity against temperature for stars Find that stars fall into definite groups…