What kind of stars are found here?

A star in this region of the Hertzsprung-Russell diagram has a temperature of about 15,000 Kelvin (15,000 K), a luminosity 600 times greater than that of the Sun (600 × L), and a radius three times larger than the Sun (R = 3 × R). This star lies along the Main Sequence, where most stars (including the Sun) are found. The fairly high temperature indicates quite yellow (moderate) colours. You may have observed the star Vega in the constellation Lyra through a telescope – this is the region in which it is located.

Try to read the values of L, T, and R for yourself from the diagram. Do you estimate values for the luminosity, temperature, and size of the star similar to those listed above?

The basic properties of the stars which lie along the Main Sequence differ from each other in predictable ways. They are all clustered tightly around a central line which runs down the middle of the Hertzsprung-Russell diagram. Because of this, for any one luminosity the range of possible radii and temperatures for Main Sequence stars is extremely small. Similarly, for any one temperature the range of possible radii and luminosities for Main Sequence stars is also fairly small. (Only at temperatures below 4,000 K does the distribution of stars along the Main Sequence begin to broaden, with many star of different luminosities and radii being found with the same temperature).

The key to understanding this property is to realize that there are families (groups) of straight lines running along and across the Hertzsprung-Russell diagram, which define all the locations in the diagram where stars have a shared property.

How can we use this narrow distribution to estimate the radii of a Main Sequence star from the temperature T alone?

Consider the case of a Main Sequence star, with a temperature T of 15,400 K. We will use the Hertzsprung-Russell diagram to estimate its radius R. Following the vertical green line draw up from 15,400 K on the x-axis, we see that quite a few yellow-coloured Main Sequence stars lie along it (we include all of the yellow-coloured stars which lie just above and just below the green cross). These are all the Main Sequence stars which have a temperature of 15,400 K. These stars are all related to one another in physical size. As stated above, diagonal lines drawn on the diagram represent the locations of stars all of the same size. Main Sequence stars with a temperature of 15,400 K thus extend over only a small range in radius.

Examine the Hertzsprung-Russell yourself, to estimate which two diagonal lines would best bound the location of these Main Sequence stars. These stars all lie above the line showing the location of all stars with the same radius as the Sun, and they lie below the the line showing the location of all stars with a radius ten times larger than that of the Sun. It is easy to see that all Main Sequence stars with a temperature of 15,400 K have a radius between one to ten times larger than the Sun – but we can do better!

To keep the diagram easy to read, we have drawn lines of constant radius at the radius of the Sun, at ten times the radius of the Sun, at one hundred times the radius of the Sun, and such, marking only powers of ten. However, we could as easily draw a line of constant radius at twice the radius of the Sun, or two-hundred times the radius of the Sun, or at any other size scale.

With your mind, fill in the region between the diagonal line at one solar radius and the diagonal line at ten solar radii with additional lines, all parallel to the two found on the diagram. These lines will mark the location of stars which are twice as large as the Sun, three times as large as the Sun, four times as large as the Sun, and such, up to nine times as large as the Sun. You can use your powers of observation to see that the Main Sequence stars with a temperature of 15,400 K will all lie above the line where stars are twice as large as the Sun, and below the line where stars are six times as large as the Sun.

We have accomplished a great deal with a small amount of information! Using only the temperature of a star and the fact that it lies on the Main Sequence, we have limited its radius to between two and six times that of the Sun.