Outline of Ch 11b: The H-R Diagram The Hertzprung-Russell (H-R) Diagram: Surface Temperature vs Luminosity Analogy: horsepower vs weight Where Stars plot in the H-R diagram Main Sequence: 90% of all stars Why? stars spend 90% of their lives fusing hydrogen Main sequence Hydrogen fusion Giants, Supergiants, White Dwarfs Main Sequence Stars (cont.)
Outline of Ch 11b: The H-R Diagram (cont.) Main Sequence Stars Stellar Masses and Densities along main sequence Mass-Luminosity Relation (L~M3.5) Lifetime on Main Sequence (TMS~ 1/M2.5) Main sequence Thermostat Star Clusters What is so special about Star Clusters? Open and Globular Clusters Ages of Clusters
Homework online due Mon.
Review of Chapter 11a Parallax and distance. Luminosity and brightness Apparent Brightness Absolute Brightness or Luminosity Inverse-Square Law Stellar Temperatures Color, Spectral lines, Spectral Classification:OBAFGKM Stellar sizes (radius) Stellar masses: binary stars, Doppler effect
What is a Hertzsprung-Russell (H-R) Diagram?
H-R diagram plots the luminosity vs. surface temperature of stars
Analogy with diagram that plots horsepower vs. weight of cars Race cars Sports cars Most Cars Horsepower Economy cars Weight
Hydrogen-fusion stars reside on the main sequence of the H-R diagram
Hydrogen-fusion stars reside on the main sequence of the H-R diagram Main Sequence: 90% of all stars Why? stars spend 90% of their lives fusing hydrogen Main sequence Hydrogen fusion
Remember Stellar sizes (radius) Luminosity proportional to surface area x temperature: L= 4R2T4 If we can measure the luminosity and the temperature of a star we can tell how large its raduis is.
H-R Diagram: Radii of stars
Stars with low temperature and high luminosity must have ____ radius SUPERGIANTS Stars with low temperature and high luminosity must have ____ radius GIANTS
SUPERGIANTS Stars with low temperature and high luminosity must have a large radius: giants and supergiants GIANTS
Stars with high temperature and low luminosity must have _____ radius
Stars with high temperature and low luminosity must have small radius: white dwarfs
Stellar Masses Mass-Luminosity Relation (L~M3.5) For main sequence stars, the larger the mass the higher the luminosity This mass-luminosity relation is valid only for the main sequence
Stellar Masses How do we know the masses of these stars? For main sequence stars, the larger the mass the higher the luminosity This mass-luminosity relation is valid only for the main sequence
Stellar Masses How do we know the masses of these stars? The study of binary stars For main sequence stars, the larger the mass the higher the luminosity This mass-luminosity relation is valid only for the main sequence
Stellar Densities Density = Mass/Volume V= 4/3(R3)
Stellar Densities Low Same as water High
Stellar Densities Giants and Supergiants: same or lower density than air M.S. same density as water W.D. very dense
Stellar_mass_and_hr_diagr.swf
Lifetime on Main Sequence TMS~ 1/M2.5 M in solar masses T in units of Sun’s total lifetime on MS (10 billion years)
TemperatureColor, Spectral Type, H-R diagram depicts: TemperatureColor, Spectral Type, Luminosity, and Radius of stars Luminosity Temperature
TemperatureColor, Spectral Type, H-R diagram depicts: TemperatureColor, Spectral Type, Luminosity, and Radius of stars (*Mass, *Lifespan, of MS stars only) Luminosity Temperature
Outline of Ch 11b: The H-R Diagram (cont.) Main Sequence Stars Stellar Masses and Densities along main sequence Mass-Luminosity Relation (L~M3.5) Lifetime on Main Sequence (TMS~ 1/M2.5) Main sequence Thermostat
Main Sequence Thermostat: In the Sun, and in all main sequence stars gravity is balanced by outward pressure due to the outflow of energy.
Which star is the hottest? 2 Which star is the hottest? 3 4 Luminosity 1 Temperature
Which star is the hottest? 2 Which star is the hottest? 3 1 4 Luminosity 1 Temperature
Which star is the most luminous? 2 Which star is the most luminous? 3 4 Luminosity 1 Temperature
Which star is the most luminous? 2 Which star is the most luminous? 3 2 4 Luminosity 1 Temperature
2 Which star is a main-sequence star? 3 4 Luminosity 1 Temperature
2 Which star is a main-sequence star? 3 4 4 Luminosity 1 Temperature
2 Which star has the largest radius? 3 4 Luminosity 1 Temperature
2 Which star has the largest radius? 3 3 4 Luminosity 1 Temperature
Which star is most like our Sun? 2 Which star is most like our Sun? 3 4 Luminosity 1 Temperature
Which star is most like our Sun? 2 Which star is most like our Sun? 3 4 4 Luminosity 1 Temperature
1 Which of these stars will have changed the least 10 billion years from now? 2 3 Luminosity 4 Temperature
1 Which of these stars will have changed the least 10 billion years from now? 2 3 Luminosity 4 4 Temperature
1 Which of these stars can be no more than a few million years old? 2 Luminosity 3 4 Temperature
1 Which of these stars can be no more than a few million years old? 2 Luminosity 3 1 4 Temperature
What have we learned? • What is a Hertzsprung-Russell diagram? An H–R diagram plots stars according to their surface temperatures and luminosities.
TemperatureColor, Spectral Type, H-R diagram depicts: TemperatureColor, Spectral Type, Luminosity, and Radius of stars (*Mass, *Lifespan, [*Age], of MS stars only) Luminosity Temperature
Outline of Ch 11b: The H-R Diagram The Hertzprung-Russell (H-R) Diagram: Surface Temperature vs Luminosity Analogy: horsepower vs weight Where Stars plot in the H-R diagram Main Sequence: 90% of all stars Why? stars spend 90% of their lives fusing hydrogen Main sequence Hydrogen fusion Giants, Supergiants, White Dwarfs Main Sequence Stars (cont.)
Outline of Ch 11 part II: The H-R Diagram (cont.) Main Sequence Stars Stellar Masses and Densities along main sequence Mass-Luminosity Relation (L~M3.5) Lifetime on Main Sequence (TMS~ 1/M2.5) Main sequence Thermostat
Which of the following correctly fills in the blank Which of the following correctly fills in the blank? A main-sequence star of spectral class B is _____ than a main-sequence star of spectral class G. 1. More massive 2. Hotter 3. Longer lived 4. More luminous The correct answer is A. 1 and 3 B. 2 and 3 C. 1, 2 and 4 D. 2, 3 and 4 E. 1, 2, 3 and 4
2. Which of the following correctly fills in the blank 2. Which of the following correctly fills in the blank? If a star is on the main-sequence and one knows its temperature, then one can estimate its ____. 1. Spectral class 2. Mass 3. Luminosity 4. Density 5. Radial velocity The correct answer is A. 1, 2, 3, 4 and 5 B. 1 and 5 C. 2 only D. 1, 3 and 5 E. 1, 2, 3 and 4
3. Which of the following correctly fills in the blank 3. Which of the following correctly fills in the blank? If a star of class O is on the main-sequence, that star must be ____. 1. Hotter than most stars 2. Very massive 3. Much denser than water 4. Very red 5. Not very old The correct answer is A. 2 and 3 B. 1, 2, 3 and 4 C. 1, 2, 3, 4 and 5 D. 1, 2 and 5 E. 4 and 5
4. Which of the following correctly fills in the blank 4. Which of the following correctly fills in the blank? If a star of class M is on the main-sequence, that star must be ____. A. Very hot B. Very massive C. Very blue D. None of the other answers are correct
Outline of Ch 11b: The H-R Diagram (cont.) Star Clusters: Confirmation of Stellar Evolution Open and Globular Clusters Ages of Clusters
Star Clusters: Confirmation of Stellar Evolution: 1. What is special about star clusters? All stars formed at same time, so plotting clusters with different ages on H-R diagrams we can see how stars evolve This confirms our theories of stellar evolution without having to wait billions of years observing how a single star evolves 2. Two types of clusters: Open and Globular 3. Ages of Clusters
Open cluster: A few thousand loosely packed stars
Useful movies to download: Globular cluster visualization http://terpsichore.stsci.edu/~summers/viz/scviz/spz.html Stellar collision simulation: http://www.ifa.hawaii.edu/faculty/barnes/research/stellar_collisions/ Globular cluster: Up to a million stars in a dense ball bound together by gravity
Two types of star clusters Open clusters: young, contain up to several thousand stars and are found in the disk of the galaxy. Globular clusters: old, contain hundreds of thousands of stars, all closely packed together. They are found mainly in the halo of the galaxy.
Our Galaxy
Which part of our galaxy is older?
How do we measure the age of a star cluster?
Theoretical Evolution of a star cluster
Massive blue stars die first, followed by white, yellow, orange, and red stars Star_cluster_evolving.swf
How do we know that this theoretical evolution is correct?
How do we know that this theoretical evolution is correct? We plot observations of actual clusters on the H-R diagram
H-R Diagram of Young Stellar Cluster
H-R Diagram of Young Stellar Cluster How do we know this cluster is Young?
H-R Diagram of Old Stellar Cluster
H-R Diagram of Old Stellar Cluster How do we know this cluster is Old?
Pleiades cluster now has no stars with life expectancy less than around 100 million years Main-sequence turnoff
Main-sequence turnoff point of a cluster tells us its age
To determine accurate ages, we compare models of stellar evolution to the cluster data Hr_diagr_and_age_of_cluster.swf
Detailed modeling of the oldest globular clusters reveals that they are about 13 billion years old (The universe is about 13.7billion years old
What have we learned? How do we measure the age of a star cluster? Because all of a cluster’s stars we born at the same time, we can measure a cluster’s age by finding the main sequence turnoff point on an H–R diagram of its stars. The cluster’s age is equal to the hydrogen-burning lifetime of the hottest, most luminous stars that remain on the main sequence.
Question 1 If the brightest main sequence star in cluster 1 is a B star and the brightest main sequence star in cluster 2 is an M star. What can we say about the age of these two clusters?
Question 1 If the brightest main sequence star in cluster 1 is a B star and the brightest main sequence star in cluster 2 is an M star. What can we say about the age of these two clusters? Nothing, there is not enough information Cluster 1 is older than cluster 2 Cluster 2 is older than cluster 1 None of the answers are correct
Chapter 12. Star Stuff Part I Birth of Stars Birth of Stars from Interstellar Clouds •Young stars near clouds of gas and dust •Contraction and heating of clouds • Hydrogen fusion stops collapse