Chapter 28 Stars and the Universe

714 CHAPTER 28: STARS AND THE UNIVERSE
HOW ARE STARS CLASSIFIED?
In the early twentieth century, astronomers in Denmark and
the United States discovered that they could classify stars on
the basis of the amount of electromagnetic energy they generate
and their temperature. The total energy output of a star
is called its luminosity, or absolute brightness. Apparent
brightness, or stellar magnitude, is how bright the star looks
as seen from Earth. The closer a star is, the brighter it appears
to us.
A good example is the sun. The sun is actually a smaller
star, and gives off less light than most of the stars you see in
the night sky. However, the sun is so close to Earth that during
the day its light drowns out the light of the other stars. If
we could see the sun at the same distance as the nighttime
stars, it would be dimmer than most of them. Therefore, the
brightness of a star depends on its absolute magnitude, or luminosity,
and its distance from the observer.
You may have noticed that when you turn off an incandescent
lightbulb the color of the hot wire briefly changes to
red before it goes dark. Red is the coolest color of light visible
to our eyes. If a material is heated beyond red-hot, it becomes
white and then blue. Continued heating would push the radiation
into the ultraviolet part of the spectrum and beyond.
These forms of electromagnetic energy are not visible to us.
However, they can affect us in other ways. Sunburn is caused
primarily by ultraviolet light, which is a part of the spectrum
of sunlight. Figure 28-2 compares the sun’s spectrum with
the spectrum of light radiated by hotter (blue) and cooler
(red) stars.
Hertzsprung-Russell Diagram
The graph used to classify stars is often called the Hertzsprung-Russell,
or H-R, diagram in honor of the two men who
developed it. This graph is printed below from the Earth Sci-