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Properties of Stars and H-R Diagrams How WE can tell the differences between stars. Objectives • When given the color of a star, determine the temperature. • To tell the difference between absolute magnitude and apparent magnitude • Read an HR diagram to determine • • • • • Stage of star Luminosity or brightness of star Temperature & color of star Relative size of star Where our sun is Surface Temperature of Stars • Surface temperature is a measure of how much energy is being given off on the outside of a star. • Most of the nuclear fusion reactions happen in the core. While nuclear fusion is happening, the core is the hottest place in a star. • Dense things are hotter. Large and airy things are cooler. • Astronomers can look at the color of light coming from a star and determine its’ surface temperature. Blue very hot 20,000 White hot Red cool Yellow warm 7,000 5,000 Temperature in Kelvin 2500 How Bright is that STAR? Luminosity is how we measure the brightness of a star. Dim Light Bright Light: There are 2 ways to measure brightness: • Apparent magnitude – when we use size, temperature, and distance to earth to calculate brightness. This is not a true measure because stars closer to Earth will appear brighter to us. • Absolute magnitude – a star’s actual or real level of brightness in comparison with other stars. Which Star is Brighter? • From Earth, both stars A and B seem like they have the same brightness. • Because we see this from earth, we say that the apparent magnitude is the same. • In reality, Star B is far brighter, but it is just farther away. • We would say that the actual magnitude of star B is far brighter than star A. Hertzsprung Russell (HR) Diagrams • We can use HR diagrams to study several characteristics about stars • Luminosity (compared to the sun) • Brightness (Absolute Magnitude as compared to the sun) • The sun is at 1 absolute magnitude • Surface Temperature • Stage of life cycle • Be careful!! HR diagrams look the same, but look at the x axis and y axis to know what the diagram is measuring!! Our sun 1 HR Diagrams show ts t. e g ho r how stars change over Once they start burning ta eHelium, ry s v e ebeocmes The star swells time. As th hand t s , like marshmallows s en bri ey g p d o t gh e t e ter old n s or en be sieor s m erg cfau e, tth y ar usge ey le byhenn the get c u l, t uc y m ho n tte lea r al a e t m r fu ke r a f s A y sio mor nd r Temperature is backwards!! e It goes n! ve from high temperature to low temperature! Key Points of H-R Diagrams • • • • • • Hottest Stars are the brightest stars. Coolest stars are the dimmest Blue stars are more dense and hotter Red stars are more airy and colder. Cooler stars live longer than hot stars. Why? Because they “cook more slowly” in a cold oven! Hot stars “cook” at a higher temperature. Diagonal Lines represent the radius of the star. The hottest stars are the brightest are blue. The coolest stars are the dimmest, or the most pale. Review What does apparent and actual magnitude measure? Luminosity or how bright the star is How can you tell temperature by color? From cold to hot: Red Yellow White Blue On the HR diagram, what stage are most stars in? Main Sequence Big Picture Review What elements are made in the following stages: Main sequence Helium Carbon Red Giant Carbon to Iron (Fe) Supergiant Supernova All elements heavier than Iron (Fe) Star X is in the Red Giant stage. How big is its’ mass? Like the Sun.