AST 101 Introduction to Astronomy: Stars & Galaxies

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1 QUIZ #3 - ISUSSION Ginger and Fred are two stars. Ginger s parallax is ½ of Fred s. Ginger s apparent brightness is 5 Magnitudes, Fred s is 10 Magnitudes. a) If Ginger is 1ly year away from us, how far is Fred? [3pt] ST 101 Introduction to stronomy: Stars & Galaxies istances scale inversely with parallax angle. Since Fred s parallax is twice that of Ginger, his distance is half that of Ginger, i.e. 0.5ly. b) Which of the two has a higher apparent brightness, and by how much? [3pt] Ginger is brighter, by a factor of 100, since the difference in magnitude is 5=2*2.5 and each factor of 2.5 in magnitudes yields a power of 10 in flux. c) Which one of the two has a higher intrinsic luminosity, and by how much? [4 pt] Lum = pparent brightness * (4*pi* ^2) [Inverse square law!] Lum(Ginger)/Lum(Fred) = 100*1^2/(1.*0.5^2)=100/0.25 = 400. Ginger's luminosity is a factor of 400 larger than Fred's. 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 later ) Top end of main sequence starts to peel off Pleiades star cluster shown! no more O and stars

2 Mainsequence turnoff point of a cluster tells us its age licker Question 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. licker Question 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. licker Question 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

3 licker Question 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) ) ) ) ) E) 100 thousand years 100 million years 1 billion years 10 billion years 100 billion years 1.) Open lusters Loose groups of 1000 s of stars This is where most stars in the Galaxy are born 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! Pleiades: an open cluster of stars about 100 million years old ompare with Sun s age of about 4.6 ILLION years old

4 epheid Variable Stars 2.) Globular lusters Some stars vary in brightness because they cannot achieve proper balance between power welling up from the core and power radiated from the surface 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 Generally much older- up to 13 ILLION years ~millions of stars, densely packed Intense gravitational interactions Which star is most like our Sun? Which star is most like our Sun? licker licker

5 stars will have changed the least 10 billion years from now? stars will have changed the least 10 billion years from now? licker licker stars can be no more than 10 million years old? stars can be no more than 10 million years old? licker licker

6 Stellar Properties Review : from brightness and distance (0.08 M 10-4 L Sun Sun ) L Sun (100 M Sun ) : from color and spectral type (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 M Sun