STR 1120 General stronomy: Stars & Galaxies HOMEWORK #3 due NEXT TUE, 09/29, by 5pm Fiske planetarium: The irth of Stars by Prof. John ally - TH 09/24-FRI 09/25, 7:30pm stronomer s Toolbox: What do we know how to do now? Measure istance: parallax good to nearby stars but not beyond Measure : measure apparent brightness and distance, infer luminosity Measure : Wien s law, or, better yet, take spectra and use spectral classification. Next: Mass Masses are much harder than distance, luminosity, or temperature Since we are only ever seeing a point source, it is hard to determine how much mass is contained. If we could see another nearby object (another star maybe?) we could use the gravity between the objects as a measure of the mass. inary Stars to the Rescue!! Types of binary star systems: Visual inary Eclipsing inary Spectroscopic inary bout half of all stars are in binary systems
Visual inary Eclipsing inary We can directly observe the orbital motions of these stars We can measure periodic eclipses Spectroscopic inary We determine the orbit by measuring oppler shifts nimation from http://www-astronomy.mps.ohio-state.edu/~pogge/st162/movies/spanim.gif
Newton s Laws of gravity provide the mass Isaac Newton irect mass measurements are possible only for stars in binary star systems Once we know: p = period a = average separation We can solve Newton s equations for mass (M) stronomer s Toolbox: What do we know how to do now? Measure istance: parallax good to nearby stars but not beyond Measure : measure apparent brightness and distance, infer luminosity Measure : Wien s law, or, better yet, take spectra and use spectral classification. Measure Mass: For stars in binary orbits, if we can get their orbital parameters, we can figure out their mass Wide range of luminosities, temperatures and masses ny correlation among these quantities? The Hertzsprung Russell iagram THIS IS N IMPORTNT IGRM TO UNERSTN. asics: Plots Stellar (not apparent brightness) Vs or olor or Spectral lass
Study this plot! re the variables plotted here related to each other?. Yes, they show a relationship. You can t be sure you don t know what they are!. They are related to each other or else both are related to a third variable. or E. None of the above They O show a relationship!!-r diagra" Emitted power per unit area= where Total luminosity from a star of radius R: For the same temperature, more luminous stars have larger radii
Main sequence stars urning hydrogen in their cores Stellar masses decrease downward s are hotter for more massive stars (more gravitational pressure! higher T, remember Equation of State) More luminous (higher T! much higher emitted power) #$%ar life&mes along 'e main sequenc( vailable hydrogen fuel is greater for the most massive stars ut luminosity (rate at which hydrogen is fused) is MUH MUH higher! More massive (more luminous) main sequence stars run out of fuel sooner Example: Most massive O star: M = 100 M Sun L = 10 6 L Sun M/L = 10 2 /10 6 = 10-4 of the Sun Life O-Star =10 10 yrs * 10-4 = 10 6 yrs Geo Metro Lifetimes of Main Sequence Stars Rock-star analogy: more massive, hotter, more luminous stars burn through the available fuel faster, leading to early burnout Lifetimes on Main Sequence (MS) Stars spend 90% of their lives on MS Lifetime on MS = amount of time star fuses hydrogen (gradually) in its core For Sun (G), this is about 10 billion years For more massive stars (OF), lifetime is (much) shorter For less massive stars (KM), lifetime is longer
George and be are two main sequence stars; George is an M star and be is a star. Which is more massive? Which is redder in color?. George is more massive and redder. be is more massive and redder. George is more massive; be is redder. be is more massive; George is redder E. They are both main sequence, they re the same mass and same color. George and be are two main sequence stars; George is an M star and be is a star. Which is more massive? Which is redder in color?. George is more massive and redder. be is more massive and redder. George is more massive; be is redder. be is more massive; George is redder E. They are both main sequence, they re the same mass and same color. Main-Sequence Star Summary High Mass: High Short-Lived Large Radius Hot lue Low Mass: Low Long-Lived Small Radius ool Red What about the other objects on the H-R diagram? s stars run out of hydrogen fuel their properties change (generally they turn into red giantsmore on why next week)
Top end of main sequence starts to peel off Mainsequence turnoff point of a cluster tells us its age Pleiades star cluster shown! no more O and stars nalogy: Your refrigerator ifferent foods have different shelf lives. ssuming you clean out food that goes bad promptly, the content of your refrigerator tells you how long it s been since you went to the store One day One week 3 weeks 3 months 30 years pplets "Picture" of an aging cluster HR iagram of an aging cluster
How do we measure the age of a stellar cluster?. Use binary stars to measure the age of stars in the cluster.. Use the spectral types of the most numerous stars in the cluster to infer their temperatures, and thus, the age of the cluster.. Find stars in the instability strip and use their variability period to measure their age.. Look for the age of stars at the main-sequence turnoff point. E. etermine if the cluster is an open cluster or globular cluster and use the average age of those types of clusters. How do we measure the age of a stellar cluster?. Use binary stars to measure the age of stars in the cluster.. Use the spectral types of the most numerous stars in the cluster to infer their temperatures, and thus, the age of the cluster.. Find stars in the instability strip and use their variability period to measure their age.. Look for the age of stars at the main-sequence turnoff point. E. etermine if the cluster is an open cluster or globular cluster and use the average age of those types of clusters. Main sequence -stars have masses about 3 times that of the Sun, and luminosities about 30 times that of the Sun. What is the age of a cluster which has a turnoff at -stars? (Remember: The Sun s lifetime ~ 10 billion years) ) 100 thousand years ) 100 million years ) 1 billion years ) 10 billion years E) 100 billion years Main sequence -stars have masses about 3 times that of the Sun, and luminosities about 30 times that of the Sun. What is the age of a cluster which has a turnoff at -stars? (Remember: The Sun s lifetime ~ 10 billion years) ) 100 thousand years ) 100 million years ) 1 billion years ) 10 billion years E) 100 billion years
Where we see this best: Star lusters Groups of 100 s to millions of stars ll about the same distance (apparent brightness tracks luminosity well) ll formed about the same time (i.e. all are same age) Range of different mass stars! Loose groups of 1000 s of stars 1.) Open lusters This is where most stars in the Galaxy are born Pleiades: an open cluster of stars about 100 million years old ompare with Sun s age of about 4.6 ILLION years old Generally much older- up to 13 ILLION years 2.) Globular lusters ~millions of stars, densely packed Intense gravitational interactions
epheid Variable Stars Some stars vary in brightness because they cannot achieve proper balance between power welling up from the core and power radiated from the surface Which star is most like our Sun? Most pulsating variable stars inhabit an instability strip on the H-R diagram The most luminous ones are known as epheid variables: important for distance measurements licker Which star is most like our Sun? Which of these stars will have changed the least 10 billion years from now? licker licker
Which of these stars will have changed the least 10 billion years from now? Which of these stars can be no more than 10 million years old? licker licker Which of these stars can be no more than 10 million years old? Stellar Properties Review : from brightness and distance (0.08 M 10-4 L Sun - 10 6 Sun ) L Sun (100 M Sun ) : from color and spectral type licker (0.08 M Sun ) 3,000 K - 50,000 K (100 M Sun ) Mass: from period (p) and average separation (a) of binary-star orbit 0.08 M Sun - 100 M Sun