http://www.ck12.org Chapter 24. Astrophysics
24.3 The Hertzsprung-Russell Diagram
- Describe the stars present in each of the significant areas on the Hertzsprung-Russell Diagram.
The star constellation “Orion, The Hunter”. Orion is one of the most beautiful of all constellations, and one of
the easiest to find. It looks like a large (slightly twisted) rectangle high in winter’s south-southeastern sky. Two of
the brightest stars in the evening sky lie at opposite corners of the rectangle: bright orange-red Betelgeuse at the
northeastern corner (upper left in the photo) and even brighter Rigel at the southwest (lower right in the photo).
Betelgeuse is at least 300 times the Sun’s diameter, and perhaps much more. It puts out about 100,000 times more
energy than the Sun does. When Betelgeuse dies, it will create a fireball that will briefly outshine billions of normal
stars. Betelgeuse is a red giant and Rigel is a blue giant.
Hertzsprung-Russell Diagram
The Hertzsprung-Russell diagram (often referred to as the H-R diagram) is a scatter graph that shows various
classes of stars in the context of properties such as theirluminosity, absolute magnitude, color, and effective
temperature. Created around 1910 by Ejnar Hertzsprung and Henry Norris Russell, the diagram provided a great
help in understanding stellar evolution.
There are several forms of the Hertzsprung-Russell diagram, and the nomenclature is not very well defined. The
original diagram displayed the spectral type of stars on the horizontal axis and the absolute magnitude on the vertical
axis. The form below shows Kelvin temperature along the horizontal axis going from high temperature on the left
to low temperature on the right and luminosity on the vertical axis. We can think of the luminosity as brightness in
multiples of the Sun. A luminosity of 100 on the axis would mean 100 times as bright as the Sun.