Stars are a long way away and even in a powerful telescope they never
look like much more than tiny points of light. How can astronomers
learn enough to ever tell the life stories of stars? How can they really
say they know about the birth, adolescence, middle age and death of
these pinpricks in the sky? Understanding the nature and history of stars
was not easy and it came slowly to humanity. Sometimes progress came
with new measurement, techniques and sometimes it came by simply finding a new way to look at the data.
By
the late 1800s, astronomers had tools that allowed them to make
quantitative measurements of stellar properties: apparent magnitude (how bright
a star appears to us on Earth), spectral type (related to temperature),
distance, radial velocity, etc. Several groups of astronomers worked
to compile big catalogs of these measurements drawing all the
information taken so far into a single resource. A big book of numbers is, of
course, not science. At first, all the catalogues could provide was a
confusing mess of data with no clear way to extract any
insights.
In
the early 1900s, two astronomers -- Ejnar Hertzsprung in Denmark and
Henry Norris Russell in America -- realized that there was a way to bring
order to the problem (notice that science has always been an
international endeavor). They used distance measurements to convert the APPARENT
magnitude of each star into its ABSOLUTE magnitude. Apparent magnitude is
a measure of how bright we on Earth perceive a star to be. Of course objects
look dimmer the farther away they are. That is why astronomers like to use absolute magnitude
when they know a star's distance since it is true measure of a star's intrinsic luminosity, its total energy output
per second. Using the big
catalogs, both men discovered a simple relationship between the absolute
magnitude of stars and their spectral type or color (Why? Because the relationship
between color and temperature came from the blackbody formula). Taken as
a whole, bluish stars tended to be luminous and reddish stars tended to
be faint. On a graph of absolute magnitude (luminosity) versus
spectral type (temperature) most of the stars fell in a diagonal stripe - a pattern that became known as the MAIN SEQUENCE. A few stars, however didn't follow the pattern, falling
instead in a clump in the upper-right of the diagram. The existence of
the main sequence, and these outliers that avoided it, soon provided
astronomers with the key insights they needed to transform thier data
into a profound understanding of how stars age.
As
time went on, this graph became known as a "Hertzsprung-Russell" (or HR) diagram
and came to be THE tool for understanding stellar history. Astronomy was
changed forever.