Transcript STARS
Review the Sun Review the Sun Review: Which of the following gases is found in the core of a star like our Sun? a. Hydrogen b. neon c. nitrogen d. oxygen Answer: a- hydrogen THE STARS Who was right? http://www.youtube.com/watch?v=5az0W4Y1nuU How do you describe how bright a star is? 2 factors: a. The star’s brightness or luminosity and b. Its distance from Earth Luminosity = Star brightness • is how much light energy a star gives off per second • we assign a luminosity of 1.0 to our Sun • if a star has a luminosity of 3.0, it gives off three times the light energy of the Sun • Alpha Centauri A, Sirius, Vega are stars ~ 30 000x brighter than the Sun Apparent magnitude • "what you see is what you get" magnitude • the brightness of a star as seen from earth • Determined by how luminous the star is and how far away it is The more negative the number the brighter the star seen from Earth (e.g. Sun is -26) Practice apparent magnitude 1. Which star is the brightest? Stars Star 1 Star 2 The more negative Star 3 the number the Star 4 brighter the star seen from Earth Therefore Star 3 Apparent magnitude + 0.8 +2 -3 +6 Absolute magnitude - a star’s "true" brightness - How bright a star is if it were 33 light year from Earth - the influence of distance on the star’s brightness is factored out - Sun = 4.7 Sun is not that bright compared to other stars Colour and Temperature of Stars A star’s colour can give us an idea of how hot that star is: – Blue 21,000-35,000C – Bluish-white – Yellow our Sun is yellow (photosphere ~ 6,000C) – Orange – Red 3,300C Which color indicates the hottest stars in the universe? Hertzsprung-Russell (HR) Diagram Hertzsprung-Russell (HR) Diagram (p310 txtbook) • Used to compare properties of stars • This diagram shows luminosity versus temperature and star colour • How bright a star is also linked to its surface temperature From the HR diagram, 1. Which star is the hottest among Alpha Centauri B, our Sun, Rigel and Betelgeuse? Answer: Rigel 2. Which star is the brightest among Vega, Sirius B, our Sun and Betelgeuse? Answer: Betelgeuse Who was right then? http://www.youtube.com/watch?v=5az0W4Y1nuU Today’s joke https://www.youtube.com/watch?v=12C4t4Gs3Bw (to 8:40min) Answers to text questions #1-7 on page 311 • Other units were too short. Light Years were the largest unit of measuring distance. • Absolute magnitude is more effective for measuring a star’s actual brightness. • Sirius (-1.5) is brighter than Deneb (+1.3). • If a star is closer to the Earth, it will appear brighter. For example…. • Our sun is hotter than Alpha Centauri B; Vega is both warmer and bigger than the Sun