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The Hertzsprung-Russell Diagram

The Hertzsprung-Russell Diagram. What is a star?. A cloud of gas, mainly hydrogen and helium The core is so hot and dense that nuclear fusion can occur. The fusion converts light elements into heavier ones. Every star is different. All the stars in the night sky are different Brightness:

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The Hertzsprung-Russell Diagram

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  1. The Hertzsprung-Russell Diagram

  2. What is a star? • A cloud of gas, mainly hydrogen and helium • The core is so hot and dense that nuclear fusion can occur. • The fusion converts light elements into heavier ones

  3. Every star is different • All the stars in the night sky are different • Brightness: • Tells us how luminous the star is, i.e. How much energy is being produced in the core • Color: • Tells us the surface temperature of the star

  4. Units of luminosity • We measure the luminosity of every day objects in Watts. • How bright is a light bulb? • By comparison, the Sun outputs: • 380,000,000,000,000,000,000,000,000 Watts • (380 million million million million Watts!) • This is easier to right as 3.8 x 1026 Watts • To make things easier we measure the brightness of stars relative to the Sun.

  5. What is the HR Diagram? • Tool that shows relationships and differences between stars. • Relative measure of luminosity & temperature. • Shows stars of different ages/stages at the same time.

  6. Measuring the temperature • The temperature of a star is indicated by its color • Blue stars are hot, and red stars are cold Red star Yellow star Blue star 3,000 K 5,000 K 10,000 K

  7. The Basics… • Each star is represented by a dot. • Vertical axis shows luminosity/absolute magnitude. • How much energy a star radiates in 1 sec. • How bright/dim a star appears, compared to Sun • Horizontal axis shows surface temperature (in K.) • NOTICE: Goes from high temps at left side to lower temps at right side!

  8. Types of Stars • Stars that fit the expected pattern – cool and dim; hot and bright – are called main sequence stars • Most stars fall in the region called main sequence. • Curved line sloping from top left to lower right of HR diagram.

  9. Why is there a “main sequence”? • Usually there is a predictable relationship between brightness and temp. Hotter things are brighter Hotter temp = more energy is radiated. Bigger stars are brighter. Bigger surface area = more energy radiated.

  10. Exception to the rule! • Some stars are cool but if they are massive enough, they can have high luminosity  red giants or supergiants

  11. Another Exception to the Rule • Some stars are very hot and shine intensely but are small so they don’t radiate much energy (appear dim)  white dwarf stars

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