Observing Stellar Evolution 1. How can we see stellar evolution in action? 1. Stellar Clusters, a group of coeval stars, I.e. all born at the same time, but with different masses (hence different life time) 2. How can one estimate the age of a stellar cluster? 1. By looking at the HR diagram of the cluster, namely at the evolutionary phase of stars of the same age but with different mass

The Hertzsprung-Russell Diagram Lifetime of Star Shorter Longer More mass, more fuel, very fast burning. Less mass, less fuel, slow, steady burning. How do we know the age of a star?

Open cluster: 10 3 stars, up to 30 pc in size, found in disk of galaxy. All have mostly young stars Globular cluster: up to 10 6 stars and 150 pc in size, in disk and halo of galaxy. All have old stars Our First Measurement of Age Star Clusters

Why are clusters useful to astronomers? All stars in a cluster are at about same distance from Earth. All stars in a cluster are at about same distance from Earth. All stars in a cluster are of about the same age. All stars in a cluster are of about the same age. Clusters therefore are natural laboratory in which mass, rather than age, of stars is only significant variable. Clusters therefore are natural laboratory in which mass, rather than age, of stars is only significant variable.

The Hertzsprung-Russell Diagram We can date a cluster by observing its population of stars. The oldest clusters known have been measured to be ~14 billion years old. All these stars in the cluster have burned themselves out! Turn-Off point: Age indicator

Variable Stars Chepeids Chepeids RR-Lyrae RR-Lyrae Variability due to Instability Variability due to Instability Variability is PERIODIC Variability is PERIODIC Instability caused by presence of ionized He Instability caused by presence of ionized He More luminous variable stars have large Period More luminous variable stars have large Period Variability is EXTREMELY USEFUL, because it is an absolute distance indicator Variability is EXTREMELY USEFUL, because it is an absolute distance indicator

Cepheid Variable Stars Cepheid variable stars have variable brightness that is very regular. Cepheid variable stars have variable brightness that is very regular. The period of the variation can be from days to weeks The period of the variation can be from days to weeks Pulsation due to instability: He ionization layer acts a energy sponge Pulsation due to instability: He ionization layer acts a energy sponge it seems to be a reliable indication of the star’s luminosity! it seems to be a reliable indication of the star’s luminosity!

Cepheids: the Period-Luminosity Relation Henrietta Leavitt Henrietta Leavitt ( ). Luminosity=4  D 2 B

Standard Candles If we know an object’s true luminosity, we can measure its distance by measuring its apparent brightness. If we know an object’s true luminosity, we can measure its distance by measuring its apparent brightness. An object that has a known luminosity is called a standard candle.

What is burning in stars? Gasoline Gasoline Nuclear fission Nuclear fission Nuclear fusion Nuclear fusion Natural gas Natural gas

Review Questions 1. How can one estimate the age of a stellar cluster?

Survey Questions How is the helium core of a star supported? What causes the expansion of a star to become a red giant? What is a supernova? When does a massive star explode as a supernova?.