Size and the H-R Diagram


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Text only 1998 - 2001
Paul J. Marquard.
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When astronomers discuss the size of the star, two parameters are possible. These are the radius of the star and the mass of the star. This is not so different from discussing the size of a person. A person designated small may either be short, or light in weight. A large star may be very massive or very large in radius. These two parameters are very different, and care needs to be taken when discussing them so there is no confusion as to which is being discussed. In most discussions, size refers to mass.

Recall that the brightness of the star depends upon both temperature and radius. A crude analogy would be the light given off by a candle. A single candle has the same temperature as a hundred candles, however the light given off by 100 candles is much brighter than the light given off by one candle. In a similar fashion, the light given off by a large diameter star with a temperature of 4000 Kelvin is much brighter than the light given off by a small radius star of the same temperature. We shall find that in the evolution of a star its radius can change dramatically, and therefore the luminosity of the star will also change. Stars with a large diameter are generally referred to as giants, while stars with a small diameter are referred to as dwarfs.

The mass of the star is the more important parameter. The mass of a star will generally not change during the stars lifetime. However the mass of the star is a key factor in determining the evolution of that star. So how do we determine the mass of a star? Consider the way we determine our own mass. If we wish to know our mass we stand on a scale. This scale effectively measures the interaction between ourselves and the Earth. This gravitational interaction is what we commonly refer to as weight. In a similar fashion there must be a gravitational interaction in order to determine the mass of a star. Therefore, the mass of the star can only be determined if the star is in a binary system, that is a system of two stars with a mutual gravitational attraction.

Fortunately for astronomers nearly half the stars in the sky are members of binary pairs. By carefully observing the stars in binary systems and applying Kepler's laws, astronomers can determine the mass of each member in the binary system. Although many of the stars in the sky are binary systems not all of these systems can be visually resolved as a binary pair.

In some binary pairs both stars can be individually seen. These types of binaries are referred to as visual binaries. In some cases it cannot be visually determined that two stars exist, however other characteristics may show that there are indeed two or more stars present. In one example, the spectral lines of one of the stars will undergo alternating red shifting and blue shifting. This indicates that the star is moving towards the Earth and then away from the Earth as it orbits a second body. This system is referred to as a spectroscopic binary.

In some cases the two stars will eclipse each other causing a regular variation in the brightness of the system. This variation can be measured from Earth and this system is referred to as an eclipsing binary. In other cases the spectra of the star will show the spectra of two different classifications or the position of the star will wobble in space. These are referred to as spectrum binaries or astrometric binaries respectively.

Once the parameters of stars are known, astronomers want to find relationships between the parameters. This is often done graphically. One such graph is referred to as the HR diagram. The HR diagram graphs temperature (or color index or spectra classification) along the horizontal axis with higher temperatures on the left and lower temperatures on the right. Luminosity (or magnitude) is graphed along the vertical axis with brighter stars towards the top and dimmer stars at the bottom. A diagram of some nearby stars are shown below.

When a graph of this nature is created the majority of the stars are found along a strip passing diagonally from the upper left to lower right. This strip is referred to as the main sequence. In general about 90 percent of all stars fall along the main sequence. These main sequence stars all have one thing in common, they are all burning hydrogen. Stars not on the main sequence may fall in one of two categories. The stars above and to the right of the main sequence are red giant stars. They are characterized by lower temperatures but higher luminosities. In order for a star to have a high luminosity and a low temperature it must have a large radius, hence the term giant. The exact opposite holds true for stars in the lower left. These stars are white dwarfs, characterized by low luminosity and high temperature.

We shall see in future sections that a star's position on the HR diagram will change as the star evolves through its life. As we discuss stellar evolution we shall continue to reference the HR diagram and the star's position on that diagram. The H-R diagram below is of globular cluster M55. It shows the H-R position of thousands of stars in various stages of evolution, but all about the same age.

M55: Color Magnitude Diagram
.J. Mochejska, J. Kaluzny (CAMK), 1m Swope Telescope


Stellar Properties

Brightness and Temperature


This page was last updated on 06/13/01.