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A Hertzsprung-Russell diagram is referred to as a H-R diagram and is a graph of stars' luminosity (or absolute magnitude) on the vertical axis and temperature (or spectral classification) on the horizontal axis.
A H-R diagram is shown below.
Two vertical scales are shown in the graph. On the left is absolute magnitude and on the right is luminosity (in units of Sun's luminosity). Both absolute magnitude and luminosity are measures of instrinsic brightness but use different units.
There are two horizontal axes shown in the graph. At the top is temperature in kelvins and at the bottom is spectral type. Though spectral type does not have units, the classificatio is based on temperature. Thus, it's like a measurement of temperature.
We will learn later that stars along the main sequence are stars that are "burning" their hydrogen fuel supply to create helium and heavier elements through a process called fusion.
The stars to the right-top part of the graph are called red giants or red supergiants. These stars have spent most of their hydrogen fuel supply and have expanded to become very large and have cooled to lower temperatures than when they were on the main sequence.
Stars to the left-bottom part of the graph are called white dwarfs. These are high temperature stars that give off very little energy because of their small size. They have shed most of atmosphere and are thus very tiny in diameter. Eventually, they will burn out and cool down and become brown dwarfs with low luminosity and low temperature.
Remember that luminosity is the energy radiated from a star per second. There are two variables that affect the luminosity of a star--(1) temperature and (2) radius. That is, a star can have a high luminosity because it is hot, big, or both big and hot.
Let's compare two stars of the same luminosity, but different temperatures. On the graph below, I've circled two stars and labeled them Star A and Star B. I've also drawn a horizontal line between them to show that they have the same luminosity (which is plotted on the vertical axis).
Which star has a larger radius? Star A | Star B
Though cool, the red giants are very, very large, as you can figure out by their large luminosity. In fact, some of them are referred to as supergiants. A well-known example is Betelgeuse.
(picture and description is from Astronomy Picture of the Day)
A. Dupree (CfA),
R. Gilliland (STScI),
Here is the first direct picture of the surface of a star other than our Sun. Taken by the Hubble Space Telescope in 1995, the atmosphere of Betelgeuse reveals some unexpected features, including a large bright hotspot visible below the center.
Betelgeuse is a red supergiant star about 600 light years distant, easily recognizable from its brightness and reddish color in the constellation of Orion. While Betelgeuse is cooler than Sun, it is more massive and over 1000 times larger.
If placed at the center of our Solar System, it would extend past the orbit of Jupiter. Betelgeuse is nearing the end of its life and will become a supernova in a perhaps a few tens of millions of years.
To get an idea of how large Betelgeuse is, check out this picture that shows the size of Betelgeuse compared to Earth's orbit and Jupiter's orbit.
(from the Hubble Site)
Let's compare two stars of the same temperature, but different luminosities. On the graph below, I've circled two stars and labeled them Star A and Star C. I've also drawn a vertical line between them to show that they have the same temperature (which is plotted on the horizontal axis).
Which star has a larger radius? Star A | Star C
Because Star C is so hot (bluish-white in color) yet so much smaller than Star A, it is called a white dwarf. Star A is in a region of the H-R diagram where stars are usually called blue giants, because they are bluish-white in color and are large (though not as large as red giants). The image below shows white dwarfs in the globular cluster M4.
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