Lecture 7: Threshold 2—The First Stars and Galaxies
they have stability, they show emergent properties, and they are sustained by
energy À 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 “mainGravity began the
process of sculpting
more complex things.