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 their luminosity, 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. 

1

www.ck12.org 

Most of the stars occupy a region in the diagram along a line called the Main Sequence. During that stage, stars are fusing hydrogen into helium in their cores. The position of the Sun in the main sequence is shown in the diagram. 

You should note that the axial scales for this diagram are not linear. The vertical scale is logarithmic, each line is 100 times greater than the previous line. On the horizontal axis, as we move to the right, the temperature reduces by between 1,000 and 10,000 degrees K between each line. 

If all other factors were the same, the highest temperature stars would also be the most luminous (the brightest). In the main sequence of stars, we see that as the temperature increases to the left, the luminosity also increases, demonstrating that the hottest stars in this grouping are also the brightest. There are stars, however, that are less bright than their temperature would predict. This group of stars is called white dwarfs. These stars are less bright than expected because of their very small size. These dwarf stars are only one one-thousandth the size of stars in the main sequence. There are also stars that are much brighter than their temperature would predict. This group of stars are called red giants. They are brighter than their temperature would predict because they are much larger than stars in the main sequence. These stars have expanded to several thousand times the size of stars in the main sequence. Stars that are reddish in color are cooler than other stars while stars that are bluish in color are hotter than other stars. 

A white dwarf is a stellar remnant that is very dense. A white dwarf’s mass is comparable to the Sun and its volume is comparable to that of Earth. The very low brightness of a white dwarf comes from the emission of stored heat energy. 

White dwarfs are thought to be the final evolutionary state of any star whose mass is not great enough to become a neutron star. Approximately 97% of the stars in our galaxy will become neutron stars. After the hydrogen–fusing lifetime of a main-sequence star of low or medium mass ends, it will expand to a red giant which fuses helium to carbon and oxygen in its core. If a red giant has insufficient mass to generate the core temperatures required to fuse carbon, around 1 billion K, an inert mass of carbon and oxygen will build up at its center. After blowing off its outer layers to form a planetary nebula, the core will be left behind to form the remnant white dwarf. White dwarfs are composed of carbon and oxygen. 

A white dwarf is very hot when it is formed, but since it has no source of energy (no further fusion reactions), it will gradually radiate away its energy and cool down. Over a very long time, a white dwarf will cool to temperatures at which it will no longer emit significant light, and it will become a cold black dwarf. 

A red giant star is a star with a mass like the Sun that is in the last phase of its life, when Hydrogen fusion reactions 2

in the core decrease due to the lack of fuel. With the gravitational collapse of the core, the fusion reactions now occur in a shell surrounding the core. The outer layer of the star expands enormously up to 1000 times the size of the Sun. When the Sun becomes a red giant, its volume will include the orbit of Mercury and Venus and maybe even Earth. The increased size increases the luminosity even though the outer layer cools to only 3000 K or so. The cooler outer layer causes it to be a red star. After a few more million years, the star evolves into a white dwarf-planetary nebula system. 

Summary 

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 their luminosity, absolute magnitude, color, and effective temperature. 

Practice Follow up questions: 

1. The scales on the HR Diagram are __________________. 

2. The spectral class is related to __________________. 

3. What color are the stars on the left side of the HR diagram? Blue - white

Review 

Questions 

1. Which of the following is a reasonable estimate for the surface temperature of the Sun as taken from the HR diagram? 

(a) 10,000 K 

(b) 8,000 K 

(c) 6,000 K 

(d) 4,000 K 

(e) 2,000 K 

2. Would the surface temperature of white dwarfs be higher or lower than that of red giants? 

(a) higher 

(b) lower 

(c) could be higher or lower 

3. What is the color of the stars with the highest surface temperature? 

(a) blue 

(b) yellow 

(c) red 

4. What is the color of the stars with the lowest surface temperature? 

(a) blue 

(b) yellow 

(c) red 

5. Most of the stars on the HR Diagram are classified as what type of stars? 

(a) white dwarf 

(b) main sequence 

(c) red giant 

(d) supergiant 

6. How is it possible for the Sun to be more luminous than a white dwarf if the Sun has a lower temperature than the white dwarf? 

luminosity – the actual brightness of an object, such as a star.

absolute magnitude – a measure of how bright a star would be if it were seen from a standard distance. 

apparent magnitude – the brightness of a star as seen from Earth