ASTR3 Week 7 Notes - Week 7-Lecture 19 Stellar Trends...

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Week 7 11/9/15 ---Lecture 19: Stellar Trends Patterns Among Stars The Hertzsprung-Russel Diagram o Plot of luminosity versus temperature o 90% of all stars lie along a line ( Main Sequence ) o Main sequence stars burn hydrogen to helium in their cores o NOTE: Temperature is “backwards.” ib H-R diagram (increases from left to right) HR Diagram o Supergiants o Giants o Main Sequence (fusion/hydrogen-burning in the core) o White Dwarfs (inert helium core after hydrogen core is exhausted → helium core contracts) HR Diagram: Stellar Radii o L = 4piR 2 constantT 4 o 1000 R Sun o 1 R Sun = radius of Sun o 0.001 R Sun Luminosity classes o …... o For stars of given temperature, brighter luminosity classes (“giants”) have narrower lines; spectral type and luminosity class locates star in H-R diagram
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o Full classification of star includes spectral type (temperature) and luminosity class (related to size of star). Examples: Sun (G2 V); Sirius (A1 V); Betelgeuse (M2 1) The Main Sequence is a Mass Sequence …. Mass - Luminosity Relation o On the main sequence high-mass stars are MUCH more luminous than low- mass stars o 0.1 M → 0.0005 L o 40 M → 400,000 L o L M 3.5 Mass-Luminosity Relation o We use binary stars to measure directly the masses of stars of every type. This leads to the mass-luminosity relation: o …. Lifetime on the Main Sequence o How long can a star shine? Time = (Amount of fuel) / (Fuel-consumption-rate) Amount of fuel: star’s mass → M Fuel-consumption rate: star’s luminosity → L o O & B Dwarfs burn fuel like a bus! o M Dwarfs burn fuel like a compact car! o So the stellar lifetime …….~ M/L ~ M/M 3.5 ~ M -2.5
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o → Lifetime…… Lifetime on the Main Sequence o So for example: o O7 star (60 Msun) lasts 3.6 x 10 5 yr o B2 star (10 Msun) lasts 3.2 x 10 7 yr o F0 star (2 Msun) lasts 1.8 x 10 9 yr o M0 star (0.5 Msun) lasts 5.6 x 10 10 yr o But the Universe is 1.38 x 10 10 yr old o ……. Our Star - the Sun (Chapter 11) Solar Statistics o Radius: 6.9 x10 8 m (109 times Earth) o Mass: 2 x 10 30 kg (300,000 Earths) o Luminosity: 3.8 x 10 26 watts o The dark limbs show that the Sun has a cooler outer “atmosphere” Gravitational equilibrium: o The Sun is a huge ball
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