The Hertzsprung-Russell Diagram
A view through a telescope will quickly convince you that there is a wide variety of stars in the sky. The
most immediate impression is one of a range of colors, from blue-white through white, yellow, orange, and into
deep reds. A comparison of stars at the same distance (as in double and multiple stars and star clusters)
shows that they come with very different intrinsic brightness.
With more careful study other differences such as variability become apparent; some stars maintain a nearly
constant luminosity, others cycle regularly through bright and dim phases, and still others vary irregularly
in brightness.
A Remarkable Regularity
A simple way to sort the different types of stars was found in 1911 by
the Danish astronomer Ejnar Hertzsprung, and independently in 1913 by the American
astronomer Henry Norris Russell.
They suggested plotting each star as a point, according to
the color
and true brightness (luminosity). Such a plot is
now known as a Hertzsprung-Russell diagram or HR diagram.
This simple plot
revealed an amazingly
regular pattern in the types of stars that exist.
Upon constructing
an HR diagram for a typical set of local stars, one doesn't find a
random scatter.
Instead most stars lie in clear groups, which we now call supergiants, giants,
main sequence stars, and white dwarfs.
HR Diagram for Local Stars
The adjacent right HR diagram plots all of the stars that are relatively close to the Sun.
As noted in the right panel, there are several choices that can be made for the horizontal and vertical axes
of an HR diagram that convey essentially the same information.
In this case we have plotted the ratio of the star's luminosity to that
of our Sun on the vertical axis and the surface temperature on the horizontal axis.
This diagram illustrates the four main main regions where we find stars
on an HR diagram: the main sequence,
white dwarfs, giants, and supergiants (though in this example there is not a
very clear boundary between giants and supergiants).
An Important Question
The adjacent diagram raises an important question: do all
populations of stars have an HR diagram like those near the Sun? When we consider star clusters a little
later, we will see that the answer is
sometimes yes and sometimes no, and that understanding the answer to this question will teach us
a lot about how stars live their lives. However, for now we will concentrate on HR diagrams that look
like the adjacent one, which is characteristic of the stars in our galaxy lying near the Sun.