LECTURE

they have stability, they show emergent properties, and they are sustained by energy ows.

How do we know about the birth and death of stars? Our understanding of star formation depends largely on our theoretical understanding of processes such as fusion (which is very well understood by nuclear physicists) and the operations of gravity. But observational astronomy can also identify regions of our own galaxy where huge clouds of matter appear to be collapsing and forming new stars, such as the famous Horsehead Nebula.

Studying the life cycle of stars is tricky because they last so long that we can never observe an individual star evolving. Instead, we have to study millions of individual stars and assume that they represent different parts of a star’s life cycle. How do we do this?

Lecture Five mentioned the invention of the spectroscope by Joseph von Fraunhofer in 1814. Spectroscopes can help us identify the relative amounts of different chemical elements within a star because of the “absorption lines” they leave in the star’s spectrum. Spectroscopic studies show that stars consist overwhelmingly of hydrogen and helium. We can measure a star’s apparent brightness, and if we know its distance, we can measure its real brightness—the amount of energy it emits. As larger stars generate greater pressures and temperatures, a star’s brightness effectively tells us its mass. We can estimate the temperature at its surface from its color.

Using such observations, astronomers built up a massive database of information on different stars. Finally, in 1910, Danish astronomer Ejnar Hertzsprung and American astronomer Henry Russell distilled this information into a simple graph that identi ed the key factors in a star’s life cycle. They plotted two features of each star: rst, its brightness (effectively a measure of its mass), and second, its surface temperature. They found a simple correlation: For most stars, the more massive they were, the higher their surface temperature. Stars in this area of the graph (the “main

Gravity began the process of sculpting more complex things.