Magnitudes. How Powerful Are the Stars? Luminosities of Different Stars

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1 How Powerful Are the Stars? Some stars are more powerful than others Power is energy output per. (Example: 00 Watts = 00 joules per second) Astronomers measure the power, or brightness of stars in ways: luminosity and. s : how bright the star appears : how bright the star actually is Sun: apparent -, absolute.8 Alpha Centauri: apparent -0.0, absolute.3 Luminosity: Amount of power a star Luminosity passing through each sphere is brightness: Amount of starlight that reaches Area of sphere: π (radius) Luminosity does not decrease with distance, but apparent brightness does brightness obeys an - relation Luminosities of Different Stars Faint M Star: L sun G Star.0 L sun A Star (Vega) : 50 L sun Giant star: 00 L sun Most luminous star known (Pistol Star): 0 Million L Sun! Example: Say two stars (A and B) have the same luminosity. Star B is four times closer to Earth than Star A. How does the apparent brightness of Star B compare to that of Star A?

2 What about size? Bigger than How do we know how big a star is? What is the range of sizes for stars? Characterize by Smaller than Miniscule compared to : Sun ( pixel) Which star would appear most red in color? Which star has the highest temperature?

3 Which star is putting off the largest total amount of energy? Radius Imagine two hot plates of the same size (but the right one is hotter): Which will cook a pot of spaghetti faster? Radius Imagine two hot plates of the same temperature (but the right one is larger): Radius Imagine two hot plates with different sizes and different temperatures: Which will cook a pot of spaghetti faster? Which will cook a pot of spaghetti faster? Size Lecture Tutorial: Page 53 Organizing the properties of stars: The H-R Diagram Work with a partner or two Read directions and answer all questions carefully. Take time to understand it now! Come to a consensus answer you all agree on before moving on to the next question. If you get stuck, ask another group for help. If you get really stuck, raise your hand and I will come around. Absolute mag. or Luminosity versus Hertzsprung-Russell Diagram Temperature Spectral type: O B A F G K M 3

4 Use of H-R Diagrams H-R Diagrams have been called the astronomers They quickly tell us the type of star They can also reveal the, & of the star. Stars of the same radius follow diagonal lines The Hertzsprung Russell (HR) Diagram Most stars are found along the H-R Diagram: es of Stars More massive stars on the Main Sequence appear to be. es in units of solar masses es of Stars Stars on the Main Sequence are more High masses es in units of solar masses An O type star on the Main Sequence has 0x the Sun s mass. Using the H-R Diagram we can read off the of stars of different mass. An M type star on the Main Sequence has only /0th of the Sun s mass. Low masses

5 H-R Diagram Lecture Tutorial: Pg Which star looks brighter from Earth? Work with a partner or two Read directions and answer all questions carefully. Take time to understand it now! Come to a consensus answer you all agree on before moving on to the next question. If you get stuck, ask another group for help. If you get really stuck, raise your hand and I will come around. Star X Y Absolute Which star is more luminous? Which star is closer to Earth? Star Absolute Star Absolute X X Y Y Which star lives the longest? Stars which are fusing hydrogen live on the in the H-R diagram. (Note: The isn t a place in space!) Stars or Cars? Stars live until they burn up a portion of their hydrogen fuel. Then they evolve, or age, and finally die. The stars which live the longest are the ones which. 5

6 Burning Hydrogen Fuel O Type Stars are the most luminous (most energy) The more quickly Hydrogen is used up, the the life of the star. O Type stars live for only million years G Type stars, like the Sun live for 5 billion years M Type stars can live for 00 billion years! Because they burn fuel. es of Stars determines the fate of the star: whether or not it will explode. -> luminosity -> lifetime But how do we measure a star s? Binary Stars Most stars are in multi-star systems Binaries, triplets, quadruplets, etc Sirius Polaris Alcor and Mizar in Big Dipper Binary Stars If bound by, both stars orbit around the center of mass Center of mass is always to the more star Center of The Sun, all by itself, is not typical m M How do we find the mass of stars? We can use Kepler s Third Law to measure the masses of binary stars. Recall Kepler s 3rd Law: Newton improved Kepler s law using the Law of gravity. as you go up the Main Sequence So does luminosity! High masses M A + M B = a AU 3 P y (M A and M B are the of the stars in units of solar masses) and do not obey the - Luminosity relation Low masses

7 The -Luminosity Relation on the When we plot and Luminosity, we find that more massive stars are much luminous. Luminosity depends on mass to the 3.5 power L ~ M 3.5 -Luminosity Relation Example: If star A is three times as massive as star B, how much more luminous is it? L ~ M 3.5 L A /L B = (M A /M B ) 3.5 L A /L B = L A /L B = L A /L B = HR Diagrams H-R Diagrams Faint, red dwarfs (low mass) are the most stars. Bright, hot, blue main-sequence stars (highmass) are. Giants and supergiants are. A Census of the Stars 7