188 Chapter 3 The Universe Was Born With a Big Bang
Even if a star does not exceed the
measurement limit of annual paral-
lax, if it has the same spectrum as the
Sun, for example, we can determine
its absolute magnitude and then esti-
mate its distance by using its apparent
magnitude (its brightness as seen from
To a certain extent, the relation-
ship between absolute magnitude
and spectrum is not that strict. If any
interstellar matter or dust blocks the
light along the way, the brightness of
a star will not accurately represent its
absolute magnitude and distance, and
significant error may occur. So astron-
omers use certain measurements and
models in their equations to correct
for this.
Interestingly enough, the spectral
classifications that Hertzsprung and
Russell both used for the vertical axis
were developed in the early 1900s by
a woman named Annie Jump Cannon. At the time, many female astronomers were work-
ing for their male counterparts collecting observational data and processing these data. The
“Harvard computers,” as these workers were then called, were often paid very little yet per-
formed much of the work that led to major discoveries by the likes of Shapley and Hubble.
Cannon was one of these computers. She was the first female astronomer named as an
officer in the American Astronomical Society, and she catalogued more stellar bodies than
any other person to date.
Stars with Varying Brightness Are “Lighthouses of
Isn’t there a more accurate method of measuring distance? The person who found the
answer to this question and brought about later significant advances in astronomy is the
American astronomer Harlow Shapley (1885–1972).
What Shapley noticed was the light from variable stars. There are several reasons why
the light from a star will vary. In some cases, it may be the result of a supernova explosion
caused by the death of a giant star, and in others, it may be coming from a seemingly vari-
able star that is actually two stars: a bright star and dark star rotating as a pair. However,
in most cases it is caused by a star whose brightness varies regularly because the surface
layer is swelling and shrinking periodically. These are called pulsating variable stars.
The pulsations are caused, of course, by the nuclear fusion reaction. For stars called
Cepheid variable stars, helium nuclei are fused together to form heavier carbon or oxygen
nuclei, causing the entire star to shrink. Since the outer layer is unstable, the star pulsates.
Cepheid variable stars with a longer period of light variation also have a greater absolute
Hertzsprung-Russell diagram
Absolute magnitude
Surface temperature
High Low

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