ASTR 1120 Astronomer’s Toolbox: General Astronomy: What do we know how to do Stars & Galaxies now? • Measure Distance: – parallax…good to nearby stars but not beyond HOMEWORK #3 due NEXT TUE, 09/29, by 5pm • Measure Luminosity: – measure apparent brightness and distance, infer luminosity • Measure Temperature: – Wien’s law, or, better yet, take spectra and use Fiske planetarium: ”The Birth of Stars” spectral classification. by Prof. John Bally - TH 09/24-FRI 09/25, 7:30pm Next: Mass Masses are much harder than Binary Stars to the Rescue!! distance, luminosity, or temperature • Since we are only ever seeing a point • Types of binary star systems: source, it is hard to determine how – Visual Binary much mass is contained. – Eclipsing Binary – If we could see another nearby object – Spectroscopic Binary (another star maybe?) we could use the gravity between the objects as a measure About half of all stars are in binary systems of the mass.
Eclipsing Binary Visual Binary We can directly observe the orbital motions of these stars We can measure periodic eclipses Spectroscopic Binary Animation from http://www-astronomy.mps.ohio-state.edu/~pogge/Ast162/Movies/spanim.gif We determine the orbit by measuring Doppler shifts
Astronomer’s Toolbox: Newton’s Laws of gravity What do we know how to do provide the mass now? • Measure Distance: Direct mass measurements are – parallax…good to nearby stars but not beyond possible only for stars in binary • Measure Luminosity: star systems – measure apparent brightness and distance, infer luminosity Once we know: • Measure Temperature: – Wien’s law, or, better yet, take spectra and use p = period spectral classification. a = average separation • Measure Mass: We can solve Newton’s equations – For stars in binary orbits, if we can get their orbital for mass (M) parameters, we can figure out their mass Isaac Newton The Hertzsprung Russell Wide range of luminosities, Diagram temperatures and masses • THIS IS AN IMPORTANT DIAGRAM TO UNDERSTAND. Any correlation • Basics: among these – Plots Stellar Luminosity (not apparent quantities? brightness) Vs – Temperature or Color or Spectral Class
Clicker Question Study this plot! Are the variables plotted here related to each other? A. Yes, they show a relationship B. You can’t be sure – you don’t know what they are! C. They are related to each other or else both are related to a third variable D. A or C E. None of the above � -R -R d dia iagra ra � They DO show a relationship! Emitted power per unit area = where Luminosity Total luminosity from a star of radius R: For the same temperature, more luminous stars have larger radii Temperature
��� ar li life � mes alo long � e main sequenc � Main sequence stars Available hydrogen fuel is greater for the most massive stars… • Burning hydrogen in their But luminosity (rate at which cores hydrogen is fused) is MUCH MUCH higher • Stellar masses decrease downward � More massive (more luminous) main sequence stars run out of • Temperatures are hotter for fuel sooner more massive stars (more gravitational pressure � Example: Most massive O star: higher T, remember M = 100 M Sun Equation of State) L = 10 6 L Sun M/L = 10 2 /10 6 = 10 -4 of the Sun • More luminous (higher T � Life O-Star =10 10 yrs * 10 -4 = 10 6 yrs much higher emitted power) Geo Metro Lifetimes on Main Sequence (MS) Lifetimes of Main Sequence Stars • Stars spend 90% of their lives on MS • Lifetime on MS = amount of time star • Rock-star analogy: fuses hydrogen (gradually) in its core more massive, hotter, more • For Sun (G), this is about 10 billion years luminous stars burn through the • For more massive stars (OBAF), lifetime is available fuel faster, (much) shorter leading to early • For less massive stars (KM), lifetime is burnout longer
Clicker Question Clicker Question George and Abe are two main George and Abe are two main sequence stars; George is an M star sequence stars; George is an M star and Abe is a B star. Which is more and Abe is a B star. Which is more massive? Which is redder in color? massive? Which is redder in color? A. George is more massive and redder A. George is more massive and redder B. Abe is more massive and redder B. Abe is more massive and redder C. George is more massive; Abe is redder C. George is more massive; Abe is redder D. Abe is more massive; George is redder D. Abe is more massive; George is redder E. They are both main sequence, they’re the E. They are both main sequence, they’re the same mass and same color. same mass and same color. Main-Sequence Star Summary High Mass: What about the other objects on High Luminosity the H-R diagram? Short-Lived Luminosity Large Radius Hot As stars run out of Blue hydrogen fuel their properties change Low Mass: (generally they turn into red giants- Low Luminosity more on why next week) Long-Lived Small Radius Cool Red Temperature
Main- sequence • Top end of main turnoff point sequence starts to of a cluster “peel off” tells us its age • Pleiades star cluster shown � no more O and B stars Analogy: Your refrigerator Applets Different foods have different shelf One day lives. One week • "Picture" of an aging cluster Assuming you clean 3 weeks out food that goes bad promptly, the 3 months • HR Diagram of an aging cluster content of your 30 years refrigerator tells you how long it’s been since you went to the store
Clicker Question Clicker Question How do we measure the age of a stellar How do we measure the age of a stellar cluster? cluster? A. Use binary stars to measure the age of stars in the A. Use binary stars to measure the age of stars in the cluster. cluster. B. Use the spectral types of the most numerous stars in B. Use the spectral types of the most numerous stars in the cluster to infer their temperatures, and thus, the the cluster to infer their temperatures, and thus, the age of the cluster. age of the cluster. C. Find stars in the instability strip and use their C. Find stars in the instability strip and use their variability period to measure their age. variability period to measure their age. D. Look for the age of stars at the main-sequence D. Look for the age of stars at the main-sequence turnoff point. turnoff point. E. Determine if the cluster is an open cluster or globular E. Determine if the cluster is an open cluster or globular cluster and use the average age of those types of cluster and use the average age of those types of clusters. clusters. Clicker Question Clicker Question Main sequence A-stars have masses about 3 Main sequence A-stars have masses about 3 times that of the Sun, and luminosities about 30 times that of the Sun, and luminosities about 30 times that of the Sun. What is the age of a cluster times that of the Sun. What is the age of a cluster which has a “turnoff” at A-stars? (Remember: The which has a “turnoff” at A-stars? (Remember: The Sun’s lifetime ~ 10 billion years) Sun’s lifetime ~ 10 billion years) A) 100 thousand years A) 100 thousand years B) 100 million years B) 100 million years C) 1 billion years C) 1 billion years D) 10 billion years D) 10 billion years E) 100 billion years E) 100 billion years
Where we see this best: Star 1.) Open Clusters Clusters • Groups of 100’s to • Loose groups of millions of stars 1000’s of stars • All about the same distance (apparent brightness tracks • This is where most luminosity well) stars in the Galaxy • All formed about the are born same time (i.e. all are same age) • Range of different mass stars! 2.) Globular Clusters • Pleiades: an “open cluster” of stars about 100 million years old • Generally much older- up to 13 • Compare with Sun’s BILLION years age of about 4.6 BILLION years old • ~millions of stars, densely packed • Intense gravitational interactions
Cepheid Variable Stars A Which star is most like our Sun? • Some stars vary in brightness because they D cannot achieve proper Luminosity balance between power welling up from the core and power radiated from B the surface • Most pulsating variable stars inhabit an instability C Clicker strip on the H-R diagram question • The most luminous ones are known as Cepheid variables: important for distance measurements Temperature A A Which star is Which of these most like our stars will have Sun? changed the least 10 billion years D D from now? B Luminosity Luminosity B B C C Clicker Clicker question question Temperature Temperature
A A Which of these Which of these stars will have stars can be no changed the least more than 10 10 billion years million years old? D D from now? Luminosity Luminosity B B C C C Clicker Clicker question question Temperature Temperature Stellar Properties Review A Which of these stars can be no Luminosity: from brightness and distance more than 10 million years old? D 10 -4 L Sun - 10 6 L Sun (0.08 M Sun ) (100 M Sun ) Luminosity A Temperature: from color and spectral type B (0.08 M Sun ) (100 M Sun ) 3,000 K - 50,000 K C Clicker Mass: from period (p) and average separation (a) question of binary-star orbit 0.08 M Sun - 100 M Sun Temperature
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