Stars

*Taught Online* At the beginning of the 20th century, two astronomers:  Ejnar Hertzprung and Henry Norris Russell independently took catalogues of stars and plotted their brightness as a function of their color. The result, now known as the HR diagram, was to become one of the most influential diagrams in astrophysics. It showed that, contrary to one's naive expectation, the distribution of stars was highly structured. The efforts to understand the HR diagram extended for the better part of the 20th century and paralleled the development of modern physics. 

In this course we will use the HR diagram as a starting point to address two fundamental questions: what is a star? And how does it evolve?

This will be a scientific journey in which we will describe the physical processes to determine the inner workings of stars. How they manage to be so hot, so bright and so remarkably long lived. We will explain how stars drive the chemical evolution of the universe by assembling heavier elements out of lighter ones. Why some stars at the end of their lives become white dwarfs and slowly fade away (die with a whimper) while others end their lives in spectacular explosions know as supernovae that are so bright that can be seen clear across the universe (die with a bang). 

The sun is as bright as 100 million, million, million, million 40 Watts light bulbs. It burns 400 million metric tons of hydrogen per second. Yet, it has been doing that for 4.5 billion years and will continue to do so for another 4.5 billion years. Pretty impressive, wouldn't you say?